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5' cap - The 5' end of a pre-mRNA molecule modified by the addition of a cap of guanine nucleotide.
A band - The broad region that corresponds to the length of the thick filaments of myofibrils.
A site - One of a ribosome’s three binding sites for tRNA during translation. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. (A stands for aminoacyl tRNA.)
ABC model - A model of flower formation identifying three classes of organ identity genes that direct formation of the four types of floral organs.
abdominal cavity- The body cavity in mammals that primarily houses parts of the digestive, excretory, and reproductive systems. It is separated from the thoracic cavity by the diaphragm.
abiotic- Nonliving.
ABO blood groups- Genetically determined classes of human blood that are based on the presence or absence of carbohydrates A and B on the surface of red blood cells. The ABa blood group phenotypes, also called blood types, are A, B, AB, and O.
abortion- The termination of a pregnancy in progress.
abscisic acid (ABA) - A plant hormone that slows down growth, often antagonizing actions of growth hormones. Two of its many effects are to promote seed dormancy and facilitate drought tolerance.
absorption- The uptake of small nutrient molecules by an organism's own body; the third main stage of food processing, following digestion.
absorption spectrum- The range of a pigment's ability to absorb various wavelengths of light.
abyssal- Referring to the very deep benthic zone of the ocean.
acanthodian- Any of a group of ancient jawed fishes from the Devonian period.
acclimatization- Physiological adjustment to a change in an environmental factor.
accommodation- The automatic adjustment of an eye to focus on near objects.
acetyl CoA- Acetyl coenzyme A; the entry compound for the citric acid cycle in cellular respiration, formed from a fragment of pyruvate attached to a coenzyme.
acetylcholine - One of the most common neurotransmitters; functions by binding to receptors and altering the permeability of the postsynaptic membrane to specific ions, either depolarizing or hyperpolarizing the membrane.
acid- A substance that increases the hydrogen ion concentration of a solution.
acid chyme - A mixture of recently swallowed food and gastric juice.
acid precipitation- Rain, snow, or fog that is more acidic than pH 5.6.
acoelomate- A solid-bodied animal lacking a cavity between the gut and outer body wall.
acquired immunity - The kind of defense that is mediated by B lymphocytes (B cells) and T lymphocytes (T cells). It exhibits specificity, memory, and self-nonself recognition. Also called adaptive immunity.
acrosomal reaction- The discharge of a sperm's acrosome when the sperm approaches an egg.
acrosome - A vesicle at the tip of a sperm cell that helps the sperm penetrate the egg.
actin - A globular protein that links into chains, two of which twist helically about each other, forming micro filaments in muscle and other contractile elements in cells.
action potential- A rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered, selective opening and closing of voltage-sensitive gates in sodium and potassium ion channels.
action spectrum- A graph that depicts the relative effectiveness of different wavelengths of radiation in driving a particular process.
activation energy- See free energy of activation.
activator- A protein that binds to DNA and stimulates transcription of a specific gene.
active immunity- Long-lasting immunity conferred by the action of a person's B cells and T cells and the resulting Band T memory cells specific for a pathogen. Active immunity can develop as a result of natural infection or immunization.
active site - The specific portion of an enzyme that attaches to the substrate by means of weak chemical bonds.
active transport- The movement of a substance across a biological membrane against its concentration or electrochemical gradient with the help of energy input and specific transport proteins.
actual evapotranspiration- The amount of water annually transpired by plants and evaporated from a landscape, usually measured in millimeters.
adaptive radiation- The emergence of numerous species from a common ancestor introduced into an environment that presents a diversity of new opportunities and problems.
adenohypophysis - See anterior pituitary.
adenylyl cyclase - An enzyme that converts ATP to cyclic AMP in response to a chemical signal.
adhesion- The attraction between different kinds of molecules.
adipose tissue- A connective tissue that insulates the body and serves as a fuel reserve; contains fat -storing cells called adipose cells.
adrenal gland - One of two endocrine glands located adjacent to the kidneys in mammals. Endocrine cells in the outer portion (cortex) respond to ACTH by secreting steroid hormones that help maintain homeostasis during longterm stress. Neurosecretory cells in the central portion (medulla) secrete epinephrine and norepinephrine in response to nervous inputs triggered by short-term stress.
adrenocorticotropic hormone (ACTH) - A tropic hormone produced and secreted by the anterior pituitary that stimulates the production and secretion of steroid hormones by the adrenal cortex.
adventitious- A term describing any plant organ that grows in an atypical location, such as roots growing from stems.
aerobic - Containing oxygen; referring to an organism, environment, or cellular process that requires oxygen.
afferent arteriole - The blood vessel supplying a nephron.
age structure- The relative number of individuals of each age in a population.
aggregate fruit- A fruit derived from a single flower that has more than one carpel.
agonistic behavior - A type of behavior involving a contest of some kind that determines which competitor gains access to some resource, such as food or mates.
AIDS (acquired immunodeficiency syndrome) - The name of the late stages of HIV infection, defined by a specified reduction of T cells and the appearance of characteristic secondary infections.
alcohol fermentation- The conversion of pyruvate to carbon dioxide and ethyl alcohol.
aldosterone - An adrenal hormone that acts on the distal tubules of the kidney to stimulate the reabsorption of sodium (Na +) and the passive flow of water from the filtrate.
alga- A photosynthetic, plant-like protist.
alimentary canal - A digestive tract consisting of a tube running between a mouth and an anus.
allantois - One of four extraembryonic membranes; serves as a repository for the embryo's nitrogenous waste.
alleles - Alternative versions of a gene that produce distinguishable ~ phenotypic effects.
allometric growth - The variation in the relative rates of growth of various parts of the body, which helps shape the organism.
allopatric speciation - A mode of speciation induced when an ancestral population becomes segregated by a geographic barrier or is itself divided into two or more geographically isolated subpopulations.
allopolyploid - A common type of polyploid species resulting from two different species interbreeding and combining their chromosomes.
allosteric regulation- The binding of a molecule to a protein that affects the function of the protein at a different site.
alpha (a) helix - A spiral shape constituting one form of the secondary structure of proteins, arising from a specific hydrogen-bonding structure.
alternation of generations- A life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte; characteristic of plants and some algae.
alternative RNA splicing- A type of regulation at the RNA-processing level in which different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns.
altruism - Behavior that reduces an individual's fitness while increasing the fitness of another individual.
alveolus - One of the dead-end, multilobed air sacs that constitute the gas exchange surface of the lungs.
Alzheimer's disease- An age-related dementia (mental deterioration) characterized by confusion, memory loss, and other symptoms.
amacrine cell - A neuron of the retina that helps integrate information before it is sent to the brain.
amino acid - An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the monomers of proteins.
amino group- A functional group that consists of a nitrogen atom bonded to two hydrogen atoms; can act as a base in solution, accepting a hydrogen ion and acquiring a charge of I + .
aminoacyl-tRNA synthetase- An enzyme that joins each amino acid to the correct tRNA
ammonia- A small, very toxic molecule made up of three hydrogen atoms and one nitrogen atom; produced by nitrogen fixation and as a metabolic waste product of protein and nucleic acid metabolism.
ammonite- A shelled cephalopod that was the dominant invertebrate predator for millions of years until the end of the Cretaceous period.
amniocentesis - A technique of prenatal diagnosis in which amniotic fluid, obtained by aspiration from a needle inserted into the uterus, is analyzed to detect certain genetic and congenital defects in the fetus.
amnion - The innermost of four extraembryonic membranes; encloses a fluid filled sac in which the embryo is suspended.
amniote- Member of a clade of tetrapods that have an amniotic egg containing specialized membranes that protect the embryo, including mammals and birds and other reptiles.
amoeba - A protist grade characterized by the presence of pseudopodia.
amoebocyte - An amoeba-like cell that moves by pseudopodia, found in most animals; depending on the species, may digest and distribute food, dispose of wastes, form skeletal fibers, fight infections, and change into other cell types.
amphibian- Member of the tetrapod class Amphibia, including salamanders, frogs, and caecilians.
amphipathic molecule - A mol-ecule that has both a hydrophilic region and a hydrophobic region.
amplification- The strengthening of stimulus energy that is otherwise too weak to be carried into the nervous system.
anabolic pathway - A metabolic pathway that synthesizes a complex molecule from simpler compounds.
anaerobic - Lacking oxygen; referring to an organism, environment, or cellular process that lacks oxygen and may be poisoned by it.
anaerobic respiration- The use of inorganic molecules other than oxygen to accept electrons at the "downhill" end of electron transport chains.
analogy- Similarity between two species that is due to convergent evolution rather than to descent from a common ancestor with the same trait.
anaphase- The fourth stage of mitosis, in which the chromatids of each chromosome have separated and the daughter chromosomes are moving to the poles of the cell.
anaphylactic shock - An acute, whole-body, life-threatening, allergic response.
anatomy- The study of the structure of an organism
anchorage dependence- The requirement to divide, a cell must be attached to the substratum.
androgen - Any steroid hormone, such as testosterone, that stimulates the development and maintenance of the male reproductive system and secondary sex characteristics.
aneuploidy - A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number.
angiosperm - A flowering plant, which forms seeds inside a protective chamber called an ovary.
angiotensin
- A hormone that stimulates constriction of precapillary arterioles and increases reabsorption of NaCl and water by the proximal tubules of the kidney, increasing blood pressure and volume.
anhydrobiosis - The ability to survive in a dormant state when an organism's habitat dries up.
animal pole- The portion of the egg where the least yolk is concentrated; opposite of vegetal pole.
Animalia - The kingdom that consists of multicellular eukaryotes that ingest their food.
anion - A negatively charged ion.
annual- A flowering plant that completes its en tire life cycle in a single year or growing season.
anterior- Referring to the head end of a bilaterally symmetrical animal.
anterior pituitary- Also called the adenohypophysis; portion of the pituitary that develops from nonneural tissue; consists of endocrine cells that synthesize and secrete several tropic and nontropic hormones.
anther - In an angiosperm, the tern nal pollen sac of a stamen, where pollen grains with male gametes form.
antheridium - In plants, the male garnet' a moist chamber in which gametes
anthropoid - A member of a primate group made up of the apes. orangutan, gorilla, chimpanzee, and bonobo), monkeys, and humans.
antibody- A protein secreted by plasma cells (differentiated B cells) that binds to a particular antigen and marks it for elimination; also called immunoglobulin. All antibody molecules have the same Y-shaped structu and in their monomer form consist of two identical heavy chains and two identical light chains joined by disulfide bridges.
anticodon - A specialized base triplet at one end of a tRNA molecule that recognizes a particular complementary codon on an mRNA molecule.
antidiuretic hormone (ADH) - A hormone duced in the hypothalamus and water retention by the kidneys as part of an elaborate feedback-scheme that helps regulate the osmolarity of the blood.
antigen - A macromolecule that elicits an immune response by lymphocytes.
antigen presentation- The process by which an MHC molecule binds to a fragment of an intracellular protein antigen and carries it to the cell surface, where it is displayed and can be recognized by a T cell.
antigen receptor- The general term for a surface protein, located on B cells and T cells, that binds to antigens, initiating acquired immune r~sponses. The antigen receptors on B cells are'called B cell receptors (or membrane immunoglobulins), and the antigen receptors on T cells are called T cell receptors.
antigen-presenting cell- A cell that ingests bacteria and viruses and destroys them, generating peptide fragments that are bound by class II MHC molecules and subsequently displayed on the cell surface to helper T cells. Macrophages, dendritic cells, and B cells are the primary antigen-presenting cells.
antiparallel- The opposite arrangement of the sugar-phosphate backbones in a DNA double helix.
aphotic zone - The part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.
apical dominance - Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth.
apical ectodermal ridge- A limb-bud organizer region consisting of a thickened area of ectoderm at the tip of a limb bud.
apical meristem - Embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length.
apicomplexan - A parasitic protozoan. Some apicomplexans cause human diseases.
apomixis - The asexual production of seeds.
apoplast- In plants, the continuum of cell walls plus the extracellular spaces.
apoptosis- The changes that occur within a cell as it undergoes programmed cell death, which is brought about by signals that trigger the activation of a cascade of suicide proteins in the cell destined to die.
aposematic coloration - The bright coloration of animals with effective physical or chemical defenses that acts as a warning to predators.
appendix- A small, fingerlike extension of the vertebrate cecum; contains a mass of white blood cells that contribute to immunity
aquaporin- A transport protein in the plasma membrane of a plant or animal cell that specifically facilitates the dillusion of water across the membrane (osmosis).
aqueous humor - Plasma-like liquid in the space between the lens and the cornea in the vertebrate eye; helps maintain the shape of the eye, supplies nutrients and oxygen to its tissues, and disposes of its wastes.
aqueous solution - A solution in which water is the solvent.
arbuscular mycorrhiza- A distinct type of endomycorrhiza formed by glomeromycete fungi, in which the tips of the fungal hyphae that invade the plant roots branch into tiny treelike structures called arbuscules.
Archaea - One of two prokaryotic domains, the other being Bacteria.
archegonium - In plants, the female gametangium, a moist chamber in which gametes develop.
archenteron - The endodermlined cavity, formed during the gastrulation process, that develops into the digestive tract of an animal.
archosaur- Member of the reptilian group that includes crocodiles, alligators, dinosaurs, and birds.
arteriole - A vessel that conveys blood between an artery and a capillary bed.
artery- A vessel that carries blood away from the heart to organs throughout the body
arthropod- A segmented coelomate with a chitinous exoskeleton, jointed appendages, and a body fonned of distinct groups of segments.
artificial selection- The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits.
ascocarp- The fruiting body of a sac fungus (ascomycete) .
ascus- A saclike spore capsule located at the tip of a dikaryotic hypha of a sac fungus.
asexual reproduction- A type of reproduction involving only one parent that produces genetically identical offspring by budding or by the division of a single cell or the entire organism into two or more parts.
aspartate- An amino acid that functions as a CNS neurotransmitter.
assisted reproductive technology (ART) - Fertilization procedures that generally involve the surgical removal of eggs (secondary oocytes) from a woman's ovaries after hormonal stimulation, fertilizing the eggs, and returning them to the woman's body
associative learning- The acquired ability to associate one stimulus with another; also called classical conditioning.
aster- A radial array of short micro tubules that extends from each centrosome toward the plasma membrane in a cell undergoing mitosis.
astrocyte- A glial cell that provides structural and metabolic support for neurons.
atherosclerosis- A cardiovascular disease in which growths called plaques develop in the inner walls of the arteries, narrowing their inner diameters.
atom- The smallest unit of matter that retains the properties of an element.
atomic mass- The total mass of an atom, which is the mass in grams of one mole of the atom.
atomic nucleus- An atom's central core, containing protons and neutrons.
atomic number- The number of protons in the nucleus of an atom, unique for each element and designated by a subscript to the left of the elemental symbol.
ATP (adenosine triphosphate) - adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed. This energy is used to drive endergonic reactions in cells.
ATP synthase- A cluster of several membrane proteins found in the mitochondrial crista (and bacterial plasma membrane) that function in chemiosmosis with adjacent electron transport chains, using the energy of a hydrogen ion concentration gradient to make ATP. ATP synthases provide a port through which hydrogen ions diffuse into the matrix of a mitrochondrion.
atrial natriuretic factor (ANF) - A peptide hormone that opposes the renin-angiotensin-aldosterone system (RAAS).
atrioventricular (AV) node- A region of specialized muscle tissue between the right atrium and right ventricle where electrical impulses are delayed for about 0.1 second before spreading to the ventricles and causing them to contract.
atrioventricular (A V) valve- A valve in the heart between each atrium and ventricle that prevents a backflow of blood when the ventricles contract.
atrium- A chamber that receives blood returning to the vertebrate heart.
autoimmune disease - An immunological disorder in which the immune system turns against self.
autonomic nervous system - A subdivision of the motor nervous system of vertebrates that regulates the internal environment; consists of the sympathetic, parasympathetic, and enteric divisions.
autopolyploid - An individual that has more than two chromosome sets, all derived from a single species.
autosome - A chromosome that is not directly involved in determining sex, as opposed to a sex chromosome.
autotroph - An organism that obtains organic food molecules without eating other organisms or substances derived from other organisms. Autotrophs use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.
auxin - A term that primarily refers to indoleacetic acid (IAA), a natural plant hormone that has a variety of effects, including cell elongation, root formation, secondary growth, and fruit growth.
average heterozygosity- The percent, on average, of a population's loci that are heterozygous in members of the population.
avirulent- A term describing a pathogen that can only mildly harm, but not kill, the host plant.
axillary bud - A structure that has the potential to form a lateral shoot, or branch. The bud appears in the angle formed between a leaf and a stem.
axon - A typically long extension, or process, from a neuron that carries nerve impulses away from the cell body toward target cells.
axon hillock- The conical region of a neuron's axon where it joins the cell body; typically the region where nerve signals are generated.
B cell receptor- The antigen receptor on B cells: a Y-shaped, membrane-bound molecule consisting of two identical heavy chains and two identical light chains linked by disulfide bridges and containing two antigen-binding sites; also called a membrane immunoglobulin or membrane antibody
B lymphocyte (B cell) - A type of lymphocyte that develops to maturity in the bone marrow. After encountering antigen, B cells differentiate into antibody-secreting plasma cells, the effector cells of humoral immunity.
Bacteria- One of two prokaryotic domains, the other being Archaea.
bacterial artificial chromosome (BAC) - An artificial version of a bacterial chromosome that can carry inserts of 100,000 to 500,000 base pairs.
bacteriophage - A virus that infects bacteria; also called a phage.
bacteroids- A form of Rhizobium contained within the vesicles formed by the root cells of a root nodule.
baculum - A bone that is contained in, and helps stiffen, the penis of rodents, raccoons, walruses, whales, and several other mammals.
balanced polymorphism- The ability of natural selection to maintain diversity in a population.
balancing selection- Natural selection that maintains stable frequencies of two or more phenotypic forms in a population (balanced polymorphism) .
bark- All tissues external to the vascular cambium, consisting mainly of the secondary phloem and layers of periderm
Barr body- A dense object lying along the inside of the nuclear envelope in female mammalian cells, representing an inactivated X chromosome.
barrier method- Contraception that relies on a physical barrier to block the passage of sperm. Examples include condoms and diaphragms.
Bartholin's glands - Glands near the vaginal opening in a human female that secrete lubricating fluid during sexual arousal.
basal angiosperms- The most primitive lineages of flowering plants, including Amborella, water lilies, and star anise and relatives.
basal body - A eukaryotic cell organelle consisting of a 9 + 0 arrangement of microtubule triplets; may organize the microtubule assembly of a cilium or flagellum; structurally identical to a centriole.
basal metabolic rate (BMR) - The metabolic rate of a resting, fasting, and nonstressed endotherm.
basal nuclei- A cluster of nuclei deep within the white matter of the cerebrum.
base- A substance that reduces the hydrogen ion concentration of a solution.
basement membrane- The floor of an epithelial membrane on which the basal cells rest.
base-pair substitution- A type of point mutation; the replacement of one nucleotide and its partner in the complementary DNA strand by another pair of nucleotides.
basidiocarp- Elaborate fruiting body of a dikaryotic mycelium of a club fungus.
basidium - A reproductive appendage that produces sexual spores on the gills of mushrooms (club fungi).
Batesian mimicry - A type of mimicry in which a harmless species looks like a species that is poisonous or otherwise harmful to predators.
behavior- Everything an animal does and how it does it, including muscular activities such as chasing prey, certain nonmuscular processes such as secreting a hormone that attracts a mate, and learning.
behavioral ecology- The scientific study of animal behavior, including how it is controlled and how it develops, evolves, and contributes to survival and reproductive success.
benign tumor- A mass of abnormal cells that remains at the site of origin.
benthic zone - The bottom surface of an aquatic environment.
benthos - The communities of organisms living in the benthic zone of an aquatic biome.
beta oxidation- A metabolic sequence that breaks fatty acids down to two-carbon fragments that enter the citric acid cycle as acetyl CoA.
beta pleated sheet- One form of the secondary structure of proteins in which the regions of the chain lie parallel to each other and are held together by hydrogen bonds.
biennial - A flowering plant that requires two years to complete its life cycle.
big-bang reproduction- A life history in which adults have but a single reproductive opportunity to produce large numbers of offspring, such as the life history of the Pacific salmon; also known as semelparity.
bilateral symmetry- Characterizing a body form with a central longitudinal plane that divides the body into two equal but opposite halves.
bilaterian - Member of the clade Bilateria, animals with bilateral symmetry.
bile- A mixture of substances that is produced in the liver, stored in the gallbladder, and acts as a detergent to aid in the digestion and absorption of fats.
binary fission- The type of cell division by which prokaryotes reproduce. Each dividing daughter cell receives a copy of the single parental chromosome.
binomial- The two-part latinized name of a species, consisting of genus and specific epithet.
biodiversity hot spot- A relatively small area with an exceptional concentration of endemic species and a large number of endangered and threatened species.
bioenergetics- The flow of energy through an animal, taking into account the energy stored in the food it consumes, the energy used for basic functions, activity, growth, reproduction, and regulation, and the energy lost to the environment as heat or in waste.
biofilm- A surface-coating colony of prokaryotes that engage in metabolic cooperation.
biogenic amine- A neurotransmitter derived from an amino acid.
biogeochemical cycle- Any of the various nutrient circuits, which involve both biotic and abiotic components of ecosystems.
biogeography- The study of the past and present distribution of species.
bioinformatics- Using computing power, software, and mathematical models to process and integrate biological information from large data sets.
biological augmentation- An approach to restoration ecology that uses organisms to add essential materials to a degraded ecosystem.
biological clock- An internal timekeeper that controls an organism's biological rhythms. The biological clock marks time with or without environmental cues but often requires signals from the environment to remain tuned to an appropriate period.
biological magnification - A trophic process in which retained substances become more concentrated with each link in the food
biological species concept- Definition of a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but are not able to produce viable, fertile offspring with members of other populations.
biology- The scientific study of life.
biomanipulation- A technique for restoring eutrophic lakes that reduces populations of algae by manipulating the higher-level consumers in the community rather than by changing nutrient levels or adding chemical treatments.
biomass - The dry weight of organic matter com: prising a group of organisms in a particular habitat.
biome - Any of the world's major ecosystems, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment.
bioremediation- The use of living organisms to detoxify and restore polluted and degraded ecosystems.
biosphere - The entire portion of Earth inhabited by life; the sum of all the planet's ecosystems.
biota- All the organisms that are part of an ecosystem.
biotechnology- The manipulation of living organisms or their components to produce useful products.
biotic - Pertaining to the living organisms in the environment.
bipolar cell- A neuron that synapses with the axon of a rod or cone in the retina of the eye.
bipolar disorder- Depressive mental illness characterized by swings of mood from high to low; also called manic-depressive disorder.
birth control pills- Chemical contraceptives that inhibit ovulation, retard follicular development, or alter a woman's cervical mucus to prevent sperm from entering the uterus.
blade- (1) A leaflike structure of a seaweed that provides most of the surface area for photosynthesis. (2) The flattened portion of a typicalleaf.
blastocoel - The fluid-filled cavity that fonns in the center of the blastula embryo.
blastocyst- An embryonic stage in mammals; a hollow ball of cells produced one week after fertilization in humans.
blastoderm- An embryonic cap of dividing cells resting on a large undivided yolk.
blastomere- A small cell of an early embryo.
blastopore - The opening of the archenteron in the gastrula that develops into the mouth in protostomes and the anus in deuterostomes.
blastula - The hollow ball of cells marking the end stage of cleavage during early embryonic development.
blood- A type of connective tissue with a fluid matrix called plasma in which blood cells are suspended.
blood pressure- The hydrostatic force that blood exerts against the wall of a vessel.
blood vessels- A set of tubes through which the blood moves through the body
blood-brain barrier- A specialized capillary arrangement in the brain that restricts the passage of most substances into the brain, thereby preventing dramatic fluctuations in the brain's environment.
blue-light photoreceptors- A class of light receptors in plants. Blue light initiates a variety of responses, such as phototropism and slowing of hypocotyl elongation.
body cavity- A fluid-containing space between the digestive tract and the body wall.
body plan- In animals, the set of morphological and developmental traits that define a grade (level of organizational complexity).
Bohr shift- A lowering of the affinity of hemoglobin for oxygen, caused by a drop in pH; facilitates the release of oxygen from hemoglobin in the vicinity of active tissues.
bolus- A lubricated ball of chewed food.
bone- A type of connective tissue, consisting of living cells held in a rigid matrix of collagen fibers embedded in calcium salts.
book lung- An organ of gas exchange in spiders, consisting of stacked plates contained in an internal chamber.
bottleneck effect- Genetic drift resulting from the reduction of a population, typically by a natural disaster, such that the surviving population is no longer genetically representative of the original population.
bottom-up model- A model of community organization in which mineral nutrients control community organization because nutrients control plant numbers, which in turn control herbivore numbers, which in turn control predator numbers.
Bowman's capsule- A cup-shaped receptacle in the vertebrate kidney that is the initial, expanded segment of the nephron where filtrate enters from the blood.
brachiopod- A marine lophophorate with a shell divided into dorsal and ventral halves. Brachiopods are also called lamp shells.
brain hormone- A hormone, produced by neurosecretory cells in the insect brain, that promotes development by stimulating the pro thoracic glands to secrete ecdysone.
brainstem- Collection of structures in the adult brain, including the midbrain, the pons, and the medulla oblongata; functions in homeostasis, coordination of movement, and conduction of information to higher brain centers.
brassinosteroids- Steroid honnones in plants that have a variety of effects, including cell elongation, retarding leaf abscission, and promoting xylem differentiation.
breathing- The process involving alternate inhalation and exhalation of air that ventilates the lungs.
breathing control center- A brain center that directs the activity of organs involved in breathing.
bronchiole- One of the fine branches of the bronchus that transport air to alveoli.
bronchus - One of a pair of breathing tubes that branch from the trachea into the lungs.
brown alga - A phaeophyte; a marine, multicellular, autotrophic protist that is the most common type of seaweed. Brown algae include the kelps.
brown fat - A tissue in some mammals, located in the neck and between the shoulders, that is specialized for rapid heat production.
bryophyte - A moss, liverwort, or hornwort; a nonvascular plant that inhabits the land but lacks many of the terrestrial adaptations of vascular plants.
budding- An asexual means of propagation in which outgrowths from the parent form and pinch off to live independently or else remain attached to eventually form extensive colonies.
buffer- A substance that consists of acid and base forms in a solution and that minimizes changes in pH when extraneous acids or bases are added to the solution.
bulbourethral gland - One of a pair of glands near the base of the penis in the human male that secretes fluid that lubricates and neutralizes acids in the urethra during sexual arousal.
bulk feeder- An animal that eats relatively large pieces of food.
bulk flow- The movement of water due to a difference in pressure between two locations.
bundle sheath- A protective covering around a leaf vein, consisting of one or more cell layers, usually parenchyma.
bundle-sheath cell- A type of photosynthetic cell arranged into tightly packed sheaths around the veins of a leaf.
C3 plant- A plant that uses the Calvin cycle for the initial steps that incorporate CO2 into organic material, forming a three-carbon compound as the first stable intermediate.
C4 plant- A plant that prefaces the Calvin cycle with reactions that incorporate CO2 into a four-carbon compound, the end product of which supplies CO2 for the Calvin cycle.
cadherins- An important class of cell-to-cell adhesion molecules.
calcitonin - A hormone secreted by the thyroid gland that lowers blood calcium levels by promoting calcium deposition in bone and calcium excretion from the kidneys.
callus- A mass of dividing, undifferentiated cells at the cut end of a shoot.
calorie (cal) - The amount of heat energy required to raise the temperature of 1 g of water by 1°C; also the amount of heat energy that I g of water releases when it cools by 1°C. The Calorie (with a capital C), usually used to indicate the energy content of food, is a kilocalorie.
Calvin cycle - The second of two major stages in photosynthesis (following the light reactions), involving atmospheric CO2 fixation and reduction of the fixed carbon into carbohydrate.
calyptra - A protective cap of gametophyte tissue that wholly or partially covers an immature capsule in many mosses.
CAM plant - A plant that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions, first discovered in the family Crassulaceae. Carbon dioxide entering open stomata during the night is converted into organic acids, which release CO2 for the Calvin cycle during the day, when stomata are closed.
Cambrian explosion- A burst of evolutionary origins when most of the major body plans of animals appeared in a relatively brief time in geologic history; recorded in the fossil record about 542-525 million years ago.
canopy - The uppermost layer of vegetation in a terrestrial biome.
capillary - A microscopic blood vessel that penetrates the tissues and consists of a single layer of endothelial cells that allows exchange between the blood and interstitial fluid.
capillary bed - A network of capillaries that infiltrate every organ and tissue in the body
capsid - The protein shell that encloses a viral genome. It may be rod-shaped, polyhedral, or more complex in shape.
capsule- (1) A sticky layer that surrounds the cell walls of some prokaryotes, protecting the cell surface and sometimes helping to glue the cell to surfaces. (2) The sporangium of a bryophyte (moss, liverwort, or hornwort).
carbohydrate - A sugar (monosaccharide) or one of its dimers (disaccharides) or polymers (polysaccharides).
carbon fixation- The incorporation of carbon from CO2 into an organic compound by an autotrophic organism (a plant, another photosynthetic organism, or a chemoautotrophic prokaryote).
carbonyl group - A functional group present in aldehydes and ketones and consisting of a carbon atom double-bonded to an oxygen atom.
carboxyl group - A functional group present in organic acids and consisting of a single carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl group.
cardiac cycle - The alternating contractions ands relaxations of the heart
cardiac muscle- A type of muscle that forms the contractile wall of the heart. lts cells are joined by intercalated disks that relay each heartbeat.
cardiac output- The volume of blood pumped per minute by the left ventricle of the heart.
cardiovascular disease - Diseases of the heart and blood vessels.
cardiovascular system- A closed circulatory system with a heart and branching network of arteries, capillaries, and veins. The system is characteristic of vertebrates.
carnivore- An animal, such as a shark, hawk, or spider, that eats other animals.
carotenoid - An accessory pigment, either yellow or orange, in the chloroplasts of plants. By absorbing wavelengths of light that chlorophyll cannot, carotenoids broaden the spectrum of colors that can drive photosynthesis.
carpel - The ovule-producing reproductive organ of a flower, consisting of the stigma, style, and ovary
carrier- In genetics, an individual who is heterozygous at a given genetic locus, with one normal allele and one potentially hannful recessive allele. The heterozygote is phenotypically normal for the character determined by the gene but can pass on the harmful allele to offspring.
carrying capacity- The maximum population size that can be supported by the available resources, symbolized as K.
cartilage - A type of flexible connective tissue with an abundance of collagenous fibers embedded in chondroitin sulfate.
Casparian strip - A waterimpermeable ring of wax in the endodermal cells of plants that blocks the passive flow of water and solutes into the stele by way of cell walls.
catabolic pathway - A metabolic pathway that releases energy by breaking down complex molecules to simpler compounds.
catalyst- A chemical agent that changes the rate of a reaction without being consumed by the reaction.
catastrophism- The hypothesis by Georges Cuvier that each boundary between strata corresponded in time to a catastrophe, such as a flood or drought, that had destroyed many of the species living there at that time.
catecholamine- Any of a class of compounds, including the hormones epinephrine and norepinephrine, that are synthesized from the amino acid tyrosine.
cation- An ion with a positive charge, produced by the loss of one or more electrons.
cation exchange- A process in which positively charged minerals are made available to a plant when hydrogen ions in the soil displace mineral ions from the clay particles
CD4- A surface protein, present on most helper T cells, that binds to class II MHC molecules on antigen-presenting cells, enhancing the interaction between the T cell and the antigen-presenting cell.
CD8- A surface protein, present on most cytotoxic cells, that binds to class I MHC molecules on target cells, enhancing the interaction between the T cell and the target cell.
cDNA library- A limited gene library using complementary DNA. The library includes only the genes that were transcribed in the cells examined.
cecum - A blind outpocket of a hollow organ such as an intestine.
cell- Life's fundamental unit of structure and function.
cell adhesion molecules (CAMs) - Glycoproteins that contribute to cell migration and stable tissue structure.
cell body- The part of a neuron that houses the nucleus and other organelles.
cell cycle- An ordered sequence of events in the life of a eukaryotic cell, from its origin in the division of a parent cell until its own division into two; composed of the M, G1, S, and G2 phases.
cell cycle control system- A cyclically operating set of molecules in the cell that triggers and coordinates key events in the cell cycle.
cell differentiation- The structural and functional divergence of cells as they become specialized during a multicellular organism's development; dependent on the control of gene expression.
cell division- The reproduction of cells.
cell fractionation- The disruption of a cell and separation of its organelles by centrifugation.
cell lineage- The ancestry of a cell.
cell-mediated immune response- The branch of acquired immunity that involves the activation of cytotoxic T cells, which defend against infected cells, cancer cells, and transplanted cells.
cell plate- A double membrane across the midline of a dividing plant cell, between which the new cell wall forms during cytokinesis.
cellular respiration- The most prevalent and efficient catabolic pathway for the production of ATp, in which oxygen is consumed as a reactant along with the organic fuel.
cellular slime mold- A type of protist that has unicellular amoeboid cells and aggregated reproductive bodies in its life cycle.
cellulose - A structural polysaccharide of cell walls, consisting of glucose monomers joined by Beta-1, 4-Glycosidic linkages
cell wall - A protective layer external to the plasma membrane in plant cells, prokaryotes, fungi, and some protists. In plant cells, the wall is formed of cellulose fibers embedded in a polysaccharide-protein matrix. The primary cell wall is thin and flexible, whereas the secondary cell wall is stronger and more rigid and is the primary constituent of wood.
Celsius scale - A temperature scale (OC) equal to 5/9 (OF - 32) that measures the freezing point of water at O°C and the boiling point of water at 100°C.
central canal- The narrow cavity in the center of the spinal cord that is continuous with the fluid-filled ventricles of the brain.
central nervous system (CNS) - In vertebrate animals, the brain and spinal cord.
central vacuole- A membranous sac in a mature plant cell with diverse roles in reproduction, growth, and development.
centriole - A structure in an animal cell composed of cylinders of microtubule triplets arranged in a 9 + 0 pattern. An animal cell usually has a pair of centrioles involved in cell division.
centromere - The centralized region joining two sister chromatids.
centrosome - Material present in the cytoplasm of all eukaryotic cells, important during cell division; the microtubuleorganizing center.
cephalization - An evolutionary trend toward the concentration of sensory equipment on the anterior end of the body.
cerebellum - Part of the vertebrate hindbrain located dorsally; functions in unconscious coordination of movement and balance.
cerebral cortex - The surface of the cerebrum; the largest and most complex part of the mammalian brain, containing sensory and motor nerve cell bodies of the cerebrum; the part of the vertebrate brain most changed through evolution.
cerebral hemisphere- The right or left side of the vertebrate brain.
cerebrospinal fluid - Blood-derived fluid that surrounds, protects against infection, nourishes, and cushions the brain and spinal cord.
cerebrum- The dorsal portion of the vertebrate forebrain, composed of right and left hemispheres; the integrating center for memory, learning, emotions, and other highly complex functions of the central nervous system.
cervix - The neck of the uterus, which opens into the vagina.
chaparral - A scrubland biome of dense, spiny evergreen shrubs found at midlatitudes along coasts where cold ocean currents circulate offshore; characterized by mild, rainy winters and long, hot, dry summers.
chaperonin - A protein molecule that assists the proper folding of other proteins.
character- An observable heritable feature.
character displacement - The tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species.
checkpoint- A critical control point in the cell cycle where stop and go-ahead signals can regulate the cycle.
chelicera- One of a pair of clawlike feeding appendages characteristic of cheliceriforms.
cheliceriform- An arthropod that has chelicerae and a body divided into a cephalothorax and an abdomen. Living chelicerifonns include sea spiders, horseshoe crabs, scorpions, ticks, and spiders.
chemical bond- An attraction between two atoms, resulting from a sharing of outer-shell electrons or the presence of opposite charges on the atoms. The bonded atoms gain complete outer electron shells.
chemical energy- Energy stored in the chemical bonds of molecules; a fonn of potential energy.
chemical equilibrium- In a reversible chemical reaction, the point at which the rate of the forward reaction equals the rate of the reverse reaction.
chemical reaction- A process leading to chemical changes in matter; involves the making and/or breaking of chemical bonds.
chemiosmosis - An energycoupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP Most ATP synthesis in cells occurs by chemiosmosis.
chemoautotroph - An organism that needs only carbon dioxide as a carbon source but that obtains energy by oxidizing inorganic substances.
chemoheterotroph - An organism that must obtain both energy and carbon by consuming organic molecules.
chemokine - Any of about 50 different proteins, secreted by many cell types near a site of injury or infection, that help direct migration of white blood cells to an injury site and induces other changes central to inflammation.
chemoreceptor- A receptor that transmits information about the total solute concentration in a solution or about individual kinds of molecules.
chiasma - The X-shaped, microscopically visible region representing homologous chromatids that have exchanged genetic material through crossing over during meiosis.
chimera- An organism with a mixture of genetically different cells.
chitin - A structural polysaccharide of an amino sugar found in many fungi and in the exoskeletons of all arthropods.
chlorophyll - A green pigment located within the chloroplasts of plants. Chlorophyll a can participate directly in the light reactions, which convert solar energy to chemical energy.
chlorophyll a- A type of blue-green photosynthetic pigment that participates directly in the light reactions.
chlorophyll b- A type of yellow-green accessory photosynthetic pigment that transfers energy to chlorophyll a.
chloroplast - An organelle found only in plants and photosynthetic protists that absorbs sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water.
choanocyte - A flagellated feeding cell found in sponges. Also called a collar cell, it has a collar-like ring that traps food particles around the base of its flagellum.
cholesterol - A steroid that forms an essential component of animal cell membranes and acts as a precursor molecule for the synthesis of other biologically important steroids.
chondrichthyan- Member of the class Chondrichthyes, vertebrates with skeletons made mostly of cartilage, such as sharks and rays.
chondrocyte- Cartilage cell that secretes collagen and chondroitin sulfate.
chordate - Member of the phylum Chordata, animals that at some point during their development have a notochord; a dorsal, hollow nerve cord; pharyngeal slits or clefts; and a muscular, post-anal tail.
chorion - The outermost of four extraembryonic membranes; contributes to the fonnation of the mammalian placenta.
chorionic villus sampling (CVS) - A technique of prenatal diagnosis in which a small sample of the fetal portion of the placenta is removed and analyzed to detect certain genetic and congenital defects in the fetus.
choroid- A thin, pigmented inner layer of the vertebrate eye.
chromatin - The complex of DNA and proteins that makes up a eukaryotic chromosome. When the cell is not dividing, chromatin exists as a mass of very long, thin fibers that are not visible with a light microscope.
chromosome - A threadlike, gene-carrying structure found in the nucleus. Each chromosome consists of one very long DNA molecule and associated proteins.
chromosome theory of inheritance- A basic principle in biology stating that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.
chylomicron - One of the small intracellular globules composed of fats that are mixed with cholesterol and coated with special proteins.
chytrid - Member of the fungal phylum Chytridiomycota, mostly aquatic fungi with flagellated zoospores that probably represent the most primitive fungal lineage.
ciliary body- A portion of the vertebrate eye associated with the lens. It produces the clear, watery aqueous humor that fills the anterior cavity of the eye.
ciliate - A type of protozoan that moves by means of cilia.
cilium - A short cellular appendage specialized for locomotion, fonned from a core of nine outer doublet microtubules and two inner single microtubules ensheathed in an extension of plasma membrane.
circadian rhythm - A physiological cycle of about 24 hours that is present in all eukaryotic organisms and that persists even in the absence of external cues.
cis- Arrangement of two noncarbon atoms, each bound to one of the carbons in a carboncarbon double bond, where the two noncarbon atoms are on the same side relative to the double bond.
citric acid cycle- A chemical cycle involving eight steps that completes the metabolic breakdown of glucose molecules to carbon dioxide; occurs within the mitochondrion; the second major stage in cellular respiration.
clade- A group of species that includes an ancestral species and all its descendants.
cladistics - The analysis of how species may be grouped into clades.
cladogram- A diagram depicting patterns of shared characteristics among species.
class- In classification, the taxonomic category above order.
class I MHC molecules- A collection of cell surface proteins encoded by a family of genes called the major histocompatibility complex. Class I MHC molecules are found on nearly all nucleated cells.
class II MHC molecules- A collection of cell surface proteins encoded by a family of genes called the major histocompatibility co~plex. Class II MHC molecules are restricted to a few specialized cell types, commonly called antigen-presenting cells (dendritic cells, macrophages, and B cells).
classical conditioning- A type of associative learning; the association of a normally irrelevant stimulus with a fixed behavioral response.
cleavage - The process of cytokinesis in animal cells, characterized by pinching of the plasma membrane. Also, the succession of rapid cell divisions without growth during early embryonic development that converts the zygote into a ball of cells.
cleavage furrow- The first sign of cleavage in an animal cell; a shallow groove in the cell surface near the old metaphase plate.
climate- The prevailing weather conditions at a locality
climograph - A plot of the temperature and precipitation in a particular region. cline A graded variation in a trait that parallels a gradient in the environment.
clitoris - An organ in the female that engorges with blood and becomes erect during sexual arousal.
cloaca- A common opening for the digestive, urinary, and reproductive tracts found in many nonmammalian vertebrates but in few mammals.
clonal selection- The process by which an antigen selectively binds to and activates only those lymphocytes bearing receptors specific for the antigen. The selected lymphocytes proliferate and differentiate into a clone of effector cells and a clone of memory cells specific for the stimulating antigen. Clonal selection accounts for the specificity and memory of acquired immune responses.
clone- (1) A lineage of genetically identical individuals or cells. (2) In popular usage, a single individual organism that is genetically identical to another individual. (3) As a verb, to make one or more genetic replicas of an individual or cell. See also gene cloning.
cloning- Using a somatic cell from a multicellular organism to make one or more genetically identical individuals.
cloning vector- An agent used to transfer DNA in genetic engineering. A plasmid that moves recombinant DNA from a test tube back into a cell is an example of a cloning vector, as is a virus that transfers recombinant DNA by infection.
closed circulatory system- A circulatory system in which blood is confined to vessels and is kept separate from the interstitial fluid.
club fungus- The common name for members of the phylum Basidiomycota. The name comes from the club-like shape of the basidium.
cnidocyte - A specialized cell for which the phylum Cnidaria is named; contains a capsule containing a fine coiled thread, which, when discharged, functions in defense and prey capture.
cochlea - The complex, coiled organ of hearing that contains the organ of Corti.
codominance- The situation in which the phenotypes of both alleles are exhibited in the heterozygote.
codon - A three-nucleotide sequence of DNA or mRNA that specifies a particular amino acid or termination signal; the basic unit ofthe genetic code.
coefficient of relatedness - The probability that a particular gene present in one individual will also be inherited from a common parent or ancestor in a second individual.
coelom- A body cavity completely lined with mesoderm.
coelomate- Animal that possesses a true coelom (fluid-filled body cavity lined by tissue completely derived from mesoderm).
coenocytic- Referring to a multinucleated condition resulting from the repeated division of nuclei without cytoplasmic division.
coenzyme - An organic molecule serving as a cofactor. Most vitamins function as coenzymes in important metabolic reactions.
coevolution- The mutual evolutionary influence between two different species interacting with each other and reciprocally influencing each other's adaptations.
cofactor- Any nonprotein molecule or ion that is required for the proper functioning of an enzyme. Co factors can be permanently bound to the active site or may bind loosely with the substrate during catalysis.
cognition- The ability of an animal's nervous system to perceive, store, process, and use information obtained by its sensory receptors.
cognitive ethology- The scientific study of cognition; the study of the connection between data processing by nervous systems and animal behavior.
cognitive map- A representation within the nervous system of spatial relations between objects in an animal's environment.
cohesion- The binding together of like molecules, often by hydrogen bonds.
cohort- A group of individuals of the same age, from birth until all are dead.
coitus - The insertion of a penis into a vagina, also called sexual intercourse.
coleoptile - The covering of the young shoot of the embryo of a grass seed.
coleorhiza - The covering of the young root of the embryo of a grass seed.
collagen- A glycoprotein in the extracellular matrix of animal cells that forms strong fibers, found extensively in connective tissue and bone; the most abundant protein in the animal kingdom.
collagenous fiber- A tough fiber of the extraellular matrix. Collagenous fibers are made of collagen, are nonelastic, and do not tear easily when pulled lengthwise.
collecting duct - The location in the kidney where filtrate from renal tubules is collected; the filtrate is now called urine.
collenchyma cell - A flexible plant cell type that occurs in strands or cylinders that support young parts of the plant without restraining growth.
colloid- A mixture made up of a liquid and pa ticles that (because of their large size) remain suspended in that liquid.
colon- See large intestine.
colony- A collection of autonomously replicating cells of the same species.
columnar- The column shape of a type of epithelial cell.
commensalism - A symbiotic relationship in which one organism benefits but the other is neither helped nor harmed
communication- Animal behavior involving transmission of, reception of, and response to signals.
community- All the organisms that inhabit a particular area; an assemblage of populations of different species living close enough together for potential interaction.
community ecology- The study of how interactions between species affect community structure and organization.
companion cell- A type of plant cell that is connected to a sieve-tube member by many plasmodesmata and whose nucleus and ribosomes may serve one or more adjacent sieve-tube members.
competitive exclusion- The concept that when populations of two similar species compete for the same limited resources, one population will use the resources more efficiently and have a reproductive advantage that will eventually lead to the elimination of the other population.
competitive inhibitor- A substance that reduces the activity of an enzyme by entering the active site in place of the substrate whose structure it mimics.
complement system- A group of about 30 blood proteins that may amplify the inflammatory response, enhance phagocytosis, or directly lyse pathogens. The complement system is activated in a cascade initiated by surface antigens on microorganisms or by antigen-antibody complexes.
complementary DNA (cDNA) - A DNA molecule made in vitro using mRNA as a template and the enzyme reverse transcriptase. A cDNA molecule therefore corresponds to a gene, but lacks the introns present in the DNA of the genome.
complete digestive tract- A digestive tube that runs between a mouth and an anus; also called an alimentary canal. An incomplete digestive tract has only one opening.
complete dominance- The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.
complete flower - A flower that has all four basic floral organs: sepals, petals, stamens, and carpels.
complete metamorphosis- The transformation of a larva into an adult that looks very different, and often functions very differently in its environment, than the larva.
compound- A substance consisting of two or more elements in a fixed ratio.
compound eye A type of multifaceted eye in insects and crustaceans consisting of up to several thousand light -detecting, focusing ommatidia; especially good at detecting movement.
concentration gradient- An increase or decrease in the density of a chemical substance in an area. Cells often maintain concentration gradients of ions across their membranes. When a gradient exists, the ions or other chemical substances involved tend to move from where they are more concentrated to where they are less concentrated.
conception - The fertilization of the egg by a sperm cell in humans.
condensation reaction- A reaction in which two molecules become covalently bonded to each other through the loss of a small molecule, usually water; also called a dehydration reaction.
condom- A thin, latex rubber or natural membrane sheath that fits over the penis to collect semen.
condont- Ancient lineage of jawless vertebrates that arose during the Cambrian period.
conduction- The direct transfer of thermal motion (heat) between molecules of objects in direct contact with each other.
cone cell - One of two types of photoreceptors in the vertebrate eye; detects color during the day
conformer- A characterization of an animal in regard to environmental variables. A confonner allows some conditions within its body to vary with certain external changes.
conidia- Naked, asexual spores produced at the ends of specialized hyphae in ascomycetes.
conifer- A gymnospenn whose reproductive structure is the cone. Conifers include pines, firs, and redwoods.
coniferous forest- A terrestrial biome characterized by long, cold winters and dominated by cone-bearing trees.
conjugation - In prokaryotes, the direct transfer of DNA between two cells that are temporarily joined. In ciliates, a sexual process in which two cells exchange haploid micronuclei.
conjunctiva - A mucous membrane that helps keep the eye moist; lines the inner surface of the eyelid and covers the front of the eyeball, except the cornea.
connective tissue- Animal tissue that functions mainly to bind and support other tissues, having a sparse population of cells scattered through an extracellular matrix.
conodont- An early, soft-bodied vertebrate with prominent eyes and dental elements.
conservation biology- The integrated study of ecology, evolutionary biology, physiology, molecular biology, genetics, and behavioral biology in an effort to sustain biological diversity at all levels.
contraception- The prevention of pregnancy.
contractile vacuole- A membranous sac that helps move excess water out of certain cells.
control element- A segment of noncoding DNA that helps regulate transcription of a gene by binding proteins called transcription factors.
controlled experiment- An experiment in which an experimental group is compared to a control group that varies only in the factor being tested.
convection- The mass movement of warmed air or liquid to or from the surface of a body or object.
convergent extension - A mechanism of cell crawling in which the cells of a tissue layer rearrange themselves in such a way that the sheet of cells becomes narrower while it becomes longer.
cooperativity - An interaction of the constituent subunits of a protein whereby a confonnational change in one subunit is transmitted to all the others.
copepod - Any of a group of small crustaceans that are important members of marine and freshwater plankton communities.
coral reef- A warm-water, tropical ecosystem dominated by the hard skeletal structures secreted primarily by the resident cnidarians.
corepressor- A small molecule that cooperates with a repressor protein to switch an operon off.
cork cambium - A cylinder of meristematic tissue in woody plants that replaces the epidermis with thicker, tougher cork cells.
cornea - The transparent frontal portion of the sclera, which admits light into the vertebrate eye.
corpus callosum - The thick band of nerve fibers that connect the right and left cerebral hemispheres in placental mammals, enabling the hemispheres to process information together.
corpus luteum - A secreting tissue in the ovary that forms from the collapsed follicle after ovulation and produces progesterone.
cortex- Ground tissue that is between the vascular tissue and dermal tissue in a root or dicot stem.
cortical granules- Vesicles located just under the plasma membrane of an egg cell that undergo exocytosis during the cortical reaction.
cortical nephrons- Nephrons located almost entirely in the renal cortex. These nephrons have a reduced loop of Henle.
cortical reaction- Exocytosis of enzymes from cortical granules in the egg cytoplasm during fertilization.
corticosteroid- Any steroid hormone produced and secreted by the adrenal cortex.
cotransport- The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.
cotyledon- A seed leaf of an angiosperm embryo. Some species have one cotyledon, others two.
countercurrent exchange- The opposite flow of adjacent fluids that maximizes transfer rates; for example, blood in the gills flows in the opposite direction in which water passes over the gills, maximizing oxygen uptake and carbon dioxide loss,
countercurrent heat exchanger- An arrangement of blood vessels that helps trap heat in the body core and is important in reducing heat loss in many endotherms.
countercurrent multiplier system- A countercurrent system in which energy is expended in active transport to facilitate exchange of materials and create concentration gradients. For example, the loop of Henle actively transports NaCl from the filtrate in the upper part of the ascending limb of the loop, making the urine-concentrating function of the kidney more effective.
covalent bond - A type of strong chemical bond in which two atoms share one or more pairs of valence electrons.
cranial nerve- A nerve that leaves the brain and innervates an organ of the head or upper body
craniate- A chordate with a head.
crassulacean acid metabolism (CAM) - A type of metabolism in which carbon dioxide is taken in at night and incorporated into a variety of organic acids.
crista - An infolding of the inner membrane of a mitochondrion that houses the electron transport chain and the enzyme catalyzing the synthesis of ATP
critical load - The amount of added nutrient, usually nitrogen or phosphorus, that can be absorbed by plants without damaging ecosystem integrity.
crop rotation- The alternation of planting a nonlegume one year and a legume the next year to restore concentration of fixed nitrogen in the soil.
crossing over- The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.
cross-pollination - In angiospenns, the transfer of pollen from an anther of a flower on one plant to the stigma of a flower on another plant of the same species.
crustacean- A member of a subphylum of arthropods t"hat includes lobsters, crayfish, crabs, shrimps, and barnacles.
cryptic coloration Camouflage, making potential prey difficult to spot against its background.
cuboidal - The cubic shape of a type of epithelial cell.
culture- The ideas, customs, skills, rituals, and similar activities of a people or group that are passed along to succeeding generations.
cuticle- (1) A waxy covering on the surface of stems and leaves that acts as an adaptation to prevent desiccation in terrestrial plants. (2) The exoskeleton of an arthropod, consisting of layers of protein and chitin that are variously modified for different functions. (3) A tough coat that covers the body of a nematode.
cyclic AMP (cAMP) - Cyclic adenosine monophosphate, a ring-shaped molecule made from ATP that is a common intracellular signaling molecule (second messenger) in eukaryotic cells (for example, in vertebrate endocrine cells). It is also a regulator of some bacterial operons.
cyclic electron flow- A route of electron flow during the light reactions of photosynthesis that involves only photosystem I and that produces ATP but not NADPH or oxygen.
cyclin - A regulatory protein whose concentration fluctuates cyclically
cyclin-dependent kinase (Cdk) - A protein kinase that is active only when attached to a particular cyclin.
cystic fibrosis- A human genetic disorder caused by a recessive allele for a chloride channel protein; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated.
cytochrome - An iron-containing protein, a component of electron transport chains in mitochondria and chloroplasts.
cytogenetic map- Chart of a chromosome that locates genes with respect to chromosomal features.
cytokine- Any of a group of proteins secreted by a number of cell types, including macrophages and helper T cells, that regulate the function of lymphocytes and other cells of the immune system.
cytokinesis - The division of the cytoplasm to form two separate daughter cells immediately after mitosis.
cytokinins - A class of related plant hormones that retard aging and act in concert with auxin to stimulate cell division, influence the pathway of differentiation, and control apical dominance.
cytoplasm - The entire contents of the cell, exclusive of the nucleus, and bounded by the plasma membrane.
cytoplasmic determinants- The maternal substances in the egg that influence the course of early development by regulating the expression of genes that affect the developmental fate of cells.
cytoplasmic streaming- A circular flow of cytoplasm, involving myosin and actin filaments, that speeds the distribution of materials within cells.
cytoskeleton- A network of microtubules, micro filaments, and intermediate filaments that branch throughout the cytoplasm and serve a variety of mechanical and transport functions.
cytosol - The semifluid portion of the cytoplasm.
cytotoxic T cell - A type of lymphocyte that, when activated, kills infected cells, cancer cells, and transplanted cells.
daily torpor- A daily decrease in metabolic activity and body temperature during times of inactivity for some small mammals and birds.
dalton- A measure of mass for atoms and subatomic particles.
data- Recorded observations.
day-neutral plant - A plant whose flowering is not affected by photoperiod.
decapod- A member of the group of crustaceans that includes lobsters, crayfish, crabs, and shrimps.
decomposer- Any of the saprobic fungi and prokaryotes that absorb nutrients from nonliving organic material such as corpses, fallen plant material, and the wastes of living organisms, and convert them into inorganic forms.
deductive reasoning- A type of logic in which specific results are predicted from a general premise.
deep-sea hydrothermal vent- A dark, hot, oxygen-deficient environment associated with volcanic activity. The food producers are chemoautotrophic prokaryotes.
de-etiolation- The changes a plant shoot undergoes in response to sunlight; also known informally as greening.
dehydration reaction- A chemical reaction in which two molecules covalently bond to each other with the removal of a water molecule.
deletion- (1) A deficiency in a chromosome resulting from the loss of a fragment through breakage. (2) A mutational loss of one or more nucleotide pairs from a gene.
demographic transition- A shift from zero population growth in which birth rates and death rates are high to zero population growth characterized instead by low birth and death rates.
demography- The study of statistics relating to births and deaths in populations.
denaturation- In proteins, a process in which a protein unravels and loses its native conformation, thereby becoming biologically inactive. In DNA, the separation of the two strands of the double helix. Denaturation occurs under extreme conditions of pH, salt concentration, and temperature.
dendrite - One of usually numerous, short, highly branched processes of a neuron that convey nerve impulses toward the cell body
dendritic cell- An antigen-presenting cell, located mainly in lymphatic tissues and skin, that is particularly efficient in presenting antigens to naive helper T cells, thereby initiating a primary immune response.
density - The number of individuals per unit area or volume.
density dependent- Referring to any characteristic that varies according to an increase in population density
density-dependent inhibition - The phenomenon observed in normal animal cells that causes them to stop dividing when they come into contact with one another.
density independent- Referring to any characteristic that is not affected by population density
deoxyribonucleic acid (DNA) - A double-stranded, helical nucleic acid molecule capable of replicating and determining the inherited structure of a cell's proteins.
deoxyribose- The sugar component of DNA, having one less hydroxyl group than ribose, the sugar component of RNA.
depolarization- An electrical state in an excitable cell whereby the inside of the cell is made less negative relative to the outside than at the resting membrane potential. A neuron membrane is depolarized if a stimulus decreases its voltage from the resting potential of -70 mV in the direction of zero voltage.
derivatives- New cells that are displaced from an apical meristem and continue to divide until the cells they produce become specialized.
dermal tissue system- The outer protective covering of plants.
descent with modification- Darwin's initial phrase for the general process of evolution.
desert- A terrestrial biome characterized by very low precipitation.
desmosome- A type of intercellular junction in animal cells that functions as an anchor.
determinate cleavage- A type of embryonic development in protostomes that rigidly casts the developmental fate of each embryonic cell very early.
determinate growth - A type of growth characteristic of most animals and some plant organs, in which growth stops after a certain size is reached.
determination- The progressive restriction of developmental potential, causing the possible fate of each cell to become more limited as the embryo develops.
detritivore- A consumer that derives its energy from nonliving organic material; a decomposer.
detritus - Dead organic matter.
deuteromycete- Traditional classification for a fungus with no known sexual stage. When a sexual stage for a so-called deuteromycete is discovered, the species is assigned to a phylum. Also called an imperfect fungus.
deuterostome development- In animals, a developmental mode distinguished by the development of the anus from the blastopore; often also characterized by enterocoelous development of the body cavity and by radial cleavage.
diabetes mellitus- An endocrine disorder marked by inability to maintain glucose homeostasis. The type I form results from autoimmune destruction of insulin-secreting cells; treatment usually requires insulin injections several times a day The type II form most commonly results from reduced responsiveness of target cells to insulin; obesity and lack of exercise are risk factors.
diacylglycerol (DAG) - A second messenger produced by the cleavage of a certain kind of phospholipid in the plasma membrane.
diaphragm- (1) A sheet of muscle that fonns the bottom wall of the thoracic cavity in mammals; active in ventilating the lungs. (2) A dome-shaped rubber cup fitted into the upper portion of the vagina before sexual intercourse. It serves as a physical barrier to block the passage of sperm.
diapsid- Member of an amniote clade distinguished by a pair of holes on each side of the skull, including the lepidosaurs and archosaurs.
diastole - The stage of the heart cycle in which the heart muscle is relaxed, allowing the chambers to fill with blood.
diastolic pressure- Blood pressure that remains between heart contractions.
diatom - A unicellular photosynthetic alga with a unique, glassy cell wall containing silica.
dicots- A term traditionally used to refer to flowering plants that have two embryonic seed leaves, or cotyledons. Recent molecular evidence indicates that dicots do not form a clade (see eudicots).
differential gene expression- The expression of different sets of genes by cells with the same genome.
diffusion- The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area.
digestion- The process of breaking down food into molecules small enough for the body to absorb.
dihybrid - An organism that is heterozygous with respect to two genes of interest. All the offspring from a cross between parents doubly homozygous for different alleles are dihybrids. For example, parents of genotypes AABB and aabb produce a dihybrid of genotype AaBb.
dikaryotic- Referring to a fungal mycelium with two haploid nuclei per cell, one from each parent.
dinoflagellate - Member of a group of mostly unicellular photosynthetic algae with two flagella situated in perpendicular grooves in cellulose plates covering the cell.
dinosaur- Member of an extremely diverse group of ancient reptiles varying in body shape, size, and habitat.
dioecious - A term typically used to describe an angiosperm species in which carpellate and staminate flowers are on separate plants.
diploblastic- Having two germ layers.
diploid cell - A cell containing two sets of chromosomes (2n), one set inherited from each parent.
diplomonad- A protist that has modified mitochondria, two equal-sized nuclei, and multiple flagella.
directional selection- Natural selection that favors individuals at one end of the phenotypic range.
disaccharide - A double sugar, consisting of two monosaccharides joined by dehydration synthesis.
discovery science- The process of scientific inquiry that focuses on describing nature.
dispersal - The distribution of individuals within geographic population boundaries.
dispersion- The pattern of spacing among individuals within geographic population boundaries.
disruptive selection- Natural selection that favors individuals on both extremes of a phenotypic range over intermediate phenotypes.
dissociation curve- A chart showing the relative amounts of oxygen bound to hemoglobin when the pigment is exposed to solutions varying in their partial pressure of dissolved oxygen, pH, or other characteristics.
distal tubule- In the vertebrate kidney, the portion of a nephron that helps refine filtrate and empties it into a collecting duct.
disturbance- A force that changes a biological community and usually removes organisms from it. Disturbances, such as fire and storms, play pivotal roles in structuring many biological communities.
disulfide bridge- A strong covalent bond formed when the sulfur of one cysteine monomer bonds to the sulfur of another cysteine monomer.
DNA fingerprint- An individual's unique collection of DNA restriction fragments, detected by electrophoresis and nucleic acid probes.
DNA ligase - A linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3' end of a new DNA fragment to the 5' end of a growing chain.
DNA microarray assay- A method to detect and measure the expression of thousands of genes at one time, Tiny amounts of a large number of single-stranded DNA fragments representing different genes are fixed to a glass slide. These fragments, ideally representing all the genes of an organism, are tested for hybridization with various samples of cDNA molecules.
DNA polymerase - An enzyme that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain.
domain- (1) A taxonomic category above the kingdom level. The three domains are Archaea, Bacteria, and Eukarya. (2) An independently folding part of a protein.
dominant allele- An allele that is fully expressed in the phenotype of a heterozygote.
dominant species- Those species in a community that have the highest abundance or highest biomass. These species exert a powerful control over the occurrence and distribution of other species.
dopamine- A biogenic amine closely related to epinephrine and norepinephrine.
dormancy- A condition typified by extremely low metabolic rate and a suspension of growth and development.
dorsal- Pertaining to the back (top) of a bilaterally symmetrical animal.
dorsal lip- The dorsal side of the blastopore.
double circulation- A circulation scheme with separate pulmonary and systemic circuits, which ensures vigorous blood flow to all organs.
double fertilization- A mechanism of fertilization in angiosperms, in which two sperm cells unite with two cells in the embryo sac to form the zygote and endosperm.
double helix- The form of native DNA, referring to its two adjacent polynucleotide strands wound into a spiral shape.
Down syndrome - A human genetic disease caused by presence of an extra chromosome 2 I; characterized by mental retardation and heart and respiratory defects,
Duchenne muscular dystrophy - A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue.
duodenum - The first section of the small intestine, where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and gland cells of the intestinal wall.
duplication- An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated.
dynamic stability hypothesis- The idea that long food chains are less stable than short chains.
dynein - A large contractile protein forming the side-anns of microtubule doublets in cilia and flagella.
E site- One of a ribosome's three binding sites for tRNA during translation. The E site is the place where discharged tRNAs leave the ribosome. (E stands for exit.)
ecdysone- A steroid hormone, secreted by the pro thoracic glands, that triggers molting in arthropods.
ecdysozoan - Member of a group of animal phyla with protostome development that some systematists hypothesize fonn a clade, including many molting animals.
echinoderm - A slow-moving or sessile marine deuterostome with a water vascular system and, in adults, radial anatomy. Echinoderms include sea stars, brittle stars, sea urchins, feather stars, and sea cucumbers.
ecological capacity- The actual resource base of a country
ecological footprint- A method of using multiple constraints to estimate the human carrying capacity of Earth by calculating the aggregate land and water area in various ecosystem categories appropriated by a nation to produce all the resources it consumes and to absorb all the waste it generates.
ecological niche - The sum total of a species' use of the biotic and abiotic resources in its environment.
ecological species concept- Defining species in tenns of ecological roles (niches).
ecological succession- Transition in the species composition of a biological community, often following ecological disturbance of the community; the establishment of a biological community in an area virtually barren of life.
ecology- The study of how organisms interact with their environment.
ecosystem- All the organisms in a given area as well as the abiotic factors with which they interact; a community and its physical environment.
ecosystem ecology- The study of energy flow and the cycling of chemicals among the various biotic and abiotic factors in an ecosystem.
ecosystem services- Functions performed by natural ecosystems that directly or indirectly benefit humans.
ecotone- The transition from one type of habitat or ecosystem to another, such as the transition from a forest to a grassland.
ectoderm - The outermost of the three primary germ layers in animal embryos; gives rise to the outer covering and, in some phyla, the nervous system, inner ear, and lens of the eye.
ectomycorrhiza - A type of mycorrhiza in which the mycelium fonns a dense sheath, or mantle, over the surface of the root. Hyphae extend from the mantle into the soil, greatly increasing the surface area for water and mineral absorption.
ectomycorrhizal fungus- A fungus that forms ectomycorrhizae with plant roots.
ectoparasite- A parasite that feeds on the external surface of a host.
ectoproct- A sessile, coloniallophophorate commonly called a bryozoan.
ectotherm - An animal, such as a reptile (other than birds), fish, or amphibian, that must use environmental energy and behavioral adaptations to regulate its body temperature.
ectothermic- Referring to organisms that do not produce enough metabolic heat to have much effect on body temperature.
Ediacaran fauna- Earliest generally accepted animal fossils, dating from about 575 million years ago.
effective population size- An estimate of the size of a population based on the numbers of females and males that successfully breed;
effector cell- A muscle cell or gland cell that performs the body's responses to stimuli; responds to signals from the brain or other processing center of the nervous system.
efferent arteriole- The blood vessel draining a nephron.
egg-polarity gene- Another name for a maternal effect gene, a gene that helps control the orientation (polarity) of the egg.
ejaculation- The propulsion of sperm from the epididymis through the muscular vas deferens, ejaculatory duct, and urethra.
ejaculatory duct- The short section of the ejaculatory route in mammals formed by the convergence of the vas deferens and a duct from the seminal vesicle. The ejaculatory duct transports sperm from the vas deferens to the urethra.
elastic fiber- A long thread made of the protein elastin. Elastic fibers provide a rubbery quality to the extracellular matrix that complements the nonelastic strength of collagenous fibers.
electrocardiogram (ECG or EKG) - A record of the electrical impulses that travel through cardiac muscle during the heart cycle.
electrochemical gradient- The diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ion across a membrane and its tendency to move relative to the membrane potential.
electrogenic pump- An ion transport protein that generates voltage across a membrane.
electromagnetic receptor- A receptor of electromagnetic energy, such as visible light, electricity, and magnetism.
electromagnetic spectrum- The entire spectrum of radiation ranging in wavelength from less than a nanometer to more than a kilometer.
electron- A subatomic particle with a single negative charge. One or more electrons move around the nucleus of an atom.
electron microscope (EM) - A microscope that focuses an electron beam through a specimen, resulting in resolving power a thousandfold greater than that of a light microscope. A transmission electron microscope (TEM) is used to study the internal structure of thin sections of cells. A scanning electron microscope (SEM) is used to study the fine details of cell surfaces.
electron shell - An energy level represented as the distance of an electron from the nucleus of an atom.
electron transport chain- A sequence of electron carrier molecules (membrane proteins) that shuttle electrons during the redox reactions that release energy used to make ATP.
electronegativity- The attraction of an atom for the electrons of a covalent bond.
electroporation- A technique to introduce recombinant DNA into cells by applying a brief a electrical pulse to a solution containing cells. The electricity creates temporary holes in the cells' plasma membranes, through which DNA can enter.
element- Any substance that cannot be broken down to any other substance.
elicitor- A molecule that induces a broad type of host defense response.
elimination- The passing of undigested material out of the digestive compartment.
embryo- New developing individual.
embryo sac - The female gametophyte of angiosperms, formed from the growth and division of the megaspore into a multicellular structure with eight haploid nuclei.
embryonic lethal- A mutation with a phenotype leading to death at the embryo or larval stage.
embryophyte- Another name for land plants, recognizing that land plants share the common derived trait of multicellular, dependent embryos.
emergent properties- New properties that emerge with each step upward in the hierarchy of life, owing to the arrangement and interactions of parts as complexity increases.
emigration- The movement of individuals out of a population.
enantiomer - One of two molecules that are mirror images of each other.
endangered species- A species that is in danger of extinction throughout all or a significant portion of its range.
endemic species - Species that are confined to a specific, relatively small geographic area.
endergonic reaction - A nonspontaneous chemical reaction, in which free energy is absorbed from the surroundings.
endocrine gland - A ductless gland that secretes hormones directly into the interstitial fluid, from which they diffuse into the bloodstream.
endocrine system- The internal system of chemical communication invoh;ing hormones, the ductless glands that secrete hormones, and the molecular receptors on or in target cells that respond to hormones; functions in concert with the nervous system to effect internal regulation and maintain homeostasis.
endocytosis - The cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surround the substance and pinch off to form an intracellular vesicle.
endoderm - The innermost of the three primary germ layers in animal embryos; lines the archenteron and gives rise to the liver, pancreas, lungs, and the lining of the digestive tract.
endodermis- The innermost layer of the cortex in plant roots; a cylinder one cell thick that fonns the boundary between the cortex and the vascular cylinder.
endomembrane system- The collection of membranes inside and around a eukaryotic cell, related either through direct physical contact or by the transfer of membranous vesicles.
endometrium - The inner lining of the uterus, which is richly supplied with blood vessels.
endomycorrhiza - A type of mycorrhiza that, unlike ectomycorrhizae, does not have a dense mantle ensheathing the root. Instead, microscopic fungal hyphae extend from the root into the soil.
endomycorrhizal fungus- A fungus that forms endomycorrhizae with plant roots.
endoparasite- A parasite that lives within a host.
endoplasmic reticulum (ER) - An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosomefree (smooth) regions.
endorphin- Any of several hormones produced in the brain and anterior pituitary that inhibits pain perception.
endoskeleton- A hard skeleton buried within the soft tissues of an animal, such as the spicules of sponges, the plates of echinoderms, and the bony skeletons of vertebrates.
endosperm- A nutrient-rich tissue formed by the union of a sperm cell with two polar nuclei during double fertiJization, which provides nourishment to the developing embryo in angiosperm seeds.
endospore- A thick-coated, resistant cell produced within a bacterial cell exposed to harsh conditions.
endothelium - The innermost, simple squamous layer of cells lining the blood vessels; the only constituent structure of capillaries.
endotherm- An animal, such as a bird or mammal, that uses metabolic heat to regulate body temperature.
endothermic- Referring to organisms with bodies that are warmed by heat generated by metabolism. This heat is usually used to maintain a relatively stable body temperature higher than that of the external environment.
endotoxin- A toxic component of the outer membrane of certain gram-negative bacteria that is released only when the bacteria die.
energetic hypothesis- The concept that the length of a food chain is limited by the inefficiency of energy transfer along the chain.
energy- The capacity to do work (to move matter against an opposing force).
energy coupling- In cellular metabolism, the use of energy released from an exergonic reaction to drive an endergonic reaction.
energy level - Any of several different states of potential energy for electrons in an atom.
enhancer- A DNA segment containing multiple control elements that may be located far away from the gene it regulates.
enteric division- Complex networks of neurons in the digestive tract, pancreas, and gallbladder; normally regulated by the sympathetic and parasympathetic divisions of the autonomic nervous system.
enterocoelous - Pattern of formation of the body cavity common in deuterostome development, in which the mesodenn buds from the wall of the archenteron and hollows, forming the body cavity
entropy - A quantitative measure of disorder or randomness, symbolized by S.
enzymatic hydrolysis - The process in digestion that splits macromolecules from food by the enzymatic addition of water.
enzyme - A protein serving as a catalyst, a chemical agent that changes the rate of a reaction without being consumed by the reaction.
enzyme-substrate complex- A temporary complex formed when an enzyme binds to its substrate molecule(s).
eosinophil- A type of white blood cell with low phagocytic activity that is thought to playa role in defense against parasitic worms by releasing enzymes toxic to these invaders.
epicotyl - In an angiosperm embryo, the embryonic axis above the point of attachment of the cotyledon(s).
epidermis- (1) The dermal tissue system of nonwoody plants, usually consisting of a single layer of tightly packed cells. (2) The outer covering of animals.
epididymis - A coiled tubule located adjacent to the testes where sperm are stored.
epigenetic inheritance- Inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence.
epiglottis- A cartilaginous flap that blocks the top of the windpipe, the glottis, during swallowing, which prevents the entry of food or fluid into the respiratory system.
epinephrine- A catecholamine hormone secreted from the adrenal medulla that mediates "fight-or-flight" responses to short-term stress; also functions as a neurotransmitter.
epiphyte- A plant that nourishes itself but grows on the surface of another plant for support, usually on the branches or trunks of tropical trees.
episome - A genetic element that can exist either as a plasmid or as part of the bacterial chromosome.
epistasis- A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited.
epithalamus- A brain region, derived from the diencephalon, that contains several clusters of capillaries that produce cerebrospinal fluid.
epithelial tissue - Sheets of tightly packed cells that line organs and body cavities.
epitope- A small, accessible region of an antigen to which an antigen receptor or antibody binds; also called an antigenic determinant. .
equilibrium potential - The magnitude of a cell's membrane voltage at equilibrium; calculated using the Nernst equation. .
erythrocyte - A red blood cell; contains hemoglobin, which functions in transporting oxygen in the circulatory system. .
erythropoietin (EPO) - A hormone produced in the kidney when tissues of the body do not receive enough oxygen. This hormone stimulates the production of erythrocytes.
esophagus - A channel that conducts food, by peristalsis, from the pharynx to the stomach. .
essential amino acid- An amino acid that an animal cannot synthesize itself and must be obtained from food. Eight amino acids are essential in the human adult. .
essential element- In plants, a chemical element that is required for the plant to grow from a seed and complete the life cycle, producing another generation of seeds. .
essential fatty acids - Certain unsaturated fatty acids that animals cannot make. .
essential nutrient- A substance that an organism must absorb in preassembled form because it cannot be synthesized from any other material. In humans, there are essential vitamins, minerals, amino acids, and fatty acids. .
estivation - Summer torpor; a physiological state that is characterized by slow metabolism and inactivity and that permits survival during long periods of elevated temperature and diminished water supplies. .
estrogen - Any steroid hormone, such as estradiol, that stimulates the development and maintenance of the female reproductive system and secondary sex characteristics. .
estrous cycle - A type of reproductive cycle in all female mammals except higher primates, in which the nonpregnant endometrium is reabsorbed rather than shed, and sexual response occurs only during mid-cycle at estrus. .
estrus- A period of sexual activity associated with ovulation. .
estuary- The area where a freshwater stream or river merges with the ocean. .
ethology - The study of animal behavior in natural conditions. .
ethylene - The only gaseous plant hormone. Among its many effects are response to mechanical stress, programmed cell death, leaf abscission, and fruit ripening. .
etiolation- Plant morphological adaptations for growing in darkness. .
euchromatin - The more open, unraveled form of eukaryotic chromatin that is available transcription. .
eudicots - A clade consisting of the vast majority of flowering plants that have two embryonic seed leaves, or cotyledons. .
euglenid - A protist, such as Euglena or its relatives, characterized by an anterior pocket, or chamber, from which one or two flagella emerge. .
Eukarya- The domain that includes all eukaryotic organisms. .
eukaryotic cell - A type of cell with a membrane-enclosed nucleus and membrane-enclosed organelles, present in protists, plants, fungi, and animals; also called eukaryote. .
eumetazoan - Member of the clade Eumetazoa, animals with true tissues (all animals except sponges). .
euryhaline- Referring to organisms that can tolerate substantial changes in external osmolarity. .
eurypterid- An extinct carnivorous cheliceriform also called a water scorpion. .
Eustachian tube- The tube that connects the middle ear to the pharynx. .
eutherian - Placental mammal; mammal whose young complete their embryonic development within the uterus, joined to the mother by the placenta. .
eutrophic lake- A nutrient-rich and oxygenpoor lake, having a high rate of biological productivity. .
eutrophication- A process by which nutrients, particularly phosphorus and nitrogen, become highly concentrated in a body of water, leading to increased growth of organisms such as algae. Cultural eutrophication refers to situations where the nutrients added to the water body originate mainly from human sources, such as agricultural drainage or sewage. .
evaporation- The removal of heat energy from the surface of a liquid that is losing some of its molecules. .
evaporative cooling- The property of a liquid whereby the surface becomes cooler during evaporation, owing to a loss of highly kinetic molecules to the gaseous state. .
evapotranspiration- The evaporation of water from soil plus the transpiration of water from plants. .
evolution- All the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today. .
evolutionary adapation- An accumulation of inherited characteristics that enhance organisms' ability to survive and reproduce in specific environments. .
excitatory postsynaptic potential (EPSP) - An electrical change (depolarization) in the membrane of a postsynaptic neuron caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more likely for a postsynaptic neuron to generate an action potential. .
excretion- The disposal of nitrogen-containing waste products of metabolism. .
exergonic reaction - A spontaneous chemical reaction, in which there is a net release of free energy. .
exocytosis - The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane. .
exoenzyme- A powerful hydrolytic enzyme secreted by a fungus outside its body to digest food. .
exon- A coding region of a eukaryotic gene. Exons, which are expressed, are separated from each other by introns. .
exoskeleton- A hard encasement on the surface of an animal, such as the shell of a mollusc or the cuticle of an arthropod, that provides protection and points of attachment for muscles. .
exotoxin- A toxic protein that is secreted by a prokaryote and that produces specific symptoms even in the absence of the prokaryote. .
expansins- Plant enzymes that break the crosslinks (hydrogen bonds) between cellulose micro fibrils and other cell wall constituents, loosening the wall's fabric. .
exponential population growth- The geometric increase of a population as it grows in an ideal, unlimited environment. .
expression vector- A cloning vector that contains the requisite prokaryotic promoter just upstream of a restriction site where a eukaryotic gene can be inserted. .
external fertilization- The fusion of gametes that parents have discharged into the environment. .
exteroreceptor- A sensory receptor that detects stimuli outside the body, such as heat, light, pressure, and chemicals. .
extinction vortex- A downward population spiral in which positive-feedback loops of inbreeding and genetic drift cause a small population to shrink and, unless reversed, become extinct. .
extracellular digestion - The breakdown of food outside cells. .
extracellular matrix (ECM) - The substance in which animal tissue cells are embedded, consisting of protein and polysaccharides. .
extraembryonic membranes- Four membranes (yolk sac, amnion, chorion, allantois) that support the developing embryo in mammals and birds and other reptiles. .
extreme halophile- A prokaryote that lives in a highly saline environment, such as the Great Salt Lake or the Dead Sea. .
extreme thermophile- A prokaryote that thrives in hot environments (often 60-80°C or hotter). .
extremophile- A prokaryote that lives in an extreme environment. Extremophiles include methanogens, extreme halophiles, and extreme thermophiles. .
F factor- A fertility factor in bacteria; a DNA segment that confers the ability to form pili for conjugation and associated functions required for the transfer of DNA from donor to recipient. It may exist as a plasmid or be integrated into the bacterial chromosome. .
F plasmid - The plasmid form of the F factor. .
Fl generation- The first filial, or hybrid, offspring in a series of genetic crosses. .
F2 generation- Offspring resulting from interbreeding of the hybrid F 1 generation. .
facilitator- A species that has a positive effect on the survival and reproduction of other species in a community and that contributes to community structure. .
facilitated diffusion- The spontaneous passage of molecules and ions, bound to specific carrier proteins, across a biological membrane down their concentration gradients. .
facultative anaerobe - An organism that makes ATP by aerobic respiration if oxygen is present but that switches to fermentation under anaerobic conditions. .
family- In classification, the taxonomic category above genus. .
fast block to polyspermy- The depolarization of the egg membrane within 1-3 seconds after sperm binding to the vitelline layer. The reaction prevents additional sperm from fusing with the egg's plasma membrane. .
fast muscle fibers- Muscle cells used for rapid, powerful contractions. .
fat (triacylglycerol) - A biological compound consisting of three fatty acids linked to one glycerol molecule. .
fate map- Territorial diagram of embryonic development that reveals the future development of individual cells and tissues. .
fatty acid- A long carbon chain carboxylic acid. Fatty acids vary in length and in the number and location of double bonds; three fatty acids linked to a glycerol molecule form fat. .
feces- The wastes of the digestive tract. .
feedback inhibition- A method of metabolic control in which the end product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway. .
fermentation- A catabolic process that makes a limited amount of ATP from glucose without an electron transport chain and that produces a characteristic end product, such as ethyl alcohol or lactic acid. .
fertilization- The union of haploid gametes to produce a diploid zygote. .
fertilization envelope- The swelling of the vitelline layer away from the plasma membrane. .
fetus- A developing human from the ninth week of gestation until birth; has all the major structures of an adult. .
fiber - A lignified cell type that reinforces the xylem of angiosperms and functions in mechanical support; a slender, tapered sclerenchyma cell that usually occurs in bundles. .
fibrin - The activated form of the blood-clotting protein fibrinogen, which aggregates into threads that form the fabric of the clot. .
fibrinogen- The inactive form of the plasma protein that is converted to the active form fibrin, which aggregates into threads that form the framework of a blood clot. .
fibroblast - A type of cell in loose connective tissue that secretes the protein ingredients of the extracellular fibers. .
fibronectin- A glycoprotein that helps cells attach to the extracellular matrix. .
fibrous connective tissue - A dense tissue with large numbers of collagenous fibers organized into parallel bundles. This is the dominant tissue in tendons and ligaments. .
fibrous root system- A root system common to monocots consisting of a mat of thin roots spreading out below the soil surface. .
filament- The stalk of a stamen. .
filtrate- Fluid extracted by the excretory system from the body fluid. The excretory system produces urine from the filtrate after extracting valuable solutes from it and concentrating it. .
filtration- The extraction of water and small solutes, including metabolic wastes, from the body fluid into the excretory system. .
fimbria - A short, hairlike prokaryotic appendage that functions in adherence to the substrate or to other cells. .
first law of thermodynamics- The principle of conservation of energy. Energy can be transferred and transformed, but it cannot be created or destroyed. .
fission- The separation of a parent into two or more individuals of approximately equal size. .
fitness- The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals. .
fixed action pattern (FAP) - A sequence of behavioral acts that is essentially unchangeable and usually carried to completion once initiated. .
flaccid - Limp. A walled cell is flaccid in surroundings where there is no tendency for water to enter. .
flagellum - A long cellular appendage specialized for locomotion. The flagella of prokaryotes and eukaryotes differ in both structure and function. .
florigen- A flowering signal, not yet chemically identified, that may be a hormone or may be a change in relative concentrations of multiple hormones. .
flower- In an angiosperm, a short stem with up to four sets of modified leaves, bearing structures that function in sexual reproduction. .
fluid feeder- An animal that lives by sucking nutrient-rich fluids from another living organism. .
fluid mosaic model- The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of individual protein molecules drifting laterally in a fluid bilayer of phospholipids. .
follicle - A microscopic structure in the ovary that contains the developing ovum and secretes estrogens. .
follicle-stimulating hormone (FSH) - A tropic hormone produced and secreted by the anterior pituitary that stimulates the production of eggs by the ovaries and sperm by the testes. .
follicular phase- That part of the ovarian cycle during which follicles are growing and oocytes maturing. .
food chain- The pathway along which food is transferred from trophic level to trophic level, beginning with producers. .
food vacuole- A membranous sac formed by phagocytosis. .
food web The elaborate, interconnected feeding relationships in an ecosystem. .
foot- (1) The portion of a bryophyte sporophyte that gathers sugars, amino acids, water, and minerals from the parent gametophyte via transfer cells. (2) One of the three main parts of a mollusc; a muscular structure usually used for movement. .
foraging- Behavior necessary to recognize, search for, capture, and consume food. .
foraminiferan (foram) - An aquatic protist that secretes a hardened shell containing calcium carbonate and extends pseudopodia through pores in the shell. .
forebrain- One of three ancestral and embryonic regions of the vertebrate brain; develops into the thalamus, hypothalamus, and cerebrum. .
fossil- A preserved remnant or impression of an organism that lived in the past. .
fossil record - The chronicle of evolution over millions of years of geologic time engraved in the order in which fossils appear in rock strata. .
founder effect- Genetic drift that occurs when a few individuals become isolated from a larger population, with the result that the new population's gene pool is not reflective of the original population. .
fovea - An eye's center of focus and the place on the retina where photoreceptors are highly concentrated. .
fragmentation- A means of asexual reproduction whereby a single parent breaks into parts that regenerate into whole new individuals. .
frameshift mutation - A mutation occurring when the number of nucleotides inserted or deleted is not a multiple of three, resulting in the improper grouping of the following nucleotides into codons.
free energy of activation- The amount of energy that reactants must absorb before a chemical reaction will start; also called activation energy
frequency-dependent selection- A decline in the reproductive success of a morph resulting from the morph's phenotype becoming too common in a population; a cause of balanced polymorphism in populations.
fruit- A mature ovary of a flower that protects dormant seeds and aids in their dispersal.
functional group- A specific configuration of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical reactions.
Fungi- The eukaryotic kingdom that includes organisms that absorb nutrients after decomposing organic material.
fusiform initials- Cells within the vascular cambrium that produce elongated cells such as tracheids, vessel elements, fibers, and sievetube members.
G protein- A GTP-binding protein that relays signals from a plasma membrane signal receptor, known as a G-protein-linked receptor, to other signal transduction proteins inside the cell. When such a receptor is activated, it in turn activates the G protein, causing it to bind a molecule of GTP in place of GDP. Hydrolysis of the bound GTP to GDP inactivates the G protein.
G0 phase- A nondividing state in which a cell has left the cell cycle.
G1 phase- The first growth phase of the cell cycle, consisting of the portion of interphase before DNA synthesis begins.
G2 phase- The second growth phase of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs.
gallbladder - An organ that stores bile and releases it as needed into the small intestine.
gametangium - Multicellular plant structures in which gametes are formed. Female gametangia are called archegonia, and male gametangia are called antheridia.
gamete - A haploid cell, such as an egg . or sperm. Gametes unite during sexual reproduction to produce a diploid zygote.
game theory- An approach to evaluating alternative strategies in situations where the outcome depends not only on each individual's strategy but also on the strategies of other individuals; a way of thinking about behavioral evolution in situations where the fitness of a particular behavioral phenotype is influenced by other behavioral phenotypes in the population.
gametogenesis- The process by which gametes are produced in the mammalian body.
gametophore- The mature gamete-producing structure of a gametophyte body of a moss.
gametophyte - In organisms undergoing alternation of generations, the multicellular haploid form that mitotically produces haploid gametes that unite and grow into the sporophyte generation.
gamma aminobutyric acid (GABA)- An amino acid that functions as a CNS neurotransmitter.
ganglion - A cluster (functional group) of nerve cell bodies in a centralized nervous system.
ganglion cell- A type of neuron in the retina that synapses with bipolar cells and transmits action potentials to the brain via axons in the optic nerve.
gap junction- A type of intercellular junction in animal cells that allows the passage of material or current between cells.
gas exchange- The uptake of molecular oxygen from the environment and the discharge of carbon dioxide to the environment.
gastric juice- A digestive fluid secreted by the stomach.
gastrovascular cavity- An extensive pouch that serves as the site of extracellular digestion and a passageway to disperse materials throughout most of an animal's body
gastrula - The three-layered, cupshaped embryonic stage.
gastrulation- The formation of a gastrula from a blastula gated channel A protein channel in a cell membrane that opens or closes in response to a particular stimulus.
gated ion channel - A gated channel for a specific ion. When ion channels are opened or closed, the membrane potential of the cell is altered.
gel electrophoresis - The separation of nucleic acids or proteins, on the basis of their size and electrical charge, by measuring their rate of movement through an electrical field in a gel.
gene- A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).
gene cloning- The production of multiple copies of a gene.
gene flow - Genetic additions to or substractions from a population resulting from the movement of fertile individuals or gametes.
gene-for-gene recognition - A widespread form of plant disease resistance involving recognition of pathogen-derived molecules by the protein products of specific plant disease resistance genes.
gene pool- The total aggregate of genes in a population at anyone time.
gene therapy - The alteration of the genes of a person afflicted with a genetic disease.
genetic annealing- The production of a new genome through the transfer of part of the genome of one organism to another organism.
genetic drift- Unpredictable fluctuations in allele frequencies from one generation to the next because of a population's finite size.
genetic engineering- The direct manipulation of genes for practical purposes.
genetic map- An ordered list of genetic loci (genes or other genetic markers) along a chromosome.
genetic polymorphism- The existence of two or more distinct alleles at a given locus in a population's gene pool.
genetic recombination- General term for the production of offspring that combine traits of the two parents.
genetically modified (GM) organism- An organism that has acquired one or more genes by artificial means; also known as a transgenic organism.
genetics - The scientific study of heredity and. hereditary variation.
genome- The complete complement of an organism's genes; an organism's genetic material.
genomic imprinting- Phenomenon in which expression of an allele in offspring depends on whether the allele is inherited from the male or female parent.
genomic library- A set of thousands of DNA segments from a genome, each carried by a plasmid, phage, or other cloning vector.
genomics - The study of whole sets of genes and their interactions.
genotype - The genetic makeup, or set of alleles, of an organism.
genus - A taxonomic category above the species level, designated by the first word of a species' two-part scientific name.
geographic variation- Differences between the gene pools of separate populations or population subgroups.
geologic record- The division of Earths history into time periods, grouped into three eras: Archaean, Proterozoic, and Phanerozoic, and further subdivided into eras and epochs.
geometric isomer - One of several organic compounds that have the same molecular formula but differ in the spatial arrangements of their atoms.
germ layers- Three main layers that form the various tissues and organs of an animal body.
gestation- Pregnancy; the state of carrying developing young within the female reproductive tract.
gibberellins - A class of related plant hormones that stimulate growth in the stem and leaves, trigger the germination of seeds and breaking of bud dormancy, and stimulate fruit development with auxin.
gill - A localized extension of the body surface of many aquatic animals, specialized for gas exchange.
gill circulation - The flow of blood through gills.
glandular epithelium- An epithelium that absorbs or secretes chemical solutions.
glans penis - The head end of the penis.
glia- Supporting cells that are essential for the structural integrity of the nervous system and for the normal functioning of neurons.
glomeromycete- Member of the fungal phylum Glomeromycota, characterized by forming a distinct branching form of endomycorrhizae (symbiotic relationships with plant roots) called arbuscular mycorrhizae.
glomerulus - A ball of capillaries surrounded by Bowman's capsule in the nephron and serving as the site of filtration in the vertebrate kidney
glucagon - A hormone secreted by pancreatic alpha cells that raises blood glucose levels. It promotes glycogen breakdown and release of glucose by the liver.
glucocorticoid- A steroid hormone secreted by the adrenal cortex that influences glucose metabolism and immune function.
glutamate- An amino acid that functions as a CNS neurotransmitter.
glyceraldehyde- 3-phosphate (G 3 P) - The carbohydrate produced directly from the Calvin cycle.
glycine - An amino acid that functions as a CNS neurotransmitter.
glycogen - An extensively branched glucose storage polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.
glycolipid- A lipid covalently attached to a carbohydrate.
glycolysis- The splitting of glucose into pyruvate. Glycolysis is the one metabolic pathway that occurs in all living cells, serving as the starting point for fermentation or aerobic respiration.
glycoprotein- A protein covalently attached to a carbohydrate.
glycosidic linkage - A covalent bond formed between two monosaccharides by a dehydration reaction.
gnathostome- Member of the vertebrate subgroup possessing jaws.
golden alga- A chrysophyte; a typically unicellular, biflagellated alga with yellow and brown carotenoid pigments.
Golgi apparatus- An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic reticulum.
gonadotropin- A hormone that stimulates the activities of the testes and ovaries. Follicle-stimulating hormone and luteinizing hormone are gonadotropins.
gonads - The male and female sex organs; the gamete-producing organs'in most animals.
G-protein-linked receptor- A signal receptor protein in the plasma membrane that responds to the binding signal molecule by activating a G protein.
grade - Group of animal species that share the same level of organizational complexity
graded potential- A local voltage change in a neuron membrane induced by stimulation of a neuron, with strength proportional to the strength of the stimulus and lasting about a millisecond.
gradualism- A view of Earth's history that attributes profound change to the cumulative product of slow but continuous processes.
graft versus host reaction- An attack against a patient's body cells by lymphocytes received in a bone marrow transplant.
gram-negative- Describing the group of bacteria with a cell wall that is structurally more complex and contains less peptidoglycan than that of gram-positive bacteria. Gramnegative bacteria are often more toxic than gram-positive bacteria.
gram-positive- Describing the group of bacteria with a cell wall that is structurally less complex and contains more peptidoglycan than that of gram-negative bacteria. Grampositive bacteria are usually less toxic than gram-negative bacteria.
Gram stain- A staining method that distinguishes between two different kinds of bacterial cell walls.
granum- A stacked portion of the thylakoid membrane in the chloroplast. Grana function in the light reactions of photosynthesis.
gravitropism - A response of a plant or animal to gravity
gray crescent- A light-gray region of cytoplasm located near the equator of the egg on the side opposite the sperm entry
gray matter - Regions of dendrites and clusters of neuron cell bodies within the CNS.
green alga- A unicellular, colonial, or multicellular photosynthetic protist that has grassgreen chloroplasts. Green algae are closely related to true plants.
greenhouse effect - The warming of planet Earth due to the atmospheric accumulation of carbon dioxide, which absorbs reflected infrared radiation and re-refJects some of it back toward Earth.
green world hypothesis - The conjecture that terrestrial herbivores consume relatively little plant biomass because they are held in check by a variety of factors, including predators, parasites, and disease.
gross primary production (GPP)- The total primary production of an ecosystem.
ground tissue system- Plant tissues that are neither vascular nor dermal, fulfilling a variety of functions, such as storage, photosynthesis, and support.
growth cone- Responsive region at the leading edge of a growing axon.
growth factor - A protein that must be present in the extracellular environment (culture medium or animal body) for the growth and normal development of certain types of cells; a local regulator that acts on nearby cells to stimulate cell proliferation and differentiation.
growth hormone (GH)- A hormone produced and secreted by the anterior pituitary that has both direct (nontropic) effects and tropic effects on a wide variety of tissues.
guard cells- The two cells that flank the stomatal pore and regulate the opening and closing of the pore,
gustatory receptor- Taste receptor.
guttation- The exudation of water droplets, caused by root pressure in certain plants.
gymnosperm - A vascular plant that bears naked seeds-seeds not enclosed in specialized chambers.
habituation- A very simple type of learning that involves a loss of responsiveness to stimuli that convey little or no information.
hair cell- A type of mechanoreceptor that detects sound waves and other forms of movement in air or water.
half-life - The number of years it takes for 50% of a sample of an isotope to decay.
Hamilton's rule- The principle that for natural selection to favor an altruistic act, the benefit to the recipient, devalued by the coefficient of relatedness, must exceed the cost to the altruist.
haploid cell- A cell containing only one set of chromosomes (n).
Hardy-Weinberg equilibrium - The condition describing a non-evolving population (one that is in genetic equilibrium).
Hardy-Weinberg theorem- The principle that frequencies of alleles and genotypes in a population remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.
haustorium - In certain symbiotic fungi, specialized hyphae that can penetrate the tissues of host organisms.
heart - A muscular pump that uses metabolic energy to elevate hydrostatic pressure of the blood. Blood then flows down a pressure gradient through blood vessels that eventually return blood to the heart.
heart attack- The death of cardiac muscle tissue resulting from prolonged blockage of one or more coronary arteries.
heart murmur- A hissing sound that occurs when blood squirts backward through a leaky valve in the heart.
heart rate- The rate of heart contraction.
heartwood- Older layers of secondary xylem, closer to the center of a stem or root, that no longer transport xylem sap.
heat- The total amount of kinetic energy due to molecular motion in a body of matter. Heat is energy in its most random form.
heat of vaporization- The quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state.
heat-shock protein- A protein that helps protect other proteins during heat stress. Heatshock proteins are found in plants, animals, and microorganisms.
heavy chain- One of the two types of polypeptide chains that make up an antibody molecule and B cell receptor; consists of a variable region, which contributes to the antigen-binding site, and a constant region.
helicase - An enzyme that untwists the double helix of DNA at the replication forks.
helper T cell - A type of T cell that, when activated, secretes cytokines that promote the response of B cells (humoral response) and cytotonic T cells (cell-mediated response) to antigens.
hemocyanin- A type of respiratory pigment that uses copper as its oxygen-binding component. Hemocyanin is found in the hemolymph of arthropods and many molluscs.
hemoglobin- An iron-containing protein in red blood cells that reversibly binds oxygen.
hemolymph- In invertebrates with an open circulatory system, the body fluid that bathes tissues.
hemophilia- A human genetic disease caused by a sex-linked recessive allele; characterized by excessive bleeding following injury
hepatic portal vein - A large circulatory channel that conveys nutrient-laden blood from the small intestine to the liver, which regulates the blood's nutrient content.
herbaceous- Referring to nonwoody plants.
herbivore- A heterotrophic animal that eats plants.
herbivory- An interaction in which an herbivore eats parts of a plant or alga.
heredity- The transmission of traits from one generation to the next.
hermaphrodite - An individual that functions as both male and female in sexual reproduction by producing both sperm and eggs.
hermaphroditism - A condition in which an individual has both female and male gonads and functions as both a male and female in sexual reproduction by producing both sperm and eggs.
heterochromatin- Nontranscribed eukaryotic chromatin that is so highly compacted that it is visible with a light microscope during interphase.
heterochrony - Evolutionary change in the timing or rate of an organism's development.
heterocyst - A specialized cell that engages in nitrogen fixation in some filamentous cyanobacteria.
heterokaryon- A fungal mycelium formed by the fusion of two hyphae that have genetically different nuclei.
heteromorphic - Referring to a condition in the life cycle of all living plants and certain algae in which the sporophyte and gametophyte generations differ in morphology
heterosporous - A term referring to a plant species that has two kinds of spores: microspores that develop into male gametophytes and megaspores that develop into female gametophytes.
heterotroph- An organism that obtains organic food molecules by eating other organisms or their by-products.
heterozygote advantage- Greater reproductive success of heterozygous individuals compared to homozygotes; tends to preserve variation in gene pools.
heterozygous- Having two different alleles for a given gene.
hexapod- An insect or closely related wingless, six-legged arthropod.
hibernation- A physiological state that allows survival during long periods of cold temperatures and reduced food supplies, in which metabolism decreases, the heart and respiratory system slow down, and body temperature is maintained at a lower level than normal.
high-density lipoprotein (HDL)- A cholesterolcarrying particle in the blood, made up of cholesterol and other lipids surrounded by a single layer of phospholipids in which proteins are embedded. An HDL particle carries less cholesterol than a related lipoprotein, LDL, and may be correlated with a decreased risk of blood vessel blockage.
hindbrain- One of three ancestral and embryonic regions of the vertebrate brain; develops into the medulla oblongata, pons, and cerebellum.
histamine - A substance released by mast cells that causes blood vessels to dilate and become more permeable during an inflammatory response.
histone - A small protein with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in its chromatin structure.
histone acetylation- The attachment of acetyl groups to certain amino acids of histone proteins.
HIV (human immunodeficiency virus)- The infectious agent that causes AIDS. HIV is a retrovirus.
holdfast A rootlike structure that anchors a seaweed.
holoblastic cleavage- A type of cleavage in which there is complete division of the egg, as in eggs having little yolk (sea urchin) or a moderate amount of yolk (frog).
homeobox - A I80-nucleotide sequence within home otic genes and some other developmental genes that is widely conserved in animals. Related sequences occur in plants and prokaryotes.
homeostasis - The steadystate physiological condition of the body
homeotic gene- Any of the genes that control the overall body plan of animals and plants by controlling the developmental fate of groups of cells.
hominid- A species on the human branch of the evolutionary tree; a member of the family Hominidae, including Homo sapiens and our ancestors.
hominoid- A term that refers to great apes and humans.
homologous chromosomes- Chromosome pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism's father, the other from the mother.
homologous structures- Structures in different species that are similar because of common ancestry
homology- Similarity in characteristics resulting from a shared ancestry
homoplasy- Similar (analogous) structure or molecular sequence that has evolved independently in two species.
homosporous- A term referring to a plant species that has a single kind of spore, which typically develops into a bisexual gametophyte.
homozygous- Having two identical alleles for a given gene.
horizon- A distinct layer of soil, such as topsoil.
horizontal cell- A neuron of the retina that helps integrate information before it is sent to the brain.
hormone- In multicellular organisms, one of many types of circulating chemical signals that are formed in specialized cells, travel in body fluids, and act on specific target cells to change their functioning.
hornwort - A small, herbaceous nonvascular plant that is a member of the phylum Anthocerophyta.
host- The larger participant in a symbiotic relationship, serving as home and feeding ground to the symbiont.
host range- The limited range of host cells that each type of virus can infect and parasitize.
human chorionic gonadotropin (HCG)- A hormone secreted by the chorion that maintains the corpus luteum of the ovary during the first three months of pregnancy
Human Genome Project- An international collaborative effort to map and sequence the DNA of the entire human genome.
humoral immune response- The branch of acquired immunity that involves the activation of B cells and that leads to the production of antibodies, which defend against bacteria and viruses in body fluids.
humus- Decomposing organic material found in topsoil.
Huntington's disease - A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.
hybridization- In genetics, the mating, or crossing, of two true-breeding varieties.
hydration shell - The sphere of water molecules around each dissolved ion.
hydrocarbon- An organic molecule consisting only of carbon and hydrogen.
hydrogen bond- A type of weak chemical bond formed when the slightly positive hydrogen atom of a polar covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent bond in another molecule.
hydrogen ion- A single proton with a charge of 1+. The dissociation of a water molecule (H2O) leads to the generation of a hydroxide ion (OH-) and a hydrogen ion (H+).
hydrolysis - A chemical process that lyses, or splits, molecules by the addition of water.
hydrophilic - Having an affinity for water.
hydrophobic- Having an aversion to water; tending to coalesce and form droplets in water.
hydrophobic interaction- A type of weak chemical bond formed when molecules that do not mix with water coalesce to exclude the water.
hydroponic culture- A method in which plants are grown without soil by using mineral solutions.
hydrostatic skeleton - A skeletal system composed of fluid held under pressure in a closed body compartment; the main skeleton of most cnidarians, flatworms, nematodes, and annelids.
hydroxide ion- A water molecule that has lost a proton; OH-.
hydroxyl group- A functional group consisting of a hydrogen atom joined to an oxygen atom by a polar covalent bond. Molecules possessing this group are soluble in water and are called alcohols.
hymen- A thin membrane that partly covers the vaginal opening in the human female. The hymen is ruptured by sexual intercourse or other vigorous activity.
hyperpolarization- An electrical state in which the inside of the cell is more negative relative to the outside than at the resting membrane potential. A neuron membrane is hyperpolarized if a stimulus increases its voltage from the resting potential of -70 mY, reducing the chance that the neuron will transmit a nerve impulse.
hypersensitive response (HR)- A plant's localized defense response to a pathogen.
hypertension- Chronically high blood pressure within the arteries.
hypertonic- In comparing two solutions, referring to the one with a greater solute concentration.
hypha - A filament that collectively makes up the body of a fungus.
hypocotyl - In an angiosperm embryo, the embryonic axis below the point of attachment of the cotyledon(s) and above the radicle.
hypothalamus- The ventral part of the vertebrate forebrain; functions in maintaining homeostasis, especially in coordinating the endocrine and nervous systems; secretes hormones of the posterior pituitary and releasing factors that regulate the anterior pituitary
hypothesis- A tentative answer to a well-framed question.
hypotonic- In comparing two solutions, referring to the one with a lower solute concentration.
I band- The area near the edge of the sarcomere where there are only thin filaments.
immigration- The influx of new individuals from other areas.
immunization- The process of generating a state of immunity by artifical means. In active immunization, a nonpathogenic version of a normally pathogenic microbe is administered, inducing Band T cell responses and immunological memory In passive immunization, antibodies specific for a particular microbe are administered, conferring immediate but temporary protection. Also called vaccination.
immunoglobulin (Ig) - Any of the class of proteins that function as antibodies. Immunoglobulins are divided into five major classes that differ in their distribution in the body and antigen disposal activities.
imperfect fungus- See deuteromycete.
imprinting- A type of learned behavior with a significant innate component, acquired during a limited critical period.
in vitro fertilization - Fertilization of ova in laboratory containers followed by artificial implantation of the early embryo in the mother's uterus.
in vitro mutagenesis- A technique to discover the function of a gene by introducing specific changes into the sequence of a cloned gene, reinserting the mutated gene into a cell, and studying the phenotype of the mutant.
inclusive fitness- The total effect an individual has on proliferating its genes by producing its own offspring and by providing aid that enables other close relatives to increase the production of their offspring.
incomplete dominance - The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.
incomplete flower- A flower in which one or more of the four basic floral organs (sepals, petals, stamens, or carpels) are either absent or nonfunctional.
incomplete metamorphosis- A type of development in certain insects, such as grasshoppers, in which the young (called nymphs) resemble adults but are smaller and have different body proportions. The nymph goes through a series of molts, each time looking more like an adult, until it reaches full size.
incus - The second of the three middle ear bones.
indeterminate cleavage- A type of embryonic development in deuterostomes, in which each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo.
indeterminate growth- A type of growth characteristic of plants, in which the organism continues to grow as long as it lives.
individualistic hypothesis- The concept, put forth by H. A. Gleason, that a plant community is a chance assemblage of species found in the same area simply because they happen to have similar biotic requirements.
induced fit- The change in shape of the active site of an enzyme so that it binds more snugly to the substrate, induced by entry of the substrate.
inducer- A specific small molecule that inactivates the repressor in an operon.
induction- The ability of one group of embryonic cells to influence the development of another.
inductive reasoning - A type of logic in which generalizations are based on a large number of specific observations.
infant mortality- The number of infant deaths per 1,000 live births.
inflammatory response- A localized innate immune defense triggered by physical injury or infection of tissue in which changes to nearby small blood vessels enhance the infiltration of white blood cells, antimicrobial proteins, and clotting elements that aid in tissue repair and destruction of invading pathogens; may also involve systemic effects such as fever and increased production of white blood cells.
inflorescence - A group of flowers tightly clustered together.
ingestion - A heterotrophic mode of nutrition in which other organisms or detritus are eaten whole or in pieces.
ingroup- In a cladistic study of evolutionary relationships among taxa of organisms, the group of taxa that is actually being analyzed.
inhibitory postsynaptic potential (IPSP) - An electrical charge (hyperpolarization) in the membrane of a postsynaptic neuron caused by the binding of an inhibitory neurotransmitter from a presynaptic cell to a postsynaptic receptor; makes it more difficult for a postsynaptic neuron to generate an action potential.
initials- Cells that remain within an apical meristem as sources of new cells.
innate behavior- Behavior that is developmentally fixed and under strong genetic control. Innate behavior is exhibited in virtually the same form by all individuals in a population despite internal and external environmental differences during development and throughout their lifetimes.
innate immunity- The kind of defense that is mediated by phagocytic cells, antimicrobial proteins, the inflammatory response, and natural killer (NK) cells. It is present before exposure to pathogens and is effective from the time of birth.
inner cell mass- A cluster of cells in a mammalian blastocyst that protrudes into one end of the cavity and subsequently develops into the embryo proper and some of the extraembryonic membranes.
inner ear- One of three main regions of the vertebrate ear; includes the cochlea, organ of Corti, and semicircular canals.
inositol trisphosphate (IP3) - A second messenger that functions as an intermediate between certain nonsteroid hormones and a third messenger, a rise in cytoplasmic Ca2+ concentration.
inquiry- The search for information and explanation, often focused by specific questions.
insertion - A mutation involving the addition of one or more nucleotide pairs to a gene.
insertion sequence- The simplest kind of transposable element, consisting of inverted repeats of DNA flanking a gene for transposase, the enzyme that catalyzes transposition.
insulin - A hormone secreted by pancreatic beta cells that lowers blood glucose levels. It promotes the uptake of glucose by most body cells and the synthesis and storage of glycogen in the liver and also stimulates protein and fat synthesis.
insulin-like growth factor- A hormone produced by the liver whose secretion is stimulated by growth hormone. It directly stimulates bone and cartilage growth.
integral protein- Typically a transmembrane protein with hydrophobic regions that completely spans the hydrophobic interior of the membrane.
integrated hypothesis - The concept, put forth by E E. Clements, that a community is an assemblage of closely linked species, locked into association by mandatory biotic interactions that cause the community to function as an integrated unit, a sort of superorganism.
integration- The interpretation of sensory signals within neural processing centers of the central nervous system.
integrin- A receptor protein built into the plasma membrane that interconnects the extracellular matrix and the cytoskeleton.
integument - Layer of sporophyte tissue that contributes to the structure of an ovule of a seed plant.
integumentary system- The outer covering of a mammal's body, including skin, hair, and nails.
intercalated disk - A specialized junction between cardiac muscle cells that provides direct electrical coupling between cells.
interferon - A protein that has antiviral or immune regulatory functions. Interferon-a and interferon-l3, secreted by virus-infected cells, help nearby cells resist viral infection; interferon-y, secreted by T cells, helps activate macrophages.
intermediate disturbance hypothesis - The concept that moderate levels of disturbance can foster greater species diversity than low or high levels of disturbance.
intermediate filament- A component of the cytoskeleton that includes all filaments intermediate in size between microtubules and microfilaments.
internal fertilization- Reproduction in which sperm are typically deposited in or near the female reproductive tract and fertilization occurs within the tract.
interneuron - An association neuron; a nerve cell within the central nervous system that forms synapses with sensory and motor neurons and integrates sensory input and motor output.
internode- A segment of a plant stem between the points where leaves are attached.
interoparity- A life history in which adults produce large numbers of offspring over many years; also known as repeated reproduction.
interoreceptor- A sensory receptor that detects stimuli within the body, such as blood pressure and body position.
interphase- The period in the cell cycle when the cell is not dividing. During interphase, cellular metabolic activity is high, chromosomes and organelles are duplicated, and cell size may increase. Interphase accounts for 90% of the cell cycle.
intersexual selection- Selection whereby individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex; also called mate choice.
interspecific competition- Competition for resources between plants, between animals, or between decomposers when resources are in short supply.
interspecific interaction - Relationships between species of a community
interstitial fluid - The internal environment of vertebrates, consisting of the fluid filling the spaces between cells.
intertidal zone- The shallow zone of the ocean where land meets water.
intracellular digestion- The joining of food vacuoles and lysosomes to allow chemical digestion to occur within the cytoplasm of a cell.
intrasexual selection- A direct competition among individuals of one sex (usually the males in vertebrates) for mates of the opposite sex.
introduced species- A species moved by humans, either intentionally or accidentally, from its native location to a new geographic region; also caned an exotic species.
intron - A noncoding, intervening sequence within a eukaryotic gene.
invagination The infolding of cells.
invasive species A species that takes hold outside of its native range; usually introduced by humans.
inversion - An aberration in chromosome structure resulting from reattachment in a reverse orientation of a chromosomal fragment to the chromosome from which the fragment originated.
invertebrate- An animal without a backbone. Invertebrates make up 95% of animal species.
involution- Cells rolling over the edge of the lip of the blastopore into the interior of the embryo during gastrulation.
ion - An atom that has gained or lost electrons, thus acquiring a charge.
ion channel- Protein channel in a cell membrane that allows passage of a specific ion down its concentration gradient.
ionic bond - A chemical bond resulting from the attraction between oppositely charged ions.
ionic compound- A compound resulting from the formation of an ionic bond; also called a salt.
islets of Langerhans- Clusters of endocrine cells within the pancreas that produce and secrete the hormones glucagon (alpha cells) and insulin (beta cells).
isomer- One of several organic compounds with the same molecular formula but different structures and therefore different properties. The three types of isomers are structural isomers, geometric isomers, and enantiomers.
isomorphic- Referring to alternating generations in plants and certain algae in which the sporophytes and gametophytes look alike, although they differ in chromosome number.
isopod- A member of one of the largest groups of crustaceans, which includes terrestrial, freshwater, and marine species. Among the terrestrial isopods are the pill bugs, or wood lice.
isotonic - Having the same solute concentration as another solution.
isotope - One of several atomic forms of an element, each containing a different number of neutrons and thus differing in atomic mass.
jasmonic acid - An important molecule in plant defense against herbivores.
joule (J)- A unit of energy: 1 J = 0.239 cal; 1 cal = 4.184 J
juvenile hormone - A hormone in arthropods, secreted by the corpora allata glands, that promotes the retention of larval characteristics.
- A specialized tissue that releases the enzyme renin when blood pressure or blood volume drops in the afferent arteriole that supplies blood to the glomerulus.
- Nephrons with welldeveloped loops of Henle that extend deeply into the renal medulla.
- The fusion of nuclei of two cells, as part of syngamy.
karyotype - A display of the chromosome pairs of a cell arranged by size and shape.
keystone species - A species that is not necessarily abundant in a community yet exerts strong control on community structure by the nature of its ecological role or niche.
kilocalorie (kcal) - A thousand calories; the amount of heat energy required to raise the temperature of 1 kg of water by 1°C.
kin selection - A phenomenon of inclusive fitness, used to explain altruistic behavior between related individuals.
kinesis - A change in activity or turning rate in response to a stimulus.
kinetic energy - The energy of motion, which is directly related to the speed of that motion. Moving matter does work by imparting motion to other matter.
kinetochore - A specialized region on the centromere that links each sister chromatid to the mitotic spindle.
kinetoplastid - A protist, such as Trypanosoma, which has a single large mitochondrion that houses extranuclear DNA.
kingdom - A taxonomic category, the second broadest after domain.
K-selection - The concept that in certain (K-selected) populations, life history is centered around producing relatively few offspring that have a good chance of survival.
labia majora - A pair of thick, fatty ridges that enclose and protect the labia minora and vestibule.
labia minora - A pair of slender skin folds that enclose and protect the vestibule.
labor - A series of strong, rhythmic contractions of the uterus that expel a baby out of the uterus and vagina during childbirth.
lactation - The continued production of milk.
lacteal - A tiny lymph vessel extending into the core of an intestinal villus and serving as the destination for absorbed chylomicrons.
lactic acid - fermentation The conversion of pyruvate to lactate with no release of carbon dioxide.
lagging strand - A discontinuously synthesized DNA strand that elongates in a direction away from the replication fork.
lancelet - Member of the subphylum Cephalochordata, small blade-shaped marine chordates that lack a backbone.
landmark - A point of reference for orientation during navigation.
landscape - Several different, primarily terrestrial ecosystems linked by exchanges of energy, materials, and organisms.
landscape ecology - The study of past, present, and future patterns of landscape use, as well as ecosystem management and the biodiversity of interacting ecosystems.
large intestine - The tubular portion of the vertebrate alimentary tract between the small intestine and the anus; functions mainly in water absorption and the formation of feces.
larva - A free-living, sexually immature form in some animal life cycles that may differ from the adult in morphology, nutrition, and habitat.
larynx - The voice box, containing the vocal cords.
lateral geniculate nuclei - The destination in the thalamus for most of the ganglion cell axons that form the optic nerves.
lateral inhibition - A process that sharpens the edges and enhances the contrast of a perceived image by inhibiting receptors lateral to those that have responded to light.
lateralization - Segregation of functions in the cortex of the left and right hemispheres of the brain.
lateral line system - A mechanoreceptor system consisting of a series of pores and receptor units (neuromasts) along the sides of the body in fishes and aquatic amphibians; detects water movements made by the animal itself and by other moving objects.
lateral meristem - A meristem that thickens the roots and shoots of woody plants. The vascular cambium and cork cambium are lateral meristems.
lateral root - A root that arises from the outermost layer of the pericycle of an established root.
law of independent assortment - Mendel's second law, stating that each pair of alleles segregates independently during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes.
law of segregation - Mendel's first law, stating that each allele in a pair separates into a different gamete during gamete formation.
leading strand - The new continuous complementary DNA strand synthesized along the template strand in the mandatory 5' -> 3' direction.
leaf - The main photosynthetic organ of vascular plants.
leaf primordia - Fingerlike projections along the flanks of a shoot apical meristem, from which leaves arise.
leaf trace - A small vascular bundle that extends from the vascular tissue of the stem through the petiole and into a leaf.
learning - A behavioral change resulting from experience.
lens - The structure in an eye that focuses light rays onto the retina.
lenticels - Small raised areas in the bark of stems and roots that enable gas exchange between living cells and the outside air.
lepidosaur - Member of the reptilian group that includes lizards, snakes, and two species of New Zealand animals called tuataras.
leukocyte - A white blood cell; typically functions in immunity, such as phagocytosis or antibody production.
Leydig cell - A cell that produces testosterone and other androgens and is located between the seminiferous tubules of the testes.
lichen - The symbiotic collective formed by the mutualistic association between a fungus and a photosynthetic alga or cyanobacterium.
life cycle - The generation-to-generation sequence of stages in the reproductive history of an organism.
life expectancy at birth - The predicted average length of life at birth.
life history - The series of events from birth through reproduction and death.
life table - A table of data summarizing mortality in a population.
ligamen - t A type of fibrous connective tissue that joins bones together at Joints.
ligand (lig'-und) A molecule that binds specifically to a receptor site of another molecule.
ligand-gated ion channel - A protein pore in the plasma membrane that opens or closes in response to a chemical signal, allowing or blocking the flow of specific ions.
light chain - One of the two types of polypeptide chains that make up an antibody molecule and B cell receptor; consists of a variable region, which contributes to the antigenbinding site, and a constant region.
light-harvesting complex - Complex of proteins associated with pigment molecules (including chlorophyll a, chlorophyll b, and carotenoids) that captures light energy and transfers it to reaction-center pigments in a photosystem.
light microscope - An optical instrument with lenses that refract (bend) visible light to magnify images of specimens.
light reactions - The steps in photosynthesis that occur on the thylakoid membranes of the chloroplast and that convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen in the process.
lignin - A hard material embedded in the cellulose matrix of vascular plant cell walls that functions as an important adaptation for support in terrestrial species.
limbic system - A group of nuclei (clusters of nerve cell bodies) in the lower part of the mammalian forebrain that interact with the cerebral cortex in determining emotions; includes the hippocampus and the amygdala.
limiting nutrient - An element that must be added for production to increase in a particular area.
limnetic zone - In a lake, the well-lit, open surface waters farther from shore.
linkage map - A genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes.
linked genes - Genes located close enough together on a chromosome to be usually inherited together.
lipid - One of a family of compounds, including fats, phospholipids, and steroids, that are insoluble in water.
littoral zone - In a lake, the shallow, well-lit waters close to shore.
liver - The largest organ in the vertebrate body. The liver performs diverse functions, such as producing bile, preparing nitrogenous wastes for disposal, and detoxifying poisonous chemicals in the blood.
liverwort - A small, herbaceous nonvascular plant that is a member of the phylum Hepatophyta.
loam - The most fertile of all soils, made up of roughly equal amounts of sand, silt, and clay.
lobe-fin - Member of the vertebrate subgroup Sarcopterygii, osteichthyans with rodshaped muscular fins, including coelacanths and lungfishes, as well as the lineage that gave rise to tetrapods.
local regulator - A chemical messenger that influences cells in the vicinity.
locomotion - Active movement from place to place.
locus - A specific place along the length of a chromosome where a given gene is located.
logistic population growth - A model describing population growth that levels off as population size approaches carrying capacity.
long-day plant - A plant that flowers (usually in late spring or early summer) only when the light period is longer than a critical length.
long-term memory - The ability to hold, associate, and recall information over one's life.
long-term potentiation (LTP) - An enhanced responsiveness to an action potential (nerve signal) by a receiving neuron.
loop of Henle - The long hairpin turn, with a descending and ascending limb, of the renal tubule in the vertebrate kidney; functions in water and salt reabsorption.
loose connective tissue - The most widespread connective tissue in the vertebrate body. It binds epithelia to underlying tissues and functions as packing material, holding organs in place.
lophophore - A horseshoe-shaped or circular fold of the body wall bearing ciliated tentacles that surround the mouth.
lophotrochozoan - Member of a group of animal phyla with protostome development that some systematists hypothesize form a clade, characterized by lophophores or trochophore larvae.
low-density lipoprotein (LDL) - A cholesterolcarrying particle in the blood, made up of cholesterol and other lipids surrounded by a single layer of phospholipids in which proteins are embedded. An LDL particle carries more cholesterol than a related lipoprotein, HDL, and high LDL levels in the blood correlate with a tendency to develop blocked blood vessels and heart disease.
lung - An invaginated respiratory surface of terrestrial vertebrates, land snails, and spiders that connects to the atmosphere by narrow tubes.
luteal phase - That portion of the ovarian cycle during which endocrine cells of the corpus luteum secrete female hormones.
luteinizing hormone (LH) - A tropic hormone produced and secreted by the anterior pituitary that stimulates ovulation in females and androgen production in males.
lycophyte - An informal name for any member of the phylum Lycophyta, which includes club mosses, spike mosses, and quillworts.
lymph - The colorless fluid, derived from interstitial fluid, in the lymphatic system of vertebrate animals.
lymph node - Organ located along a lymph vessel. Lymph nodes filter lymph and help attack viruses and bacteria.
lymphatic system - A system of vessels and lymph nodes, separate from the circulatory system, that returns fluid, proteins, and cells to the blood.
lymphocyte - A type of white blood cell that mediates acquired immunity. Lymphocytes that complete their development in the bone marrow are called B cells, and those that mature in the thymus are called T cells.
lysogenic cycle - A phage replication cycle in which the viral genome becomes incorporated into the bacterial host chromosome as a prophage and does not kill the host.
lysosome - A membrane-enclosed sac of hydrolytic enzymes found in the cytoplasm of eukaryotic cells.
lysozyme - An enzyme in sweat, tears, and saliva that attacks bacterial cell walls.
lytic cycle - A type of viral (phage) replication cycle resulting in the release of new phages by lysis (and death) of the host cell.
M phase - See mitotic (M) phase.
macroclimate - Large-scale variations in climate; the climate of an entire region.
macroevolution - Evolutionary change above the species level, including the appearance of major evolutionary developments, such as flight, that we use to define higher taxa.
macromolecule - A giant molecule formed by the joining of smaller molecules, usually by a condensation reaction. Polysaccharides, proteins, and nucleic acids are macromolecules.
macronutrient - A chemical substance that an organism must obtain in relatively large amounts. See also micronutrient.
macrophage - A phagocytic cell present in many tissues that functions in innate immunity by destroying microbes and in acquired immunity as an antigenpresenting cell.
magnetic reversal - A reversal of the polarity of Earth's magnetic field.
magnoliids - A flowering plant clade that evolved later than basal angiosperms but before mono cots and eudicots. Extant examples are magnolias, laurels, and black pepper plants.
major depression - Depressive mental illness characterized by experiencing a low mood most of the time.
major histocompatibility complex (MHC) - A family of genes that encode a large set of cell surface proteins called MHC molecules. Class I and class II MHC molecules function in antigen presentation to T cells. Foreign MHC molecules on transplanted tissue can trigger T cell responses that may lead to rejection of the transplant.
malignant tumor - A cancerous tumor that is invasive enough to impair the functions of one or more organs.
malleus - The first of the three middle ear bones.
malnourished - Referring to an animal whose diet is missing one or more essential nutrients.
Malpighian tubule - A unique excretory organ of insects that empties into the digestive tract, removes nitrogenous wastes from the hemolymph, and functions in osmoregulation.
mammal Member of the class Mammalia, amniotes with mammary glands that produce milk.
mammary glands - Exocrine glands that secrete milk to nourish the young. These glands are characteristic of mammals.
mandible - One of a pair of jaw-like feeding appendages found in myriapods, hexapods, and crustaceans.
mantle - A fold of tissue in molluscs that drapes over the visceral mass and may secrete a shell.
mantle cavity - A water-filled chamber that houses the gills, anus, and excretory pores of a mollusc.
map unit - A unit of measurement of the distance between genes. One map unit is equivalent to a I % recombination frequency.
marine benthic zone - The ocean floor.
mark-recapture method - A sampling technique used to estimate wildlife populations.
marsupial - A mammal, such as a koala, kangaroo, or opossum, whose young complete their embryonic development inside a maternal pouch called the marsupium.
mass number - The sum of the number of protons and neutrons in an atom's nucleus.
mast cell - A vertebrate body cell that produces histamine and other molecules that trigger the inflammatory response.
mate choice copying - Behavior in which individuals in a population copy the mate choice of others, apparently as a result of social learning.
maternal effect gene - A gene that, when mutant in the mother, results in a mutant phenotype in the offspring, regardless of the genotype.
matter - Anything that takes up space and has mass.
maximum likelihood - A principle that states that when considering multiple phylogenetic hypotheses, one should take into account the one that reflects the most likely sequence of evolutionary events, given certain rules about how DNA changes over time.
maximum parsimony - A principle that states that when considering multiple explanations for an observation, one should first investigate the simplest explanation that is consistent with the facts.
mechanoreceptor - A sensory receptor that detects physical deformations in the body's environment associated with pressure, touch, stretch, motion, and sound.
medulla oblongata - The lowest part of the vertebrate brain, commonly called the medulla; a swelling of the hindbrain dorsal to the anterior spinal cord that controls autonomic, homeostatic functions, including breathing, heart and blood vessel activity, swallowing, digestion, and vomiting.
medusa - The floating, flattened, mouth-down version of the cnidarian body plan. The alternate form is the polyp.
megapascal - A unit of pressure equivalent to 10 atmospheres of pressure.
megaphyll - A leaf with a highly branched vascular system, characteristic of the vast majority of vascular plants.
megaspore - A spore from a heterosporous plant species that develops into a female gametophyte.
meiosis - A two-stage type of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell.
meiosis I - The first division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell.
meiosis II - The second division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell.
melanocyte-stimulating hormone (MSH) - A hormone produced and secreted by the anterior pituitary that regulates the activity of pigment-containing cells in the skin of some vertebrates.
melatonin - A hormone secreted by the pineal gland that regulates body functions related to seasonal day length.
membrane attack complex (MAC) - A molecular complex consisting of a set of complement proteins that forms a pore in the membrane of bacterial and transplanted cells, causing the cells to die by lysis.
membrane potential - The charge difference between a cell's cytoplasm and the extracellular fluid, due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances.
memory cell - One of a clone of long-lived lymphocytes, formed during the primary immune response, that remains in a lymphoid organ until activated by exposure to the same antigen that triggered its formation. Activated memory cells mount the secondary immune response.
menopause - The cessation of ovulation and menstruation.
menstrual cycle - A type of reproductive cycle in higher female primates, in which the nonpregnant endometrium is shed as a bloody discharge through the cervix into the vagina.
menstrual flow phase - That portion of the uterine (menstrual) cycle when menstrual bleeding occurs.
menstruation - The shedding of portions of the endometrium during a uterine (menstrual) cycle.
meristem - Plant tissue that remains embryonic as long as the plant lives, allowing for indeterminate growth.
meristem identity gene - A plant gene that promotes the switch from vegetative growth to flowering.
meroblastic cleavage - A type of cleavage in which there is incomplete division of yolk-rich egg, characteristic of avian development.
mesentery - A membrane that suspends many of the organs of vertebrates inside fluid-filled body cavities.
mesoderm - The middle primary germ layer of an early embryo that develops into the notochord, the lining of the coelom, muscles, skeleton, gonads, kidneys, and most of the circulatory system.
mesohyl - A gelatinous region between the two layers of cells of a sponge.
mesophyll - The ground tissue of a leaf, sandwiched between the upper and lower epidermis and specialized for photosynthesis.
mesophyll cell - A loosely arranged photosynthetic cell located between the bundle sheath and the leaf surface.
messenger RNA (mRNA) - A type of RNA, synthesized from DNA, that attaches to ribosomes in the cytoplasm and specifies the primary structure of a protein.
metabolic pathway - A series of chemical reactions that either builds a complex molecule (anabolic pathway) or breaks down a complex molecule into simpler compounds (catabolic pathway).
metabolic rate - The total amount of energy an animal uses in a unit of time.
metabolism - The totality of an organism's chemical reactions, consisting of catabolic and anabolic pathways.
metamorphosis - The resurgence of development in an animal larva that transforms it into a sexually mature adult.
metanephridium - In annelid worms, a type of excretory tubule with internal openings called nephrostomes that collect body fluids and external openings called nephridiopores.
metaphase - The third stage of mitosis, in which the spindle is complete and the chromosomes, attached to micro tubules at their kinetochores, are all aligned at the metaphase plate.
metaphase plate - An imaginary plane during metaphase in which the centromeres of all the duplicated chromosomes are located midway between the two poles.
metapopulation - A subdivided population of a single species.
metastasis - The spread of cancer cells to locations distant from their original site.
methanogen - A microorganism that obtains energy by using carbon dioxide to oxidize hydrogen, producing methane as a waste product.
microclimate - Very fine scale variations of climate, such as the specific climatic conditions underneath a log.
microevolution - Evolutionary change below the species level; change in the genetic makeup of a population from generation to generation.
microfilament - A solid rod of actin protein in the cytoplasm of almost all eukaryotic cells, making up part of the cytoskeleton and acting alone or with myosin to cause cell contraction.
micronutrient - An element that an organism needs in very small amounts and that functions as a component or cofactor of enzymes. See also macronutrient.
microphyll - In lycophytes, a small leaf with a single unbranched vein.
micropyle - A pore in the integument(s) of an ovule.
micro-RNA (miRNA) - A small, single-stranded RNA molecule that binds to a complementary sequence in mRNA molecules and directs associated proteins to degrade or prevent translation of the target mRNA.
microspore - A spore from a heterosporous plant species that develops into a male gametophyte.
microsporidia - Unicellular parasites of animals and protists that molecular comparisons suggest may be most closely related to zygomycete fungi.
microtubule - A hollow rod of tubulin protein in the cytoplasm of all eukaryotic cells and in cilia, flagella, and the cytoskeleton.
microvillus - One of many fine, fingerlike projections of the epithelial cells in the lumen of the small intestine that increase its surface area.
midbrain - One of three ancestral and embryonic regions of the vertebrate brain; develops into sensory integrating and relay centers that send sensory information to the cerebrum.
middle ear - One of three main regions of the vertebrate ear; a chamber containing three small bones (the hammer, anvil, and stirrup) that convey vibrations from the eardrum to the oval window.
middle lamella - A thin layer of adhesive extracellular material, primarily pectins, found between the primary walls of adjacent young plant cells.
mineral - In nutrition, a chemical element other than hydrogen, oxygen, or nitrogen that an organism requires for proper body functioning.
mineral nutrient - An essential chemical element absorbed from the soil in the form of inorganic ions.
mineralocorticoid - A steroid hormone secreted by the adrenal cortex that regulates salt and water homeostasis.
minimum viable population (MVP) - The smallest population size at which a species is able to sustain its numbers and survive.
mismatch repair - The cellular process that uses special enzymes to fix incorrectly paired nucleotides.
missense mutation - The most common type of mutation, a base-pair substitution in which the new codon makes sense in that it still codes for an amino acid.
mitochondrial matrix - The compartment of the mitochondrion enclosed by the inner membrane and containing enzymes and substrates for the Krebs cycle.
mitochondrion - An organelle in eukaryotic cells that serves as the site of cellular respiration.
mitosis - A process of nuclear division in eukaryotic cells conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis conserves chromosome number by equally allocating replicated chromosomes to each of the daughter nuclei.
mitotic (M) phase - The phase of the cell cycle that includes mitosis and cytokinesis.
mitotic spindle - An assemblage of microtubules and associated proteins that is involved in the movements of chromosomes during mitosis.
mixotroph - An organism that is capable of both photosynthesis and heterotrophy.
model - A representation of a theory or process.
model organism - An organism chosen to study broad biological principles.
modern synthesis - A comprehensive theory of evolution emphasizing populations as units of evolution and integrating ideas from many fields, including genetics, statistics, paleontology, taxonomy, and biogeography.
molarity - A common measure of solute concentration, referring to the number of moles of solute per liter of solution.
mold - A rapidly growing fungus that reproduces asexually by producing spores.
mole (mol) - The number of grams of a substance that equals its molecular weight in daltons and contains Avogadro's number of molecules.
molecular clock - An evolutionary timing method based on the observation that at least some regions of genomes evolve at constant rates.
molecular formula - A type of molecular notation indicating only the quantity of the constituent atoms.
molecular mass - The sum of the masses of all the atoms in a molecule; sometimes called molecular weight.
molecular systematics - The comparison of nucleic acids or other molecules in different species to infer relatedness.
molecule - Two or more atoms held together by covalent bonds.
molting - A process in arthropods in which the exoskeleton is shed at intervals, allowing growth by the production of a larger exoskeleton.
monoclonal antibody - Any of a preparation of antibodies that have been produced by a single clone of cultured cells and thus are all specific for the same epitope.
monocots - A clade consisting of flowering plants that have one embryonic seed leaf, or cotyledon.
monocyte - A type of white blood cell that migrates into tissues and develops into a macrophage.
monoecious - A term typically used to describe an angiosperm species in which carpellate and staminate flowers are on the same plant.
monogamous - A type of relationship in which one male mates with just one female.
monohybrid - An organism that is heterozygous with respect to a single gene of interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids. For example, parents of genotypes AA and aa produce a monohybrid of genotype Aa.
monomer - The subunit that serves as the building block of a polymer.
monophyletic - Pertaining to a grouping of species consisting of an ancestral species and all its descendants; a clade.
monosaccharide - The simplest carbohydrate, active alone or serving as a monomer for disaccharides and polysaccharides. Also known as simple sugars, the molecular formulas of monosaccharides are generally some multiple of CH2O.
monosomic - Referring to a cell that has only one copy of a particular chromosome, instead of the normal two.
monotreme - An egg-laying mammal, represented by the platypus and echidna.
morphogen - A substance, such as Bicoid protein, that provides positional information in the form of a concentration gradient along an embryonic axis.
morphogenesis - The development of body shape and organization.
morphological species concept - Defining species by measurable anatomical criteria.
morula - A solid ball of blastomeres formed by early cleavage.
mosaic evolution - The evolution of different features of organisms at different rates.
moss - A small, herbaceous nonvascular plant that is a member of the phylum Bryophyta.
motor neuron - A nerve cell that transmits signals from the brain or spinal cord to muscles or glands.
motor unit - A single motor neuron and all the muscle fibers it controls.
movement corridor - A series of small clumps or a narrow strip of quality habitat (usable by organisms) that connects otherwise isolated patches of quality habitat.
mucous membrane - Smooth moist epithelium that lines the digestive tract and air tubes leading to the lungs.
Mullerian mimicry - A mutual mimicry by two unpalatable species.
multifactorial - Referring to a phenotypic character that is influenced by multiple genes and environmental factors.
multigene family - A collection of genes with similar or identical sequences, presumably of common origin.
multiple fruit - A fruit derived from an inflorescence, a group of flowers tightly clustered together.
muscle spindle - A mechanoreceptor stimulated by mechanical distortion.
muscle tissue - Tissue consisting of long muscle cells that are capable of contracting when stimulated by nerve impulses.
mutagen - A chemical or physical agent that interacts with DNA and causes a mutation.
mutation- A change in the DNA of a gene, ultimately creating genetic diversity.
mutualism - A symbiotic relationship in which both participants benefit.
mycelium - The densely branched network of hyphae in a fungus.
mycorrhizae - Mutualistic associations of plant roots and fungi.
mycosis - The general term for a fungal infection.
myelin sheath - In a neuron, an insulating coat of cell membrane from Schwann cells that is interrupted by nodes of Ranvier, where saltatory conduction occurs.
myofibril - A fibril collectively arranged in longitudinal bundles in muscle cells (fibers); composed of thin filaments of actin and a regulatory protein and thick filaments of myosin.
myofilaments - The thick and thin filaments that form the myofibrils.
myogenic heart - A type of heart, such as in vertebrate animals, in which the pacemaker is made up of specialized muscle tissues and located within the heart itself.
myoglobin - An oxygenstoring, pigmented protein in muscle cells.
myosin - A type of protein filament that interacts with actin filaments to cause cell contraction.
myotonia - Increased muscle tension.
myriapod - A terrestrial arthropod with many body segments and one or two pairs of legs per segment. Millipedes and centipedes comprise the two classes of living myriapods.
NAD+ - Nicotinamide adenine dinucleotide, a coenzyme present in all cells that helps enzymes transfer electrons during the redox reactions of metabolism.
NADP+ - Nicotinamide adenine dinucleotide phosphate, an acceptor that temporarily stores energized electrons produced during the light reactions.
natural family planning - A form of contraception that relies on refraining from sexual intercourse when conception is most likely to occur; also called the rhythm method.
natural killer (NK) cell - A type of white blood cell that can kill tumor cells and virusinfected cells; an important component of innate immunity.
natural selection - Differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. Evolution occurs when natural selection causes changes in relative frequencies of alleles in the gene pool.
negative feedback - A primary mechanism of homeostasis, whereby a change in a physiological variable that is being monitored triggers a response that counteracts the initial fluctuation.
negative pressure breathing - A breathing system in which air is pulled into the lungs.
nematocyst - A stinging, capsule-like organelle in a cnidocyte.
neocortex - In the mammalian brain, the outermost region of the cerebral cortex.
nephron - The tubular excretory unit of the vertebrate kidney.
neritic zone - The shallow region of the ocean overlying the continental shelf.
nerve - A ropelike bundle of neuron fibers (axons and dendrites) tightly wrapped in connective tissue.
nerve cord - A ropelike arrangement of neurons characteristic of animals with bilateral symmetry and cephalization.
nerve net - A weblike system of neurons, characteristic of radially symmetrical animals, such as hydra.
nervous tissue - Tissue made up of neurons and supportive cells.
net primary production (NPP) - The gross primary production of an ecosystem minus the energy used by the producers for respiration.
neural crest - A band of cells along the border where the neural tube pinches off from the ectoderm. The cells migrate to various parts of the embryo and form the pigment cells in the skin, bones of the skull, the teeth, the adrenal glands, and parts of the peripheral nervous system.
neural tube - A tube of cells running along the dorsal axis of the body, just dorsal to the notochord. It will give rise to the central nervous system.
neurogenic heart - A type of heart, such as in insects, in which the pacemakers originate in motor nerves arising from outside the heart.
neurohypophysis - See posterior pituitary.
neuron - A nerve cell; the fundamental unit of the nervous system, having structure and properties that allow it to conduct signals by taking advantage of the electrical charge across its cell membrane.
neuropeptide - A relatively short chain of amino acids that serves as a neurotransmitter.
neurosecretory cell - A specialized nerve cell that releases a hormone into the bloodstream in response to signals from other nerve cells; located in the hypothalamus and adrenal medulla.
neurotransmitter - A chemical messenger released from the synaptic terminal of a neuron at a chemical synapse that diffuses across the synaptic cleft and binds to and stimulates the postsynaptic cell.
neutral theory - The hypothesis that much evolutionary change in genes and proteins has no effect on fitness and therefore is not influenced by Darwinian natural selection.
neutral variation - Genetic diversity that confers no apparent selective advantage.
neutron - An electrically neutral particle (a particle having no electrical charge), found in the nucleus of an atom.
neutrophil - The most abundant type of white blood cell. Neutrophils are phagocytic and tend to self-destruct as they destroy foreign invaders, limiting their life span to a few days.
nitric oxide (NO) - A gas produced by many types of cells that functions as a local regulator, a neurotransmitter, and an antibacterial agent.
nitrogen fixation - The assimilation of atmospheric nitrogen by certain prokaryotes into nitrogenous compounds that can be directly used by plants.
nitrogenase - An enzyme complex, unique to certain prokaryotes, that reduces N2 to NH3'
nitrogen-fixing bacteria - Microorganisms that restock nitrogenous minerals in the soil by converting nitrogen to ammonia.
nociceptor - A class of naked dendrites in the epidermis of the skin.
node - A point along the stem of a plant at which leaves are attached.
nodule - A swelling on the root of a legume. Nodules are composed of plant cells that contain nitrogen-fixing bacteria of the genus Rhizobium.
noncompetitive inhibitor - A substance that reduces the activity of an enzyme by binding to a location remote from the active site, changing its conformation so that it no longer binds to the substrate.
noncyclic electron flow - A route of electron flow during the light reactions of photosynthesis that involves both photosystems and produces ATp, NADPH, and oxygen. The net electron flow is from water to NADP+.
nondisjunction - An error in meiosis or mitosis, in which both members of a pair of homologous chromosomes or both sister chromatids fail to move apart properly.
nonequilibrium model - The model of communities that emphasizes that they are not stable in time but constantly changing after being buffeted by disturbances.
nonpolar covalent bond - A type of covalent bond in which electrons are shared equally between two atoms of similar electronegativity.
nonsense mutation - A mutation that changes an amino acid codon to one of the three stop codons, resulting in a shorter and usually nonfunctional protein.
nonshivering thermogenesis (NST) - The increased production of heat in some mammals by the action of certain hormones that cause mitochondria to increase their metabolic activity and produce heat instead of ATP
norepinephrine - A hormone that is chemically and functionally similar to epinephrine.
norm of reaction - The range of phenotypes produced by a single genotype, due to environmental influences.
notochord - A longitudinal, flexible rod that runs along the dorsal axis of an animal's body in the future position of the vertebral column.
nuclear envelope - The membrane in eukaryotes that encloses the nucleus, separating it from the cytoplasm.
nuclear lamina - A netlike array of protein filaments that maintains the shape of the nucleus.
nuclease - An enzyme that hydrolyzes DNA and RNA into their component nucleotides.
nucleic acid A polymer (polynucleotide) consisting of many nucleotide monomers; serves as a blueprint for proteins and, through the actions of proteins, for all cellular activities. The two types are DNA and RNA.
nucleic acid hybridization - Base pairing between a gene and a complementary sequence on another nucleic acid molecule.
nucleic acid probe - In DNA technology, a labeled single-stranded nucleic acid molecule used to tag a specific nucleotide sequence in a nucleic acid sample. Molecules of the probe hydrogen-bond to the complementary sequence wherever it occurs; radioactive or other labeling of the probe allows its location to be detected.
nucleoid - A dense region of DNA in a prokaryotic cell.
nucleoid region - The region in a prokaryotic cell consisting of a concentrated mass of DNA.
nucleolus - A specialized structure in the nucleus, formed from various chromosomes and active in the synthesis of ribosomes.
nucleosome - The basic, beadlike unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a protein core composed of two copies of each of four types of histone.
nucleotide - The building block of a nucleic acid, consisting of a five-carbon sugar covalently bonded to a nitrogenous base and a phosphate group.
nucleotide excision repair - The process of removing and then correctly replacing a damaged segment of DNA using the undamaged strand as a guide.
nucleus - (1) An atom's central core, containing protons and neutrons. (2) The chromosomecontaining organelle of a eukaryotic cell. (3) A cluster of neurons.
obligate aerobe - An organism that requires oxygen for cellular respiration and cannot live without it.
obligate anaerobe - An organism that cannot use oxygen and is poisoned by it.
oceanic pelagic biome Most of the ocean's waters far from shore, constantly mixed by ocean currents.
oceanic zone - The region of water lying over deep areas beyond the continental shelf.
Okazaki fragment - A short segment of DNA synthesized on a template strand during DNA replication. Many Okazaki fragments make up the lagging strand of newly synthesized DNA.
olfactory receptor - Smell receptor.
oligodendrocyte - A type of glial cell that forms insulating myelin sheaths around the axons of neurons in the central nervous system.
oligosaccharin - A type of elicitor (molecule that induces a broad defense response in plants) that is derived from cellulose fragments released by cell wall damage.
oligotrophic lake - A nutrient-poor, oxygen-rich clear, deep lake with few phytoplankton.
ommatidium - One of the facets of the compound eye of arthropods and some polychaete worms.
omnivore - A heterotrophic animal that consumes both meat and plant material.
oncogene - A gene found in viruses or as part of the normal genome that is involved in triggering cancerous characteristics.
one gene-one polypeptide hypothesis - The premise that a gene is a segment of DNA that codes for one polypeptide.
oogenesis - The process in the ovary that results in the production of female gametes.
oogonia - Ovary-specific stem cells.
oomycete - A protist with flagellated cells, such as a water mold, white rust, or downy mildew, that acquires nutrition mainly as a decomposer or plant parasite.
open circulatory system - A circulatory system in which fluid called hemolymph bathes the tissues and organs directly and there is no distinction between the circulating fluid and the interstitial fluid.
operant conditioning - A type of associative learning in which an animal learns to associate one of its own behaviors with a reward or punishment and then tends to repeat or avoid that behavior; also called trial-and-error learning.
operator - In prokaryotic DNA, a sequence of nucleotides near the start of an operon to which an active repressor can attach. The binding of the repressor prevents RNA polymerase from attaching to the promoter and transcribing the genes of the operon.
operculum - In aquatic osteichthyans, a protective bony flap that covers and protects the gills.
operon - A unit of genetic function common in bacteria and phages, consisting of coordinately regulated clusters of genes with related functions.
opisthokont - Member of the clade Opisthokonta, organisms that descended from an ancestor with a posterior flagellum, including fungi, animals, and certain protists.
opposable thumb - An arrangement of the fingers such that the thumb can touch the ventral surface of the fingertips of all four fingers.
opsin - A membrane protein bonded to a lightabsorbing pigment molecule.
optic chiasm - The arrangement of the nerve tracts of the eye such that the visual sensations from the left visual field of both eyes are transmitted to the right side of the brain and the sensations from the right visual field of both eyes are transmitted to the left side of the brain.
optimal foraging theory - The basis for analyzing behavior as a compromise of feeding costs versus feeding benefits.
oral cavity - The mouth of an animal.
orbital - The three-dimensional space where an electron is found 90% of the time.
order - In classification, the taxonomic category above family
organ - A specialized center of body function composed of several different types of tissues.
organ identity genes - Plant homeotic genes that use positional information to determine which emerging leaves develop into which types of floral organs.
organ of Corti - The actual hearing organ of the vertebrate ear, located in the floor of the cochlear canal in the inner ear; contains the receptor cells (hair cells) of the ear.
organ system - A group of organs that work together in performing vital body functions.
organelle - One of several formed bodies with specialized functions, suspended in the cytoplasm of eukaryotic cells.
organic chemistry - The study of carbon compounds (organic compounds).
organism - An individual living thing.
organismal ecology - The branch of ecology concerned with the morphological, physiological, and behavioral ways in which individual organisms meet the challenges posed by their biotic and abiotic environments.
organogenesis - The development of organ rudiments from the three germ layers.
orgasm Rhythmic, involuntary contractions of certain reproductive structures in both sexes during the human sexual response cycle.
origin of replication - Site where the replication of a DNA molecule begins.
orthologous genes - Homologous genes that are passed in a straight line from one generation to the next, but have ended up in different gene pools because of speciation.
osculum - A large opening in a sponge that connects the spongocoel to the environment.
osmoconformer - An animal that does not actively adjust its internal osmolarity because it is isoosmotic with its environment.
osmolarity - Solute concentration expressed as molarity.
osmoregulation - How organisms regulate solute concentrations and balance the gain and loss of water.
osmoregulator - An animal whose body fluids have a different osmolarity than the environment and that must either discharge excess water if it lives in a hypoosmotic environment or take in water if it inhabits a hyperosmotic environment.
osmosis - The diffusion of water across a selectively permeable membrane.
osmotic potential- A component of water potential that is proportional to the number of dissolved solute molecules in a solution and measures the effect of solutes on the direction of water movement; also called solUte potential, it can be either zero or negative.
osteichthyan- Member of a vertebrate subgroup with jaws and mostly bony skeletons.
osteoblast- A bone-forming cell that deposits collagen.
osteon - The repeating organizational unit forming the microscopic structure of hard mammalian bone.
outer ear - One of three main regions of the ear in reptiles, birds, and mammals; made up of the auditory canal and, in many birds and mammals, the pinna.
outgroup- A species or group of species that is closely related to the group of species being studied, but clearly not as closely related as any study-group members are to each other.
oval window- In the vertebrate ear, a membrane-covered gap in the skull bone, through which sound waves pass from the middle ear to the inner ear.
ovarian cycle- The cyclic recurrence of the follicular phase, ovulation, and the luteal phase in the mammalian ovary, regulated by hormones.
ovary - In flowers, the portion of a carpel in which the egg-containing ovules develop. (2) In animals, the structure that produces female gametes and reproductive hormones.
overexploitation- Harvesting by humans of wild plants or animals at rates exceeding the ability of populations of those species to rebound.
overnourishment- A diet that is chronically excessive in calories.
oviduct - A tube passing from the ovary to the vagina in invertebrates or to the uterus in vertebrates.
oviparous - Referring to a type of development in which young hatch from eggs laid outside the mother's body.
ovoviviparous- Referring to a type of development in which young hatch from eggs that are retained in the mother's uterus.
ovulation- The release of an egg from ovaries. In humans, an ovarian follicle releases an egg during each uterine (menstrual) cycle.
ovule - A structure that develops within the ovary of a seed plant and contains the female gametophyte.
ovum - The female gamete; the haploid, unfertilized egg, which is usually a relatively large, nonmotile cell.
oxidation- The loss of electrons from a substance involved in a redox reaction.
oxidative phosphorylation - The production of ATP using energy derived from the redox reactions of an electron transport chain.
oxidizing agent- The electron acceptor in a redox reaction.
oxytocin- A hormone produced by the hypothalamus and released from the posterior pituitary. It induces contractions of the uterine muscles and causes the mammary glands to eject milk during nursing.
P generation- The parent individuals from which offspring are derived in studies of inheritance; P stands for "parental."
P site - One of a ribosome's three binding sites for tRNA during translation. The P site holds the tRNA carrying the growing polypeptide chain. (P stands for peptidyl tRNA.)
p53 gene- The "guardian angel of the genome," a gene that is expressed when a cell's DNA is damaged. Its product, p53 protein, functions as a transcription factor for several genes.
pacemaker- A specialized region of the right atrium of the mammalian heart that sets the rate of contraction; also called the sinoatrial (SA) node.
paedomorphosis - The retention in an adult organism of the juvenile features of its evolutionary ancestors.
pain receptor- A kind of interoreceptor that detects pain; also called a nociceptor.
paleoanthropology- The study of human origins and evolution.
paleontological species concept- Definition of species based on morphological differences known only from the fossil record.
paleontology - The scientific study of fossils.
palisade mesophyll - One or more layers of elongated photosynthetic cells on the upper pan of a leaf; also called palisade parenchyma.
pancreas- A gland with dual functions: The nonendocrine portion secretes digestive enzymes and an alkaline solution into the small intestine via a duct; the endocrine portion secretes the hormones insulin and glucagon into the blood.
Pangaea - The supercontinent formed near the end of the Paleozoic era when plate movements brought all the landmasses of Earth together.
parabasalid- A protist such as a trichomonad, with modified mitochondria.
parabronchus- A site of gas exchange in bird lungs. Parabronchi allow air to flow past the respiratory surface in just one direction.
paralogous genes- Homologous genes that are found in the same genome due to gene duplication.
paraphyletic - Pertaining to a grouping of species that consists of an ancestral species and some, but not all, of its descendants.
parareptile- First major group of reptiles to emerge, mostly large, stocky quadrupedal herbivores; died out in the late Triassic period.
parasite - An organism that benefits by living in or on another organism at the expense of the host.
parasitism - A symbiotic relationship in which the symbiont (parasite) benefits at the expense of the host by living either within the host (as an endoparasite) or outside the host (as an ectoparasite).
parasitoidism - A type of parasitism in which an insect lays eggs on or in a living host; the larvae then feed on the body of the host, eventually killing it.
parasympathetic division- One of three divisions of the autonomic nervous system; generally enhances body activities that gain and conserve energy, such as digestion and reduced heart rate.
parathyroid gland- Any o{ {our smarr encfocrine glands, embedded in the surface of the thyroid gland, that secrete parathyroid hormone. parathyroid hormone (PTH) A hormone secreted by the parathyroid glands that raises blood calcium level by promoting calcium release from bone and calcium retention by the kidneys.
parazoan- Animal belonging to a grade of organization lacking true tissues (collections of specialized cells isolated from other tissues by membranes); a sponge (phylum Porifera).
parenchyma cell- A relatively unspecialized plant cell type that carries out most of the metabolism, synthesizes and stores organic products, and develops into a more differentiated cell type.
parental type- An offspring with a phenotype that matches one of the parental phenotypes.
Parkinson's disease- A motor disorder caused by a progressive brain disease and characterized by difficulty in initiating movements, slowness of movement, and rigidity.
parthenogenesis - A type of reproduction in which females produce offspring from unfertilized eggs.
partial pressure - A measure of the concentration of one gas in a mixture of gases; the pressure exerted by a particular gas in a mixture of gases (for instance, the pressure exerted by oxygen in air).
parturition- The expulsion of a baby from the mother; also called birth.
passive immunity- Short-term immunity conferred by the administration of ready-made antibodies or the transfer of maternal antibodies to a fetus or nursing infant; lasts only a few weeks or months because the immune system has not been stimulated by antigens.
passive transport- The diffusion of a substance across a biological membrane.
patchiness- Localized variation in environmental conditions within an ecosystem, arranged spatially into a complex of discrete areas that may be characterized by distinctive groups of species or ecosystem processes.
pathogen- A disease-causing agent.
pattern formation- The ordering of cells into specific three-dimensional structures, an essential part of shaping an organism and its individual parts during development.
peat- Extensive deposits of undecayed organic material formed primarily from the wetland moss Sphagnum.
pedigree- A diagram of a family tree showing the occurrence of heritable characters in parents and offspring over multiple generations.
pelagic zone - The area of the ocean past the continental shelf, with areas of open water often reaching to very great depths.
penis- The copulatory structure of male mammals.
PEP carboxylase- An enzyme that adds carbon dioxide to phosphoenolpyruvate (PEP) to form oxaloacetate.
pepsin- An enzyme present in gastric juice that begins the hydrolysis of proteins.
pepsinogen - The inactive form of pepsin that is first secreted by specialized (chiel) cells located in gastric pits of the stomach.
peptide bond - The covalent bond between two amino acid units, formed by a dehydration reaction.
peptidoglycan - A type of polymer in bacterial cell walls consisting of modified sugars cross-linked by short polypeptides.
perception - The interpretation of sensations by the brain.
perennial- A flowering plant that lives for many years.
pericarp- The thickened wall of a fruit.
pericycle - The outermost layer of the vascular cylinder of a root, where lateral roots originate.
periderm- The protective coat that replaces the epidermis in plants during secondary growth, formed of the cork and cork cambium.
periodic table of the elements- A chart of the chemical elements, arranged in three rows, corresponding to the number of electron shells in their atoms.
peripheral nervous system (PNS)- The sensory and motor neurons that connect to the central nervous system.
peripheral protein - A protein appendage loosely bound to the surface of a membrane and not embedded in the lipid bilayer.
peripheral resistance- The impedance of blood flow by the arterioles.
peristalsis- (1) Rhythmic waves of contraction of smooth muscle that push food along the digestive tract. (2) A type of movement on land produced by rhythmic waves of muscle contractions passing from front to back, as in many annelids.
peristome- The upper part of the moss capsule (sporangium) often specialized for gradual spore discharge.
peritubular capillaries- The network of tiny blood vessels that surrounds the proximal and distal tubules in the kidney.
permafrost - A permanently frozen stratum below the arctic tundra.
peroxisome - A microbody containing enzymes that transfer hydrogen from various substrates to oxygen, producing and then degrading hydrogen peroxide.
petal- A modified leaf of a flowering plant. Petals are the often colorful parts of a flower that advertise it to insects and other pollinators.
petiole - The stalk of a leaf, which joins the leaf to a node of the stem.
pH- A measure of hydrogen ion concentration equal to -log [H+] and ranging in value from 0 to 14.
phage- A virus that infects bacteria; also called a bacteriophage.
phagocytosis- A type of endocytosis involving large, particulate substances, accomplished mainly by macrophages, neutrophils, and dendritic cells.
pharyngeal clefts- In chordate embryos, grooves that separate a series of pouches along the sides of the pharynx and may develop into pharyngeal slits.
pharyngeal slits- In chordate embryos, slits that form from the pharyngeal clefts and communicate to the outside, later developing into gill slits in many vertebrates.
pharynx- An area in the vertebrate throat where air and food passages cross; in flatworms, the muscular tube that protrudes from the ventral side of the worm and ends in the mouth.
phase change - A shift from one developmental phase to another.
phenotype- The physical and physiological traits of an organism, which are determined by its genetic makeup.
phenotypic polymorphism- The existence of two or more distinct morphs (discrete forms), each represented in a population in high enough frequencies to be readily noticeable.
pheromone- In animals and fungi, a small, volatile chemical that functions in communication and that in animals acts much like a hormone in influencing physiology and behavior.
phloem - Vascular plant tissue consisting of living cells arranged into elongated tubes that transport sugar and other organic nutrients throughout the plant.
phoronids- A tube-dwelling marine lophophorate.
phosphate group- A functional group important in energy transfer.
phospholipid - A molecule that is a constituent of the inner bilayer of biological membranes, having a polar, hydrophilic head and a nonpolar, hydrophobic tail.
phosphorylated- Referring to a molecule that has been the recipient of a phosphate group.
photic zone- The narrow top slice of the ocean, where light permeates sufficiently for photosynthesis to occur.
photoautotroph - An organism that harnesses light energy to drive the synthesis of organic compounds from carbon dioxide.
photoheterotroph- An organism that uses light to generate ATP but that must obtain carbon in organic form.
photomorphogenesis- Effects of light on plant morphology.
photon - A quantum, or discrete amount, of light energy.
photoperiodism - A physiological response to photoperiod, the relative lengths of night and day. An example of photoperiodism is flowering.
photophosphorylation - The process of generating ATP from ADP and phosphate by means of a protonmotive force generated by the thylakoid membrane of the chloroplast during the light reactions of photosynthesis.
photopsin - One of a family of visual pigments in the cones of the vertebrate eye that absorb bright, colored light.
photoreceptor- An electromagnetic receptor that detects the radiation known as visible light.
photorespiration - A metabolic pathway that consumes oxygen, releases carbon dioxide, generates no ATp, and decreases photosynthetic output; generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide.
photosynthesis- The conversion of light energy to chemical energy that is stored in glucose or other organic compounds; occurs in plants, algae, and certain prokaryotes.
photosystem- Light-capturing unit located in the thylakoid membrane of the chloroplast, consisting of a reaction center surrounded by numerous light-harvesting complexes. There are two types of photosystems, I and II; they absorb light best at different wavelengths.
photosystem I- One of two light-capturing units in a chloroplast's thylakoid membrane; it has two molecules of P700 chlorophyll a at its reaction center.
photosystem II- One of two light-capturing units in a chloroplast's thylakoid membrane; it has two molecules of P680 chlorophyll a at its reaction center.
phototropism - Growth of a plant shoot toward or away from light.
phragmoplast- An alignment of cytoskeletal elements and Golgi-derived vesicles across the midline of a dividing plant cell.
phylogenetic tree- A branching diagram that represents a hypothesis about evolutionary relationships.
phylogeny- The evolutionary history of a species or group of related species.
phylogram- A phylogenetic tree in which the lengths of the branches reflect the number of genetic changes that have taken place in a particular DNA or RNA sequence in the various lineages.
phylum- In classification, the taxonomic category above class.
physical map- A genetic map in which the actual physical distances between genes or other genetic markers are expressed, usually as the number of base pairs along the DNA.
physiology- The study of the functions of an organism.
phytoalexin - An antibiotic, produced by plants, that destroys microorganisms or inhibits their growth.
phytochromes- A class of light receptors in plants. Mostly absorbing red light, these photoreceptors regulate many plant responses, including seed germination and shade avoidance.
phytoplankton- Algae and photosynthetic bacteria that drift passively in the peJagic zone of an aquatic environment.
phytoremediation- An emerging nondestructive technology that seeks to cheaply reclaim contaminated areas by taking advantage of the remarkabJe ability of some plant species to extract heavy metals and other pollutants from the soil and to concentrate them in easily harvested portions of the plant.
pilus - A long, hairlike prokaryotic appendage that functions in adherence or in the transfer of DNA during conjugation.
pineal gland - A small gland on the dorsal surface of the vertebrate forebrain that secretes the hormone melatonin.
pinocytosis- A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.
pistil- A single carpel or a group of fused carpels.
pitch- A function of a sound wave's frequency, or number of vibrations per second, expressed in hertz.
pith- Ground tissue that is internal to the vascular tissue in a stem; in many mono cot roots, parenchyma cells that form the central core of the vascular cylinder.
pit- A thinner region in the walls of tracheids and vessels where only primary wall is present.
pituitary gland - An endocrine gland at the base of the hypothalamus; consists of a posterior lobe (neurohypophysis), which stores and releases two hormones produced by the hypothalamus, and an anterior lobe (adenohypophysis), which produces and secretes many hormones that regulate diverse body functions.
placenta- A structure in the pregnant uterus for nourishing a viviparous fetus with the mother's blood supply; formed from the uterine lining and embryonic membranes.
placental transfer cell - A plant cell that enhances the transfer of nutrients from parent to embryo.
placoderm - A member of an extinct class of fishlike vertebrates that had jaws and were enclosed in a tough, outer armor.
planarian- A free-living flatworm found in unpolluted ponds and streams.
plankton- Mostly microscopic organisms that drift passively or swim weakly near the surface of oceans, ponds, and lakes.
Plantae - The kingdom that consists of multicellular eukaryotes that carry out photosynthesis.
plasma - The liquid matrix of blood in which the cells are suspended.
plasma cell - The antibody-secreting effector cell of humoral immunity; arises from antigenstimulated B cells.
plasma membrane- The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell's chemical composition.
plasmid- A small ring of DNA that carries accessory genes separate from those of a bacterial chromosome; also found in some eukaryotes, such as yeast.
plasmodesma- An open channel in the cell wall of a plant through which strands of cytosol connect from an adjacent cell.
plasmodial slime mold- A type of protist that has amoeboid cells, flagellated cells, and a plasmodial feeding stage in its life cycle.
plasmodium- A single mass of cytoplasm containing many diploid nuclei that forms during the life cycle of some slime molds.
plasmogamy- The fusion of the cytoplasm of cells from two individuals; occurs as one stage of syngamy.
plasmolysis - A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall when the cell loses water to a hypertonic environment.
plasmolyze- To shrink and pull away from a cell wall, or when a plant cell protoplast pulls away from the cell wall as a result of water loss.
plasticity- An organism's ability to alter or "mold" itself in response to local environmental conditions.
plastid- One of a family of closely related plant organelles that includes chloroplasts, chromoplasts, and amyloplasts (leucoplasts).
platelet- A small enucleated blood cell important in blood clotting; derived from large cells in the bone marrow.
pleiotropy- The ability of a single gene to have multiple effects.
pluripotent - Describing a stem cell, from an embryo or adult organism, that can give rise to multiple but not all differentiated cell types.
point mutation - A change in a gene at a single nucleotide pair.
polar covalent bond - A covalent bond between atoms that differ in electronegativity. The shared electrons are pulled closer to the more electronegative atom, making it slightly negative and the other atom slightly positive.
polar molecule- A molecule (such as water) with opposite charges on opposite sides.
polarity- A lack of symmetry. Structural differences in opposite ends of an organism or structure, such as the root end and shoot end of a plant.
pollen grains - The structures that contain the male gametophyte of seed plants.
pollination- The transfer of pollen to the part of a seed plant containing the ovules, a process that is a prerequisite for fertilization.
poly-A tail- The modified end of the 3' end of an mRNA molecule consisting of the addition of some 50 to 250 adenine nucleotides.
polyandry - A polygamous mating system involving one female and many males.
polygamous- A type of relationship in which an individual of one sex mates with several of the other.
polygenic inheritance - An additive effect of two or more gene loci on a single phenotypic character.
polygyny - A polygamous mating system involving one male and many females.
polymer- A long molecule consisting of many simi1ar or identical monomers linked together.
polymerase chain reaction (PCR) - A technique for amplifying DNA in vitro by incubating with special primers, DNA polymerase molecules, and nucleotides.
polymorphism - The coexistence of two or more distinct forms in the same population.
polynucleotide- A polymer consisting of many nucleotide monomers; serves as a blueprint for proteins and, through the actions of proteins, for all cellular activities. The two types are DNA and RNA.
polyp- The sessile variant of the cnidarian body plan. The alternate form is the medusa.
polypeptide (pol'-e-pep'-tfd) A polymer (chain) of many amino acids linked together by peptide bonds.
polyphyletic- Pertaining to a grouping of species derived from two or more different ancestral forms.
polyploidy- A chromosomal alteration in which the organism possesses more than two complete chromosome sets.
polyribosome (polysome) - An aggregation of several ribosomes attached to one messenger RNA molecule.
polysaccharide - A polymer of up to over a thousand monosaccharides, formed by dehydration reactions.
pons- Portion of the brain that participates in certain automatic, homeostatic functions, such as regulating the breathing centers in the medulla.
population- A localized group of individuals that belong to the same biological species (that are capable of interbreeding and producing fertile offspring).
population dynamics- The study of how complex interactions between biotic and abiotic factors influence variations in population size.
population ecology- The study of populations in relation to the environment, including environmental influences on population density and distribution, age structure, and variations in population size.
population genetics - The study of how populations change genetically over time.
population viability analysis (PVA)- A method of predicting whether or not a population will persist.
positional information- Signals to which genes regulating development respond, indicating a cell's location relative to other cells in an embryonic structure.
positive feedback- A physiological control mechanism in which a change in some variable triggers mechanisms that amplify the change.
positive pressure breathing- A breathing system in which air is forced into the lungs.
posterior - Pertaining to the rear, or tail end, of a bilaterally symmetrical animal.
posterior pituitary- Also called the neurohypophysis; an extension of the hypothalamus composed of nervous tissue that secretes oxytocin and antidiuretic hormone made in the hypothalamus; a temporary storage site for these hormones.
postsynaptic cell- The target cell at a synapse.
postzygotic barrier- Any of several species-isolating mechanisms that prevent hybrids produced by two different species from developing into viable, fertile adults.
potential energy- The energy stored by matter as a result of its location or spatial arrangement.
precautionary principle- A guiding principle in making decisions about the environment, cautioning to consider carefully the potential consequences of actions.
predation- An interaction between species in which one species, the predator, eats the other, the prey.
pregnancy- The condition of carrying one or more embryos in the uterus.
preprophase band - Microtubules in the cortex (outer cytoplasm) of a cell that are concentrated into a ring.
prepuce - A fold of skin covering the head of the clitoris and penis.
pressure potential - A component of water potential that consists of the physical pressure on a solution, which can be positive, zero, or negative.
presynaptic cell- The transmitting cell at a synapse.
prezygotic barrier - A reproductive barrier that impedes mating between species or hinders fertilization of ova if interspecific mating is attempted.
primary cell wall - A relatively thin and flexible layer first secreted by a young plant cell.
primary consumer- An herbivore; an organism in the trophic level of an ecosystem that eats plants or algae.
primary electron acceptor- A specialized molecule sharing the reaction center with the pair of reaction-center chlorophyll a molecules; it accepts an electron from one of these two chlorophylls.
primary growth - Growth produced by apical meristems, lengthening stems and roots.
primary immune response - The initial acquired immune response to an antigen, which appears after a lag of about 10 to 17 days.
primary oocyte- A diploid cell, in prophase I of meiosis, that can be hormonally triggered to develop into an ovum.
primary plant body- The tissues produced by apical meristems, which lengthen stems and roots.
primary producer- An autotroph, usually a photosynthetic organism. Collectively, autotrophs make up the trophic level of an ecosystem that ultimately supports all other levels.
primary production- The amount of light energy converted to chemical energy (organic compounds) by autotrophs in an ecosystem during a given time period.
primary structure - The level of protein structure referring to the specific sequence of amino acids.
primary succession- A type of ecological succession that occurs in a virtually lifeless area, where there were originally no organisms and where soil has not yet formed.
primary transcript- An initial RNA transcript; also called pre-mRNA when transcribed from a protein-coding gene.
primary visual cortex- The destination in the occipital lobe of the cerebrum for most of the axons from the lateral geniculate nuclei.
primase- An enzyme that joins RNA nucleotides to make the primer.
primer - A polynucleotide with a free 3' end, bound by complementary base pairing to the template strand, that is elongated during DNA replication.
primitive streak - A groove on the surface of an early avian embryo along the future long axis of the body.
prion- An infectious form of protein that may increase in number by converting related proteins to more prions.
product- An ending material in a chemical reaction.
production efficiency - The fraction of energy stored in food that is not used for respiration.
progestin- One of a family of steroid hormones, including progesterone, that prepare the uterus for pregnancy.
progymnosperms- Extinct seedless vascular plants that may be ancestral to seed plants.
prokaryotic cell - A type of cell lacking a membrane-enclosed nucleus and membrane-enclosed organelles; found only in the domains Bacteria and Archaea.
prolactin (PRL)- A hormone produced and secreted by the anterior pituitary with a great diversity of effects in different vertebrate species. In mammals, it stimulates growth of and milk production by the mammary glands.
proliferative phase- That portion of the uterine (menstrual) cycle when the endometrium regenerates and thickens.
prometaphase- The second stage of mitosis, in which discrete chromosomes consisting of identical sister chromatids appear, the nuclear envelope fragments, and the spindle micro tubules attach to the kinetochores of the chromosomes.
promiscuous- A type of relationship in which mating occurs with no strong pair-bonds or lasting relationships.
promoter- A specific nucleotide sequence in DNA that binds RNA polymerase and indicates where to start transcribing RNA.
prophage - A phage genome that has been inserted into a specific site on the bacterial chromosome.
prophase- The first stage of mitosis, in which the chromatin is condensing and the mitotic spindle begins to form, but the nucleolus and nucleus are still intact.
prostaglandin (PG) - One of a group of modified fatty acids secreted by virtually all tissues and performing a wide variety of functions as local regulators.
prostate gland - A gland in human males that secretes an acid-neutralizing component of semen.
proteasome- A giant protein complex that recognizes and destroys proteins tagged for elimination by the small protein ubiquitin.
protein - A three-dimensional biological polymer constructed from a set of 20 different monomers called amino acids.
protein kinase- An enzyme that transfers phosphate groups from ATP to a protein.
protein phosphatase- An enzyme that removes phosphate groups from proteins, often functioning to reverse the effect of a protein kinase.
proteoglycan - A glycoprotein in the extracellular matrix of animal cells, rich in carbohydrate.
proteomics- The systematic study of the full protein sets (proteomes) encoded by genomes.
protist- An informal term applied to any eukaryote that is not a plant, animal, or fungus. Most protists are unicellular, though some are colonial or multicellular.
protobiont- An aggregate of abiotically produced molecules surrounded by a membrane or membrane-like structure.
proton- A subatomic particle with a single positive electrical charge, found in the nucleus of an atom.
proton pump- An active transport mechanism in cell membranes that uses ATP to force hydrogen ions out of a cell, generating a membrane potential in the process.
protonema- A mass of green, branched, onecell-thick filaments produced by germinating moss spores.
protonephridium- An excretory system, such as the flame-bulb system of flatworms, consisting of a network of closed tubules having external openings called nephridiopores and lacking internal openings.
proton-motive force - The potential energy stored in the form of an electrochemical gradient, generated by the pumping of hydrogen ions across biological membranes during chemiosmosis.
proto-oncogene- A normal cellular gene corresponding to an oncogene; a gene with a potential to cause cancer but that requires some alteration to become an oncogene.
protoplast- The contents of a plant cell exclusive of the cell wall.
protoplast fusion- The fusing of two protoplasts from different plant species that would otherwise be reproductively incompatible.
protostome development- In animals, a developmental mode distinguished by the development of the mouth from the blastopore; often also characterized by schizocoelous development of the body cavity and by spiral cleavage.
protozoan - A protist that lives primarily by ingesting food, an animal-like mode of nutrition.
provirus- Viral DNA that inserts into a host genome.
proximal tubule- In the vertebrate kidney, the portion of a nephron immediately downstream from Bowman's capsule that conveys and helps refine filtrate.
proximate question- In animal behavior, an inquiry that focuses on the environmental stimuli, if any, that trigger a particular behavioral act, as well as the genetic, physiological, and anatomical mechanisms underlying it.
PR protein - A protein involved in plant responses to pathogens (PR = pathogenesisrelated) .
pseudocoelomate - An animal whose body cavity is not completely lined by mesoderm.
pseudogene- A DNA segment very similar to a real gene but which does not yield a functional product; a gene that has become inactivated in a particular species because of mutation.
pseudopodium - A cellular extension of amoeboid cells used in moving and feeding.
pterophyte - An informal name for any member of the phylum Pterophyta, which includes ferns, horsetails, whisk ferns, and the genus T mesipteris.
pterosaur - Winged reptile that lived during the time of dinosaurs.
pulmocutaneous circuit - The route of circulation that directs blood to the skin and lungs.
pulmonary circuit- The branch of the circulatory system that supplies the lungs.
pulse- The rhythmic stretching of the arteries caused by the pressure of blood forced through the arteries by contractions of the ventricles during systole.
punctuated equilibrium- In evolutionary theory, long periods of apparent stasis (no change) interrupted by relatively brief periods of sudden change.
Punnett square - A diagram used in the study of inheritance to show the results of random fertilization in genetic crosses.
pupil- The opening in the iris, which admits light into the interior of the vertebrate eye. Muscles in the iris regulate its size.
purine- One of two types of nitrogenous bases found in nucleotides. Adenine (A) and guanine (G) are purines.
pyloric sphincter- In the vertebrate digestive tract, a muscular ring that regulates the passage of food out of the stomach and into the small intestine.
pyrimidine- One of two types of nitrogenous bases found in nucleotides. Cytosine (C), thymine (T), and uracil (U) are pyrimidines.
quantitative character- A heritable feature that varies continuously over a range rather than in an either-or fashion.
quaternary structure- The particular shape of a complex, aggregate protein, defined by the characteristic threedimensional arrangement of its constituent subunits, each a polypeptide.
R plasmid- A bacterial plasmid carrying genes that confer resistance to certain antibiotics.
radial cleavage- A type of embryonic development in deuterostomes in which the planes of cell division that transform the zygote into a ball of cells are either parallel or perpendicular to the polar axis, thereby aligning tiers of cells one above the other.
radial glia- In an embryo, supporting cells that form tracks along which newly formed neurons migrate from the neural tube; can also act as stem cells that give rise to neurons and other glia.
radial symmetry- Characterizing a body shaped like a pie or barrel, with many equal parts radiating outward like the spokes of a wheel; present in cnidarians and echinoderms; also can refer to flower structure.
radiation- The emission of electromagnetic waves by all objects warmer than absolute zero.
radicle - An embryonic root of a plant.
radioactive isotope- An isotope (an atomic form of a chemical element) that is unstable; the nucleus decays spontaneously, giving off detectable particles and energy
radiolarian- A protist, usually marine, with a shell generally made of silica and pseudopodia that radiate from the central body.
radiometric dating - A method paleontologists use for determining the ages of rocks and fossils on a scale of absolute time, based on the half-life of radioactive isotopes.
radula- A straplike rasping organ used by many molluscs during feeding.
ras gene- A gene that codes for Ras protein, a G protein that relays a growth signal from a growth factor receptor on the plasma membrane to a cascade of protein kinases that ultimately results in the stimulation of the cell cycle. Many ras oncogenes have a point mutation that leads to a hyperactive version of the Ras protein that can lead to excessive cell division.
ratite - Member of the group of flightless birds.
ray initials - Cells within the vascular cambrium that produce xylem and phloem rays, radial files that consist mostly of parenchyma cells.
ray-finned fish- Member of the class Actinopterygii, aquatic osteichthyans with fins supported by long, flexible rays, including tuna, bass, and herring.
reactant- A starting material in a chemical reaction.
reaction center - Complex of proteins associated with two special chlorophyll a molecules and a primary electron acceptor. Located centrally in a photosystem, this complex triggers the light reactions of photosynthesis. Excited by light energy, one of the chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain.
reading frame - The way a cell's mRNAtranslating machinery groups the mRNA nucleotides into codons.
receptacle - The base of a flower; the part of the stem that is the site of attachment of the floral organs.
reception- In cellular communication, the target cell's detection (by binding to a receptor protein) of a signal molecule from outside the cell.
receptor-mediated endocytosis- The movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances.
receptor potential - An initial response of a receptor cell to a stimulus, consisting of a change in voltage across the receptor membrane proportional to the stimulus strength. The intensity of the receptor potential determines the frequency of action potentials traveling to the nervous system.
receptor tyrosine kinase - A receptor protein in the plasma membrane that responds to the binding of a signal molecule by catalyzing the transfer of phosphate groups from ATP to tyrosines on the cytoplasmic side of the receptor. The phosphorylated tyrosines activate other signal transduction proteins within the cell.
recessive allele - An allele whose phenotypic effect is not observed in a heterozygote.
reciprocal altruism- Altruistic behavior between unrelated individuals, whereby the current altruistic individual benefits in the future when the current beneficiary reciprocates.
recombinant- An offspring whose phenotype differs from that of the parents; also called recombinant type.
recombinant chromosome - A chromosome created when crossing over combines the DNA from two parents into a single chromosome.
recombinant DNA - A DNA molecule made in vitro with segments from different sources.
recruitment- The process of progressively increasing the tension of a muscle by activating more and more of the motor neurons controlling the muscle.
rectum- The terminal portion of the large intestine where the feces are stored until they are eliminated.
red alga - A photosynthetic marine protist that contains the accessory pigment phycoerythrin. Most are multicellular.
red blood cell - A blood cell containing hemoglobin, which transports O2; also called an erythrocyte.
redox reaction- A chemical reaction involving the transfer of one or more electrons from one reactant to another; also called oxidation-reduction reaction.
reducing agent- The electron donor in a redox reaction.
reduction - The addition of electrons to a substance involved in a redox reaction.
reductionism- Reducing complex systems to simpler components that are more manage
redundancy model - The concept, put forth by Henry Gleason and Brian Walker, that most of the species in a community are not tightly coupled with one another (that is, the web of life is very loose). According to this model, an increase or decrease in one species in a community has little effect on other species, which operate independently
reflex- An automatic reaction to a stimulus, mediated by the spinal cord or lower brain.
refractory period- The short time immediately after an action potential in which the neuron cannot respond to another stimulus, owing to an increase in potassium permeability.
regeneration- The regrowth of body parts from pieces of an organism.
regulator- A characterization of an animal in regard to environmental variables. A regulator uses mechanisms of homeostasis to moderate internal changes in the face of external fluctuations.
regulatory gene - A gene that codes for a protein, such as a repressor, that controls the transcription of another gene or group of genes.
relative abundance - Differences in the abundance of different species within a community.
relative fitness - The contribution of one genotype to the next generation compared to that of alternative genotypes for the same locus.
renal artery- The blood vessel bringing blood to the kidney
renal cortex- The outer portion of the vertebrate kidney
renal medulla- The inner portion of the vertebrate kidney, beneath the renal cortex.
renal pelvis- Funnel-shaped chamber that receives processed filtrate from the vertebrate kidney's collecting ducts and is drained by the ureter.
renal vein - The blood vessel draining the kidney
renin-angiotensin-aldosterone system (RAAS)- A part of a complex feedback circuit that helps regulate blood pressure and blood volume.
repeated reproduction- A life history in which adults produce large numbers of offspring over many years; also known as iteroparity.
repetitive DNA- Nucleotide sequences, usually noncoding, that are present in many copies in a eukaryotic genome. The repeated units may be short and arranged tandemly (in series) or long and dispersed in the genome.
replication fork- A Y-shaped region on a replicating DNA molecule where new strands are growing.
repressor- A protein that suppresses the transcription of a gene.
reproductive isolation - The existence of biological factors (barriers) that impede members of two species from producing viable, fertile hybrids.
reproductive table - An age-specific summary of the reproductive rates in a population
reptile- Member of the clade of amniotes that includes tuatara, lizards, snakes, turtles, crocodilians, and birds.
residual volume- The amount of air that remains in the lungs after forcefully exhaling.
resource partitioning- The division of environmental resources by coexisting species such that the niche of each species differs by one or more significant factors from the niches of all coexisting species.
respiratory medium- The source of oxygen. It is typically air for terrestrial animals and water for aquatic organisms.
respiratory pigment - A protein that transports most of the oxygen in blood.
respiratory surface - The part of an animal where gases are exchanged with the environment.
response- In cellular communication, the change in a specific cellular activity brought about by a transduced signal from outside the cell.
resting potential- The membrane potential characteristic of a nonconducting, excitable cell, with the inside of the cell more negative than the outside.
restoration ecology - A goal-directed science that applies ecological principles in an effort to return degraded ecosystems to conditions as similar as possible to their natural, predegraded state.
restriction enzyme- A degradative enzyme that recognizes and cuts up DNA (including that of certain phages) that is foreign to a bacterium.
restriction fragment - DNA segment resulting from cutting of DNA by a restriction enzyme.
restriction fragment length polymorphisms (RFLPs)- Differences in DNA sequence on homologous chromosomes that can result in different patterns of restriction fragment lengths (DNA segments resulting from treatment with restriction enzymes); useful as genetic markers for making linkage maps.
restriction site - A specific sequence on a DNA strand that is recognized as a "cut site" by a restriction enzyme.
reticular fiber- A very thin and branched fiber made of collagen. Reticular fibers form a tightly woven fabric that is continuous with the collagenous fibers of the extracellular matrix.
reticular formation- A system of neurons, containing over 90 separate nuclei, that passes through the core of the brainstem.
retina - The innermost layer of the vertebrate eye, containing photoreceptor cells (rods and cones) and neurons; transmits images formed by the lens to the brain via the optic nerve.
retinal - The light-absorbing pigment in rods and cones of the vertebrate eye.
retrotransposon- A transposable element that moves within a genome by means of an RNA intermediate, a transcript of the retrotransposon DNA
retrovirus- An RNA virus that reproduces by transcribing its RNA into DNA and then inserting the DNA into a cellular chromosome; an important class of cancer-causing viruses.
reverse transcriptase- An enzyme encoded by some certain viruses (retroviruses) that uses RNA as a template for DNA synthesis.
Rh factor- A protein antigen on the surface of red blood cells designated Rh-positive. If an Rh-negative mother is exposed to blood from an Rh-positive fetus, she produces anti-Rh antibodies of the IgG class.
rhizoid- Long tubular single cell or filament of cells that anchors bryophytes to the ground. Rhizoids are not composed of tissues, lack specialized conducting cells, and do not playa primary role in water and mineral absorption.
rhodopsin- A visual pigment consisting of retinal and opsin. When rhodopsin absorbs light, the retinal changes shape and dissociates from the opsin, after which it is converted back to its original form.
rhythm method- A form of contraception that relies on refraining from sexual intercourse when conception is most likely to occur; also called natural family planning.
ribonucleic acid (RNA)- A type of nucleic acid consisting of nucleotide monomers with a ribose sugar and the nitrogenous bases adenine (A), cytosine (C), guanine (G), and uracil (U); usually singlestranded; functions in protein synthesis and as the genome of some viruses.
ribose- The sugar component of RNA.
ribosomal RNA - The most abundant type of RNA, which together with proteins forms the structure of ribosomes. Ribosomes coordinate the sequential coupling of tRNA molecules to mRNA codons.
ribosome - A cell organelle constructed in the nucleolus and functioning as the site of protein synthesis in the cytoplasm; consists of rRNA and protein molecules, which make up two subunits.
ribozyme - An enzyme-like RNA molecule that catalyzes reactions during RNA splicing.
river - A flowing body of water.
rivet model - The concept, put forth by Paul and Anne Ehrlich, that many or most of the species in a community are associated tightly with other species in a web of life. According to this model, an increase or decrease in one species in a community affects many other species.
RNA interference (RNAi) - A technique to silence the expression of selected genes in nonmammalian organisms. The method uses synthetic double-stranded RNA molecules matching the sequence of a particular gene to trigger the breakdown of the gene's messenger RNA.
RNA polymerase- An enzyme that links together the growing chain of ribonucleotides during transcription.
RNA processing- Modification of RNA before it leaves the nucleus, a process unique to eukaryotes.
RNA splicing - The removal of noncoding portions (introns) of the RNA molecule after initial synthesis.
rod cell- One of two kinds of photoreceptors in the vertebrate retina; sensitive to black and white and enables night vision.
root - An organ in vascular plants that anchors the plant and enables it to absorb water and nutrients from the soil.
root cap- A cone of cells at the tip of a plant root that protects the apical meristem.
root hair- A tiny extension of a root epidermal cell, growing just behind the root tip and increasing surface area for absorption of water and minerals.
root pressure- The upward push of xylem sap in the vascular tissue of roots.
root system - All of a plant's roots that anchor it in the soil, absorb and transport minerals and water, and store food.
rosette cellulose-synthesizing complex Roseshaped array of proteins that synthesize the cellulose micro fibrils of the cell walls of charophyceans and land plants.
rough ER- That portion of the endoplasmic reticulum studded with ribosomes.
round window- The point of contact between the stapes and the cochlea. It is where the vibrations of the stapes create a traveling series of pressure waves in the fluid of the cochlea.
r-selection- The concept that in certain (r-selected) populations, a high reproductive rate is the chief determinant of life history
rubisco- Ribulose carboxylase, the enzyme that catalyzes the first step of the Calvin cycle (the addition of CO2 to RuBp' or ribulose bisphosphate).
ruminant- An animal, such as a cow or a sheep, with an elaborate, multicompartmentalized stomach specialized for an herbivorous diet.
S phase- The synthesis phase of the cell cycle; the portion of interphase during which DNA is replicated.
sac fungus- Member of the phylum Ascomycota. Sac fungi range in size and complexity from unicellular yeasts to minute leafspot fungi to elaborate cup fungi and morels. About half of the sac fungi live with algae or cyanobacteria in the mutualistic associations called lichens.
saccule- A chamber in the vestibule behind the oval window that participates in the sense of balance.
salicylic acid - A plant hormone that may be partially responsible for activating systemic acquired resistance to pathogens.
salivary amylase- A salivary gland enzyme that hydrolyzes starch and glycogen.
salivary glands- Exocrine glands associated with the oral cavity. The secretions of salivary glands contain substances to lubricate food, adhere together chewed pieces into a bolus, and begin the process of chemical digestion.
salt - A compound resulting from the formation of an ionic bond; also called an ionic compound.
saltatory conduction- Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.
sapwood- Outer layers of secondary xylem that still transport xylem sap.
sarcomere- The fundamental, repeating unit of striated muscle, delimited by the Z lines.
sarcoplasmic reticulum (SR)- A specialized endoplasmic reticulum that regulates the calcium concentration in the cytosol.
saturated fatty acid- A fatty acid in which all carbons in the hydrocarbon tail are connected by single bonds, thus maximizing the number of hydrogen atoms that can attach to the carbon skeleton.
savanna - A tropical grassland biome with scattered individual trees, large herbivores, and three distinct seasons based primarily on rainfall, maintained by occasional fires and drought.
scaffolding protein- A type of large relay protein to which several other relay proteins are simultaneously attached to increase the efficiency of signal transduction.
scanning electron microscope (SEM)- A microscope that uses an electron beam to scan the surface of a sample to study details of its topography
schizocoelous- Pattern of formation of the body cavity common in protostome development, in which initially solid masses of mesoderm split, forming the body cavity
schizophrenia- Severe mental disturbance characterized by psychotic episodes in which patients lose the ability to distinguish reality from hallucination.
Schwann cell- A type of glial cell that forms insulating myelin sheaths around the axons of neurons in the peripheral nervous system.
scion- The twig grafted onto the stock when making a graft.
sclera - A tough, white outer layer of connective tissue that forms the globe of the vertebrate eye.
sclereid- A short, irregular sclerenchyma cell in nutshells and seed coats and scattered through the parenchyma of some plants.
sclerenchyma cell - A rigid, supportive plant cell type usually lacking pro top lasts and possessing thick secondary walls strengthened by lignin at maturity
scrotum - A pouch of skin outside the abdomen that houses a testis; functions in cooling sperm, thereby keeping them viable.
scutellum - A specialized type of cotyledon found in the grass family.
seascape - Several different, primarily aquatic ecosystems linked by exchanges of energy, materials, and organisms.
second law of thermodynamics- The principle whereby every energy transfer or transformation increases the entropy of the universe. Ordered forms of energy are at least partly converted to heat, and in spontaneous reactions, the free energy of the system also decreases.
second messenger - A small, nonprotein, watersoluble molecule or ion, such as calcium ion or cyclic AMP, that relays a signal to a cell's interior in response to a signal received by a signal receptor protein.
secondary cell wall- A strong and durable matrix often deposited in several laminated layers for plant cell protection and support.
secondary consumer - A member of the trophic level of an ecosystem consisting of carnivores that eat herbivores.
secondary endosymbiosis- A process in eukaryotic evolution in which a heterotrophic eukaryotic cell engulfed a photosynthetic eukaryotic cell, which survived in a symbiotic relationship inside the heterotrophic cell.
secondary growth- Growth produced by lateral meristems, thickening the roots and shoots of woody plants.
secondary immune response- The acquired immune response elicited on second or subsequent exposures to a particular antigen. The secondary immune response is more rapid, of greater magnitude, and of longer duration than the primary immune response.
secondary oocyte - A haploid cell resulting from meiosis I in oogenesis, which will become an ovum after meiosis II.
secondary plant body- The tissues produced by the vascular cambium and cork cambium, which thicken the stems and roots of woody plants.
secondary production - The amount of chemical energy in consumers' food that is converted to their own new biomass during a given time period.
secondary structure- The localized, repetitive coiling or folding of the polypeptide backbone of a protein due to hydrogen bond formation between peptide linkages.
secondary succession - A type of succession that occurs where an existing community has been cleared by some disturbance that leaves the soil intact.
secretion- (1) The discharge of molecules synthesized by a cell. (2) The discharge of wastes from the body fluid into the filtrate.
secretory phase- That portion of the uterine (menstrual) cycle when the endometrium continues to thicken, becomes more vascularized, and develops glands that secrete a fluid rich in
glycogen.
sedimentary rock- Rock formed from sand and mud that once settled in layers on the bottom of seas, lakes, and marshes. Sedimentary rocks are often rich in fossils.
seed - An adaptation for terrestrial plants consisting of an embryo packaged along with a store of food within a resistant coat.
seed coat- A tough outer covering of a seed, formed from the outer coat of an ovule. In a flowering plant, the seed coat encloses and protects the embryo and endosperm.
seedless vascular plants- The informal collective name for the phyla Lycophyta (club mosses and their relatives) and Pteridophyta (ferns and their relatives).
segmentation gene - A gene of the embryo that directs the actual formation of segments after the embryo's axes are defined.
selective permeability - A property of biological membranes that allows some substances to cross more easily than others.
selective reabsorption- The selective uptake of solutes from a filtrate of blood, coelomic fluid, or hemolymph in the excretory organs of animals.
self-incompatibility - The ability of a seed plant to reject its own pollen and sometimes the pollen of closely related individuals.
semelparity - A life history in which adults have but a single reproductive opportunity to produce large numbers of offspring, such as the life history of the Pacific salmon; also known as big-bang reproduction.
semen - The fluid that is ejaculated by the male during orgasm; contains sperm and secretions from several glands of the male reproductive tract.
semicircular canals- A three-part chamber of the inner ear that functions in maintaining equilibrium.
semiconservative model- Type of DNA replication in which the replicated double helix consists of one old strand, derived from the old molecule, and one newly made strand.
semilunar valve- A valve located at the two exits of the heart, where the aorta leaves the left ventricle and the pulmonary artery leaves the right ventricle.
seminal vesicle - A gland in males that secretes a fluid component of semen that lubricates and nourishes sperm.
seminiferous tubule- A highly coiled tube in the testis in which sperm are produced.
sensation- An impulse sent to the brain from activated receptors and sensory neurons.
sensitive period - A limited phase in an individual animal's development when learning of particular behaviors can take place
sensory adaptation - The tendency of sensory neurons to become less sensitive when they are stimulated repeatedly.
sensory neuron- A nerve cell that receives information from the internal and external environments and transmits the signals to the central nervous system.
sensory reception- The detection of the energy of a stimulus by sensory cells.
sensory receptor- A cellular system that collects information about the physical world outside the body and inside the organism.
sensory transduction - The conversion of stimulus energy to a change in the membrane potential of a sensory receptor.
sepal - A modified leaf in angiosperms that helps enclose and protect a flower bud before it opens.
septum- One of the cross-walls that divide a fungal hypha into cells. Septa generally have pores large enough to allow ribosomes, mitochondria, and even nuclei to flow from cell to cell.
sequential hermaphroditism- A reproductive pattern in which an individual reverses its sex during its lifetime.
serial endosymbiosis- A model of the origin of eukaryotes consisting of a sequence of endosymbiotic events in which mitochondria, chloroplasts, and perhaps other cellular structures were derived from small prokaryotes that had been engulfed by larger cells.
serotonin- A biogenic amine synthesized from the amino acid tryptophan.
seta - The elongated stalk of a bryophyte sporophyte, such as in a moss.
sex chromosome - One of the pair of chromosomes responsible for determining the sex of an individual.
sex-linked gene- A gene located on a sex chromosome.
sexual dimorphism - A special case of polymorphism based on the distinction between the secondary sex characteristics of males and females.
sexual reproduction - A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from the gametes of the two parents.
sexual selection - Natural selection for mating success.
shared derived character- An evolutionary novelty that evolved within a particular clade.
shared primitive character- A character displayed in species outside a particular taxon.
shoot system - The aerial portion of a plant body, consisting of stems, leaves, and (in angiosperms) flowers.
short-day plant- A plant that flowers (usually in late summer, fall, or winter) only when the light period is shorter than a critical length.
short-term memory - The ability to hold information, anticipations, or goals for a time and then release them if they become irrelevant.
sickle-cell disease - A human genetic disease caused by a recessive allele that results in the substitution of a single amino acid in the hemoglobin protein; characterized by deformed red blood cells that can lead to numerous symptoms.
sieve plate - An end wall in a sieve-tube member, which facilitates the flow of phloem sap in angiosperm sieve tubes.
sieve-tube member - A living cell that conducts sugars and other organic nutrients in the phloem of angiosperms. They form chains called sieve tubes.
sign stimulus - An external sensory stimulus that triggers a fixed action pattern.
signal - A behavior that causes a change in behavior in another animal.
signal peptide- A stretch of amino acids on a polypeptide that targets the protein to a specific destination in a eukaryotic cell.
signal-recognition particle (SRP) - A proteinRNA complex that recognizes a signal peptide as it emerges from the ribosome.
signal transduction pathway- A mechanism linking a mechanical or chemical stimulus to a specific cellular response.
simple fruit - A fruit derived from a single carpel or several fused carpels.
simple epithelium- An epithelium consisting of a single layer of cells that all touch the basal lamina.
single nucleotide polymorphisms (SNPs) - One base-pair variation in the genome sequence.
single-lens eye- The camera-like eye found in some jellies, polychaetes, spiders, and many molluscs.
single-strand binding protein - During DNA replication, molecules that line up along the unpaired DNA strands, holding them apart while the DNA strands serve as templates for the synthesis of complementary strands of DNA.
sinoatrial (SA) node - A region of the heart composed of specialized muscle tissue that sets the rate and timing at which all cardiac muscle cells contract; the pacemaker.
sinus - Any of the spaces surrounding the organs of the body in animals with open circulatory systems.
sister chromatids- Replicated forms of a chromosome joined together by the centromere and eventUally separated during mitosis or meiosis II.
skeletal muscle (striated muscle)- Muscle generally responsible for the voluntary movements of the body.
sliding-filament model- The theory explaining how muscle contracts, based on change within a sarcomere, the basic unit of muscle organization, stating that thin (actin) filaments slide across thick (myosin) filaments, shortening the sarcomere. The shortening of all sarcomeres in a myofibril shortens the entire myofibril.
slow block to polyspermy- The formation of the fertilization envelope and other changes in the egg's surface that prevent fusion of the egg with more than one sperm.
slow muscle fibers - Muscle cells that can sustain long contractions.
small intestine - The longest section of the alimentary canal; the principal site of the enzymatic hydrolysis of food macromolecules and the absorption of nutrients.
smooth ER - That portion of the endoplasmic reticulum that is free of ribosomes.
smooth muscle- A type of muscle lacking the striations of skeletal and cardiac muscle because of the uniform distribution of myosin filaments in the cell; responsible for involuntary body activities.
snowball Earth hypothesis- The hypothesis that glaciers covered the planet's landmasses from pole to pole 750-570 million years ago, confining life to very limited areas.
social learning - Modification of behavior through the observation of other individuals.
sociobiology- The study of social behavior based on evolutionary theory.
sodium-potassium pump- A special transport protein in the plasma membrane of animal cells that transports sodium out of the cell and potassium into the cell against their concentration gradients.
solute- A substance that is dissolved in a solution.
solute potential - A component of water potential that is proportional to the number of dissolved solute molecules in a solution and measures the effect of solutes on the direction of water movement; also called osmotic potential, it can be either zero or negative.
solution- A liquid that is a homogeneous mixture of two or more substances.
solvent - The dissolving agent of a solution. Water is the most versatile solvent known.
somatic cell - Any cell in a multicellular organism except a sperm or egg cell.
somatic nervous system- The branch of the motor division of the vertebrate peripheral nervous system composed of motor neurons that carry signals to skeletal muscles in response to external stimuli.
somites- Paired blocks of mesoderm just lateral to the notochord of a vertebrate embryo.
soredia- In lichens, small clusters of fungal hyphae with embedded algae.
sorus - A cluster of sporangia on a fern sporophyll. Sori may be arranged in various patterns, such as parallel lines or dots, that are useful in fern identification.
Southern blotting - A hybridization technique that enables researchers to determine the presence of certain nucleotide sequences in a sample of DNA.
spatial learning- Modification of behavior based on experience of the spatial structure of the environment.
spatial summation- A phenomenon of neural integration in which the membrane potential of the postsynaptic cell is determined by the combined effect of EPSPs or IPSPs produced nearly simultaneously by different synapses.
speciation - The origin of new species in evolution.
species- A group whose members possess similar anatomical characteristics and have the ability to interbreed.
species-area curve- The biodiversity pattern, first noted by Alexander von Humboldt, that illustrates that the larger the geographic area of a community, the greater the number of species.
species diversity- The number and relative abundance of species in a biological community.
species richness- The number of species in a biological community
species selection- A theory maintaining that species living the longest and generating the greatest number of species determine the direction of major evolutionary trends.
specific epithet - The second part of a binomial, referring to one species within a genus.
spectrophotometer An instrument that measures the proportions of light of different wavelengths absorbed and transmitted by a pigment solution.
sperm- The male gamete.
spermatheca- A sac in the female reproductive system where sperm are stored.
spermatogenesis- The continuous and prolific production of mature sperm cells in the testis.
spermatogonia- Stem cells that give rise to sperm.
sphincter - A ringlike valve, consist, ing of modified muscles in a muscular tube, such as a digestive tract; closes off the tube like a drawstring.
spinal nerve - In the vertebrate peripheral nervous system, a nerve that carries signals to or from the spinal cord.
spiral cleavage- A type of embryonic development in protostomes, in which the planes of cell division that transform the zygote into a ball of cells occur obliquely to the polar axis, resulting in cells of each tier sitting in the grooves between cells of adjacent tiers.
spiral valve- A corkscrew-shaped ridge that increases surface area and prolongs the passage of food along the short digestive tract.
spliceosome- A complex assembly that interacts with the ends of an RNA intron in splicing RNA, releasing the intron and joining the two adjacent exons.
spongocoel- The central cavity of a sponge.
spongy mesophyll - Loosely arranged photosynthetic cells located below the palisade mesophyll cells in a leaf.
sporangium- A capsule in fungi and plants in which meiosis occurs and haploid spores develop.
spore- In the life cycle of a plant or alga undergoing alternation of generations, a meiotically produced haploid cell that divides mitotically, generating a multicellular individual, the gametophyte, without fusing with another cell.
sporocyte- A diploid cell, also known as a spore mother cell, that undergoes meiosis and generates haploid spores.
sporophyll - A leaf specialized for reproduction.
sporophyte - In organisms undergoing alternation of generations, the multicellular diploid form that results from a union of gametes and that meiotically produces haploid spores that grow into the gametophyte generation.
sporopollenin - A duarable polymer that covers exposed zygotes of charophycean algae and forms walls of plant spores, preventing them from drying out.
sporozoite - A tiny infectious cell that represents a stage in the apicomplexan life cycle.
squamous- The flat, tile-like shape of a type of epithelial cell.
stabilizing selection - Natural selection that favors intermediate variants by acting against extreme phenotypes.
stamen - The pollen-producing reproductive organ of a flower, consisting of an anther and filament.
standard metabolic rate (SMR)- The metabolic rate of a resting, fasting, and nonstressed ectotherm.
stapes- The third of the three middle ear bones.
starch- A storage polysaccharide in plants consisting entirely of glucose.
statocyst - A type of mechanoreceptor that functions in equilibrium in invertebrates through the use of statoliths, which stimulate hair cells in relation to gravity.
statolith- (1) In plants, a specialized plastid that contains dense starch grains and may playa role in detecting gravity (2) In invertebrates, a grain or other dense granule that settles in response to gravity and is found in sensory organs that function in equilibrium.
stele- The vascular tissue of a stem or root. stem A vascular plant organ consisting of an alternating system of nodes and internodes that support the leaves and reproductive structures.
stem cell - Any relatively unspecialized cell that can divide during a single division into one identical daughter cell and one more specialized daughter cell, which can undergo further differentiation.
stenohaline- Referring to organisms that cannot tolerate substantial changes in external osmolarity.
steroid - A type of lipid characterized by a carbon skeleton consisting of four rings with various functional groups attached.
sticky end - A single-stranded end of a doublestranded DNA restriction fragment.
stigma - The sticky part of a flower's carpel, which traps pollen grains.
stipe- A stemlike structure of a seaweed.
stock- The plant that provides the root system when making a graft.
stoma - A microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange between the environment and the interior of the plant.
stomach- An organ of the digestive system that stores food and performs preliminary steps of digestion.
stratified epithelium - An epithelium consisting of more than one layer of cells in which some but not all cells touch the basal lamina.
stream - A flowing body of water that is generally small, cold, and clear.
stress-induced proteins - Molecules, including heat-shock proteins, that are produced within cells in response to exposure to marked increases in temperature and to other forms of severe stress, such as toxins, rapid pH changes, and viral infections.
stretch-gated ion channel - Protein pore in a cell's plasma membrane that opens when the membrane is mechanically deformed, allowing the passage of certain ions.
striated muscle - See skeletal muscle.
strobili - The technical term for clusters of sporophylls known commonly as cones, found in most gymnosperms and some seedless vascular plants.
stroke- The death of nervous tissue in the brain, usually resulting from rupture or blockage of arteries in the head.
stroke volume - The amount of blood pumped by the left ventricle in each contraction.
stroma- The fluid of the chloroplast surrounding the thylakoid membrane; involved in the synthesis of organic molecules from carbon dioxide and water.
stromatolite- Rocklike structure composed of layers of prokaryotes and sediment.
structural formula - A type of molecular notation in which the constituent atoms are joined by lines representing covalent bonds.
structural isomer - One of several organic compounds that have the same molecular formula but differ in the covalent arrangements of their atoms.
style- The stalk of a flower's carpel, with the ovary at the base and the stigma at the top.
substance P- A neuropeptide that is a key excitatory signal that mediates our perception of pain.
substrate- The reactant on which an enzyme
substrate feeder- An organism that lives in or on its food source, eating its way through the food.
substrate-level phosphorylation- The formation of ATP by directly transferring a phosphate group to ADP from an intermediate substrate in catabolism.
sugar sink - A plant organ that is a net consumer or storer of sugar. Growing roots, shoot tips, stems, and fruits are sugar sinks supplied by phloem.
sugar source - A plant organ in which sugar is being produced by either photosynthesis or the breakdown of starch. Mature leaves are the primary sugar sources of plants.
sulfhydryl group - A functional group consisting of a sulfur atom bonded to a hydrogen atom (-SH).
suprachiasmatic nuclei (SCN)- A pair of structures in the hypothalamus of mammals that functions as a biological clock.
surface tension - A measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension because of the hydrogen bonding of surface molecules.
survivorship curve - A plot of the number of members of a cohort that are still alive at each age; one way to represent age-specific mortality.
suspension feeder - An aquatic animal, such as a clam or a baleen whale, that sifts small food particles from the water.
sustainable agriculture - Long-term productive farming methods that are environmentally safe.
sustainable development- The long-term prosperity of human societies and the ecosystems that support them.
swim bladder - In aquatic osteichthyans, an air sac that enables the animal to control its buoyancy in the water.
symbiont- The smaller participant in a symbiotic relationship, living in or on the host.
symbiosis- An ecological relationship between organisms of two different species that live together in direct contact.
sympathetic division - One of three divisions of the autonomic nervous system of vertebrates; generally increases energy expenditUre and prepares the body for action.
sympatric speciation- A mode of speciation occurring as a result of a radical change in the genome of a subpopulation, reproductively isolating the subpopulation from the parent population.
symplast- In plants, the continuum of cytoplasm connected by plasmodesmata between cells.
synapse - The locus where one neuron communicates with another neuron in a neural pathway; a narrow gap between a synaptic terminal of an axon and a signal receiving portion (dendrite or cell body) of another neuron or effector cell. Neurotransmitter molecules released by synaptic terminals diffuse across the synapse, relaying messages to the dendrite or effector.
synapsid - Member of an amniote clade distinguished by a single hole on each side of the skull, including the mammals.
synapsis- The pairing of replicated homologous chromosomes during prophase I of meiosis.
synaptic cleft - A narrow gap separating the synaptic knob of a transmitting neuron from a receiving neuron or an effector cell.
synaptic terminal- A bulb at the end of an axon in which neurotransmitter molecules are stored and released.
synaptic vesicle- Membranous sac containing neurotransmitter molecules at the tip of the presynaptic axon.
system- A more complex organization formed from a combination of components.
systematics- The analytical study of the diversity and relationships of organisms, both present -day and extinct.
systemic acquired resistance (SAR)- A defensive response in infected plants that helps protect healthy tissue from pathogenic invasion.
systemic circuit- The branch of the circulatory system that supplies all body organs and then returns oxygen-poor blood to the right atrium via the veins.
systemic circulation - Movement of blood through the systemic circuit.
systems biology- An approach to studying biology that aims to model the dynamic behavior of whole biological systems.
systole - The stage of the heart cycle in which the heart muscle contracts and the chambers pump blood.
systolic pressure- Blood pressure in the arteries during contraction of the ventricles.
T cell receptor - The antigen receptor on T cells; a membrane-bound molecule consisting of one a chain and one f3 chain linked by a disulfide bridge and containing one antigenbinding site.
T lymphocyte (T cell) - A type of lymphocyte, including the helper T cells and cytotoxic T cells, that develops to maturity in the thymus. After encountering antigen, T cells are responsible for cell-mediated immunity.
taproot system- A root system common to eudicots, consisting of one large, vertical root (the taproot) that produces many smaller lateral, or branch, roots.
taste buds- Collections of modified epithelial cells that are scattered in several areas of the tongue and mouth and are receptors for taste in humans.
TATA box- A promoter DNA sequence crucial in forming the transcription initiation complex.
taxis - Movement toward or away from a stimulus.
taxon - The named taxonomic unit at any given level of classification.
taxonomy - Ordered division of organisms into categories based on a set of characteristics used to assess similarities and differences, leading to a classification scheme; the branch of biology concerned with naming and classifying the diverse forms of life.
Tay-Sachs disease- A human genetic disease caused by a recessive allele for a dysfunctional enzyme, leading to accumulation of certain lipids in the brain. Seizures, blindness, and degeneration of motor and mental performance usually become manifest a few months after birth.
technology- The application of scientific knowledge for a specific purpose.
telomerase- An enzyme that catalyzes the lengthening of telomeres. The enzyme includes a molecule of RNA that serves as a template for new telomere segments.
telomere - The protective structure at each end of a eukaryotic chromosome. Specifically, the tandemly repetitive DNA at the end of the chromosome's DNA molecule. See also repetitive DNA.
telophase - The fifth and final stage of mitosis, in which daughter nuclei are forming and cytokinesis has typically begun.
temperate broadleaf forest- A biome located throughout midlatitude regions where there is sufficient moisture to support the growth of large, broadleaf deciduous trees.
temperate grassland - A terrestrial biome dominated by grasses and forbs.
temperate phage- A phage that is capable of reproducing by either the lytic or lysogenic cycle.
temperature- A measure of the intensity of heat in degrees, reflecting the average kinetic energy of the molecules.
template strand - The DNA strand that provides the template for ordering the sequence of nucleotides in an RNA transcript.
temporal summation - A phenomenon of neural integration in which the membrane potential of the postsynaptic cell in a chemical synapse is determined by the combined effect of EPSPs or IPSPs produced in rapid succession.
tendon- A type of fibrous connective tissue that attaches muscle to bone.
terminal bud- Embryonic tissue at the tip of a shoot, made up of developing leaves and a compact series of nodes and internodes.
terminator- In prokaryotes, a special sequence of nucleotides in DNA that marks the end of a gene. It signals RNA polymerase to release the newly made RNA molecule, which then departs from the gene.
territoriality- A behavior in which an animal defends a bounded physical space against encroachment by other individuals, usually of its own species. Territory defense may involve direct aggression or indirect mechanisms such as scent marking or singing.
tertiary consumer- A member of the trophic level of an ecosystem consisting of carnivores that eat mainly other carnivores.
tertiary structure- Irregular contortions of a protein molecule due to interactions of side chains involved in hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges.
test - The hardened shell of some protists, including forams and rediolarians, or the rigid endoskeleton of a sea urchin or sand dollar.
testcross - Breeding of an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype. The ratio of phenotypes in the offspring determines the unknown genotype.
testes - The male reproductive organ, or gonad, in which sperm and reproductive hormones are produced.
testosterone - The most abundant androgen hormone in the male body.
tetanus - The maximal, sustained contraction of a skeletal muscle, caused by a very fast frequency of action potentials elicited by continual stimulation.
tetrad - A paired set of homologous chromosomes, each composed of two sister chromatids. Tetrads form during prophase I of meiosis.
tetrapod - A vertebrate with two pairs of limbs, including mammals, amphibians, and birds and other reptiles.
thalamus - One of two integrating centers of the vertebrate forebrain. Neurons with cell bodies in the thalamus relay neural input to specific areas in the cerebral cortex and regulate what information goes to the cerebral cortex.
thallus - A seaweed body that is plantlike but lacks true roots, stems, and leaves.
theory - An explanation that is broad in scope, generates new hypotheses, and is supported by a large body of evidence.
thermal energy- See heat.
thermocline- A narrow stratum of rapid temperature change in the ocean and in many temperate-zone lakes.
thermodynamics - (1) The study of energy transformations that occur in a collection of matter. See first law of thermodynamics and second law of thermodynamics. (2) A phenomenon in which external DNA is taken up by a cell and functions there.
thermoreceptor- An interoreceptor stimulated by either heat or cold.
thermoregulation - The maintenance of internal body temperature within a tolerable range.
theropod- A member of an ancient group of dinosaurs that were bipedal carnivores.
thick filament- A filament composed of staggered arrays of myosin molecules; a component of myofibrils in muscle fibers.
thigmomorphogenesis - A response in plants to chronic mechanical stimulation, resulting from increased ethylene production. An example is thickening stems in response to strong winds.
thigmotropism - A directional growth of a plant in response to touch.
thin filament - The smaller of the two myofilaments consisting of two strands of actin and two strands of regulatory protein coiled around one another.
thoracic cavity- The body cavity in mammals that houses the lungs and heart. It is surrounded in part by ribs and separated from the lower abdominal cavity by the diaphragm.
threatened species- A species that is considered likely to become endangered in the foreseeable future.
three-domain system - A system of taxonomic classification based on three "superkingdoms": Bacteria, Archaea, and Eukarya.
threshold - The potential an excitable cell membrane must reach for an action potential to be initiated.
thrombus- A clump of platelets and fibrin that blocks the flow of blood through a blood vessel.
thylakoid - A flattened membrane sac inside the chloroplast, used to convert light energy to chemical energy.
thymus- A small organ in the thoracic cavity of vertebrates where maturation of T cells is completed.
thyroid gland - An endocrine gland, located on the ventral surface of the trachea, that secretes two iodine-containing hormones, triiodothyronine (T3) and thyroxine (T4), and cacitonin.
thyroid-stimulating hormone (TSH)- A tropic hormone produced and secreted by the anterior pituitary that regulates the release of thyroid hormones.
thyroxine (T4) - One of two iodine-containing hormones that are secreted by the thyroid gland and help regulate metabolism, development, and maturation in vertebrates.
Ti plasmid - A plasmid of a tumor-inducing bacterium that integrates a segment of its DNA into the host chromosome of a plant; frequently used as a carrier for genetic engineering in plants.
tidal volume- The volume of air an animal inhales and exhales with each breath.
tight junction- A type of intercellular junction in animal cells that prevents the leakage of material between cells.
tissue - An integrated group of cells with a common function, structure, or both.
tissue system- One or more tissues organized into a functional unit connecting the organs of a plant.
tonicity - The ability of a solution to cause a cell within it to gain or lose water.
tonoplast - A membrane that encloses the central vacuole in a plant cell, separating the cytosol from the vacuolar contents, called cell sap; also known as the vacuolar membrane.
top-down model - A model of community organization in which predation controls community organization because predators control herbivores, which in turn control plants, which in turn control nutrient levels; also called the trophic cascade model.
topoisomerase - A protein that functions in DNA replication, helping to relieve strain in the double helix ahead of the replication fork.
topsoil - A mixture of particles derived from rock, living organisms, and humus.
torpor- In animals, a physiological state that conserves energy by slowing down metabolism.
torsion- A characteristic of gastropods in which the visceral mass rotates during development.
totipotent- Describing a cell that can give rise to all parts of an organism.
trace element- An element indispensable for life but required in extremely minute amounts.
trachea - The windpipe; that portion of the respiratory tube that has C-shaped cartilagenous rings and passes from the larynx to two bronchi.
tracheal system- A gas exchange system of branched, chitin-lined tubes that infiltrate the body and carry oxygen directly to cells in insects.
tracheid - A long, tapered waterconducting cell that is dead at maturity and is found in the xylem of all vascular plants.
trait - Any detectable variation in a genetic character.
trans- Arrangement of two noncarbon atoms, each bound to one of the carbons in a carbon-carbon double bond, where the two noncarbon atoms are on opposite sides relative to the double bond.
transcription - The synthesis of RNA on a DNA template.
transcription factor - A regulatory protein that binds to DNA and stimulates transcription of specific genes.
transcription initiation complex - The completed assembly of transcription factors and RNA polymerase bound to the promoter.
transcription unit - A region of a DNA molecule that is transcribed into an RNA molecule.
transduction- (1) A DNA transfer process in which phages carry bacterial genes from one host cell to another. (2) In cellular communication, the conversion of a signal from outside the cell to a form that can bring about a specific cellular response.
transfer cell - A companion cell with numerous ingrowths of its wall, increasing the cell's surface area and enhancing the transfer of solutes between apoplast and symplast.
transfer RNA (tRNA)- An RNA molecule that functions as an interpreter between nucleic acid and protein language by picking up specific amino acids and recognizing the appropriate codons in the mRNA.
transformation- (1) The conversion of a normal animal cell to a cancerous cell. (2) A change in genotype and phenotype due to the assimilation of external DNA by a cell.
transgenic - Pertaining to an individual plant or animal whose genome contains a gene introduced from another organism, either from the same or a different species.
translation- The synthesis of a polypeptide using the genetic information encoded in an mRNA molecule. There is a change of "language" from nucleotides to amino acids.
translocation- (1) An aberration in chromosome structure resulting from attachment of a chromosomal fragment to a nonhomologous chromosome. (2) During protein synthesis, the third stage in the elongation cycle when the RNA carrying the growing polypeptide moves from the A site to the P site on the ribosome. (3) The transport of organic nutrients in the phloem of vascular plants.
transmission- The conduction of impulses to the central nervous system.
transmission electron microscope (TEM) - A microscope that passes an electron beam through very thin sections; primarily used to study the internal ultrastructure of cells.
transpiration - The evaporative loss of water from a plant.
transport epithelium - One or more layers of specialized epithelial cells that regulate solute movements.
transport protein - A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane.
transport vesicle- A tiny membranous sac in a cell's cytoplasm carrying molecules produced by the cell.
transposable element A segment of DNA that can move within the genome of a cell by means of a DNA or RNA intermediate; also called a transposable element.
transposon- A transposable element that moves within a genome by means of a DNA intermediate.
transverse (T) tubules - Infoldings of the plasma membrane of skeletal muscle cells.
triacylglycerol - Three fatty acids linked to one glycerol molecule.
triiodothyronine (T 3) - One of two iodine-containing hormones that are secreted by the thyroid gland and help regulate metabolism, development, and maturation in vertebrates.
trilobite - An extinct arthropod with pronounced segmentation and appendages that varied little from segment to segment.
trimester - In human development, one of three 3-month-1ong periods of pregnancy.
triple response - A plant growth maneuver in response to mechanical stress, involving slowing of stem elongation, a thickening of the stem, and a curvature that causes the stem to start growing horizontally.
triplet code - A set of three-nucleotide-Iong words that specify the amino acids for polypeptide chains.
triploblastic - Possessing three germ layers: the endoderm, mesoderm, and ectoderm. Most eumetazoans are triploblastic.
trisomic- Referring to a cell that has three copies of a particular chromosome, instead of the normal two.
trochophore larva- Distinctive larval stage observed in certain invertebrates, including some annelids and molluscs.
trophic efficiency - The percentage of production transferred from one trophic level to the next.
trophic structure - The different feeding relationships in an ecosystem, which determine the route of energy flow and the pattern of chemical cycling.
trophoblast - The outer epithelium of the blastocyst, which forms the fetal part of the placenta.
tropical rainforest- A terrestrial biome characterized by high levels of precipitation and warm temperatures year-round.
tropic hormone- A hormone that has another endocrine gland as a target.
tropics - Latitudes between 23.50 north and south.
tropism- A growth response that results in the curvature of whole plant organs toward or away from stimuli owing to differential rates of cell elongation.
tropomyosin - The regulatory protein that blocks the myosin-binding sites on the actin molecules.
troponin complex- The regulatory proteins that control the position of tropomyosin on the thin filament.
true-breeding - Referring to plants that produce offspring of the same variety when they selfpollinate.
tubal ligation- A means of sterilization in which a woman's two oviducts (Fallopian tubes) are tied closed to prevent eggs from reaching the uterus. A segment of each oviduct is removed.
tube foot - One of numerous extensions of an echinoderm's water vascular system. Tube feet function in locomotion, feeding, and gas exchange.
tumor-suppressor gene - A gene whose protein products inhibit cell division, thereby preventing uncontrolled cell growth (cancer).
tundra- A biome at the extreme limits of plant growth. At the northernmost limits, it is called arctic tundra, and at high altitudes, where plant forms are limited to low shrubby or matlike vegetation, it is called alpine tundra.
tunicate - Member of the subphylum Urochordata, sessile marine chordates that lack a backbone.
turgid - Very firm. A walled cell becomes turgid if it has a greater solute concentration than its surroundings, resulting in entry of water.
turgor pressure - The force directed against a cell wall after the influx of water and the swelling of a walled cell due to osmosis.
turnover - The mixing of waters as a result of changing water-temperature profiles in a lake.
turnover time - The time required to replace the standing crop of a population or group of populations (for example, of phytoplankton), calculated as the ratio of standing crop biomass to production.
tympanic membrane- Another name for the eardrum.
tyrosine kinase - An enzyme that catalyzes the transfer of phosphate groups from ATP to the amino acid tyrosine on a substrate protein.
ultimate question - In animal behavior, an inquiry that focuses on the evolutionary significance of a behavioral act.
ultrametric tree- A phylogenetic tree in which the lengths of the branches reflect measurements of geologic time.
undernourishment - A diet that is chronically deficient in calories.
uniformitarianism - Charles Lyell's idea that geologic processes have not changed throughout Earth's history.
unsaturated fatty acid - A fatty acid possessing one or more double bonds between the carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms attached to the carbon skeleton.
urea - A soluble nitrogenous waste excreted by mammals, most adult amphibians, sharks, and some marine bony fishes and turtles; produced in the liver by a metabolic cycle that combines ammonia with carbon dioxide.
ureter - A duct leading from the kidney to the urinary bladder.
urethra- A tube that releases urine from the body near the vagina in females and through the penis in males; also serves in males as the exit tube for the reproductive system.
uric acid - An insoluble precipitate of nitrogenous waste excreted by land snails, insects, and many reptiles, including birds.
urinary bladder - The pouch where urine is stored prior to elimination.
uterine cycle - The changes that occur in the uterus during the reproductive cycle of the human female; also called the menstrual cycle.
uterus- A female organ where eggs are fertilized and/or development of the young occurs.
utricle- A chamber behind the oval window that opens into the three semicircular canals.
vaccination - See immunization.
vaccine - A harmless variant or derivative of a pathogen that stimulates a host's immune system to mount defenses against the pathogen.
vacuolar membrane - A membrane that encloses the central vacuole in a plant cell, separating the cytosol from the vacuolar contents, called cell sap; also known as the tonoplast.
vagina - Part of the female reproductive system between the uterus and the outside opening; the birth canal in mammals; also accommodates the male's penis and receives sperm during copulation.
valence- The bonding capacity of an atom, generally equal to the number of unpaired electrons in the atom's outermost shell.
valence electron- An electron in the outermost electron shell.
valence shell- The outermost energy shell of an atom, containing the valence electrons involved in the chemical reactions of that atom.
van der Waals interactions- Weak attractions between molecules or parts of molecules that are brought about by localized charge fluctuations.
variation - Differences between members of the same species.
vas deferens - The tube in the male reproductive system in which sperm travel from the epididymis to the urethra.
vasa recta - The capillary system that serves the loop of Henle.
vascular bundle - A strand of vascular tissues (both xylem and phloem) in a stem or leaf.
vascular cambium - A cylinder of meristematic tissue in woody plants that adds layers of secondary vascular tissue called secondary xylem (wood) and secondary phloem.
vascular cylinder- The central cylinder of vascular tissue in a root.
vascular plant- A plant with vascular tissue. Vascular plants include all living species except mosses, liverworts, and hornworts.
vascular tissue - Plant tissue consisting of cells joined into tubes that transport water and nutrients throughout the plant body.
vascular tissue system - A system formed by xylem and phloem throughout a vascular plant, serving as a transport system for water and nutrients, respectively.
vasectomy - The cutting of each vas deferens to prevent sperm from entering the urethra.
vasocongestion- The filling of a tissue with blood, caused by increased blood flow through the arteries of that tissue.
vasoconstriction - A decrease in the diameter of superficial blood vessels triggered by nerve signals that contract the muscles of the vessel walls.
vasodilation - An increase in the diameter of superficial blood vessels triggered by nerve signals that relax the muscles of the vessel walls.
vegetal pole- The portion of the egg where most yolk is concentrated; opposite of animal pole.
vegetative reproduction- Cloning of plants by asexual means.
vein- (1) In animals, a vessel that returns blood to the heart. (2) In plants, a vascular bundle in a leaf.
ventilation - Any method of increasing contact between the respiratory medium and the respiratory surface.
ventral - Pertaining to the underside, or bottom, of a bilaterally symmetrical animal.
ventricle- (1) A heart chamber that pumps blood out of a heart. (2) A space in the vertebrate brain, filled with cerebrospinal fluid.
venule- A vesse1 that conveys blood between a capillary bed and a vein.
vernalization- The use of cold treatment to induce a plant to flower.
vertebrate - A chordate animal with a backbone: the mammals, reptiles (including birds), amphibians, sharks and rays, ray-finned fishes, and lobe-fins.
vesicle- A sac made of membrane inside of cells.
vessel element - A short, wide, water-conducting cell found in the xylem of most angiosperms and a few nonflowering vascular plants. Dead at maturity, vessel elements are aligned end to end to form micropipes called vessels.
vessels- Continuous water-conducting micropipes found in most angiosperms and a few nonflowering vascular plants.
vestibule - The cavity enclosed by the labia minora; the space into which the vagina and urethral opening empty.
vestigial organ- A structure of marginal, if any, importance to an organism. Vestigial organs are historical remnants of structures that had important functions in ancestors.
villus- (1) A fingerlike projection of the inner surface of the small intestine. (2) A fingerlike projection of the chorion of the mammalian placenta. Large numbers of villi increase the surface areas of these organs.
viral envelope- A membrane that cloaks the capsid that in turn encloses a viral genome.
viroid - A plant pathogen composed of molecules of naked circular RNA only several hundred nucleotides long.
virulent - A term describing a pathogen against which a plant has little specific defense.
virulent phage - A phage that reproduces only by a lytic cycle.
visceral mass- One of the three main parts of a mollusc, containing most of the internal organs.
visible light - That portion of the electromagnetic spectrum detected as various colors by the human eye, ranging in wavelength from about 380 nm to about 750 nm.
vital capacity - The maximum volume of air that a respiratory system can inhale and exhale.
vitamin - An organic molecule required in the diet in very small amounts. Vitamins serve primarily as coenzymes or parts of coenzymes.
vitamin D- One of the fat-soluble vitamins. The active form functions as a hormone, acting in concert with parathyroid hormone in bone and promoting the uptake of calcium from food within the intestines.
vitreous humor - The jellylike material that fills the posterior cavity of the vertebrate eye.
viviparous - Referring to a type of development in which the young are born alive after having been nourished in the uterus by blood from the placenta.
vocal cord - One of a pair of stringlike tissues in the larynx. Air rushing past the tensed vocal cords makes them vibrate, producing sounds.
voltage-gated ion channel - A specialized ion channel that opens or closes in response to changes in membrane potential.
vulva - Collective term for the female external genitalia.
water potential - The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure.
water vascular system - A network of hydraulic canals unique to echinoderms that branches into extensions called tube feet, which function in locomotion, feeding, and gas exchange.
wavelength - The distance between crests of waves, such as those of the electromagnetic spectrum.
wetland - An ecosystem intermediate between an aquatic one and a terrestrial one. Wetland soil is saturated with water permanently or periodically.
white blood cell - A blood cell that functions in defending the body against infections and cancer cells; also called a leukocyte.
white matter - Tracts of axons within the CNS.
wild type- An individual with the normal (most common) phenotype.
wilting- The drooping of leaves and stems as a result of plant cells becoming flaccid.
wobble- A violation of the base-pairing rules in that the third nucleotide (5' end) of a tRNA anticodon can form hydrogen bonds with more than one kind of base in the third position (3' end) of a codon.
xerophyte - A plant adapted to an arid climate.
X-ray crystallography - A technique that depends on the diffraction of an X-ray beam by the individual atoms of a molecule to study the three-dimensional structure of the molecule.
xylem- Vascular plant tissue consisting mainly of tubular dead cells that conduct most of the water and minerals upward from roots to the rest of the plant.
yeast - Single-celled fungi that inhabit liquid or moist habitats and reproduce asexually by simple cell division or by the pinching of small buds off a parent cell.
yeast artificial chromosome (YAC) - A vector that combines the essentials of a eukaryotic chromosome-an origin for DNA replication, a centromere, and two telomeres-with foreign DNA.
yolk - Nutrients stored in an egg.
yolk plug- Large food-laden endodermal cells surrounded by the blastopore of an amphibian gastrula.
yolk sac- One of four extraembryonic membranes that support embryonic development; the first site of blood cells and circulatory system function.
Z lines- The borders of a sarcomere.
zero population growth (ZPG) - A period of stability in population size, when the per capita birth rate and death rate are equal.
zona pellucida - The extracellular matrix of a mammalian egg.
zone of cell division- The zone of primary growth in roots consisting of the root apical meristem and its derivatives. New root cells are produced in this region.
zone of elongation- The zone of primary growth in roots where new cells elongate, sometimes up to ten times their original length.
zone of maturation- The zone of primary growth in roots where cells complete their differentiation and become functionally mature.
zone of polarizing activity (ZPA)- A limb-bud organizer region consisting of a block of mesoderm located where the posterior side of the bud is attached to the body.
zoned reserve - An extensive region of land that includes one or more areas undisturbed by humans surrounded by lands that have been changed by human activity and are used for economic gain.
zoospore- Flagellated spore occurring in chytrid fungi.
zygomycete - Member of the fungal phylum Zygomycota, characterized by forming a sturdy structure called a zygosporangium during sexual reproduction.
zygosporangium- In zygomycete fungi, a sturdy multinucleate structure in which karyogamy and meiosis occur.
zygote - The diploid product of the union of haploid gametes in conception; a fertilized egg.
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