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1651 Cards in this Set

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In vertebrates, the portion of the trunk containing visceral organs other than heart and lungs; in arthropods, the posterior portion of the body, made up of similar segments and containing the reproductive organs and part of the digestive tract.
Nonliving; specifically, the nonliving components of an ecosystem, such as temperature, humidity, the mineral content of the soil, etc.
abscisic acid (ABA)
A plant hormone that generally acts to inhibit growth, promote dormancy, and help the plant tolerate stressful conditions.
In plants, the dropping of leaves, flowers, fruits, or stems at the end of a growing season, as the result of formation of a two-layered zone of specialized cells (the abscission zone) and the action of a hormone (ethylene).
The movement of water and dissolved substances into a cell, tissue, or organism.
absorption spectrum
The range of a pigment's ability to absorb various wavelengths of light.
abyssal zone
The portion of the ocean floor where light does not penetrate and where temperatures are cold and pressures intense.
Physiological adjustment to a change in an environmental factor.
The automatic adjustment of an eye to focus on near objects.
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.
acetyl CoA
The entry compound for the Krebs cycle in cellular respiration; formed from a fragment of pyruvate attached to a coenzyme.
A substance that increases the hydrogen ion concentration in a solution.
acid precipitation
Rain, snow, or fog that is more acidic than pH 5.6.
A solid-bodied animal lacking a cavity between the gut and outer body wall.
An organelle at the tip of a sperm cell that helps the sperm penetrate the egg.
Abbreviation of adrenocorticotropic hormone.
A globular protein that links into chains, two of which twist helically about each other, forming microfilaments 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.
activation energy
The energy that must be possessed by atoms or molecules in order to react.
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.
(1) The evolution of features that make a group of organisms better suited to live and reproduce in their environment. (2) A peculiarity of structure, physiology, or behavior that aids the organism in its environment.
adaptive peak
An equilibrium state in a population when the gene pool has allele frequencies that maximize the average fitness of a population's members.
adaptive radiation
The emergence of numerous species from a common ancestor introduced into an environment, presenting a diversity of new opportunities and problems.
adenosine diphosphate (ADP)
A nucleotide consisting of adenine, ribose, and two phosphate groups; formed by the removal of one phosphate from an ATP molecule.
adenosine monophosphate (AMP)
A nucleotide consisting of adenine, ribose, and one phosphate group; can be formed by the removal of two phosphates from an ATP molecule; in its cyclic form, functions as a "second messenger" for a number of vertebrate hormones and neurotransmitters.
adenosine triphosphate (ATP)
An adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed. This energy is used to drive endergonic reactions in cells.
adenylyl cyclase
An enzyme that converts ATP to cyclic AMP in response to a chemical signal.
Abbreviation of antidiuretic hormone.
The tendency of different kinds of molecules to stick together.
Abbreviation of adenosine diphosphate.
adrenal gland
An endocrine gland located adjacent to the kidney in mammals; composed of two glandular portions: an outer cortex, which responds to endocrine signals in reacting to stress and effecting salt and water balance, and a central medulla, which responds to nervous inputs resulting from stress.
A hormone, produced by the medulla of the adrenal gland, that increases the concentration of glucose in the blood, raises blood pressure and heartbeat rate, and increases muscular power and resistance to fatigue; also a neurotransmitter across synaptic junctions. Also called epinephrine. See Epinephrine.
adrenocorticotropic hormone (ACTH)
A hormone, produced by the anterior lobe of the pituitary gland, that stimulates the production of cortisol by the adrenal cortex.
Referring to a structure arising from an unusual place, such as roots growing from stems or leaves.
Containing oxygen; referring to an organism, environment, or cellular process that requires oxygen.
Bringing inward to a central part, applied to nerves and blood vessels.
A gelatinous material prepared from certain red algae that is used to solidify nutrient media for growing microorganisms.
age structure
The relative number of individuals of each age in a population.
A member of a jawless class of vertebrates represented today by the lampreys and hagfishes.
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.
An organic molecule with a carbonyl group located at the end of the carbon skeleton.
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.
aleurone layer
The outermost cell layer of the endosperm of the grains (seeds) of wheat and other grasses; when acted upon by gibberellin, the aleurone layer releases enzymes that digest the stored food of the endosperm into small nutrient molecules that can be taken up by the embryo.
alga pl. algae
A photosynthetic, plantlike protist.
Pertaining to substances that increase the relative number of hydroxide ions (OH–) in a solution; having a pH greater than 7; basic; opposite of acidic.
all-or-none event
An action that occurs either completely or not at all, such as the generation of an action potential by a neuron.
One of four extraembryonic membranes; serves as a repository for the embryo's nitrogenous waste.
An alternative form of a gene.
allele frequency
The proportion of a particular allele in a population.
allergic reaction
An inflammatory response triggered by a weak antigen (an allergen) to which most individuals do not react; involves the release of large amounts of histamine from mast cells.
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 the ancestral population becomes segregated by a geographical barrier.
A common type of polyploid species resulting from two different species interbreeding and combining their chromosomes.
allosteric site
A specific receptor site on an enzyme molecule remote from the active site. Molecules bind to the allosteric site and change the shape of the active site, making it either more or less receptive to the substrate.
Slightly different versions of the same enzyme, distinguishable via gel electrophoresis.
alpha 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.
alternative splicing
In alternative splicing, the same pre-mRNA molecule, which consists of introns and exons, is spliced in different ways to produce mature mRNAs of different lengths and different functionality.
altruistic behavior
The aiding of another individual at one's own risk or expense.
alveolus pl. alveoli
(1) One of the deadend, multilobed air sacs that constitute the gas exchange surface of the lungs. (2) One of the milk-secreting sacs of epithelial tissue in the mammary glands.
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 +1.
aminoacyl—tRNA synthetases
A family of enzymes, at least one for each amino acid, that catalyze the attachment of an amino acid to its specific tRNA molecule.
The process by which decomposers break down proteins and amino acids, releasing the excess nitrogen in the form of ammonia (NH3) or ammonium ion (NH4+).
A technique for determining genetic abnormalities in a fetus by the presence of certain chemicals or defective fetal cells in the amniotic fluid, obtained by aspiration from a needle inserted into the uterus.
The innermost of four extraembryonic membranes; encloses a fluid-filled sac in which the embryo is suspended.
A vertebrate possessing an amnion surrounding the embryo; reptiles, birds, and mammals are amniotes.
amniotic egg
A shelled, water-retaining egg that enables reptiles, birds, and egg-laying mammals to complete their life cycles on dry land.
Moving or feeding by means of pseudopodia (temporary cytoplasmic protrusions from the cell body).
Abbreviation of adenosine monophosphate.
The vertebrate class of amphibians, represented by frogs, salamanders, and caecilians.
amphipathic molecule
A molecule that has both a hydrophilic region and a hydrophobic region.
anabolic steroids
Synthetic chemical variants of the male sex hormone testosterone; they produce increased muscle mass but also suppress testosterone production, leading to shrinkage of the testes, growth of the breasts, and premature baldness; long-term use increases the risk of kidney and liver damage and of liver cancer.
Within a cell or organism, the sum of all biosynthetic reactions (that is, chemical reactions in which larger molecules are formed from smaller ones).
Lacking oxygen; referring to an organism, environment, or cellular process that lacks oxygen and may be poisoned by it.
A pattern of evolutionary change involving the transformation of an entire population, sometimes to a state different enough from the ancestral population to justify renaming it as a separate species; also called phyletic evolution.
The similarity of structure between two species that are not closely related; attributable to convergent evolution.
Applied to structures similar in function but different in evolutionary origin, such as the wing of a bird and the wing of an insect.
The third stage of mitosis, beginning when the centromeres of duplicated chromosomes divide and sister chromotids separate from each other, and ending when a complete set of daughter chromosomes are located at each of the two poles of the cell.
The principal male steroid hormones, such as testosterone, which stimulate the development and maintenance of the male reproductive system and secondary sex characteristics.
A chromosomal aberration in which certain chromosomes are present in extra copies or are deficient in number.
A flowering plant, which forms seeds inside a protective chamber called an ovary.
A negatively charged ion.
A plant that completes its entire life cycle in a single year or growing season.
Long, paired sensory appendages on the head of many arthropods.
Referring to the head end of a bilaterally symmetrical animal.
The terminal pollen sac of a stamen, inside which pollen grains with male gametes form in the flower of an angiosperm.
antheridium pl. antheridia
In plants, the male gametangium, a moist chamber in which gametes develop.
Natural water-soluble pigments of blue, purple or red which are dissolved in the cell-sap vacuole of plant cells.
A higher primate; includes monkeys, apes, and humans.
A chemical that kills bacteria or inhibits their growth.
An antigen-binding immunoglobulin, produced by B cells, that functions as the effector in an immune response.
A specialized base triplet on one end of a tRNA molecule that recognizes a particular complementary codon on an mRNA molecule.
antidiuretic hormone (ADH)
A hormone important in osmoregulation.
A foreign macromolecule that does not belong to the host organism and that elicits an immune response.
The major artery in blood-circulating systems; the aorta sends blood to the other body tissues.
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 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.
apomorphic character
A derived phenotypic character, or homology, that evolved after a branch diverged from a phylogenetic tree.
In plants, the nonliving continuum formed by the extracellular pathway provided by the continuous matrix of cell walls.
Programmed cell death brought about by signals that trigger the activation of a cascade of "suicide" proteins in the cells destined to die.
aposematic coloration
The bright coloration of animals with effective physical or chemical defenses that acts as a warning to predators.
A transport protein in the plasma membranes of a plant or animal cell that specifically facilitates the diffusion of water across the membrane (osmosis).
aqueous solution
A solution in which water is the solvent.
One of two prokaryotic domains, the other being the Bacteria.
archegonium pl. archegonia
In plants, the female gametangium, a moist chamber in which gametes develop.
The endoderm-lined cavity, formed during the gastrulation process, that develops into the digestive tract of an animal.
Primitive eukaryotic group that includes diplomonads, such as Giardia; some systematists assign kingdom status to archezoans.
A very small artery. See also artery.
A vessel that carries blood away from the heart to organs throughout the body.
A cardiovascular disease caused by the formation of hard plaques within the arteries.
artificial selection
The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits.
ascus pl. asci
A saclike spore capsule located at the tip of the ascocarp in dikaryotic hyphae; defining feature of the Ascomycota division of fungi.
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.
The energy-requiring process by which plant cells convert nitrate ions (NO3–) taken up by the roots of plants into ammonium ions (NH4+), which can then be used in the synthesis of amino acids and other nitrogenous compounds.
associative learning
The acquired ability to associate one stimulus with another; also called classical conditioning.
assortative mating
A type of nonrandom mating in which mating partners resemble each other in certain phenotypic characters.
asymmetric carbon
A carbon atom covalently bonded to four different atoms or groups of atoms.
atmospheric pressure
The weight of the Earth's atmosphere over a unit area of the Earth's surface.
The smallest unit of matter that retains the properties of an element.
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.
atomic weight
The total atomic mass, which is the mass in grams of one mole of the atom.
Abbreviation of adenosine triphosphate, the principal energy-carrying compound of the cell.
ATP synthase
A cluster of several membrane proteins found in the mitochondrial cristae (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.
atrioventricular node
A group of slow-conducting fibers in the atrium of the vertebrate heart that are stimulated by impulses originating in the sinoatrial node (the pacemaker) and that conduct impulses to the bundle of His, a group of fibers that stimulate contraction of the ventricles.
atrioventricular valve
A valve in the heart between each atrium and ventricle that prevents a backflow of blood when the ventricles contract.
atrium pl. atria
A chamber that receives blood returning to the vertebrate heart.
autogenesis model
According to this model, eukaryotic cells evolved by the specialization of internal membranes originally derived from prokaryotic plasma membranes.
autoimmune disease
An immunological disorder in which the immune system turns against itself.
autonomic nervous system
A subdivision of the motor nervous system of vertebrates that regulates the internal environment; consists of the sympathetic and parasympathetic divisions.
A type of polyploid species resulting from one species doubling its chromosome number to become tetraploid, which may self-fertilize or mate with other tetraploids.
A chromosome that is not directly involved in determining sex, as opposed to the sex chromosomes.
An organism that obtains organic food molecules without eating other organisms. Autotrophs use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.
A class of plant hormones, including indoleacetic acid (IAA), having a variety of effects, such as phototropic response through the stimulation of cell elongation, stimulation of secondary growth, and the development of leaf traces and fruit.
A nutritional mutant that is unable to synthesize and that cannot grow on media lacking certain essential molecules normally synthesized by wild-type strains of the same species.
The vertebrate class of birds, characterized by feathers and other flight adaptations.
axillary bud
An embryonic shoot present in the angle formed by a leaf and stem.
An imaginary line passing through a body or organ around which parts are symmetrically aligned.
A typically long extension, or process, from a neuron that carries nerve impulses away from the cell body toward target cells.
B cell
A type of lymphocyte that develops in the bone marrow and later produces antibodies, which mediate humoral immunity.
One of two prokaryotic domains, the other being the Archaea.
A virus that parasitizes a bacterial cell.
bacterium pl. bacteria
A prokaryotic microorganism in Domain Bacteria.
balanced polymorphism
A type of polymorphism in which the frequencies of the coexisting forms do not change noticeably over many generations.
All tissues external to the vascular cambium in a plant growing in thickness, consisting of phloem, phelloderm, cork cambium, and cork.
Barr body
A dense object lying along the inside of the nuclear envelope in female mammalian cells, representing an inactivated X chromosome.
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 minimal number of kilocalories a resting animal requires to fuel itself for a given time.
A substance that reduces the hydrogen ion concentration in a solution. See Alkaline.
basement membrane
The floor of an epithelial membrane on which the basal cells rest.
base-pair substitution
A point mutation; the replacement of one nucleotide and its partner from the complementary DNA strand by another pair of nucleotides.
base-pairing principle
In the formation of nucleic acids, the requirement that adenine must always pair with thymine (or uracil) and guanine with cytosine.
basidium pl. basidia
A reproductive appendage that produces sexual spores on the gills of mushrooms. The fungal division Basidiomycota is named for this structure.
Batesian mimicry
A type of mimicry in which a harmless species looks like a different species that is poisonous or otherwise harmful to predators.
All of the acts an organism performs, as in, for example, seeking a suitable habitat, obtaining food, avoiding predators, and seeking a mate and reproducing.
behavioral ecology
A heuristic approach based on the expectation that Darwinian fitness (reproductive success) is improved by optimal behavior.
benthic zone
The bottom surfaces of aquatic environments.
A plant that requires two years to complete its life cycle.
bilateral symmetry
Characterizing a body form with a central longitudinal plane that divides the body into two equal but opposite halves.
Members of the branch of eumetazoans possessing bilateral symmetry.
A yellow secretion of the vertebrate liver, temporarily stored in the gallbladder and composed of organic salts that emulsify fats in the small intestine.
binary fission
The type of cell division by which prokaryotes reproduce; each dividing daughter cell receives a copy of the single parental chromosome.
The two-part Latinized name of a species, consisting of genus and specific epithet.
biochemical pathway
An ordered series of chemical reactions in a living cell, in which each step is catalyzed by a specific enzyme; different biochemical pathways serve different functions in the life of the cell.
biodiversity hotspot
A relatively small area with an exceptional concentration of species.
The study of how organisms manage their energy resources.
biogeochemical cycles
The various nutrient circuits, which involve both biotic and abiotic components of ecosystems.
The study of the past and present distribution of species.
biological clock
Proposed internal factor(s) in organisms that governs functions that occur rhythmically in the absence of external stimuli.
biological magnification
A trophic process in which retained substances become more concentrated with each link in the food chain.
biological species
A population or group of populations whose members have the potential to interbreed.
The dry weight of organic matter comprising a group of organisms in a particular habitat.
One of the world's major communities, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment.
The entire portion of Earth that is inhabited by life; the sum of all the planet's communities and ecosystems.
Formation by living organisms of organic compounds from elements or simple compounds.
The industrial use of living organisms or their components to improve human health and food production.
Pertaining to the living organisms in the environment.
Walking upright on two feet.
(1) The broad, expanded part of a leaf. (2) The broad, expanded photosynthetic part of the thallus of a multicellular alga or a simple plant.
The fluid-filled cavity that forms in the center of the blastula embryo.
An embryonic stage in mammals; a hollow ball of cells produced one week after fertilization in humans.
Disklike area on the surface of a large, yolky egg that undergoes cleavage and gives rise to the embryo.
The opening of the archenteron in the gastrula that develops into the mouth in protostomes and the anus in deuterostomes.
The hollow ball of cells marking the end stage of cleavage during early embryonic development.
A type of connective tissue with a fluid matrix called plasma in which blood cells are suspended.
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.
blood pressure
The hydrostatic force that blood exerts against the wall of a vessel.
bond energy
The quantity of energy that must be absorbed to break a particular kind of chemical bond; equal to the quantity of energy the bond releases when it forms.
bond strength
The strength with which a chemical bond holds two atoms together; conventionally measured in terms of the amount of energy, in kilocalories per mole, required to break the bond.
book lungs
Organs of gas exchange in spiders, consisting of stacked plates contained in an internal chamber.
The study of plants.
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.
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.
A small portion of a gene or protein that appears in many genes or proteins that are related in structure; the box usually has some specific function, sometimes called a "motif", like binding DNA or interacting with specific proteins or other molecules.
The master control center in an animal; in vertebrates, the brain and spinal cord make up the central nervous system.
The hindbrain and midbrain of the vertebrate central nervous system. In humans, it forms a cap on the anterior end of the spinal cord, extending to about the middle of the brain.
The mosses, liverworts, and hornworts; a group of nonvascular plants that inhabit the land but lack many of the terrestrial adaptations of vascular plants.
bronchus pl. bronchi
One of a pair of respiratory tubes branching into either lung at the lower end of the trachea; it subdivides into progressively finer passageways, the bronchioles, culminating in the alveoli.
(1) In plants, an embryonic shoot, including rudimentary leaves, often protected by special bud scales. (2) In animals, an asexually produced outgrowth that develops into a new individual.
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.
A substance that consists of acid and base forms in solution and that minimizes changes in pH when extraneous acids or bases are added to the solution.
A modified bud with thickened leaves adapted for underground food storage.
bulbourethral gland
One of a pair of glands near the base of the penis in the human male that secrete fluid that lubricates and neutralizes acids in the urethra during sexual arousal.
bulk flow
The movement of water due to a difference in pressure between two locations.
bundle of His
In the vertebrate heart, a group of muscle fibers that carry impulses from the atrioventricular node to the walls of the ventricles; the only electrical bridge between the atria and the ventricles.
C3 pathway
See Calvin cycle.
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 pathway
The set of reactions by which some plants initially fix carbon in the four-carbon compound oxaloacetic acid; the carbon dioxide is later released in the interior of the leaf and enters the Calvin cycle.
C4 plant
A plant that prefaces the Calvin cycle with reactions that incorporate CO2 into four-carbon compounds, the end-product of which supplies CO2 for the Calvin cycle.
A mammalian thyroid hormone that lowers blood calcium levels.
In plants, undifferentiated tissue; a term used in tissue culture, grafting, and wound healing.
An intracellular protein to which calcium binds in its function as a second messenger in hormone action.
calorie (cal)
The amount of heat energy required to raise the temperature of 1 g of water 1°C; the amount of heat energy that 1 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.
Collectively, the sepals of a flower.
CAM photosynthesis
See Crassulacean acid metabolism.
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 geological history; recorded in the fossil record about 545 to 525 million years ago.
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 action
The movement of water or any liquid along a surface; results from the combined effect of cohesion and adhesion.
The protein shell that encloses the viral genome; rod-shaped, polyhedral, or more completely shaped.
(1) A slimy layer around the cells of certain bacteria. (2) The sporangium of a bryophyte.
A sugar (monosaccharide) or one of its dimers (disaccharides) or polymers (polysaccharides).
carbon cycle
Worldwide circulation and reutilization of carbon atoms, chiefly due to metabolic processes of living organisms. Inorganic carbon, in the form of carbon dioxide, is incorporated into organic compounds by photosynthetic organisms; when the organic compounds are broken down in respiration, carbon dioxide is released. Large quantities of carbon are "stored" in the seas and the atmosphere, as well as in fossil fuel deposits.
carbon fixation
The incorporation of carbon from CO2 into an organic compound by an autotrophic organism (a plant, another photosynthetic organism, or a chemoautotrophic bacterium).
carbonyl group
A functional group present in aldehydes and ketones, consisting of a carbon atom double-bonded to an oxygen atom.
carboxyl group
A functional group present in organic acids, consisting of a single carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl group.
A chemical agent that causes cancer.
cardiac muscle
A type of muscle that forms the contractile wall of the heart; its cells are joined by intercalated discs that relay each heartbeat.
cardiac output
The volume of blood pumped per minute by the left ventricle of the heart.
cardiovascular system
A closed circulatory system with a heart and branching network of arteries, capilleries, and veins.
An animal, such as a shark, hawk, or spider, that eats other animals.
Accessory pigments, yellow and orange, in the chloroplasts of plants; by absorbing wavelengths of light that chlorophyll cannot, they broaden the spectrum of colors that can drive photosynthesis.
The female reproductive organ of a flower, consisting of the stigma, style, and ovary.
carrying capacity
The maximum population size that can be supported by the available resources, symbolized as K.
A type of flexible connective tissue with an abundance of collagenous fibers embedded in chondrin.
Casparian strip
A water-impermeable ring of wax around endodermal cells in 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 into simpler compounds.
Within a cell or organism, the sum of all chemical reactions in which large molecules are broken down into smaller parts.
catabolite activator protein (CAP)
In E. coli, a helper protein that stimulates gene expression by binding within the promoter region of an operon and enhancing the promoter's ability to associate with RNA polymerase.
A substance that lowers the activation energy of a chemical reaction by forming a temporary association with the reacting molecules; as a result, the rate of the reaction is accelerated. Enzymes are catalysts.
In a hierarchical classification system, the level at which a particular group is ranked.
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.
A basic unit of living matter separated from its environment by a plasma membrane; the fundamental structural unit of life.
cell center
A region in the cytoplasm near the nucleus from which microtubules originate and radiate.
cell cycle
An ordered sequence of events in the life of a dividing eukaryotic cell, composed of the M, G1, S, and G2 phases.
cell-cycle control system
A cyclically operating set of proteins that triggers and coordinates events in the eukaryotic cell cycle.
cell fractionation
The disruption of a cell and separation of its organelles by centrifugation.
cell-mediated immunity
The type of immunity that functions in defense against fungi, protists, bacteria, and viruses inside host cells and against tissue transplants, with highly specialized cells that circulate in the blood and lymphoid tissue.
cell membrane
The outer membrane of the cell; the plasma membrane.
cell plate
A double membrane across the midline of a dividing plant cell, between which the new cell wall forms during cytokinesis.
cell theory
All living things are composed of cells; cells arise only from other cells. No exception has been found to these two principles since they were first proposed well over a century ago.
cell wall
A protective layer external to the plasma membrane in plant cells, bacteria, fungi, and some protists. In the case of 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.
cellular 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.
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.
A structural polysaccharide of cell walls, consisting of glucose monomers joined by (1-4) glycosidic linkages.
Celsius scale
A temperature scale (°C) equal to 5/9 (°F – 32) that measures the freezing point of water at 0°C and the boiling point of water at 100°C.
central nervous system (CNS)
In vertebrate animals, the brain and spinal cord.
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, which are involved in cell division.
The centralized region joining two sister chromatids.
Material present in the cytoplasm of all eukaryotic cells and important during cell division; also called microtubule-organizing center.
A chordate without a backbone, represented by lancelets, tiny marine animals.
Part of the vertebrate hindbrain (rhombencephalon) 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.
The dorsal portion, composed of right and left hemispheres, of the vertebrate forebrain; the integrating center for memory, learning, emotions, and other highly complex functions of the central nervous system.
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.
character displacement
A phenomenon in which species that live together in the same environment tend to diverge in those characteristics that overlap; exemplified by Darwin's finches.
chemical bond
An attraction between two atoms resulting from a sharing of outer-shell elctrons or the presence of opposite charges on the atoms; the bonded atoms gain complete outer electron shells.
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.
The production of ATP using the energy of hydrogen-ion gradients across membranes to phosphorylate ADP; powers most ATP synthesis in cells.
chemiosmotic coupling
The mechanism by which ADP is phosphorylated to ATP in mitochondria and chloroplasts. The energy released as electrons pass down an electron transport chain is used to establish a proton gradient across an inner membrane of the organelle; when protons subsequently flow down this electrochemical gradient, the potential energy released is captured in the terminal phosphate bonds of ATP.
An organism that needs only carbon dioxide as a carbon source but that obtains energy by oxidizing inorganic substances.
An organism that must consume organic molecules for both energy and carbon.
A receptor that transmits information about the total solute concentration in a solution or about individual kinds of molecules.
Applied to autotrophic bacteria that use the energy released by specific inorganic reactions to power their life processes, including the synthesis of organic molecules.
chiasma pl. chiasmata
The X-shaped, microscopically visible region representing homologous chromatids that have exchanged genetic material through crossing over during meiosis.
A structural polysaccharide of an amino sugar found in many fungi and in the exoskeletons of all arthropods.
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.
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.
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.
The vertebrate class of cartilaginous fishes, represented by sharks and their relatives.
A protein-carbohydrate complex secreted by chondrocytes; chondrin and collagen fibers form cartilage.
A member of a diverse phylum of animals that possess a notochord; a dorsal, hollow nerve cord; pharyngeal gill slits; and a postanal tail as embryos.
The outermost of four extraembryonic membranes; contributes to the formation of the mammalian placenta.
chorionic villus sampling (CVS)
A technique for diagnosing genetic and congenital defects while the fetus is in the uterus. A small sample of the fetal portion of the placenta is removed and analyzed.
Either of the two strands of a replicated chromosome, which are joined at the centromere.
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.
In some classification systems, a kingdom consisting of brown algae, golden algae, and diatoms.
A threadlike, gene-carrying structure found in the nucleus. Each chromosome consists of one very long DNA molecule and associated proteins. See chromatin.
chromosome map
A diagram of the linear order of the genes on a chromosome.
Fungus with flagellated stage; possible evolutionary link between fungi and protists.
cilium pl. cilia
A short cellular appendage specialized for locomotion, formed from a core of nine outer doublet microtubules and two inner single microtubules ensheathed in an extension of plasma membrane.
circadian rhythms
A physiological cycle of about 24 hours, present in all eukaryotic organisms, that persists even in the absence of external cues.
A taxonomic approach that classifies organisms according to the order in time at which branches arise along a phylogenetic tree, without considering the degree of morphological divergence.
A pattern of evolutionary change that produces biological diversity by budding one or more new species from a parent species that continues to exist; also called branching evolution.
A dichotomous phylogenetic tree that branches repeatedly, suggesting a classification of organisms based on the time sequence in which evolutionary branches arise.
A taxonomic grouping of related, similar orders; category above order and below phylum.
classical conditioning
A type of associative learning; the association of a normally irrelevant stimulus with a fixed behavioral response.
The process of cytokinesis in animal cells, characterized by pinching of the plasma membrane; specifically, 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.
Variation in features of individuals in a population that parallels a gradient in the environment.
A common opening for the digestive, urinary, and reproductive tracts in all vertebrates except most mammals.
clonal selection
The mechanism that determines specificity and accounts for antigen memory in the immune system; occurs because an antigen introduced into the body selectively activates only a tiny fraction of inactive lymphocytes, which proliferate to form a clone of effector cells specific for the stimulating antigen.
(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. Also, see gene cloning.
cloning vector
An agent used to transfer DNA in genetic engineering, such as a plasmid that moves recombinant DNA from a test tube back into a cell, or a virus that transfers recombinant DNA by infection.
closed circulatory system
A type of internal transport in which blood is confined to vessels.
A stinging cell containing a nematocyst; characteristic of cnidarians.
The complex, coiled organ of hearing that contains the organ of Corti.
A phenotypic situation in which both alleles are expressed in the heterozygote.
A three-nucleotide sequence of DNA or mRNA that specifies a particular amino acid or termination signal; the basic unit of the genetic code.
A body cavity completely lined with mesoderm.
An animal whose body cavity is completely lined by mesoderm, the layers of which connect dorsally and ventrally to form mesenteries.
Referring to a multinucleated condition resulting from the repeated division of nuclei without cytoplasmic division.
An organic molecule serving as a cofactor. Most vitamins function as coenzymes in important metabolic reactions.
The mutual influence on the evolution of two different species interacting with each other and reciprocally influencing each other's adaptations.
Any nonprotein molecule or ion that is required for the proper functioning of an enzyme. Cofactors can be permanently bound to the active site or may bind loosely with the substrate during catalysis.
The binding together of like molecules, often by hydrogen bonds.
cohesion species concept
The idea that specific evolutionary adaptations and discrete complexes of genes define species.
cohesion-tension theory
A theory accounting for the upward movement of water in plants. According to this theory, transpiration of a water molecule results in a negative (below 1 atmosphere) pressure in the leaf cells, inducing the entrance from the vascular tissue of another water molecule, which, because of the cohesive property of water, pulls with it a chain of water molecules extending up from the cells of the root tip.
cold aclimation response
The process by which plants increase their tolerance to freezing by exposure to low, nonfreezing temperatures.
The sheath enclosing the apical meristem and leaf primordia of a germinating monocot.
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.
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.
A group of organisms of the same species living together in close association.
A symbiotic relationship in which the symbiont benefits but the host is neither helped nor harmed. See Symbiosis.
All the organisms that inhabit a particular area; an assemblage of populations of different species living close enough together for potential interaction.
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.
Interaction between members of the same population or of two or more populations using the same resource, often present in limited supply.
competitive exclusion principle
The concept that when the populations of two 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 fixation
An immune response in which antigen-antibody complexes activate complement proteins.
complement system
A group of at least 20 blood proteins that cooperate with other defense mechanisms; may amplify the inflammatory response, enhance phagocytosis, or directly lyse pathogens; activated by the onset of the immune response or by surface antigens on microorganisms or other foreign cells.
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 alimentary canal. An incomplete digestive tract has only one opening.
complete flower
A flower that has sepals, petals, stamens, and carpels.
A chemical combination, in a fixed ratio, of two or more elements.
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
A regular increase of decrease in the intensity or density of a chemical substance. Cells often maintain concentration gradients of H+ 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.
See Dehydration synthesis.
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 dehydration reaction.
cone cell
(1) In plants, the reproductive structure of a conifer. (2) In vertebrates, a type of photoreceptor cell in the retina, concerned with the perception of color and with the most acute discrimination of detail.
conidium pl. conidia
A naked, asexual spore produced at the ends of hyphae in ascomycetes.
A gymnosperm whose reproductive structure is the cone. Conifers include pines, firs, redwoods, and other large trees.
In bacteria, the transfer of DNA between two cells that are temporarily joined.
connective tissues
Animal tissue that functions mainly to bind and support other tissues, having a sparse population of cells scattered through an extracellular matrix.
conservation biology
A goal-oriented science that seeks to counter the biodiversity crisis, the current rapid decrease in Earth's variety of life.
consumer, in ecological systems
A heterotroph that derives its energy from living or freshly killed organisms or parts thereof. Primary consumers are herbivores; higher-level consumers are carnivores.
continental drift
The gradual movement of the Earth's continents that has occurred over hundreds of millions of years.
continuous variation
A gradation of small differences in a particular trait, such as height, within a population; occurs in traits that are controlled by a number of genes.
The mass movement of warmed air or liquid to or from the surface of a body or object.
convergent evolution
The independent development of similarity between species as a result of their having similar ecological roles and selection pressures.
An interaction of the constituent subunits of a protein causing a conformational change in one subunit to be transmitted to all the others.
A secondary tissue that is a major constituent of bark in woody and some herbaceous plants; made up of flattened cells, dead at maturity; restricts gas and water exchange and protects the vascular tissues from injury.
cork cambium
A cylinder of meristematic tissue in plants that produces cork cells to replace the epidermis during secondary growth.
Petals, collectively; usually the conspicuously colored flower parts.
corpus callosum
In the vertebrate brain, a tightly packed mass of myelinated nerve fibers connecting the two cerebral hemispheres.
corpus lutuem
A secreting tissue in the ovary that forms from the collapsed follicle after ovulation and produces progesterone.
(1) The outer, as opposed to the inner, part of an organ, as in the adrenal gland. (2) In a stem or root, the primary tissue bounded externally by the epidermis and internally by the central cylinder of vascular tissue.
A steroid hormone, produced by the adrenal cortex, that promotes the formation of glucose from protein and fat; also suppresses the inflammatory and immune responses.
The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.
The one (monocot) or two (dicot) seed leaves of an angiosperm embryo.
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.
coupled reactions
In cells, the linking of endergonic (energy-requiring) reactions to exergonic (energy-releasing) reactions that provide enough energy to drive the endergonic reactions forward.
covalent bond
A chemical bond formed as a result of the sharing of one or more pairs of electrons.
Crassulacean acid metabolism
A process by which some species of plants in hot, dry climates take in carbon dioxide during the night, fixing it in organic acids; the carbon dioxide is released during the day and used immediately in the Calvin cycle.
crista pl. cristae
An infolding of the inner membrane of a mitochondrion that houses the electron transport chain and the enzyme catalyzing the synthesis of ATP.
Fusion of gametes formed by different individuals; as opposed to self-fertilization.
crossing over
The reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I.
cryptic coloration
A type of camouflage that makes potential prey difficult to spot against its background.
(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.
Photosynthetic, oxygen-producing bacteria (formerly know as blue-green algae).
cyclic AMP
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 produces ATP but not NADPH or oxygen.
A regulatory protein whose concentration fluctuates cyclically.
cyclin-dependent kinase (Cdk)
A protein kinase that is active only when attached to a particular cyclin.
An iron-containing protein, a component of electron transport chains in mitochondria and chloroplasts.
In the vertebrate immune system, protein factors secreted by macrophages and helper T cells as regulators of neighboring cells.
The division of the cytoplasm to form two separate daughter cells immediately after mitosis.
A class of related plant hormones that retard aging and act in concert with auxins to stimulate cell division, influence the pathway of differentiation, and control apical dominance.
The entire contents of the cell, exclusive of the nucleus, and bounded by the plasma membrane.
cytoplasmic determinants
In animal development, substances deposited by the mother in the eggs she produces that regulate the expression of genes affecting the early development of the embryo.
cytoplasmic streaming
A circular flow of cytoplasm, involving myosin and actin filaments, that speeds the distribution of materials within cells.
A network of microtubules, microfilaments, and intermediate filaments that branch throughout the cytoplasm and serve a variety of mechanical and transport functions.
The semifluid portion of the cytoplasm.
cytotoxic T cell (TC)
A type of lymphocyte that kills infected cells and cancer cells.
The atomic mass unit; a measure of mass for atoms and subatomic particles.
Darwinian fitness
A measure of the relative contribution of an individual to the gene pool of the next generation.
daughter cell
A cell that is the offspring of a cell that has undergone mitosis or meiosis. The term "daughter" does not indicate the sex of the cell.
day-neutral plant
A plant whose flowering is not affected by photoperiod.
Refers to plants that shed their leaves at a certain season.
Saprotrophic fungi and bacteria 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.
dehydration reaction
A chemical reaction in which two molecules covalently bond to one another with the removal of a water molecule.
(1) A deficiency in a chromosome resulting from the loss of a fragment through breakage. (2) A mutational loss of a nucleotide from a gene.
The study of statistics relating to births and deaths in populations.
For proteins, a process in which a protein unravels and loses its native conformation, thereby becoming biologically inactive. For DNA, the separation of the two strands of the double helix. Denaturation occurs under extreme conditions of pH, salt concentration, and temperature.
One of usually numerous, short, highly branched processes of a neuron that conveys nerve impulses toward the cell body.
The process by which certain bacteria living in poorly aerated soils break down nitrates, using the oxygen for their own respiration and releasing nitrogen back into the atmosphere.
The number of individuals per unit area or volume.
density-dependent factor
Any factor influencing population regulation that has a greater impact as population density increases.
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 factors
Any factor influencing population regulation that acts to reduce population by the same percentage, regardless of size.
deoxyribonucleic acid (DNA)
A double-stranded, helical nucleic acid molecule capable of replicating and determining the inherited structure of a cell's proteins.
The sugar component of DNA, having one less hydroxyl group than ribose, the sugar component of RNA.
dependent variable
In an experiment, the dependent variable is the factor that responds when another factor is manipulated.
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.
A heterotroph, such as an earthworm, that eats its way through detritus, salvaging bits and pieces of decaying organic matter.
dermal tissue system
The protective covering of plants; generally a single layer of tightly packed epidermal cells covering young plant organs formed by primary growth.
The inner layer of the skin, beneath the epidermis.
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 animals, in which the organism stops growing after it reaches a certain size.
The progressive restriction of developmental potential, causing the possible fate of each cell to become more limited as the embryo develops.
Dead organic matter.
Organisms that live on dead and discarded organic matter; include large scavengers, smaller animals such as earthworms and some insects, as well as decomposers (fungi and bacteria).
One of two distinct evolutionary lines of coelomates, consisting of the echinoderms and chordates and characterized by radial, indeterminate cleavage, enterocoelous formation of the coelom, and development of the anus from the blastopore.
The progressive production of the phenotypic characteristics of a multicellular organism, beginning with the fertilization of an egg.
A sheet of muscle that forms the bottom wall of the thoracic cavity in mammals; active in ventilating the lungs.
The stage of the heart cycle in which the heart muscle is relaxed, allowing the chambers to fill with blood.
diastolic pressure
The pressure in an artery during the ventricular relaxation phase of the heart cycle.
A subdivision of flowering plants whose members possess two embryonic seed leaves, or cotyledons.
A member of the class of flowering plants having two seed leaves, or cotyledons, among other distinguishing features; often abbreviated as dicot.
See cellular differentiation.
The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area.
The process of breaking down food into molecules small enough for the body to absorb.
A hybrid individual that is heterozygous for two genes or two characters.
dihybrid cross
A breeding experiment in which parental varieties differing in two traits are mated.
A mycelium of certain septate fungi that possesses two separate haploid nuclei per cell.
Referring to a plant species that has staminate and carpellate flowers on separate plants.
Displaying two separate growth forms.
diploid cell
A cell containing two sets of chromosomes (2n), one set inherited from each parent.
directed molecular evolution
A laboratory version of evolution at the molecular level that can produce "designer molecules." A large starting population of molecules (typically nucleic acids) that varies randomly in base sequence and shape is subjected to replication with variation, followed by selection. After several cycles of replication and selection, the population of molecules will evolve toward one containing a high proportion of molecules well adapted to the selection criterion applied.
directional selection
Natural selection that favors individuals on one end of the phenotypic range.
A double sugar, consisting of two monosaccharides joined by dehydration synthesis.
Applied to organisms that are active during the daylight hours.
The distribution of individuals within geographical population boundaries.
diversifying selection
Natural selection that favors extreme over intermediate phenotypes.
A taxonomic grouping of related, similar classes; a high- level category below kingdom and above class. Division is generally used in the classification of prokaryotes, algae, fungi, and plants, whereas an equivalent category, phylum, is used in the classification of protozoa and animals.
Abbreviation of deoxyribonucleic acid.
DNA ligase
width="400" valign="TOP"> 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 methylation
The addition of methyl groups (–CH3) to bases of DNA after DNA synthesis; may serve as a long-term control of gene expression.
DNA polymerase
An enzyme that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain.
DNA probe
A chemically synthesized, radioactively labeled segment of nucleic acid used to find a gene of interest by hydrogen-bonding to a complementary sequence.
A taxonomic category above the kingdom level; the three domains are Archaea, Bacteria, and Eukarya.
dominance hierarchy
A linear "pecking order" of animals, where position dictates characteristic social behaviors.
dominant allele
In a heterozygote, the allele that is fully expressed in the phenotype.
A period during which growth ceases and metabolic activity is greatly reduced; dormancy is broken when certain requirements, for example, of temperature, moisture, or day length, are met.
Pertaining to or situated near the back; opposite of ventral.
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 resulting from having an extra chromosome 21, characterized by mental retardation and heart and respiratory defects.
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.
An aberration in chromosome structure resulting from an error in meiosis or mutagens; duplication of a portion of a chromosome resulting from fusion with a fragment from a homologous chromosome.
A large contractile protein forming the sidearms of microtubule doublets in cilia and flagella.
A steroid hormone that triggers molting in arthropods.
ecological efficiency
The ratio of net productivity at one trophic level to net productivity at the next lower level.
ecological niche
The sum total of an organism's utilization of the biotic and abiotic resources of its environment.
ecological pyramid
A graphic representation of the quantitative relationships of numbers of organisms, biomass, or energy flow between the trophic levels of an ecosystem. Because large amounts of energy and biomass are dissipated at every trophic level, these diagrams nearly always take the form of pyramids.
ecological species concept
The idea that ecological roles (niches) define species.
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.
The study of how organisms interact with their environments.
A level of ecological study that includes all the organisms in a given area as well as the abiotic factors with which they interact; a community and its physical environment.
A locally adapted variant of a species, differing genetically from other ecotypes of the same species.
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.
An animal such as a reptile, fish, or amphibian, that must use environmental energy and behavioral adaptations to regulate its body temperature.
effector cell
Carrying away from a center, applied to nerves and blood vessels.
A female gamete, which usually contains abundant cytoplasm and yolk; nonmotile and often larger than a male gamete.
ejaculatory duct
In the male, a duct from each testis that join to form the urethra.
electric potential
The difference in the amount of electric charge between a region of positive charge and a region of negative charge. The establishment of electric potentials across the plasma membrane and across organelle membranes makes possible a number of phenomena, including the chemiosmotic synthesis of ATP, the conduction of nerve impulses, and muscle contraction.
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 generating voltage across the membrane.
electromagnetic spectrum
The entire spectrum of radiation; ranges in wavelength from less than a nanometer to more than a kilometer.
A particle with a single negative charge; one or more electrons orbit the nucleus of the atom.
electron acceptor
Substance that accepts or receives electrons in an oxidation-reduction reaction, becoming reduced in the process.
electron carrier
A molecule that conveys electrons; one of several membrane proteins in electron transport chains in cells. Electron carriers shuttle electrons during the redox reactions that release energy used to make ATP.
electron donor
Substance that donates or gives up electrons in an oxidation-reduction reaction, becoming oxidized in the process.
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 at which an electron orbits 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.
The tendency for an atom to pull electrons toward itself.
Any substance that cannot be broken down to any other substance.
A developing stage of multicellular organisms; in humans, the stage in the development of offspring from the first division of the zygote until body structures begin to appear; about the ninth week of gestation. See Fetus.
embryo sac
The female gametophyte of angiosperms, formed from the growth and division of the megaspore into a multicellular structure with eight haploid nuclei.
One of a pair of molecules that are mirror-image isomers 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.
An organism found only in one particular location.
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 bloodstream.
endocrine system
The internal system of chemical communication involving 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.
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.
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.
The innermost layer of the cortex in plant roots; a cylinder one cell thick that forms the boundary between the cortex and the stele.
Arising from internal structures or functional causes.
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.
The inner lining of the uterus, which is richly supplied with blood vessels.
endoplasmic reticulum
An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions.
A hormone produced in the brain and anterior pituitary that inhibits pain perception.
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.
A nutrient-rich tissue formed by the union of a sperm cell with two polar nuclei during double fertilization, which provides nourishment to the developing embryo in angiosperm seeds.
A thick-coated, resistant cell produced within a bacterial cell exposed to harsh conditions.
endosymbiotic theory
A hypothesis about the origin of the eukaryotic cell, maintaining that the forerunners of eukaryotic cells were symbiotic associations of prokaryotic cells living inside larger prokaryotes.
The innermost, simple squamous layer of cells lining the blood vessels; the only constituent structure of capillaries.
An animal that uses metabolic energy to maintain a constant body temperature, such as a bird or mammal. See also Homeotherm.
A component of the outer membranes of certain gram-negative bacteria responsible for generalized symptoms of fever and ache.
The capacity to do work by moving matter against an opposing force.
energy of activation (EA)
The amount of energy that reactants must absorb before a chemical reaction will start.
A DNA sequence that recognizes certain transcription factors that can stimulate transcription of nearby genes.
A quantitative measure of disorder or randomness, symbolized by S.
environmental grain
An ecological term for the effect of spatial variation, or patchiness, relative to the size and behavior of an organism.
A class of proteins serving as catalysts, chemical agents that change the rate of a reaction without being consumed by the reaction.
(1) The dermal tissue system in plants. (2) The outer covering of animals.
The progressive development of form in an embryo.
A long coiled tube into which sperm pass from the testis and are stored until mature and ejaculated.
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.
A hormone produced as a response to stress; also called adrenaline. See Adrenaline.
A plant that nourishes itself but grows on the surface of another plant for support, usually on the branches or trunks of tropical trees.
A plasmid capable of integrating into the bacterial chromosome.
A phenomenon in which one gene alters the expression of another gene that is independently inherited.
epithelial tissue
Sheets of tightly packed cells that line organs and body cavities.
A localized region on the surface of an antigen that is chemically recognized by antibodies; also called antigenic determinant.
The state of a system in which no further net change is occurring; result of counterbalancing forward and backward processes.
equilibrium species
Species characterized by low reproduction rates, long development times, large body size, and long adult life with repeated reproductions.
A red blood cell; contains hemoglobin, which functions in transporting oxygen in the circulatory system.
A channel that conducts food, by peristalsis, from the pharynx to the stomach.
essential amino acids
The amino acids that an animal cannot synthesize itself and must obtain from food. Eight amino acids are essential in the human adult.
A physiological state characterized by slow metabolism and inactivity, which permits survival during long periods of elevated temperature and diminished water supplies.
The primary female steroid sex hormones, which are produced in the ovary by the developing follicle during the first half of the cycle and in smaller quantities by the corpus luteum during the second half. Estrogens stimulate 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 midcycle at estrus.
The comparative study of patterns of animal behavior, with emphasis on their adaptive significance and evolutionary origin.
The only gaseous plant hormone, responsible for fruit ripening, growth inhibition, leaf abscission, and aging.
In plants, a condition characterized by stem elongation, poor leaf development, and lack of chlorophyll; occurs in plants growing in the dark or with greatly reduced light.
The more open, unraveled form of eukaryotic chromatin, which is available for transcription.
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.
An organism whose cells contain membraine-bound organelles and whose DNA is enclosed in a cell nucleus and is associated with proteins.
Members of the subkingdom that includes all animals except sponges.
Applied to animal societies, such as those of certain insects, in which sterile individuals work on behalf of reproductive individuals.
eutherian mammals
Placental mammals; those whose young complete their embryonic development within the uterus, joined to the mother by the placenta.
eutrophic lake
A highly productive lake, having a high rate of biological productivity supported by a high rate of nutrient cycling.
A process in which an aquatic environment accumulates high nutrient levels due to factors such as industrial or urban pollution or run-off of fertilizers from nearby agricultural lands. The nutrients lead to dense blooms of algae and aquatic plants that cloud lake water, deplete specific minerals and dissolved gases, and can cause natural plant and animal populations to decline.
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.
All the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today.
evolutionary species concept
The idea that evolutionary lineages and ecological roles can form the basis of species identification.
A structure that evolves and functions in one environmental context but that can perform additional functions when placed in some new environment.
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.
The disposal of nitrogen-containing waste products of metabolism.
excretory system
The organ system that disposes of nitrogen-containing metabolic wastes.
exergonic reaction
A spontaneous chemical reaction in which there is a net release of free energy.
exocrine glands
Glands, such as sweat glands and digestive glands, that secrete their products into ducts that empty onto surfaces, such as the skin, or into cavities, such as the interior of the stomach.
The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.
The coding region of a eukaryotic gene that is expressed. Exons are separated from each other by introns.
A hard encasement on the surface of an animal, such as the shells of mollusks or the cuticles of arthropods, that provides protection and points of attachment for muscles.
A toxic protein secreted by a bacterial cell that produces specific symptoms even in the absence of the bacterium.
exponential growth
In populations, the increasingly accelerated rate of growth due to the increasing number of individuals being added to the reproductive base. Exponential growth is very seldom approached or sustained in natural populations.
expression vector
A vector that allows a DNA sequence cloned into it to be transcribed when the vector is introduced into a cell.
In genetics, the degree to which a particular genotype is expressed in the phenotype of individuals with that genotype.
No longer existing.
extracellular matrix (ECM)
The substance in which animal tissue cells are embedded; consists of protein and polysaccharides.
extraembryonic membranes
Four membranes (yolk sac, amnion, chorion, allantois) that support the developing embryo in reptiles, birds, and mammals.
F1 (first filial generation)
The first filial or hybrid offspring in a genetic cross-fertilization.
F2 (second filial generation)
Offspring resulting from interbreeding of the hybrid F1 generation.
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. May exist as a plasmid or integrated into the bacterial chromosome.
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.
Abbreviation of flavin adenine dinucleotide, a coenzyme that functions as an electron acceptor in the Krebs cycle.
Fallopian tube
See Oviduct.
A taxonomic grouping of related, similar genera; the category below order and above genus.
fat (triacylglycerol)
A biological compound consisting of three fatty acids linked to one glycerol molecule.
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.
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.
feedback systems
Control mechanisms whereby an increase or decrease in the level of a particular factor inhibits or stimulates the production, utilization, or release of that factor; important in the regulation of enzyme and hormone levels, ion concentrations, temperature, and many other factors.
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.
The union of haploid gametes to produce a diploid zygote.
An unborn or unhatched vertebrate that has passed through the earliest developmental stages; a developing human from about the second month of gestation until birth.
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.
Any minute, threadlike structure within a cell.
The activated form of the blood-clotting protein fibrinogen, which aggregates into threads that form the fabric of the clot.
A type of cell in loose connective tissue that secretes the protein ingredients of the extracellular fibers.
fibrous protein
Insoluble structural protein in which the polypeptide chain is coiled along one dimension. Fibrous proteins constitute the main structural elements of many animal tissues.
(1) A chain of cells. (2) In flowers, the stalk of a stamen.
Fluid extracted by the excretory system from the blood or body cavity. The excretory system produces urine from the filtrate after extracting valuable solutes from it and concentrating it.
The first stage of kidney function; blood plasma is forced, under pressure, out of the glomerular capillaries into Bowman's capsule, through which it enters the renal tubule.
first law of thermodynamics
The principle of conservation of energy. Energy can be transferred and transformed, but it cannot be created or destroyed.
The genetic contribution of an individual to succeeding generations relative to the contributions of other individuals in the population.
fixed action pattern
A highly stereotypical behavior that is innate and must be carried to completion once initiated.
Limp; walled cells are flaccid in isotonic surroundings, where there is no tendency for water to enter.
flagellum pl. flagella
A long cellular appendage specialized for locomotion, formed from a core of nine outer doublet microtubules and two inner single microtubules, ensheathed in an extension of plasma membrane.
The reproductive structure of angiosperms; a complete flower includes sepals, petals, stamens (male structures), and carpels (female structures).
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 individually inserted protein molecules drifting laterally in a fluid bilayer of phospholipids.
A microscopic structure in the ovary that contains the developing ovum and secretes estrogens.
follicle-stimulating hormone (FSH)
A protein hormone secreted by the anterior pituitary that stimulates the production of eggs by the ovaries and sperm by the testes.
food chain
The pathway along which food is transferred from trophic level to trophic level, beginning with producers.
food web
The elaborate, interconnected feeding relationships in an ecosystem.
The remains of an organism, or direct evidence of its presence (such as tracks). May be an unaltered hard part (tooth or bone), a mold in a rock, petrification (wood or bone), unaltered or partially altered soft parts (a frozen mammoth).
founder effect
A cause of genetic drift attributable to colonization by a limited number of individuals from a parent population.
A small area in the center of the retina in which cones are concentrated; the area of sharpest vision.
fragile X syndrome
A hereditary mental disorder, partially explained by genomic imprinting and the addition of nucleotides to a triplet repeat near the end of an X chromosome.
frameshift mutation
A mutation occurring when the number of nucleotides inserted or deleted is not a multiple of 3, thus resulting in improper grouping into codons.
free energy
A quantity of energy that interrelates entropy (S) and the system's total energy (H); symbolized by G. The change in free energy of a system is calculated by the equation G = ΔH – T ΔS, where T is absolute temperature.
free energy of activation
The initial investment of energy necessary to start a chemical reaction; 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.
A mature ovary of a flower that protects dormant seeds and aids in their dispersal.
Characteristic role or action of a structure or process in the normal metabolism or behavior of an organism.
functional group
A specific configuration of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical reactions.
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.
G-protein linked receptor
A signal receptor protein in the plasma membrane that responds to the binding of a signal molecule by activating a G protein.
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.
gametangium pl. gametangia
The reproductive organ of bryophytes, consisting of the male antheridium and female archegonium; a multichambered jacket of sterile cells in which gametes are formed.
A haploid egg or sperm cell; gametes unite during sexual reproduction to produce a diploid zygote.
The multicellular haploid form in organisms undergoing alternation of generations, which mitotically produces haploid gametes that unite and grow into the sporophyte generation.
ganglion pl. ganglia
A cluster (functional group) of nerve cell bodies in a centralized nervous system.
gap junction
A type of intercellular junction in animal cells that allows the passage of material or current between cells.
gap phases
In the cell cycle, the phases that precede (G1) and follow (G2) the synthesis (S) phase in which DNA is replicated; in the G1 phase, the cell doubles in size, and its enzymes, ribosomes, and other cytoplasmic molecules and structures increase in number; in the G2 phase, the replicated chromosomes begin to condense and the structures required for mitosis or meiosis are assembled.
Pertaining to the stomach.
A digestive hormone, secreted by the stomach, that stimulates the secretion of gastric juice.
gastrovascular cavity
The central digestive compartment, usually with a single opening that functions as both mouth and anus.
The two-layered, cup-shaped embryonic stage.
The formation of a gastrula from a blastula.
gated ion channel
A specific ion channel that opens and closes to allow the cell to alter its membrane potential.
Gause's principle
See Competitive exclusion principle.
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.
A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).
gene amplification
The selective synthesis of DNA, which results in multiple copies of a single gene, thereby enhancing expression.
gene cloning
The production of multiple copies of a gene.
gene flow
The loss or gain of alleles from a population due to the emigration or immigration of fertile individuals, or the transfer of gametes, between populations.
gene pool
The total aggregate of genes in a population at any one time.
genetic code
The system of nucleotide triplets in DNA and RNA that carries genetic information; referred to as a code because it determines the amino acid sequence in the enzymes and other protein molecules synthesized by the organism.
genetic drift
Changes in the gene pool of a small population due to chance.
genetic isolation
The absence of genetic exchange between populations or species as a result of geographic separation or of premating or postmating mechanisms (behavioral, anatomical, or physiological) that prevent reproduction.
genetic map
An ordered list of genetic loci (genes or other genetic markers) along a chromosome.
genetic recombination
The general term for the production of offspring that combine traits of the two parents.
The complete complement of an organism's genes; an organism's genetic material.
genomic imprinting
The parental effect on gene expression. Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
genomic library
A set of thousands of DNA segments from a genome, each carried by a plasmid, phage, or other cloning vector.
The genetic makeup of an organism.
genus pl. genera
A taxonomic category above the species level, designated by the first word of a species' binomial Latin name.
geographical range
The geographic area in which a population lives.
geological time scale
A time scale established by geologists that reflects a consistent sequence of historical periods, grouped into four eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
germ cells
Gametes or the cells that give rise to gametes.
In plants, the resumption of growth or the development from seed or spore.
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.
A localized extension of the body surface of many aquatic animals, specialized for gas exchange.
A structure composed of modified epithelial cells specialized to produce one or more secretions that are discharged to the outside of the gland.
glial cell
A nonconducting cell of the nervous system that provides support, insulation, and protection for the neurons.
globular protein
A polypeptide chain folded into a roughly spherical shape.
A ball of capillaries surrounded by Bowman's capsule in the nephron and serving as the site of filtration in the vertebrate kidney.
A peptide hormone secreted by pancreatic endocrine cells that raises blood glucose levels; an antagonistic hormone to insulin.
A corticosteroid hormone secreted by the adrenal cortex that influences glucose metabolism and immune function.
A six-carbon sugar (C6H12O6); the most common monosaccharide in animals.
A three-carbon molecule with three hydroxyl ( ) groups attached; a glycerol molecule can combine with three fatty acid molecules to form a fat or an oil.
A fuzzy coat on the outside of animal cells, made of sticky oligosaccharides.
An extensively branched glucose storage polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.
Organic molecules similar in structure to fats, but in which a short carbohydrate chain rather than a fatty acid is attached to the third carbon of the glycerol molecule; as a result, the molecule has a hydrophilic "head" and a hydrophobic "tail." Glycolipids are important constituents of the plasma membrane and of organelle membranes.
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.
A protein with covalently attached carbohydrate.
Golgi apparatus
An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic reticulum.
Hormones that stimulate the activities of the testes and ovaries; a collective term for follicle-stimulating and luteinizing hormones.
The male and female sex organs; the gamete-producing organs in most animals.
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.
A view of Earth's history that attributes profound change to the cumulative product of slow but continuous processes.
Gram stain
A staining method that distinguishes between two different kinds of bacterial cell walls.
granum pl. grana
A stacked portion of the thylakoid membrane in the chloroplast. Grana function in the light reactions of photosynthesis.
A response of a plant or animal in relation to gravity.
greenhouse effect
The warming of planet Earth due to the atmospheric accumulation of carbon dioxide, which absorbs infrared radiation and slows its escape from the irradiated Earth.
gross primary productivity (GPP)
The total primary productivity of an ecosystem.
gross productivity
A measure of the rate at which energy is assimilated by the organisms in a trophic level, a community, or an ecosystem.
ground meristem
A primary meristem that gives rise to ground tissue in plants.
ground tissue system
A tissue of mostly parenchyma cells that makes up the bulk of a young plant and fills the space between the dermal and vascular tissue systems.
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.
guard cell
A specialized epidermal plant cell that forms the boundaries of the stomata.
The exudation of water droplets caused by root pressure in certain plants.
A vascular plant that bears naked seeds not enclosed in any specialized chambers.
The place in which individuals of a particular species can usually be found.
A simple kind of learning involving a loss of sensitivity to unimportant stimuli, allowing an animal to conserve time and energy.
The average time required for the disappearance or decay of one-half of any amount of a given substance.
haploid cell
A cell containing only one set of chromosomes (n).
Hardy-Weinberg equilibrium
The steady-state relationship between relative frequencies of two or more alleles in an idealized population; both the allele frequencies and the genotype frequencies will remain constant from generation to generation in a population breeding at random in the absence of evolutionary forces.
Hardy-Weinberg theorem
An axiom maintaining that the sexual shuffling of genes alone cannot alter the overall genetic makeup of a population.
haustorium pl. haustoria
In parasitic fungi, a nutrient-absorbing hyphal tip that penetrates the tissues of the host but remains outside the host cell membranes.
Haversian system
One of many structural units of vertebrate bone, consisting of concentric layers of mineralized bone matrix surrounding lacunae, which contain osteocytes, and a central canal, which contains blood vessels and nerves.
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 amount of heat required to change a given amount of a liquid into a gas; 540 calories are required to change 1 gram of liquid water into vapor.
heat-shock protein
A protein that helps protect other proteins during heat stress, found in plants, animals, and microorganisms.
helper T cell (TH)
A type of T cell that is required by some B cells to help them make antibodies or that helps other T cells respond to antigens or secrete lymphokines or interleukins.
The iron-containing group of heme proteins such as hemoglobin and the cytochromes.
An iron-containing protein in red blood cells that reversibly binds oxygen.
A group of hereditary disorders characterized by failure of the blood to clot and consequent excessive bleeding from even minor wounds.
In invertebrates with an open circulatory system, the body fluid that bathes tissues.
Pertaining to the liver.
hepatic portal vessel
A large circulatory channel that conveys nutrient-laden blood from the small intestine to the liver, which regulates the blood's nutrient content.
In plants, nonwoody.
A heterotrophic animal that eats plants.
The transmission of characteristics from parent to offspring.
An individual that functions as both male and female in sexual reproduction by producing both sperm and eggs.
Nontranscribed eukaryotic chromatin that is so highly compacted that it is visible with a light microscope during interphase.
Evolutionary changes in the timing or rate of development.
A specialized cell that engages in nitrogen fixation on some filamentous cyanobacteria.
A condition in the life cycle of all modern plants in which the sporophyte and gametophyte generations differ in morphology.
Referring to plants in which the sporophyte produces two kinds of spores that develop into unisexual gametophytes, either female or male.
An organism that obtains organic food molecules by eating other organisms or their by-products.
A diploid organism that carries two different alleles at one or more genetic loci.
heterozygote advantage
A mechanism that preserves variation in eukaryotic gene pools by conferring greater reproductive success on heterozygotes over individuals homozygous for any one of the associated alleles.
Having two different alleles for a given genetic character.
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.
A substance released by injured cells that causes blood vessels to dilate during an inflammatory response.
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.
Abbreviation of human immunodeficiency virus, the infectious agent that causes AIDS; HIV is an RNA retrovirus.
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).
A 180-nucleotide sequence within a homeotic gene encoding the part of the protein that binds to the DNA of the genes regulated by the protein.
Evolutionary alteration in the placement of different body parts.
The steady-state physiological condition of the body.
An organism, such as a bird or mammal, capable of maintaining a stable body temperature independent of the environment.
homeotic genes
Genes that control the overall body plan of animals by controlling the developmental fate of groups of cells.
Humans and closely related primates; includes modern and fossil forms, such as the australopithecines, but not the apes.
Hominids and the apes.
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.
Similarity in characteristics resulting from a shared ancestry.
Referring to plants in which a single type of spore develops into a bisexual gametophyte having both male and female sex organs.
A diploid organism that carries identical alleles at one or more genetic loci.
Having two identical alleles for a given trait.
One of many types of circulating chemical signals in all multicellular organisms that are formed in specialized cells, travel in body fluids, and coordinate the various parts of the organism by interacting with target cells.
(1) An organism on or in which a parasite lives. (2) A recipient of grafted tissue.
Human Genome Project
An international collaborative effort to map and sequence the DNA of the entire human genome.
human immunodeficiency virus (HIV)
The infectious agent that causes AIDS; HIV is an RNA retrovirus.
humoral immunity
The type of immunity that fights bacteria and viruses in body fluids with antibodies that circulate in blood plasma and lymph, fluids formerly called humors.
(1) Offspring of two parents that differ in one or more inheritable characteristics. (2) Offspring of two different varieties or of two different species.
hybrid zone
A region where two related populations that diverged after becoming geographically isolated make secondary contact and interbreed where their geographical ranges overlap.
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.
A chemical process that lyses or splits molecules by the addition of water; an essential process in digestion.
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+).
Having an aversion to water; tending to coalesce and form droplets in water.
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.
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.
An electrical state whereby the inside of the cell is made 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 mV, reducing the chance that the neuron will transmit a nerve impulse.
hypertonic solution
A solution with a greater solute concentration than another, a hypotonic solution.
A filament that collectively makes up the body of a fungus.
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, which regulate the anterior pituitary.
A temporary working explanation or supposition based on accumulated facts and suggesting some general principle or relation of cause and effect; a postulated solution to a scientific problem that must be tested and if not validated, discarded.
hypotonic solution
A solution with a lesser solute concentration than another, a hypertonic solution.
Biologists who specialize in the study of fish behavior, anatomy, physiology, and evolution.
imaginal disk
An island of undifferentiated cells in an insect larva, which are committed (determined) to form a particular organ during metamorphosis to the adult.
The capillary movement of water into germinating seeds and into substances such as wood and gelatin, which swell as a result.
immune response
A highly specific defensive reaction of the body to invasion by a foreign substance or organism; consists of a primary response in which the invader is recognized as foreign, or "not-self," and eliminated and a secondary response to subsequent attacks by the same invader. Mediated by two types of lymphocytes B cells, which mature in the bone marrow and are responsible for antibody production, and T cells, which mature in the thymus and are responsible for cell-mediated immunity.
immunoglobulin (Ig)
One of the class of proteins comprising the antibodies.
A type of learned behavior with a significant innate component, acquired during a limited critical period.
The mating of individuals that are closely related genetically.
inclusive fitness
The relative number of an individual's alleles that are passed on from generation to generation, either as a result of his or her own reproductive success, or that of related individuals.
incomplete dominance
A type of inheritance in which F1 hybrids have an appearance that is intermediate between the phenotypes of the parental varieties.
incomplete flower
A flower lacking sepals, petals, stamens, or carpels.
incomplete metamorphosis
A type of development in certain insects, such as grasshoppers, in which the larvae resemble adults but are smaller and have different body proportions. The animal goes through a series of molts, each time looking more like an adult, until it reaches full size.
independent assortment
See Mendel's second law.
independent variable
In an experiment, when one factor is manipulated, a second factor responds. The independent variable is the factor that is manipulated.
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.
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.
(1) The ability of one group of embryonic cells to influence the development of another. (2) In genetics, the phenomenon in which the presence of a substrate (the inducer) initiates transcription and translation of the genes coding for the enzymes required for its metabolism.
inflammatory response
A line of defense triggered by penetration of the skin or mucous membranes, in which small blood vessels in the vicinity of an injury dilate and become leakier, enhancing the infiltration of leukocytes; may also be widespread in the body.
A heterotrophic mode of nutrition in which other organisms or detritus are eaten whole or in pieces.
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.
innate releasing mechanism
In ethology, a circuit within an animal's brain that is hypothesized to respond to a specific stimulus, setting in motion, or "releasing," the sequence of movements that constitute a fixed action pattern.
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.
inositol trisphosphate (IP3)
The second messenger, which functions as an intermediate between certain nonsteroid hormones and the third messenger, a rise in cytoplasmic Ca2+ concentration.
A mutation involving the addition of one or more nucleotide pairs to a gene.
insertion sequence
The simplest kind of a transposon, consisting of inserted repeats of DNA flanking a gene for transposase, the enzyme that catalyzes transposition.
insight learning
The ability of an animal to perform a correct or appropriate behavior on the first attempt in a situation with which it has had no prior experience.
The stage of an arthropod's life cycle between molts (shedding of the exoskeleton). As an example, the third instar is the stage of the life cycle between the second and third molting cycles. Some arthropods molt throughout their entire life and may have as many as 30 instars. Insects tend to have a set number of instars, the number varying by species. The instars succeed one another until the final mature instar, when the organism stops molting.
A vertebrate hormone that lowers blood glucose levels by promoting the uptake of glucose by most body cells and the synthesis and storage of glycogen in the liver; also stimulates protein and fat synthesis; secreted by endocrine cells of the pancreas called islets of Langerhans.
A chemical messenger of the immune system, produced by virus-infected cells and capable of helping other cells resist the virus.
Interleukin-1, a chemical regulator (cytokin) secreted by macrophages that have ingested a pathogen or foreign molecule and have bound with a helper T cell; stimulates T cells to grow and divide and elevates body temperature. Interleukin-2, secreted by activated T cells, stimulates helper T cells to proliferate more rapidly.
intermediate filament
A component of the cytoskeleton that includes all filaments intermediate in size between microtubules and microfilaments.
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.
The segment of a plant stem between the points where leaves are attached.
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 time of each cell cycle.
interstitial cells
Cells scattered among the seminiferous tubules of the vertebrate testis that secrete testosterone and other androgens, the male sex hormones.
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.
intrinsic rate of increase
The difference between the number of births and the number of deaths, symbolized as rmax; the maximum population growth rate.
The transplantation of genes between species resulting from fertile hybrids mating successfully with one of the parent species.
A noncoding, intervening sequence within a eukaryotic gene.
The local infolding of a layer of tissue, especially in animal embryos, so as to form a depression or pocket opening to the outside.
An aberration in chromosome structure resulting from an error in meiosis or from mutagens; reattachment in a reverse orientation of a chromosomal fragment to the chromosome from which the fragment originated.
An animal without a backbone; invertebrates make up 95% of animal species.
in vitro fertilization
Fertilization of ova in laboratory containers followed by artificial implantation of the early embryo in the mother's uterus.
An atom that has gained or lost electrons, thus acquiring a charge.
ionic bond
A chemical bond resulting from the attraction between oppositely charged ions.
A condition in which male and female gametes are morphologically indistinguishable.
isolating mechanisms
Mechanisms that prevent genetic exchange between individuals of different populations or species; they prevent mating or successful reproduction even when mating occurs; may be behavioral, anatomical, or physiological.
One of several organic compounds with the same molecular formula but different structures and therefore different properties. The three types are structural isomers, geometric isomers, and enantiomers.
isomorphic generations
Alternating generations in which the sporophytes and gametophytes look alike, although they differ in chromosome number.
isotonic solution
Solutions of equal solute concentration.
One of several atomic forms of an element, each containing a different number of neutrons and thus differing in atomic mass.
joule (J)
A unit of energy: 1 J = 0.239 cal; 1 cal = 4.184 J.
juvenile hormone (JH)
A hormone in arthropods, secreted by the corpora allata glands, that promotes the retention of larval characteristics.
juxtaglomerular apparatus (JGA)
Specialized tissue located near the afferent arteriole that supplies blood to the kidney glomerulus; the JGA raises blood pressure by producing renin, which activates angiotensin.
The concept that in certain (K-selected) populations, life history is centered around producing relatively few offspring that have a good chance of survival.
The fusion of nuclei of two cells, as part of syngamy.
Division of the nucleus during the cell cycle.
A method of organizing the chromosomes of a cell in relation to number, size, and type.
One of a group of tough, fibrous proteins formed by certain epidermal tissues and especially abundant in skin, claws, hair, feathers, and hooves.
keystone predator
A predatory species that helps maintain species richness in a community by reducing the density of populations of the best competitors so that populations of less competitive species are maintained.
keystone species
A species that is of exceptional importance in maintaining the species diversity of a community; when a keystone species is lost, the diversity of the community decreases and its structure is significantly altered.
In vertebrates, the organ that regulates the balance of water and solutes in the blood and the excretion of nitrogenous wastes in the form of urine.
kilocalorie (kcal)
A thousand calories; the amount of heat energy required to raise the temperature of 1 kg of water 1°C.
kin selection
A phenomenon of inclusive fitness, used to explain altruistic behavior between related individuals.
A change in activity 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 transferring some of its kinetic energy to other matter.
A specialized region on the centromere that links each sister chromatid to the mitotic spindle.
A taxonomic category, the second broadest after domain.
Koch's postulates
A set of four criteria for determining whether a specific pathogen is the cause of a disease.
Krebs 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.
A tiny lymph vessel extending into the core of an intestinal villus and serving as the destination for absorbed chylomicrons.
lagging strand
A discontinuously synthesized DNA strand that elongates in a direction away from the replication fork.
Layer, thin sheet.
larva pl. larvae
A free-living, sexually immature form in some animal life cycles that may differ from the adult in morphology, nutrition, and habitat.
lateral line system
A mechanoreceptor system consisting of a series of pores and receptor units (neuromasts) along the sides of the body of fishes and aquatic amphibians; detects water movements made by an animal itself and by other moving objects.
lateral meristem
The vascular and cork cambium, a cylinder of dividing cells that runs most of the length of stems and roots and is responsible for secondary growth.
law of independent assortment
Mendel's second law, stating that each allele pair segregates independently during gamete formation; applies when genes for two traits are located on different pairs of homologous chromosomes.
law of segregation
Mendel's first law, stating that allele pairs separate during gamete formation, and then randomly re-form pairs during the fusion of gametes at fertilization.
The dissolving of minerals and other elements in soil or rocks by the downward movement of water.
leading strand
The new continuous complementary DNA strand synthesized along the template strand in the mandatory 5' to 3' direction.
The main site of photosynthesis in a plant; consists of a flattened blade and a stalk (petiole) that joins the leaf to the stem.
The process that leads to modification in individual behavior as the result of experience.
A white blood cell; typically functions in immunity, such as phagocytosis or antibody production.
A type of prostaglandin produced by various white blood cells involved in the inflammatory and immune responses and in allergic reactions.
An organism formed by the symbiotic association between a fungus and a photosynthetic alga.
life cycle
The entire sequence of stages in the life of an organisms, from the adults of one generation to the adults of the next.
life-history pattern
A group of traits, such as size and number of offspring, length of maturation, age at first reproduction, and the number of times reproduction occurs, that affect reproduction, survival, and the rate of population growth.
life table
A table of data summarizing mortality in a population.
A type of fibrous connective tissue that joins bones together at joints.
A molecule that binds specifically to a receptor site of another molecule.
ligand-gated ion channel receptor
A signal receptor protein in a cell membrane that can act as a channel for the passage of a specific ion across the membrane. When activated by a signal molecule, the receptor either allows or blocks passage of the ion, resulting in a change in ion concentration that usually affects cell functioning.