Zoology Glossary



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.


Upsied down the aboral surface of a starfish. Pertaining to away from the mouth in organisms with no distinct front or back sides


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.


The spiny-headed worms, a phylum of helminths; adults are parasitic in the alimentary canal of vertebrates.

Accessory cells

Any nonlymphocytic cell that helps in the induction of the immune response by presenting antigen to a helper T lymphocyte.


An animal that lacks a coelom. Acoelomates, which include the flatworm, fluke, tapeworm, and ribbon worm, exhibit bilateral symmetry and possess one internal space, the digestive cavity.


Physiological adjustment to a change in an environmental factor. Accommodation The automatic adjustment of an eye to focus on near objects.


Containg no cells; not made of cells.

Accessory Cell

A cell which is associated with the guard cell of a stoma


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.


Not having a floral envelope or perianth.


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.


Having the centromere located near one end of the chromosome so that one chromosomal arm is long and the other is short.


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.


The evolution of features that make a group of organisms better suited to live and reproduce in their 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

A nucleotide consisting of adenine, ribose, and two phosphate groups; formed by the removal of one phosphate from an ATP molecule.

Adenosine monophosphate

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

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.


In botany terminology adaxial describes a side or surface nearest or facing toward the axis of an organ or organism, such as the upper surface of a leaf lamina


Any of various cells found in adipose tissue that are specialized for the storage of fat. Also called adipocyte.

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.

Adrenocorticotropic hormone (ACTH)

A hormone, produced by the anterior lobe of the pituitary gland, that stimulates the production of cortisol by the adrenal cortex.


Containing oxygen; referring to an organism, environment, or cellular process that requires oxygen.


Exsiting or living in the air.


Cessation from or slowing of activity during the summer; especially slowing of metabolism in some animals


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.


The outer zone of wood in a tree, next to the bark. Sapwood is generally lighter than heartwood.

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 air-filled space in the body of a bird that forms a connection between the lungs and bone cavities and aids in breathing and temperature regulation.


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.


Either of two large reptiles, Alligator mississipiensis of the southeast United States or A. sinensis of China, having sharp teeth and powerful jaws. They differ from crocodiles in having a broader, shorter snout.

Alimentary canal

The tube or passage of the digestive system through which food passes, nutrients are absorbed, waste is eliminated.


The alkalinity of water is a measure of how much acid it can neutralize. If any changes are made to the water that could raise or lower the pH value, alkalinityacts as a buffer, protecting the water and its life forms from sudden shifts in pH.



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.

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.


One of the deadend, multilobed air sacs that constitute the gas exchange surface of the lungs. Or One of the milk-secreting sacs of epithelial tissue in the mammary glands.

Amino acids

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.


A family of enzymes, at least one for each amino acid, that catalyze the attachment of an amino acid to its specific tRNA molecule.


Direct cell devision, that is, the cell divides by simple cleavage of the nucles without formation of spireme spindle figures or chromosomes.


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.


Living or able to live both land and water.

Amphipathic molecule

A molecule that has both a hydrophilic region and a hydrophobic region.


Any of various small, flattened marine organisms of the subphylum Cephalochordata, structurally similar to the vertebrates but having a notochord rather than a true vertebral column. Also called amphioxus.


Having the characteristics of an acid and a base and capable of reacting chemically either as an acid or a base.


Of a leaf, having stomata on both surfaces


The outer protion of a starch granule consisting of insoluble, highly branched polysaccharides of high molecular weight.

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 morphological structure of a plant or an animal or of any of its parts.


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.


Animals are a major group of multicellular organisms, of the kingdom Animalia or metazoa. animal starch


A union between two gametes that differ in size of form.


A negatively charged ion.


The bone in the ankle that articulates with the leg bones to form the ankle joint


Long, paired sensory appendages on the head of many arthropods.


Referring to the head end of a bilaterally symmetrical animal.


A higher primate; includes monkeys, apes, and humans.


A chemical that kills bacteria or inhibits their growth.

Antibiotic resistance

Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic. It is a specific type of drug resistance.


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 opening at the lower end of the digestive tract through which solid waste is excreated.


Any of numerous tailless, aquatic, semiaquatic, or terrestrial amphibians of the order Anura and especially of the family Ranidae, characteristically having a smooth moist skin, webbed feet, and long hind legs adapted for leaping.


The major artery in blood-circulating systems; the aorta sends blood to the other body tissues.


Elating to, or consisting of an appendage or appendages, especially the limbs: the appendicular skeleton.

Aphotic zone

The part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.


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).


Consisting of, relating to, or being in water; an aquatic environment.

Aqueous solution

A solution in which water is the solvent.




One of two prokaryotic domains, the other being the Bacteria.


Arches are curved structures, arch-like in profile, which span the foot.


The central cavity of the gastrula, which ultimately becomes the intestinal or digestive cavity.


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.



Arteries are blood vessels that carry blood away from the heart


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.


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 Aschelminthes (also known as Aeschelminthes, Nemathelminthes, or Pseudocoelomata), closely associated with the Platyhelminthes, are an obsolete phylum of pseudocoelomate and other similar animals that are no longer considered closely related and have been promoted to phyla in their own right.

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.


The bone of the ankle which articulates with the bones of the leg. Also known as talus.


A star-shaped cell, especially a neuroglial cell of nervous tissue.


Irregular in shape or outline.

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 mass

The mass of an atom of a chemical element expressed in atomic mass units.\

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 Theory

The physical theory of the structure, properties, and behavior of the atom.

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.


A chamber that receives blood returning to the vertebrate heart.


The flap of the ear in the form of a funnel-like organ which collects the sound waves. Called also pinna.


A single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the catalyst for that reaction.


Having all stages of a life cycle occurring on the same host. Eg.; fungi

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.

Autumn wood

The season of the year between summer and winter, lasting from the autumnal equinox to the winter solstice and from September to December in the Northern Hemisphere; fall.


The vertebrate class of birds, characterized by feathers and other flight adaptations.

Axial filaments

The central filament of a flagellum or cilium. Also called axoneme.


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.

Bacilli (pl.) Bacillus (Sin.)

Aerobic rod-shaped spore-producing bacterium; often occurring in chainlike formations


One of two prokaryotic domains, the other being the Archaea.


A virus that parasitizes a bacterial cell.

Bacterium (Sin.) Bacteria (Pl.)

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.

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.

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.

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.


Not life-threatening or severe, and likely to respond to treatment, as a tumor that is not malignant.


Occurring twice a year.

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.


Having both male and female reproductive organs; hermaphroditic.


Causing a stinging sensation; nipping: biting cold.


Having a shell consisting of twohinged valves.


A pair of homologous, synapsed chromosomes associated together during meiosis.


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 flukes

A blood fluke species related to schistosoma haematobium which lives in the blood of host like human

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.

Blood vascular system

When blood vessels connect to form a region of diffuse vascular supply it is called an anastomosis


The scientific study of life and of living organisms. Botany, zoology, and ecology are all branches of biology.

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.


Having a internal skeleton of bones.

Book lungs

Organs of gas exchange in spiders, consisting of stacked plates contained in an internal chamber.

Bottleneck effect

Genetic drift resulting from the reduction of a population, typically by a natural disaster, such that the surviving population is no longer genetically representative of the original population.

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.


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.


Pertaining to the cheek and the pharynx or to the mouth and the pharynx. (In amphibians respiration takes place by mouth cavity also )


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 cap

A bundle cap is a a cluster of fibers that covers the top of the top or the phloem side of the vascular bundle.

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.



A tough, elastic, fibrous connective tissue found in various parts of the body, such as the joints, outer ear, and larynx. A major constituent of the embryonic and young vertebrate skeleton, it is converted largely to bone with maturation.


The quadrangular bone at the back of the tarsus. Also called heel bone.


A mammalian thyroid hormone that lowers blood calcium levels.

Calcium oxalate

A small, colorless crystal that may be present in urine or may be a component of real calculi.


An intracellular protein to which calcium binds in its function as a second messenger in hormone action.


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.


A serious disease resulting from a malignant growth or tumour, caused by abnormal and uncontrolled cell division.


One of the four pointed conical teeth located between the incisors and the premolars.

Canal system

A tube, duct, or passageway.


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.



Capillaries are the smallest of a body's blood vessels.


The protein shell that encloses the viral genome; rod-shaped, polyhedral, or more completely shaped.


A slimy layer around the cells of certain bacteria.


Coupled; joined


A hard bony or chitinous outer covering, such as the fused dorsal plates of a turtle or the portion of the exoskeleton covering the head and thorax of a crustacean.


A sugar (monosaccharide) or one of its dimers (disaccharides) or polymers (polysaccharides).


Compounds,, such as cellulose, sugar, and starch, that contain only carbon, hydrogen, and oxygen, and are a major part of the diets of people and other animals.

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.

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.

Cardiac muscle

The specialized striated muscle tissue of the heart; the myocardium.

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.

Carotid arteries

Either of the two principal arteries on both sides of the neck that supply blood to the head and neck. Also known as common carotid artery.

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.


Having a skeleton consisting mainly of cartilage.

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.

Cdc genes

A gene that regulates the cell cycle. Also known as CDC gene.


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 division

The process in reproduction and growth by which a cell divides to form daughter cells.

Cell fractionation

The disruption of a cell and separation of its organelles by centrifugation.

Cell inclusions

The act of including or the state of being included.

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 organelles

A specialized part of a cell; “the first organelle to be identified was the nucleus”.

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.


Consisting of or containing a cell or cells

Cellular level

Process that occurs in cellular

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.

Centrifugation process

An apparatus in which humans or animals are enclosed and which is revolved to simulate the effects of acceleration in a spacecraft.


A small wormlike creature with many legs.


One of two cylindrical cellular structures that are composed of nine triplet microtubules and form the asters during mitosis.

Central nervous system (CNS)

In vertebrate animals, the brain and spinal cord.


Centrifugation is a process that involves the use of the centrifugal force for the separation of mixtures.


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.

Cerebral ganglia

They transmit nerve pulse activity so nerve cells can 'talk' to each other.

Cerebral hemispheres

Either of the two symmetrical halves of the cerebrum, as divided by the longitudinal cerebral fissure.


Any of various lipid compounds containing glucose or galactose and glucose, and found in the brain and other nerve tissue.


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.

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.


An order of the Reptilia, subclass Anapsida, including the turtles, terrapins, and tortoises.

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.


The movements of the mandible during mastication; controlled by neuromuscular action and limited by the anatomic structure of the temporomandibular joints.


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 tough, protective, semitransparent substance, primarily a nitrogen-containing polysaccharide, forming the principal component of arthropod exoskeletons and the cell walls of certain fungi.


Any of the flagellated cells in sponges having a collar of cytoplasm around the flagellum; they maintain a flow of water through the body.


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 plastid contains pigments other than chlorophyll, usually yellow or orange carotenoids.


A threadlike, gene-carrying structure found in the nucleus. Each chromosome consists of one very long DNA molecule and associated proteins.

Chromosome map

A diagram of the linear order of the genes on a chromosome.


A pigment-containing or pigment-producing cell, especially in certain lizards, that by expansion or contraction can change the color of the skin. Also called pigment cell.


Fungus with flagellated stage; possible evolutionary link between fungi and protists.


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.


The flow of blood from the heart through the arteries, and then back through the veins to the heart, where the cycle is renewed.


Relating to blood circulation or circulatory system.


A slender flexible appendage, such as the fused cilia of certain protozoans.


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 photosynthetic branch or portion of a stem that resembles and functions as a leaf, as in the asparagus. Also called cladode.


Any of the appendages of the male of certain insects and crustaceans that are used during copulation to hold the female.


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 systematic grouping of organisms into categories on the basis of evolutionary or structural relationships between them.

A division or category in a classifying system.


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 small elongated erectile organ at the anterior part of the vulva, homologous with the penis, present in female


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.


  • A lineage of genetically identical individuals or cells.
  • In popular usage, a single individual organism that is genetically identical to another individual.

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.


Phylum Cnidaria includes animals like hydras; polyps; jellyfishes; sea anemones; corals


A stinging cell containing a nematocyst; characteristic of cnidarians.


Cnidarians are the group of invertebrate animals which possess stinging cells called cnidocytes


A cell in the epidermis of coelenterates in which a nematocyst is developed.


Any spherical or nearly spherical bacteria


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.


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.

Cholesterol metabolism

A white crystalline substance, C27H45OH, found in animal tissues and various foods, that is normally synthesized by the liver and is important as a constituent of cell membranes and a precursor to steroid hormones. Its level in the bloodstream can influence the pathogenesis of certain conditions, such as the development of atherosclerotic plaque and coronary artery disease.


Living in, consisting of, or forming a colony. An inhabitant of a colony.


A group of organisms of the same species living together in close association.


In biology, columnar refers to the shape of epithelial cells that are taller than they are wide


A symbiotic relationship in which the symbiont benefits but the host is neither helped nor harmed.


All the organisms that inhabit a particular area; an assemblage of populations of different species living close enough together for potential interaction.


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.


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.

Compound leaf

a leaf that is composed of two or more leaflets on a common stalk. Clover, roses, sumac, and walnut trees have compound leaves.


Having a common center.