Biology Campbell Glossary Flashcards

Terms Definitions
The broad region that corresponds to the length of the thick filaments.
A band
One of three binding sites for tRNA during translation, it holds the tRNA carrying the next amino acid to be added to the polypeptide chain; A stands for aminoacyl-tRNA site.
A site
The body cavity in mammals that primarily houses parts of the digestive, excretory, and reproductive systems. It is separated from the more cranial thoracic cavity by the diaphragm.
abdominal cavity
Nonliving chemical and physical factors in the environment.
abiotic components
Genetically determined classes of human blood that are based on the presence or absence of carbohydrates A and B on the surface of red blood cells. The ABO blood group phenotypes, also called blood types, are A, B, AB, and O.
ABO blood groups
A plant hormone that generally acts to inhibit growth, promote dormancy, and help the plant tolerate stressful conditions.
abscisic acid (ABA)
The uptake of small nutrient molecules by an organism's own body; the third main stage of food processing, following digestion.
absorption
The range of a pigment's ability to absorb various wavelengths of light.
absorption spectrum
The very deep benthic communities near the bottom of the ocean. This region is characterized by continuous cold, extremely high water pressure, low nutrients, and near or total absence of light.
abyssal zone
A group of ancient jawed fishes from the Devonian period.
acanthodians
Physiological adjustment to a change in an environmental factor.
acclimatization
The automatic adjustment of an eye to focus on near objects.
accommodation
The entry compound for the Krebs cycle in cellular respiration; formed from a fragment of pyruvate attached to a coenzyme.
acetyl CoA (acetyl coenzyme A)
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.
acetylcholine
A substance that increases the hydrogen ion concentration of a solution.
acid
A mixture of recently swallowed food and gastric juice.
acid chyme
Rain, snow, or fog that is more acidic than pH 5.6.
acid precipitation
A solid-bodied animal lacking a cavity between the gut and outer body wall.
acoelomate
The discharge of a sperm's acrosome when the sperm approaches an egg.
acrosomal reaction
An organelle at the tip of a sperm cell that helps the sperm penetrate the egg.
acrosome
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.
actin
The class of lobe-finned fishes.
Actinistia
The class of ray-finned fishes.
Actinopterygii
A rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered, selective opening and closing of voltage-sensitive gates in sodium and potassium ion channels.
action potential
A profile of the relative performance of different wavelengths of light.
action spectrum
The amount of energy that reactants must absorb before a chemical reaction will start.
activation energy
A transcription factor that binds to an enhancer and stimulates transcription of a gene.
activator
Immunity conferred by recovering from an infectious disease.
active immunity
The specific portion of an enzyme that attaches to the substrate by means of weak chemical bonds.
active site
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.
active transport
Inherited characteristics that enhance the ability of an organism to survive and reproduce in a particular environment.
adaptations
An equilibrium state in a population when the gene pool has allele frequencies that maximize the average fitness of a population's members.
adaptive peak
The emergence of numerous species from a common ancestor introduced into an environment, presenting a diversity of new opportunities and problems.
adaptive radiation
Also called the anterior pituitary, it consists of endocrine cells that synthesize and secrete several hormones directly into the blood.
adenohypophysis
An enzyme that converts ATP to cyclic AMP in response to a chemical signal.
adenylyl cyclase
The attraction between different kinds of molecules.
adhesion
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 ner
adrenal gland
The central portion of an adrenal gland, controlled by nerve signals, that secretes the fight-or-flight hormones epinephrine and norepinephrine.
adrenal medulla
A peptide hormone released from the anterior pituitary, it stimulates the production and secretion of steroid hormones by the adrenal cortex.
adrenocorticotropic hormone
Roots extending from stems and leaves above ground.
adventitious
Containing oxygen; referring to an organism, environment, or cellular process that requires oxygen.
aerobic
The blood vessel supplying a nephron.
afferent arteriole
A branch of mammals that includes sloths, anteaters, and armadillos.
Afrotheria
The relative number of individuals of each age in a population.
age structure
An antibody-mediated immune response in which bacteria or viruses are clumped together, effectively neutralized, and opsonized.
agglutination
A fruit such as a blackberry that develops from a single flower that has several carpels.
aggregate fruit
A member of a jawless class of vertebrates represented today by the lampreys and hagfishes.
agnathan
A type of behavior involving a contest of some kind that determines which competitor gains access to some resource, such as food or mates.
agonistic behavior
The name of the late stages of HIV infection; defined by a specified reduction of T cells and the appearance of characteristic secondary infections.
AIDS (acquired immunodeficiency syndrome)
The conversion of pyruvate to carbon dioxide and ethyl alcohol.
alcohol fermentation
Organic compounds containing hydroxyl groups.
alcohols
An organic molecule with a carbonyl group located at the end of the carbon skeleton.
aldehyde
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.
aldosterone
A photosynthetic, plantlike protist.
alga
A digestive tract consisting of a tube running between a mouth and an anus.
alimentary canal
One of four extra-embryonic membranes; serves as a repository for the embryo's nitrogenous waste.
allantois
Alternate versions of a gene.
alleles
The variation in the relative rates of growth of various parts of the body, which helps shape the organism.
allometric growth
A mode of speciation induced when the ancestral population becomes segregated by a geographic barrier.
allopatric speciation
A common type of polyploid species resulting from two different species interbreeding and combining their chromosomes.
allopolyploid
An action that occurs either completely or not at all, such as the generation of an action potential by a neuron.
all-or-none event
A specific receptor site on some part of an enzyme molecule remote from the active site.
allosteric site
A spiral shape constituting one form of the secondary structure of proteins, arising from a specific hydrogen-bonding structure.
alpha helix
A life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte; characteristic of plants.
alternation of generations
A type of regulation at the RNA-processing level in which different mRNA molecules are produced from the same primary transcript depending on which RNA segments are treated as exons and which as introns.
alternative RNA splicing
Behavior that reduces an individual's fitness while increasing the fitness of another individual.
altruism
The aiding of another individual at one's own risk or expense.
altruistic behavior
A protistan clade that includes dinoflagellates, apicomplexans, and the ciliates. Alveolates have small membrane-bounded cavities called alveoli under their cell surfaces. The function of alveoli is unknown.
Alveolata
(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.
alveolus
Neurons of the retina that help integrate information before it is sent to the brain.
amacrine cell
An organic compound with one or more amino groups.
amine
An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the monomers of proteins.
amino acid
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.
amino group
An enzyme that joins each amino acid to the correct tRNA.
aminoacyl-tRNA synthetase
An enzyme found within the small intestine that splits off one amino acid at a time, beginning at the opposite end of the polypeptide containing a free carboxyl group.
aminopeptidase
A small and very toxic nitrogenous waste produced by metabolism.
ammonia
Shelled cephalopod animals that were the dominant invertebrate predators for millions of years ending with the mass extinctions at the end of the Cretaceous period.
ammonites
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.
amniocentesis
The innermost of four extraembryonic membranes; encloses a fluid-filled sac in which the embryo is suspended.
amnion
A vertebrate possessing an amnion surrounding the embryo; reptiles, birds, and mammals are amniotes.
amniote
A shelled, water-retaining egg that enables reptiles, birds, and egg-laying mammals to complete their life cycles on dry land.
amniotic egg
A type of protist characterized by great flexibility and the presence of pseudopodia.
amoeba
An amoebalike cell that moves by pseudopodia, found in most animals; depending on the species, may digest and distribute food, dispose of wastes, form skeletal fibers, fight infections, and change into other cell types.
amoebocyte
The vertebrate class of amphibians, represented by frogs, salamanders, and caecilians.
Amphibia
A molecule that has both a hydrophilic region and a hydrophobic region.
amphipathic molecule
The strengthening of stimulus energy that is otherwise too weak to be carried into the nervous system.
amplification
Lacking oxygen; referring to an organism, environment, or cellular process that lacks oxygen and may be poisoned by it.
anaerobic
A metabolic pathway that synthesizes a complex molecule from simpler compounds.
anabolic pathway
The use of inorganic molecules other than oxygen to accept electrons at the "downhill" end of electron transport chains.
anaerobic respiration
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.
anagenesis
The similarity of structure between two species that are not closely related; attributable to convergent evolution.
analogy
The fourth subphase of mitosis, in which the chromatids of each chromosome have separated and the daughter chromosomes are moving to the poles of the cell.
anaphase
An acute, life-threatening, allergic response.
anaphylactic shock
One of three groups of amniotes based on key differences between their skulls.
anapsids
Fully modern humans.
anatomically modern humans
The study of the structure of an organism.
anatomy
The requirement that to divide, a cell must be attached to the substratum.
anchorage dependence
Adhesive junctions that link cells together into tissues.
anchoring junctions
The principal male steroid hormones, such as testosterone, which stimulate the development and maintenance of the male reproductive system and secondary sex characteristics.
androgens
A chromosomal aberration in which certain chromosomes are present in extra copies or are deficient in number.
aneuploidy
A flowering plant, which forms seeds inside a protective chamber called an ovary.
angiosperm
The ability to survive in a dormant state when an organism's habitat dries up. Also called cryptobiosis.
anhydrobiosis
The portion of the egg where the least yolk is concentrated. Opposite of vegetal pole.
animal pole
A negatively charged ion.
anion
A plant that completes its entire life cycle in a single year or growing season.
annual
Sensory appendages found in uniramians and crustaceans.
antennae
Also called the adenohypophysis, it consists of endocrine cells that synthesize and secrete several hormones directly into the blood.
anterior pituitary
Referring to the head end of a bilaterally symmetrical animal.
anterior
The terminal pollen sac of a stamen, inside which pollen grains with male gametes form in the flower of an angiosperm.
anther
In plants, the male gametangium, a moist chamber in which gametes develop.
antheridium
The phylum of hornworts, small herbaceous (non-woody) plants.
Anthocerophyta
The phylum containing all angiosperms.
Anthophyta
A member of a primate group made up of the apes (gibbon, orangutan, gorilla, chimpanzee, and bonobo), monkeys, and humans.
anthropoid
A chemical that kills bacteria or inhibits their growth.
antibiotic
An antigen-binding immunoglobulin, produced by B cells, that functions as the effector in an immune response.
antibody
A specialized base triplet at one end of a tRNA molecule that recognizes a particular complementary codon on an mRNA molecule.
anticodon
A hormone that is part of an elaborate feedback scheme that helps regulate the osmolarity of the blood.
antidiuretic hormone
A foreign macromolecule that does not belong to the host organism and that elicits an immune response.
antigen
The process by which an MHC molecule cradles a fragment of an intracellular protein antigen in its hammocklike groove, carries it to the cell surface, and "presents" the protein to an antigen receptor on a nearby T cell.
antigen presentation
Transmembrane versions of antibody molecules that B cells and T cells use to recognize specific antigens. Also called membrane antibodies.
antigen receptor
Cells that ingest bacteria and viruses and then destroy them. Class II MHC molecules in these cells collect peptide remnants of this degradation and present them to helper T cells.
antigen-presenting cell (APCs)
The order of frogs and toads that includes tailless tetrapod amphibians.
anura
The group of frogs and toads.
anurans
The part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.
aphotic zone
Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth.
apical dominance
A limb-bud organizing region consisting of a thickened area of ectoderm at the tip of a limb bud.
apical ectodermal ridge
Embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length.
apical meristem
One of a group of parasitic protozoans, some of which cause human diseases.
apicomplexan
The order of caecilians that includes legless amphibians.
apoda
The group of caecilians.
apodans
The asexual production of seeds.
apomixis
A derived phenotypic character, or homology, that evolved after a branch diverged from a phylogenetic tree.
apomorphic character
In plants, the nonliving continuum formed by the extracellular pathway provided by the continuous matrix of cell walls.
apoplast
Programmed cell death brought about by signals that trigger the activation of a cascade of "suicide" proteins in the cells destined to die.
apoptosis
The bright coloration of animals with effective physical or chemical defenses that acts as a warning to predators.
aposematic coloration
A small, fingerlike extension of the vertebrate cecum; contains a mass of white blood cells that contribute to immunity.
appendix
A transport protein in the plasma membrane of a plant or animal cell that specifically facilitates the diffusion of water across the membrane (osmosis).
aquaporin
Plasmalike liquid in the space between the lens and the cornea in the vertebrate eye; helps maintain the shape of the eye, supplies nutrients and oxygen to its tissues, and disposes of its wastes.
aqueous humor
A solution in which water is the solvent.
aqueous solution
The animal class that includes scorpions, spiders, ticks, and mites.
Arachnida
One of two prokaryotic domains, the other being the Bacteria.
Archaea
Primitive eukaryotic group that includes diplomonads, such as Giardia; some systematists assign kingdom status to archezoans.
Archaezoa
In plants, the female gametangium, a moist chamber in which gametes develop.
archegonium
The endoderm-lined cavity, formed during the gastrulation process, that develops into the digestive tract of an animal.
archenteron
The reptilian group that includes crocodiles, alligators, dinosaurs, and birds.
archosaurs
A vessel that conveys blood between an artery and a capillary bed.
arteriole
A cardiovascular disease caused by the formation of hard plaques within the arteries.
arteriosclerosis
A vessel that carries blood away from the heart to organs throughout the body.
artery
Segmented coelomates with exoskeletons and jointed appendages.
arthropod
The most diverse phylum in the animal kingdom; includes the horseshoe crab, arachnids (e.g., spiders, ticks, scorpions, and mites), crustaceans (e.g., crayfish, lobsters, crabs, barnacles), millipedes, centipedes, and insects. Arthropods are characterized
Arthropoda
The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits.
artificial selection
Macroscopic fruiting bodies of sac fungi.
ascocarps
A saclike spore capsule located at the tip of the ascocarp in dikaryotic hyphae; defining feature of the Ascomycota division of fungi.
ascus
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.
asexual reproduction
An amino acid that functions as a CNS neurotransmitter.
aspartate
The acquired ability to associate one stimulus with another; also called classical conditioning.
associative learning
A type of nonrandom mating in which mating partners resemble each other in certain phenotypic characters.
assortative mating
Glial cells that provide structural and metabolic support for neurons.
astrocytes
A carbon atom covalently bonded to four different atoms or groups of atoms.
asymmetric carbon
Cell division in which one daughter cell receives more cytoplasm than the other during mitosis.
asymmetric cell division
A cardiovascular disease in which growths called plaques develop on the inner walls of the arteries, narrowing their inner diameters.
atherosclerosis
The smallest unit of matter that retains the properties of an element.
atom
An atom's central core, containing protons and neutrons.
atomic nucleus
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 number
The total atomic mass, which is the mass in grams of one mole of the atom.
atomic weight
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.
ATP (adenosine triphosphate)
A cluster of several membrane proteins found in the mitochondrial crista (and bacterial plasma membrane) that function in chemiosmosis with adjacent electron transport chains, using the energy of a hydrogen ion concentration gradient to make ATP. ATP synt
ATP synthase
A peptide hormone that opposes the renin-angiotensin-aldosterone system (RAAS).
atrial natriuretic factor
A region of specialized muscle tissue between the right atrium and right ventricle. It generates electrical impulses that primarily cause the ventricles to contract.
atrioventricular (AV) node
A valve in the heart between each atrium and ventricle that prevents a backflow of blood when the ventricles contract.
atrioventricular valve
A chamber that receives blood returning to the vertebrate heart.
atrium
According to this model, eukaryotic cells evolved by the specialization of internal membranes originally derived from prokaryotic plasma membranes.
autogenesis model
An immunological disorder in which the immune system turns against itself.
autoimmune disease
A subdivision of the motor nervous system of vertebrates that regulates the internal environment; consists of the sympathetic and parasympathetic divisions.
autonomic nervous system
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.
autopolyploid
A chromosome that is not directly involved in determining sex, as opposed to a sex chromosome.
autosome
An organism that obtains organic food molecules without eating other organisms or substances derived from other organisms. Autotrophs use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.
autotroph
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.
auxins
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.
auxotroph
The vertebrate class of birds, characterized by feathers and other flight adaptations.
Aves
An embryonic shoot present in the angle formed by a leaf and stem.
axillary bud
A typically long extension, or process, from a neuron that carries nerve impulses away from the cell body toward target cells.
axon
A type of lymphocyte that develops in the bone marrow and later produces antibodies, which mediate humoral immunity.
B lymphocyte (B cell)
One of two prokaryotic domains, the other being the Archaea.
Bacteria
An artificial version of a bacterial chromosome that can carry inserts of 100,000–500,000 base pairs.
bacterial artificial chromosome (BAC)
A virus that infects bacteria; also called a phage. See phage.
bacteriophage
A photosynthetic pigment found in halophiles. It is very similar to the visual pigments in the retinas of our eyes.
bacteriorhodopsin
A prokaryotic microorganism in Domain Bacteria.
bacterium
A form of Rhizobium contained within the vesicles formed by the root cells of a root nodule.
bacteroids
A bone that is contained in, and helps stiffen, the penis of rodents, raccoons, walruses, and several other mammals.
baculum
The ability of natural selection to maintain diversity in a population.
balanced polymorphism
All tissues external to the vascular cambium in a plant growing in thickness, consisting of phloem, phelloderm, cork cambium, and cork.
bark
A dense object lying along the inside of the nuclear envelope in female mammalian cells, representing an inactivated X chromosome.
Barr body
Contraception that relies upon a physical barrier to block the passage of sperm. Examples include condoms and diaphragms.
barrier methods
Glands near the vaginal opening in a human female that secrete lubricating fluid during sexual arousal.
Bartholin's glands
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 body
The minimal number of kilocalories a resting animal requires to fuel itself for a given time.
basal metabolic rate (BMR)
A cluster of nuclei deep within the white matter of the cerebrum.
basal nuclei
A substance that reduces the hydrogen ion concentration of a solution.
base
The floor of an epithelial membrane on which the basal cells rest.
basement membrane
A point mutation; the replacement of one nucleotide and its partner in the complementary DNA strand by another pair of nucleotides.
base-pair substitution
Elaborate fruiting bodies of a dikaryotic mycelium of a club fungus.
basidiocarps
A reproductive appendage that produces sexual spores on the gills of mushrooms. The fungal division Basidiomycota is named for this structure.
basidium
A circulating leukocyte that produces histamine.
basophil
A type of mimicry in which a harmless species looks like a species that is poisonous or otherwise harmful to predators.
Batesian mimicry
What an animal does and how it does it.
behavior
A heuristic approach based on the expectation that Darwinian fitness (reproductive success) is improved by optimal behavior.
behavioral ecology
A mass of abnormal cells that remains at the site of origin.
benign tumor
The bottom surfaces of aquatic environments.
benthic zone
The communites of organisms living in the benthic zone of an aquatic biome.
benthos
The source of insulin within the islets of Langerhans, nestled within the pancreas.
beta cell
A metabolic sequence that breaks fatty acids down to two-carbon fragments which enter the Krebs cycle as acetyl CoA.
beta oxidation
One form of the secondary structure of proteins in which the polypeptide chain folds back and forth, or where two regions of the chain lie parallel to each other and are held together by hydrogen bonds.
beta (b) pleated sheet
A plant that requires two years to complete its life cycle.
biennial
A life history in which adults have but a single reproductive opportunity to produce large numbers of offspring, such as the life history of the Pacific salmon. Also known as semelparity.
big-bang reproduction
Characterizing a body form with a central longitudinal plane that divides the body into two equal but opposite halves.
bilateral symmetry
Members of the branch of eumetazoans possessing bilateral symmetry.
bilateria
A mixture of substances that is produced in the liver, stored in the gall bladder, and acts as a detergent to aid in the digestion and absorption of fats.
bile
The type of cell division by which prokaryotes reproduce. Each dividing daughter cell receives a copy of the single parental chromosome.
binary fission
The two-part latinized name of a species, consisting of genus and specific epithet.
binomial
All of the variety of life; usually refers to the variety of species that make up a community; concerns both species richness (the total number of different species) and the relative abundance of the different species.
biodiversity
The current rapid decline in the variety of life on Earth, largely due to the effects of human culture.
biodiversity crisis
A relatively small area with an exceptional concentration of endemic species.
biodiversity hot spot
The study of how organisms manage their energy resources.
bioenergetics
The principle that all life arises by the reproduction of preexisting life.
biogenesis
Neurotransmitters derived from amino acids.
biogenic amines
Any of the various nutrient circuits, which involve both biotic and abiotic components of ecosystems.
biogeochemical cycles
The study of the past and present distribution of species.
biogeography
An internal timekeeper that controls an organism's biological rhythms; marks time with or without environmental cues but often requires signals from the environment to remain tuned to an appropriate period. See also circadian rhythm.
biological clock
A trophic process in which retained substances become more concentrated with each link in the food chain.
biological magnification
The definition of a species as a population or group of populations whose members have the potential in nature to interbreed and produce fertile offspring; a biological species is also called a sexual species.
biological species concept
The dry weight of organic matter comprising a group of organisms in a particular habitat.
biomass
One of the world's major ecosystems, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment.
biome
The use of living organisms to detoxify and restore polluted and degraded ecosystems.
bioremediation
The entire portion of Earth inhabited by life; the sum of all the planet's ecosystems.
biosphere
The manipulation of living organisms or their components to produce useful products.
biotechnology
Pertaining to the living organisms in the environment.
biotic
All the organisms that are part of the environment.
biotic components
Neurons that synapse with the axons of rods and cones in the retina of the eye.
bipolar cell
Any of a class of warm-blooded vertebrates distinguished by having the body more or less completely covered with feathers and the forelimbs modified as wings.
birds
Chemical contraceptives that inhibit ovulation, retard follicular development, or alter a woman's cervical mucus to prevent sperm from entering the uterus.
birth control pills
A flower equipped with both stamens and carpels.
bisexual flower
A leaflike structure of a seaweed that provides most of the surface area for photosynthesis.
blade
The fluid-filled cavity that forms in the center of the blastula embryo.
blastocoel
An embryonic stage in mammals; a hollow ball of cells produced one week after fertilization in humans.
blastocyst
An embryonic cap of dividing cells resting on a large undivided yolk.
blastodisc
Small cells of an early embryo.
blastomeres
The opening of the archenteron in the gastrula that develops into the mouth in protostomes and the anus in deuterostomes.
blastopore
The hollow ball of cells marking the end stage of cleavage during early embryonic development.
blastula
A type of connective tissue with a fluid matrix called plasma in which blood cells are suspended.
blood
The hydrostatic force that blood exerts against the wall of a vessel.
blood pressure
A set of tubes through which the blood moves through the body.
blood vessel
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-brain barrier
A fluid-containing space between the digestive tract and the body wall.
body cavity
A lubricated ball of chewed food.
bolus
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 energy
A type of connective tissue, consisting of living cells held in a rigid matrix of collagen fibers embedded in calcium salts.
bone
Organs of gas exchange in spiders, consisting of stacked plates contained in an internal chamber.
book lungs
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.
bottleneck effect
A model of community organization in which mineral nutrients control community organization because nutrients control plant numbers, which in turn control herbivore numbers, which in turn control predator numbers.
bottom-up model
A cup-shaped receptacle in the vertebrate kidney that is the initial, expanded segment of the nephron where filtrate enters from the blood.
Bowman's capsule
Also called lamp shells, these animals superficially resemble clams and other bivalve mollusks, but the two halves of the brachiopod shell are dorsal and ventral to the animal rather than lateral, as in clams.
brachiopod
A hormone produced by neurosecretory cells in the insect brain. It promotes development by stimulating the prothoracic glands to secrete ecdysone.
brain hormone
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.
brainstem
A brain center that directs the activity of organs involved in breathing.
breathing control center
Fine branches of the bronchus that transport air to alveoli.
bronchioles
One of a pair of breathing tubes that branch from the trachea into the lungs.
bronchus
One of a group of marine, multicellular, autotrophic protists, the most common type of seaweed. Brown algae include the kelps.
brown algae
A special tissue in some mammals, located in the neck and between the shoulders, that is specialized for rapid heat production.
brown fat
The phylum of mosses. Note that the term "bryophyte " refers instead to the informal group of mosses, liverworts, and hornworts, nonvascular plants that inhabit the land but lack many of the terrestrial adaptations of vascular plants.
Bryophyta
A moss, liverwort, or hornwort; a nonvascular plant that inhabits the land but lacks many of the terrestrial adaptations of vascular plants.
bryophytes
Colonial animals that superficially resemble mosses.
bryozoan
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.
budding
Used in reference to the gains and losses of various materials and energy. Most energy and materials budgets are interconnected, with changes in the flux of one component affecting the exchanges of other components.
budgets
A substance that consists of acid and base forms in a solution and that minimizes changes in pH when extraneous acids or bases are added to the solution.
buffer
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.
bulbourethral gland
The movement of water due to a difference in pressure between two locations.
bulk flow
Animals that eat relatively large pieces of food.
bulk-feeder
A type of photosynthetic cell arranged into tightly packed sheaths around the veins of a leaf.
bundle-sheath cell
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.
C3 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.
C4 plant
An important class of cell-to-cell adhesion molecules.
cadherins
A mammalian thyroid hormone that lowers blood calcium levels.
calcitonin
A mass of dividing, undifferentiated cells at the cut end of a shoot.
callus
An intracellular protein to which calcium binds in its function as a second messenger in hormone action.
calmodulin
The amount of heat energy required to raise the temperature of 1 g of water by 1°C; also the amount of heat energy that 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
calorie (cal)
The second of two major stages in photosynthesis (following the light reactions), involving atmospheric CO2 fixation and reduction of the fixed carbon into carbohydrate.
Calvin cycle
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
CAM plant
A burst of evolutionary origins when most of the major body plans of animals appeared in a relatively brief time in geologic history; recorded in the fossil record about 545 to 525 million years ago.
Cambrian explosion
A regulatory protein that directly stimulates gene expression.
cAMP receptor protein (CRP)
Minute canals in a bodily structure.
canaliculi
The uppermost layer of vegetation in a terrestrial biome.
canopy
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
A network of capillaries that infiltrate every organ and tissue in the body.
capillary bed
The protein shell that encloses a viral genome. It may be rod-shaped, polyhedral, or more complete in shape.
capsid
A sticky layer that surrounds the cell walls of some bacteria, protecting the cell surface and sometimes helping to glue the cell to surfaces.
capsule
A sugar (monosaccharide) or one of its dimers (disaccharides) or polymers (polysaccharides).
carbohydrate
The incorporation of carbon from CO2 into an organic compound by an autotrophic organism (a plant, another photosynthetic organism, or a chemoautotrophic bacterium).
carbon fixation
A functional group present in aldehydes and ketones and consisting of a carbon atom double-bonded to an oxygen atom.
carbonyl group
A functional group present in organic acids and consisting of a single carbon atom double-bonded to an oxygen atom and also bonded to a hydroxyl group.
carboxyl group
An organic compound containing a carboxyl group.
carboxylic acid
An enzyme found within the small intestine that splits off one amino acid at a time, beginning at the end of the polypeptide that has a free carboxyl group.
carboxypeptidase
A chemical agent that causes cancer.
carcinogen
The alternating contractions and relaxations of the heart.
cardiac cycle
A type of muscle that forms the contractile wall of the heart; its cells are joined by intercalated discs that relay each heartbeat.
cardiac muscle
The volume of blood pumped per minute by the left ventricle of the heart.
cardiac output
Diseases of the heart and blood vessels.
cardiovascular disease
A closed circulatory system with a heart and branching network of arteries, capillaries, and veins; the system is characteristic of vertebrates.
cardiovascular system
The group of birds with a carina, or sternal keel, supporting their large breast muscles.
carnites
An animal, such as a shark, hawk, or spider, that eats other animals.
carnivore
An accessory pigment, either yellow or orange, in the chloroplasts of plants. By absorbing wavelengths of light that chlorophyll cannot, carotenoids broaden the spectrum of colors that can drive photosynthesis.
carotenoid
The female reproductive organ of a flower, consisting of the stigma, style, and ovary.
carpel
In human genetics, an individual who is heterozygous at a given genetic locus, with one normal allele and one potentially harmful recessive allele. The heterozygote is phenotypically normal for the character determined by the gene but can pass on the harm
carrier
The maximum population size that can be supported by the available resources, symbolized as K.
carrying capacity
A type of flexible connective tissue with an abundance of collagenous fibers embedded in chondrin.
cartilage
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.
Casparian strip
A metabolic pathway that releases energy by breaking down complex molecules to simpler compounds.
catabolic pathway
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.
catabolite activator protein (CAP)
A chemical agent that changes the rate of a reaction without being consumed by the reaction.
catalyst
The hypothesis by Georges Cuvier that each boundary between strata corresponded in time to a catastrophe, such as a flood or drought, that had destroyed many of the species living there at that time.
catastrophism
A class of compounds, including epinephrine and norepinephrine, that are synthesized from the amino acid tyrosine.
catecholamines
An ion with a positive charge, produced by the loss of one or more electrons.
cation
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.
cation exchange
A T cell surface protein, present on most helper T cells, CD4 binds to part of the class II MHC protein.
CD4
A T cell surface protein that enhances the interaction between the antigen-presenting infected cell and a cytotoxic T cell.
CD8
A limited gene library using complementary DNA. The library includes only the genes that were transcribed in the cells examined.
cDNA library
A blind outpocket of a hollow organ such as an intestine.
cecum
The simplest collection of matter that can live.
cell
Glycoproteins that contribute to cell migration and stable tissue structure.
cell adhesion molecules
The part of a cell, such as a neuron, that houses the molecules.
cell body
A region in the cytoplasm near the nucleus from which microtubules originate and radiate.
cell center
An ordered sequence of events in the life of a eukaryotic cell, from its origin in the division of a parent cell until its own division into two; composed of the M, G1, S, and G2 phases.
cell cycle
A cyclically operating set of molecules in the cell that triggers and coordinates key events in the cell cycle.
cell cycle control system
The reproduction of cells.
cell division
The disruption of a cell and separation of its organelles by centrifugation.
cell fractionation
The ancestry of a cell.
cell lineage
A double membrane across the midline of a dividing plant cell, between which the new cell wall forms during cytokinesis.
cell plate
A protective layer external to the plasma membrane in plant cells, bacteria, fungi, and some protists. In plant cells, the wall is formed of cellulose fibers embedded in a polysaccharide-protein matrix. The primary cell wall is thin and flexible, whereas
cell wall
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-mediated immunity
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 differentiation
The most prevalent and efficient catabolic pathway for the production of ATP, in which oxygen is consumed as a reactant along with the organic fuel.
cellular respiration
A type of protist that has unicellular amoeboid cells and multicellular reproductive bodies in its life cycle.
cellular slime mold
A structural polysaccharide of cell walls, consisting of glucose monomers joined by b-1, 4-glycosidic linkages.
cellulose
A temperature scale (°C) equal to 5/9 (°F 232) that measures the freezing point of water at 0°C and the boiling point of water at 100°C.
Celsius scale
The narrow cavity in the center of the spinal cord that is continuous with the fluid-filled ventricles of the brain.
central canal
In vertebrate animals, the brain and spinal cord.
central nervous system (CNS)
A membranous sac in a mature plant cell with diverse roles in reproduction, growth, and development.
central vacuole
A structure in an animal cell composed of cylinders of microtubule triplets arranged in a 9 + 0 pattern. An animal cell usually has a pair of centrioles involved in cell division.
centriole
The centralized region joining two sister chromatids.
centromere
Material present in the cytoplasm of all eukaryotic cells, important during cell division; the microtubule-organizing center.
centrosome
An evolutionary trend toward the concentration of sensory equipment on the anterior end of the body.
cephalization
A chordate without a backbone, represented by lancelets, tiny marine animals.
cephalochordate
Part of the vertebrate hindbrain (rhombencephalon) located dorsally; functions in unconscious coordination of movement and balance.
cerebellum
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 cortex
The right or left side of the vertebrate brain.
cerebral hemisphere
Blood-derived fluid that surrounds, protects, against infection, nourishes, and cushions the brain and spinal cord.
cerebrospinal fluid
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.
cerebrum
The neck of the uterus, which opens into the vagina.
cervix
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.
chaparral
Protein molecules that assist the proper folding of other proteins.
chaperonin
A heritable feature.
character
The green algal group that shares two ultrastructural features with land plants. They are considered to be the closest relatives of land plants.
Charophyceans
A critical control point in the cell cycle where stop and go-ahead signals can regulate the cycle.
checkpoint
Clawlike feeding appendages characteristic of the chelicerate group.
chelicerae
The animal phylum that includes horseshoe crabs, scorpions, ticks, spiders, and an extinct group called the eurypterids.
Chelicerata
Members of the animal phylum that includes horseshoe crabs, scorpions, ticks, spiders, and an extinct group called the eurypterids.
chelicerates
An attraction between two atoms resulting from a sharing of outer-shell electrons or the presence of opposite charges on the atoms; the bonded atoms gain complete outer electron shells.
chemical bond
Energy stored in the chemical bonds of molecules; a form of potential energy.
chemical energy
In a reversible chemical reaction, the point at which the rate of the forward reaction equals the rate of the reverse reaction.
chemical equilibrium
A process leading to chemical changes in matter; involves the making and/or breaking of chemical bonds.
chemical reaction
Specialized ion channels that open or close in response to a chemical stimulus.
chemically-gated ion channels
An energy-coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP. Most ATP synthesis in cells occurs by chemiosmosis.
chemiosmosis
An organism that needs only carbon dioxide as a carbon source but that obtains energy by oxidizing inorganic substances.
chemoautotroph
An organism that must consume organic molecules for both energy and carbon.
chemoheterotroph
A group of about 50 different proteins secreted by blood vessel endothelial cells and monocytes. These molecules bind to receptors on many types of leukocytes and induce numerous changes central to inflammation.
chemokine
A receptor that transmits information about the total solute concentration in a solution or about individual kinds of molecules.
chemoreceptor
The X-shaped, microscopically visible region representing homologous chromatids that have exchanged genetic material through crossing over during meiosis.
chiasma
The animal class that includes centipedes.
Chilopoda
A structural polysaccharide of an amino sugar found in many fungi and in the exoskeletons of all arthropods.
chitin
A green pigment located within the chloroplasts of plants. Chlorophyll a can participate directly in the light reactions, which convert solar energy to chemical energy.
chlorophyll
A type of blue-green photosynthetic pigment that participates directly in the light reactions.
chlorophyll a
A type of yellow-green accessory photosynthetic pigment that transfers energy to chlorophyll a.
chlorophyll b
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.
chloroplast
A flagellated feeding cell found in sponges. Also called a collar cell, it has a collarlike ring that traps food particles around the base of its flagellum.
choanocyte
A hormone released from the walls of the duodenum in response to the presence of amino acids or fatty acids.
cholecystokinin (CCK)
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.
cholesterol
The vertebrate class of cartilaginous fishes, represented by sharks and their relatives.
Chondrichthyes
A protein-carbohydrate complex secreted by chondrocytes; chondrin and collagen fibers form cartilage.
chondrin
Cartilage cells.
chondrocytes
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 an embryo.
chordate
The outermost of four extraembryonic membranes; contributes to the formation of the mammalian placenta.
chorion
A technique for diagnosing genetic and congenital defects in a fetus by removing and analyzing a small sample of the fetal portion of the placenta.
chorionic villus sampling (CVS)
A thin, pigmented inner layer of the vertebrate eye.
choroid
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.
chromatin
In some classification systems, a kingdom consisting of brown algae, golden algae, and diatoms.
chromista
A threadlike, gene-carrying structure found in the nucleus. Each chromosome consists of one very long DNA molecule and associated proteins. See chromatin.
chromosome
A basic principle in biology stating that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.
chromosome theory of inheritance
A DNA mapping technique that begins with a gene or other sequence that has already been cloned, mapped, and sequenced and "walks" along the chromosomal DNA from that locus, producing a map of overlapping restriction fragments.
chromosome walking
Small intracellular globules composed of fats that are mixed with cholesterol and coated with special proteins.
chylomicron
An enzyme found in the duodenum. It is specific for peptide bonds adjacent to certain amino acids.
chymotrypsin
Mainly aquatic primitive fungi that form uniflagellated spores (zoospores). The chytrids and fungi are now thought to form a monophyletic branch of the eukaryotic tree.
chytrid
A type of protozoan that moves by means of cilia.
ciliate
A portion of the vertebrate eye associated with the lens. It produces the clear, watery aqueous humor that fills the anterior cavity of the eye.
ciliary body
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.
cilium
A physiological cycle of about 24 hours that is present in all eukaryotic organisms and that persists even in the absence of external cues.
circadian rhythm
Each evolutionary branch in a cladogram.
clade
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.
cladistics
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.
cladogenesis
A dichotomous phylogenetic tree that branches repeatedly, suggesting a classification of organisms based on the time sequence in which evolutionary branches arise.
cladogram
In classification, the taxonomic category above order.
class
The animal group that includes scorpions, spiders, ticks, and mites.
class Arachnida
The centipede group of animals.
class Chilopoda
The millipede group of animals.
class Diplopoda
A collection of cell surface glycoproteins encoded by a family of genes called the major histocompatibility complex. In humans, these glycoproteins are also known as the HLA, human leukocyte antigens. Class I MHC molecules are found on all nucleated cells
class I MHC molecules
A collection of cell surface glycoproteins encoded by a family of genes called the major histocompatibility complex. In humans, these glycoproteins are also known as the HLA, human leukocyte antigens. Class II MHC molecules are restricted to a few special
class II MHC molecules
A type of associative learning; the association of a normally irrelevant stimulus with a fixed behavioral response.
classical conditioning
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
The first sign of cleavage in an animal cell; a shallow groove in the cell surface near the old metaphase plate.
cleavage furrow
The prevailing weather conditions at a locality.
climate
Graded variation in some traits of individuals that parallels a gradient in the environment.
cline
An organ in the female that engorges with blood and becomes erect during sexual arousal.
clitoris
A common opening for the digestive, urinary, and reproductive tracts in all vertebrates except most mammals.
cloaca
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 eff
clonal selection
(1) A lineage of genetically identical individuals or cells. (2) In popular usage, a single individual organism that is genetically identical to another individual. (3) As a verb, to make one or more genetic replicas of an individual or cell. See also gen
clone
Using a somatic cell from a multicellular organism to make one or more genetically identical individuals.
cloning
An agent used to transfer DNA in genetic engineering. A plasmid that moves recombinant DNA from a test tube back into a cell is an example of a cloning vector, as is a virus that transfers recombinant DNA by infection.
cloning vector
Circulatory systems in which blood is confined to vessels and is kept separate from the interstitial fluid.
closed circulatory system
The common name for members of the phylum Basidiomycota. The name comes from the clublike shape of the basidium.
club fungus
Describing a dispersion pattern in which individuals are aggregate in patches.
clumped
A specialized cell for which the phylum Cnidaria is named; consists of a capsule containing a fine coiled thread, which, when discharged, functions in defense and prey capture.
cnidocyte
The complex, coiled organ of hearing that contains the organ of Corti.
cochlea
A phenotypic situation in which the two alleles affect the phenotype in separate, distinguishable ways.
codominance
A three-nucleotide sequence of DNA or mRNA that specifies a particular amino acid or termination signal; the basic unit of the genetic code.
codon
The probability that a particular gene present in one individual will also be inherited from a common parent or ancestor in a second individual.
coefficient of relatedness
A body cavity completely lined with mesoderm.
coelom
An animal whose body cavity is completely lined by mesoderm, the layers of which connect dorsally and ventrally to form mesenteries.
coelomate
Referring to a multinucleated condition resulting from the repeated division of nuclei without cytoplasmic division.
coenocytic
An organic molecule serving as a cofactor. Most vitamins function as coenzymes in important metabolic reactions.
coenzyme
The mutual influence on the evolution of two different species interacting with each other and reciprocally influencing each other's adaptations.
coevolution
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.
cofactor
The ability of an animal's nervous system to perceive, store, process, and use information obtained by its sensory receptors.
cognition
The scientific study of cognition; the study of the connection between data processing by nervous systems and animal behavior.
cognitive ethology
A representation within the nervous system of spatial relations among objects in an animal's environment.
cognitive map
The binding together of like molecules, often by hydrogen bonds.
cohesion
The idea that specific evolutionary adaptations and discrete complexes of genes define species.
cohesion species concept
A group of individuals of the same age, from birth until all are dead.
cohort
The insertion of a penis into a vagina, also called sexual intercourse.
coitus
The covering of the young shoot of the embryo of a grass seed.
coleoptile
The covering of the young root of the embryo of a grass seed.
coleorhiza
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.
collagen
Tough fibers of the extracellular matrix. They are made of collagen that are nonelastic and do not tear easily when pulled lengthwise.
collagenous fibers
The location in the kidney where filtrate from renal tubules is collected; the filtrate is now called urine.
collecting duct
A flexible plant cell type that occurs in strands or cylinders that support young parts of the plant without restraining growth.
collenchyma cell
Adhesive structures on the tentacles of ctenophores.
colloblasts
The tubular portion of the vertebrate alimentary tract between the small intestine and the anus; functions mainly in water absorption and the formation of feces.
colon
The column shape of one type of epithelial cell.
columnar
A symbiotic relationship in which the symbiont benefits but the host is neither helped nor harmed.
commensalism
All the organisms that inhabit a particular area; an assemblage of populations of different species living close enough together for potential interaction.
community
The study of how interactions between species affect community structure and organization.
community ecology
A type of plant cell that is connected to a sieve-tube cell by many plasmodesmata and whose nucleus and ribosomes may serve one or more adjacent sieve-tube cells.
companion cell
Active demand by two or more organisms or kinds of organisms for some environmental resource in short supply.
competition
The concept that when populations of two similar species compete for the same limited resources, one population will use the resources more efficiently and have a reproductive advantage that will eventually lead to the elimination of the other population.
competitive exclusion principle
A substance that reduces the activity of an enzyme by entering the active site in place of the substrate whose structure it mimics.
competitive inhibitor
A set of about 20 serum proteins that carry out a cascade of steps leading to the lysis of microbes.
complement
An immune response in which antigen-antibody complexes activate complement proteins.
complement fixation
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 microorgan
complement system
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.
complementary DNA (cDNA)
A digestive tube that runs between a mouth and an anus; also called alimentary canal. An incomplete digestive tract has only one opening.
complete digestive tract
A type of inheritance in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.
complete dominance
A flower that has sepals, petals, stamens, and carpels.
complete flower
The transformation of a larva into an adult that looks very different, and often functions very differently in its environment, than the larva.
complete metamorphosis
A substance consisting of two or more elements in a fixed ratio.
compound
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 eye
A small molecule that cooperates with a repressor protein to switch an operon off.
compressor
An increase or decrease in the density of a chemical substance in an area. Cells often maintain concentration gradients of ions across their membranes. When a gradient exists, the ions or other chemical substances involved tend to move from where they are
concentration gradient
The fertilization of the egg by a sperm cell.
conception
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.
condensation reaction
The direct transfer of thermal motion (heat) between molecules of objects in direct contact with each other.
conduction
One of two types of photoreceptors in the vertebrate eye; detects color during the day.
cone cell
A characterization of an animal in regard to environmental variables. The animal is a conformer if it allows some conditions within its body to vary with certain external changes.
conformer
A naked, asexual spore produced at the ends of hyphae in ascomycetes.
conidium
A gymnosperm whose reproductive structure is the cone. Conifers include pines, firs, redwoods, and other large trees.
conifer
The largest of the four gymnosperm phyla, the reproductive structure is the cone. Conifers include pines, firs, redwoods, and other large trees.
Coniferophyta
In bacteria, the direct transfer of DNA between two cells that are temporarily joined.
conjugation
A mucous membrane that helps keep the eye moist; lines the inner surface of the eyelid and covers the front of the eyeball, except the cornea.
conjunctiva
Animal tissue that functions mainly to bind and support other tissues, having a sparse population of cells scattered through an extracellular matrix.
connective tissue
The group of ancient vertebrates that date back as far as 510 million years.
conodonts
A goal-oriented science that seeks to counter the biodiversity crisis, the current rapid decrease in Earth's variety of life.
conservation biology
Organisms that obtain their food by eating plants or by eating animals that have eaten plants.
consumers
The prevention of pregnancy.
contraception
A membranous sac that helps move excess water out of the cell.
contractile vacuole
Segments of noncoding DNA that help regulate transcription of a gene by binding proteins called transcription factors.
control elements
The mass movement of warmed air or liquid to or from the surface of a body or object.
convection
The independent development of similarity between species as a result of their having similar ecological roles and selection pressures.
convergent evolution
A mechanism of cell crawling in which the cells of a tissue layer rearrange themselves in such a way that the sheet of cells becomes narrower while it becomes longer.
convergent extension
An interaction of the constituent subunits of a protein whereby a conformational change in one subunit is transmitted to all the others.
cooperativity
A group of small crustaceans that are important members of marine and freshwater plankton communities.
copepods
Warm water, tropical, ecosystems dominated by the hard skeletal structures secreted primarily by the resident cnidarians.
coral reefs
A cylinder of meristematic tissue in plants that produces cork cells to replace the epidermis during secondary growth.
cork cambium
The transparent frontal portion of the sclera, which admits light into the vertebrate eye.
cornea
The thick band of nerve fibers that connect the right and left cerebral hemispheres in placental mammals, enabling the hemispheres to process information together.
corpus callosum
A secreting tissue in the ovary that forms from the collapsed follicle after ovulation and produces progesterone.
corpus luteum
The region of the root between the stele and epidermis filled with ground tissue.
cortex
Vesicles that begin just under the egg plasma membrane prior to their involvement in the cortical reaction.
cortical granules
Nephrons located almost entirely in the renal cortex. These nephrons have a reduced loop of Henle.
cortical nephrons
A series of changes in the cortex of the egg cytoplasm during fertilization.
cortical reaction
A family of steroids synthesized by and released from the adrenal cortex.
corticosteroids
The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.
cotransport
The one (monocot) or two (dicot) seed leaves of an angiosperm embryo.
cotyledons
The opposite flow of adjacent fluids that maximizes transfer rates; for example, blood in the gills flows in the opposite direction in which water passes over the gills, maximizing oxygen uptake and carbon dioxide loss.
countercurrent exchange
A special arrangement of blood vessels that helps trap heat in the body core and is important in reducing heat loss in many endotherms.
countercurrent heat exchanger
Behavior patterns that lead up to copulation or gamete release.
courtship
A type of strong chemical bond in which two atoms share one pair of valence electrons.
covalent bond
Nerves that leave the brain and innervate organs of the head and upper body.
cranial nerves
The chordate subgroup that possess a cranium.
craniata
A type of metabolism in which carbon dioxide is taken in at night and incorporated into a variety of organic acids.
crassulacean acid metabolism (CAM)
An infolding of the inner membrane of a mitochondrion that houses the electron transport chain and the enzyme catalyzing the synthesis of ATP.
crista
The reptile group that includes crocodiles and alligators.
crocodilia
The reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I.
crossing over
The transfer of pollen from flowers of one plant to flowers of another plant of the same species.
cross-pollination
The animal phylum that includes mostly aquatic animals such as crabs, lobsters, crayfish, and shrimp.
Crustacea
A member of a major arthropod phylum that includes lobsters, crayfish, crabs, shrimps, and barnacles.
crustacean
Camouflage, making potential prey difficult to spot against its background.
cryptic coloration
The cubic shape of a type of epithelial cell.
cuboidal
(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 funct
cuticle
Photosynthetic, oxygen-producing bacteria (formerly known as blue-green algae).
cyanobacteria
A phylum of gymnosperms that superficially resemble palms. Cycads bear naked seeds on sporophylls, leaves specialized for reproduction.
Cycadophyta
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 AMP (cAMP)
A route of electron flow during the light reactions of photosynthesis that involves only photosystem I and that produces ATP but not NADPH or oxygen.
cyclic electron flow
The generation of ATP by cyclic electron flow.
cyclic photophosphorylation
A regulatory protein whose concentration fluctuates cyclically.
cyclin
A protein kinase that is active only when attached to a particular cyclin.
cyclin-dependent kinase (Cdk)
A genetic disorder that occurs in people with two copies of a certain recessive allele; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated.
cystic fibrosis
An iron-containing protein, a component of electron transport chains in mitochondria and chloroplasts.
cytochrome
In the vertebrate immune system, protein factors secreted by macrophages and helper T cells as regulators of neighboring cells.
cytokines
The division of the cytoplasm to form two separate daughter cells immediately after mitosis.
cytokinesis
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.
cytokinins
Charts of chromosomes that locate genes with respect to chromosomal features.
cytological maps
The entire contents of the cell, exclusive of the nucleus, and bounded by the plasma membrane.
cytoplasm
The maternal substances in the egg that influences the course of early development by regulating the expression of genes that affect the developmental fate of cells.
cytoplasmic determinants
A circular flow of cytoplasm, involving myosin and actin filaments, that speeds the distribution of materials within cells.
cytoplasmic streaming
A network of microtubules, microfilaments, and intermediate filaments that branch throughout the cytoplasm and serve a variety of mechanical and transport functions.
cytoskeleton
The semifluid portion of the cytoplasm.
cytosol
A type of lymphocyte that kills infected cells and cancer cells.
cytotoxic T cell (TC)
A daily decrease in metabolic activity and corresponding body temperature during times of inactivity for some small mammals and birds. The physiological changes during resting periods enable these organisms to survive on energy stores in their tissues.
daily torpor
A measure of mass for atoms and subatomic particles.
dalton
The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.
Darwinian fitness
Either of the two cells that result when a cell divides.
daughter cells
A plant whose flowering is not affected by photoperiod.
day-neutral plant
A relatively large group of crustaceans that includes lobsters, crayfish, crabs, and shrimp.
decapod
Any of the 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.
decomposers
The breakdown of organic materials into inorganic ones.
decomposition
An international initiative focusing on the deepest phylogenetic branching within the plant kingdom to identify and name the major plant clades.
deep green
A dark, hot, oxygen-deficient environment associated with volcanic activity. The food producers are chemoautotrophic prokaryotes.
deep-sea hydrothermal vents
A chemical reaction in which two molecules covalently bond to each other with the removal of a water molecule.
dehydration reaction
(1) A deficiency in a chromosome resulting from the loss of a fragment through breakage. (2) A mutational loss of one or more nucleotide pairs from a gene.
deletion
A shift from zero population growth in which birth rates and death rates are high to zero population growth characterized instead by low birth and death rates.
demographic transition
The study of statistics relating to births and deaths in populations.
demography
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 concent
denaturation
One of usually numerous, short, highly branched processes of a neuron that conveys nerve impulses toward the cell body.
dendrite
The number of individuals per unit area or volume.
density
Any characteristic that varies according to an increase in population density.
density dependent
Any factor that has a greater impact on a population as the population density increases.
density-dependent factor
The phenomenon observed in normal animal cells that causes them to stop dividing when they come into contact with one another.
density-dependent inhibition
Any factor that affects a population by the same percentage, regardless of density.
density-independent factor
A double-stranded, helical nucleic acid molecule capable of replicating and determining the inherited structure of a cell's proteins.
deoxyribonucleic acid (DNA)
The sugar component of DNA, having one less hydroxyl group than ribose, the sugar component of RNA.
deoxyribose
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 o
depolarization
The condition of a membrane that is more negatively charged on one side than on the other.
depolarized
A heterotroph, such as an earthworm, that eats its way through detritus, salvaging bits and pieces of decaying organic matter.
deposit-feeder
The protective covering of plants; generally a single layer of tightly packed epidermal cells covering young plant organs formed by primary growth.
dermal tissue system
Darwin's initial phrase for the general process of evolution.
descent with modification
A type of intercellular junction in animal cells that functions as an anchor.
desmosome
A type of embryonic development in protostomes that rigidly casts the developmental fate of each embryonic cell very early.
determinate cleavage
A type of growth characteristic of animals, in which the organism stops growing after it reaches a certain size.
determinate growth
The progressive restriction of developmental potential, causing the possible fate of each cell to become more limited as the embryo develops.
determination
A consumer that derives its energy from nonliving organic material.
detritivore
Dead organic matter.
detritus
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.
deuterostomes
The sum of all of the changes that progressively elaborate an organism's body.
development
A second messenger produced by the cleavage of a certain kind of phospholipid in the plasma membrane.
diacylglycerol (DAG)
(1) A sheet of muscle that forms the bottom wall of the thoracic cavity in mammals; active in ventilating the lungs. (2) A dome-shaped rubber cup fitted into the upper portion of the vagina before sexual intercourse. It serves as a physical barrier to blo
diaphragm
One of three groups of amniotes based on key differences between their skulls.
diapsids
The stage of the heart cycle in which the heart muscle is relaxed, allowing the chambers to fill with blood.
diastole
Blood pressure that remains between heart contractions.
diastolic pressure
A unicellular photosynthetic alga with a unique, glassy cell wall containing silica.
diatom
A subdivision of flowering plants whose members possess two embryonic seed leaves, or cotyledons.
dicot
The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area.
diffusion
The process of breaking down food into molecules small enough for the body to absorb.
digestion
An organism that is heterozygous with respect to two genes of interest. A dihybrid results from a cross between parents doubly homozygous for different alleles. For example, parents of genotype AABB and aabb produce a dihybrid of genotype AaBb.
dihybrid
A mycelium of certain septate fungi that possesses two separate haploid nuclei per cell.
dikaryon
A mycelium with two haploid nuclei per cell, one from each parent.
dikaryotic
A unicellular photosynthetic alga with two flagella situated in perpendicular grooves in cellulose plates covering the cell.
dinoflagellate
An extremely diverse group of ancient reptiles varying in body shape, size, and habitat.
dinosaurs
Referring to a plant species that has staminate and carpellate flowers on separate plants.
dioecious
An enzyme found attached to the intestinal lining. It splits small peptides.
dipeptidase
Having two germ layers.
diploblastic
A cell containing two sets of chromosomes (2n), one set inherited from each parent.
diploid cell
The animal class that includes millipedes.
Diplopoda
The class of lungfishes.
Dipnoi
Natural selection that favors individuals at one end of the phenotypic range.
directional selection
A double sugar, consisting of two monosaccharides joined by dehydration synthesis.
disaccharide
The distribution of individuals within geographic population boundaries.
dispersal
The pattern of spacing among individuals within geographic population boundaries.
dispersion
A chart showing the relative amounts of oxygen bound to hemoglobin when the pigment is exposed to solutions varying in their partial pressure of dissolved oxygen.
dissociation curve
In the vertebrate kidney, the portion of a nephron that helps refine filtrate and empties it into a collecting duct.
distal tubule
A force that changes a biological community and usually removes organisms from it. Disturbances, such as fire and storms, play pivotal roles in structuring many biological communities.
disturbance
Strong covalent bonds formed when the sulfur of one cysteine monomer bonds to the sulfur of another cysteine monomer.
disulfide bridge
Natural selection that favors extreme over intermediate phenotypes.
diversifying selection
Variety in organisms.
diversity
An individual's unique collection of DNA restriction fragments, detected by electrophoresis and nucleic acid probes.
DNA fingerprint
A linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 39 end of a new DNA fragment to the 59 end of a growing chain.
DNA ligase
The addition of methyl groups (—CH3) to bases of DNA after DNA synthesis; may serve as a long-term control of gene expression.
DNA methylation
A method to detect and measure the expression of thousands of genes at one time. Tiny amounts of a large number of single-stranded DNA fragments representing different genes are fixed to a glass slide. These fragments, ideally representing all the genes o
DNA microarray assays
An enzyme that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain.
DNA polymerase
A chemically synthesized, radioactively labeled segment of nucleic acid used to find a gene of interest by hydrogen-bonding to a complementary sequence.
DNA probe
A part of the three-dimensional structure of a transcription factor that binds to DNA.
DNA-binding domain
A taxonomic category above the kingdom level. The three domains are Archaea, Bacteria, and Eukarya.
domain
A linear "pecking order" of animals, where position dictates characteristic social behaviors.
dominance hierarchy
In a heterozygote, the allele that is fully expressed in the phenotype.
dominant allele
The phenotype that results from having at least one dominant allele.
dominant phenotype
Those species in a community that have the highest abundance or highest biomass. These species exert a powerful control over the occurrence and distribution of other species.
dominant species
A biogenic amine closely related to epinephrine and norepinephrine.
dopamine
A condition typified by extremely low metabolic rate and a suspension of growth and development.
dormancy
Pertaining to the back of a bilaterally symmetrical animal.
dorsal
The dorsal side of the blastopore.
dorsal lip
A circulation scheme with separate pulmonary and systemic circuits, which ensures vigorous blood flow to all organs.
double circulation
A type of covalent bond in which two atoms share two pairs of electrons; symbolized by a pair of lines between the bonded atoms.
double covalent bond
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 fertilization
The form of native DNA, referring to its two adjacent polynucleotide strands wound into a spiral shape.
double helix
A human genetic disease resulting from having an extra chromosome 21, characterized by mental retardation and heart and respiratory defects.
Down syndrome
Members of the group Oomycota, they are heterotrophic stramenopiles that lack chloroplasts, typically have cell walls made of cellulose, and generally live on land as parasites of plants.
downy mildews
Fruit fly.
Drosophila
A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue.
Duchenne muscular dystrophy
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.
duodenum
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.
duplication
A large contractile protein forming the side-arms of microtubule doublets in cilia and flagella.
dynein
One of three binding sites for tRNA during translation, it is the place where discharged tRNAs leave the ribosome; E stands for exit site.
E site
A steroid hormone that triggers molting in arthropods.
ecdysone
One of two distinct clades within the protostomes. It includes the arthropods.
ecdysozoa
Sessile or slow-moving animals that include sea stars, sea urchins, brittle stars, crinoids, and basket stars.
echinoderms
The ratio of net productivity at one trophic level to net productivity at the next lower level.
ecological efficiency
A method to use multiple constraints to estimate the human carrying capacity of Earth by calculating the aggregate land and water area in various ecosystem categories that is appropriated by a nation to produce all the resources it consumes and to absorb
ecological footprint
The sum total of a species' use of the biotic and abiotic resources of its environment.
ecological niche
The idea that ecological roles (niches) define species.
ecological species concept
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.
ecological succession
The study of how organisms interact with their environments.
ecology
All the organisms in a given area as well as the abiotic factors with which they interact; a community and its physical environment.
ecosystem
The study of energy flow and the cycling of chemicals among the various biotic and abiotic factors in an ecosystem.
ecosystem ecology
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.
ectoderm
A type of mycorrhizae in which the mycelium forms a dense sheath, or mantle, over the surface of the root. Hyphae extend from the mantle into the soil, greatly increasing the surface area for water and mineral absorption.
ectomycorrhizae
Parasites that feed on the external surface of a host.
ectoparasites
An animal, such as a reptile, fish, or amphibian, that must use environmental energy and behavioral adaptations to regulate its body temperature.
ectotherm
Organisms that do not produce enough metabolic heat to have much effect on body temperature.
ectothermic
The last period of the Precambrian era.
Ediacaran period
A muscle cell or gland cell that performs the body's responses to stimuli; responds to signals from the brain or other processing center of the nervous system.
effector cell
The blood vessel draining a nephron.
efferent arteriole
Another name for maternal effect genes, these genes control the orientation (polarity) of the egg.
egg-polarity genes
The short section of the ejaculatory route in mammals formed by the convergence of the vas deferens and a duct from the seminal vesicle. The ejaculatory duct transports sperm from the vas deferens to the urethra.
ejaculatory duct
Long threads made of the protein elastin. Elastic fibers provide a rubbery quality to the extracellular matrix that complements the nonelastic strength of collagenous fibers.
elastic fibers
A record of the electrical impulses that travel through cardiac muscle during the heart cycle.
electrocardiogram (ECG or EKG)
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.
electrochemical gradient
A medical test that measures different patterns in the electrical activity of the brain.
electroencephalogram (EEG)
An ion transport protein generating voltage across the membrane.
electrogenic pump
Receptors of electromagnetic energy, such as visible light, electricity, and magnetism.
electromagnetic receptor
The entire spectrum of radiation ranging in wavelength from less than a nanometer to more than a kilometer.
electromagnetic spectrum
A subatomic particle with a single negative charge; one or more electrons move around the nucleus of an atom.
electron
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.
electron microscope (EM)
An energy level representing the distance of an electron from the nucleus of an atom.
electron shell
A sequence of electron carrier molecules (membrane proteins) that shuttle electrons during the redox reactions that release energy used to make ATP.
electron transport chain
The attraction of an atom for the electrons of a covalent bond.
electronegativity
A technique to introduce recombinant DNA into cells by applying a brief electrical pulse to a solution containing cells. The electricity creates temporary holes in the cells' plasma membranes, through which DNA can enter.
electroporation
Any substance that cannot be broken down to any other substance.
element
The passing of undigested material out of the digestive compartment.
elimination
New developing individuals.
embryo
The female gametophyte of angiosperms, formed from the growth and division of the megaspore into a multicellular structure with eight haploid nuclei.
embryo sac
Mutations with phenotypes leading to death at the embryo or larval stage.
embryonic lethals
Another name for land plants, recognizing that land plants share the common derived trait of multicellular, dependent embryos.
embryophyte
The process that keeps tiny fat droplets from coalescing.
emulsification
Molecules that are mirror images of each other.
enantiomer
A species that is in danger of extinction throughout all or a significant portion of its range.
endangered species
Species that are confined to a specific, relatively small geographic area.
endemic species
A nonspontaneous chemical reaction in which free energy is absorbed from the surroundings.
endergonic reaction
A ductless gland that secretes hormones directly into the bloodstream.
endocrine gland
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 regula
endocrine system
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.
endocytosis
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.
endoderm
The innermost layer of the cortex in plant roots; a cylinder one cell thick that forms the boundary between the cortex and the stele.
endodermis
The collection of membranes inside and around a eukaryotic cell, related either through direct physical contact or by the transfer of membranous vesicles.
endomembrane system
The inner lining of the uterus, which is richly supplied with blood vessels.
endometrium
A type of mycorrhizae that, unlike ectomycorrhizae, do not have a dense mantle ensheathing the root. Instead, microscopic fungal hyphae extend from the root into the soil.
endomycorrhizae
Parasites that live within a host.
endoparasites
An extensive membranous network in eukaryotic cells, continuous with the outer nuclear membrane and composed of ribosome-studded (rough) and ribosome-free (smooth) regions.
endoplasmic reticulum (ER)
A hormone produced in the brain and anterior pituitary that inhibits pain perception.
endorphin
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.
endoskeleton
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.
endosperm
A thick-coated, resistant cell produced within a bacterial cell exposed to harsh conditions.
endospore
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.
endosymbiotic theory
The innermost, simple squamous layer of cells lining the blood vessels; the only constituent structure of capillaries.
endothelium
An animal that uses metabolic energy to maintain a constant body temperature, such as a bird or mammal.
endotherm
Organisms with bodies that are warmed by heat generated by metabolism. This heat is usually used to maintain a relatively stable body temperature higher than that of the external environment.
endothermic
A component of the outer membranes of certain gram-negative bacteria responsible for generalized symptoms of fever and ache.
endotoxin
The concept that the length of a food chain is limited by the inefficiency of energy transfer along the chain.
energetic hypothesis
The capacity to do work (to move matter against an opposing force).
energy
In cellular metabolism, the use of energy released from an exergonic reaction to drive an endergonic reaction.
energy coupling
The different states of potential energy for electrons in an atom.
energy level
A DNA sequence that recognizes certain transcription factors that can stimulate transcription of nearby genes.
enhancer
The type of development found in deuterostomes. The coelomic cavities form when mesoderm buds from the wall of the archenteron and hollows out.
enterocoelous
A category of hormones secreted by the wall of the duodenum.
enterogastrones
An intestinal enzyme that directly or indirectly triggers activation of other enzymes within the intestinal lumen.
enteropeptidase
The study of insects.
entomology
A quantitative measure of disorder or randomness, symbolized by S.
entropy
An ecological term for the effect of spatial variation, or patchiness, relative to the size and behavior of an organism.
environmental grain
A protein serving as a catalyst, a chemical agent that changes the rate of a reaction without being consumed by the reaction.
enzyme
The embryonic axis above the point at which the cotyledons are attached.
epicotyl
(1) The dermal tissue system in plants. (2) The outer covering of animals.
epidermis
A coiled tubule located adjacent to the testes where sperm are stored.
epididymis
The progressive development of form in an embryo.
epigenesis
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.
epiglottis
A hormone produced as a response to stress; also called adrenaline.
epinephrine
A plant that nourishes itself but grows on the surface of another plant for support, usually on the branches or trunks of tropical trees.
epiphyte
A genetic element that can exist either as a plasmid or as part of the bacterial chromosome.
episome
A phenomenon in which one gene alters the expression of another gene that is independently inherited.
epistasis
A brain region, derived from the diencephalon, that contains several clusters of capillaries that produce cerebrospinal fluid.
epithalamus
Sheets of tightly packed cells that line organs and body cavities.
epithelial tissue
A localized region on the surface of an antigen that is chemically recognized by antibodies; also called antigenic determinant.
epitope
A red blood cell; contains hemoglobin, which functions in transporting oxygen in the circulatory system.
erythrocyte
A hormone produced in the kidney when tissues of the body do not receive enough oxygen. This hormone stimulates the production of erythrocytes.
erythropoietin
A channel that conducts food, by peristalsis, from the pharynx to the stomach.
esophagus
The amino acids that an animal cannot synthesize itself and must obtain from food. Eight amino acids are essential in the human adult.
essential amino acids
Certain unsaturated fatty acids that animals cannot make.
essential fatty acids
A chemical element that is required for a plant to grow from a seed and complete the life cycle, producing another generation of seeds.
essential nutrient
A physiological state characterized by slow metabolism and inactivity, which permits survival during long periods of elevated temperature and diminished water supplies.
estivation
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 maintenan
estrogens
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.
estrous cycle
A period of sexual activity associated with ovulation.
estrus
The area where a freshwater stream or river merges with the ocean.
estuary
The study of animal behavior in natural conditions.
ethology
The only gaseous plant hormone, responsible for fruit ripening, growth inhibition, leaf abscission, and aging.
ethylene
The more open, unraveled form of eukaryotic chromatin that is available for transcription.
euchromatin
A large subgroup of traditionally dicot angiosperms including roses, peas, buttercups, sunflowers, oaks, and maples.
eudicots
A group of protistans, including Euglena and its relatives, characterized by an anterior pocket or chamber from which one or two flagella emerge.
euglenoid
A type of cell with a membrane-enclosed nucleus and membrane-enclosed organelles, present in protists, plants, fungi, and animals; also called eukaryote.
eukaryotic cell
Members of the subkingdom that includes all animals except sponges.
eumetazoa
Organisms that can tolerate substantial changes in external osmolarity.
euryhaline
Mainly marine and freshwater, extinct, chelicerates. These predators, also called water scorpions, ranged up to 3 meters long.
eurypterids
The tube that connects the middle ear to the pharynx.
eustachian tube
Placental mammals; those whose young complete their embryonic development within the uterus, joined to the mother by the placenta.
eutherian mammals
Pertaining to a highly productive lake, having a high rate of biological productivity supported by a high rate of nutrient cycling.
eutrophic
The removal of heat energy from the surface of a liquid that is losing some of its molecules.
evaporation
The property of a liquid whereby the surface becomes cooler during evaporation, owing to a loss of highly kinetic molecules to the gaseous state.
evaporative cooling
All the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today.
evolution
The idea that evolutionary lineages and ecological roles can form the basis of species identification.
evolutionary species concept
A structure that evolves and functions in one environmental context but that can perform additional functions when placed in some new environment.
exaptation
Cells that have the ability to generate changes in their membrane potentials.
excitable cells
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
excitatory postsynaptic potential (EPSP)
The disposal of nitrogen-containing waste products of metabolism.
excretion
A spontaneous chemical reaction in which there is a net release of free energy.
exergonic reaction
The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.
exocytosis
Powerful hydrolytic enzymes secreted by a fungus outside its body to digest food.
exoenzymes
A coding region of a eukaryotic gene. Exons, which are expressed, are separated from each other by introns.
exon
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.
exoskeleton
A toxic protein secreted by a bacterial cell that produces specific symptoms even in the absence of the bacterium.
exotoxin
The geometric increase of a population as it grows in an ideal, unlimited environment.
exponential population growth
A cloning vector that contains the requisite prokaryotic promoter just upstream of a restriction site where a eukaryotic gene can be inserted.
expression vector
The fusion of gametes that parents have discharged into the environment.
external fertilization
Sensory receptors that detect stimuli outside the body, such as heat, light, pressure, and chemicals.
exteroreceptor
The breakdown of food outside cells.
extracellular digestion
The substance in which animal tissue cells are embedded consisting of protein and polysaccharides.
extracellular matrix (ECM)
Four membranes (yolk sac, amnion, chorion, allantois) that support the developing embryo in reptiles, birds, and mammals.
extraembryonic membranes
Microorganisms that live in unusually highly saline environments such as the Great Salt Lake or the Dead Sea.
extreme halophile
Microorganisms that thrive in hot environments (often 60–80 degrees C).
extreme thermophiles
Microorganisms that live in extreme environments. They are further classified as either methanogens, extreme halophiles, or extreme thermophiles.
extremeophile
The 5' end of a pre-mRNA molecule modified by the addition of a cap of guanine nucleotide.
5' cap
A fertility factor in bacteria, a DNA segment that confers the ability to form pili for conjugation and associated functions required for the transfer of DNA from donor to recipient. It may exist as a plasmid or integrated into the bacterial chromosome.
F factor
The first filial, or hybrid, offspring in a genetic cross-fertilization.
F1 generation
The plasmid form of an F factor.
F plasmid
Offspring resulting from interbreeding of the hybrid F1 generation.
F2 generation
The positive effect of early species on the appearance of later species in ecological succession.
facilitate
The spontaneous passage of molecules and ions, bound to specific carrier proteins, across a biological membrane down their concentration gradients.
facilitated diffusion
An organism that makes ATP by aerobic respiration if oxygen is present but that switches to fermentation under anaerobic conditions.
facultative anaerobe
In classification, the taxonomic category above genus.
family
The depolarization of the egg membrane within 1–3 seconds after sperm binding to the vitelline layer. The reaction prevents additional sperm from fusing with the egg's plasma membrane.
fast block to polyspermy
Muscle cells used for rapid, powerful contractions.
fast muscle fibers
A biological compound consisting of three fatty acids linked to one glycerol molecule.
fat (triacylglycerol)
Territorial diagrams of embryonic development that reveal the future development of individual cells and tissues.
fate maps
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.
fatty acid
Light epidermal outgrowths that form the external covering of the body of birds.
feathers
The wastes of the digestive tract.
feces
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 inhibition
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.
fermentation
The union of haploid gametes to produce a diploid zygote.
fertilization
The swelling of the vitelline layer away from the plasma membrane.
fertilization envelope
A developing human from the ninth week of gestation until birth; has all the major structures of an atult.
fetus
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.
fiber
The activated form of the blood-clotting protein fibrinogen, which aggregates into threads that form the fabric of the clot.
fibrin
The inactive form of the plasma protein that is converted to the active form fibrin, which aggregates into threads that form the framework of a blood clot.
fibrinogen
A type of cell in loose connective tissue that secretes the protein ingredients of the extracellular fibers.
fibroblast
A glycoprotein that helps cells attach to the extracellular matrix.
fibronectin
A dense tissue with large numbers of collagenous fibers organized into parallel bundles. This is the dominant tissue in tendons and ligaments.
fibrous connective tissue
Root systems common to monocots consisting of a mat of thin roots that spread out below the soil surface.
fibrous root systems
The stalk of a stamen.
filament
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.
filtrate
In the vertebrate kidney, the extraction of water and small solutes, including metabolic wastes, from the blood by the nephrons.
filtration
The principle of conservation of energy. Energy can be transferred and transformed, but it cannot be created or destroyed.
first law of thermodynamics
A sequence of behavioral acts that is essentially unchangeable and usually carried to completion once initiated.
fixed action pattern (FAP)
Limp. Walled cells are flaccid in isotonic surroundings, where there is no tendency for water to enter.
flaccid
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.
flagellum
In an angiosperm, a short stem with four sets of modified leaves, bearing structures that function in sexual reproduction.
flower
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.
fluid mosaic model
An animal that lives by sucking nutrient-rich fluids from another living organism.
fluid-feeder
A microscopic structure in the ovary that contains the developing ovum and secretes estrogens.
follicle
A protein hormone secreted by the anterior pituitary that stimulates the production of eggs by the ovaries and sperm by the testes.
follicle-stimulating hormone (FSH)
That portion of the ovarian cycle during which several follicles in the ovary begin to grow.
follicular phase
The pathway along which food is transferred from trophic level to trophic level, beginning with producers.
food chain
A membranous sac formed by phagocytosis.
food vacuole
The elaborate, interconnected feeding relationships in an ecosystem.
food web
The portion of a moss sporophyte that gathers sugars, amino acids, water, and minerals from the parent gametophyte via transfer cells.
foot
Behavior necessary to recognize, search for, capture, and consume food.
foraging
A marine protozoan that secretes a shell and extends pseudopodia through pores in its shell.
foram
One of three ancestral and embryonic regions of the vertebrate brain; develops into the thalamus, hypothalamus, and cerebrum.
forebrain
A preserved remnant or impression of an organism that lived in the past.
fossil
Energy deposits formed from the remains of extinct organisms; fossil fuels include coal, oil, and natural gas.
fossil fuels
The chronicle of evolution over millions of years of geologic time engraved in the order in which fossils appear in rock strata.
fossil record
Genetic drift attributable to colonization by a limited number of individuals from a parent population.
founder effect
An eye's center of focus and the place on the retina where photoreceptors are highly concentrated.
fovea
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.
fragile X syndrome
A means of asexual reproduction whereby a single parent breaks into parts that regenerate into whole new individuals.
fragmentation
A mutation occurring when the number of nucleotides inserted or deleted is not a multiple of three, resulting in the improper grouping of the following nucleotides into codons.
frameshift mutation
The portion of a system's energy that can perform work when temperature is uniform throughout the system.
free energy
The initial investment of energy necessary to start a chemical reaction; also called activation energy.
free energy of activation
frequency-dependent selection
A mature ovary of a flower that protects dormant seeds and aids in their dispersal.
fruit
A specific configuration of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical reactions.
functional group
The kingdom that contains the fungi.
Fungi
A heterotrophic eukaryote that digests its food externally and absorbs the resulting small nutrient molecules. Most fungi consist of a netlike mass of filaments called hyphae. Molds, mushrooms, and yeasts are examples of fungi.
fungus
The cambium cells within the vascular bundles. The name refers to the tapered ends of these elongated cells.
fusiform initials
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 i
G protein
A nondividing state in which a cell has left the cell cycle.
G0phase
The first growth phase of the cell cycle, consisting of the portion of interphase before DNA synthesis begins.
G1 phase
The second growth phase of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs.
G2 phase
An organ that stores bile and releases it as needed into the small intestine.
gallbladder
The reproductive organ of bryophytes, consisting of the male antheridium and female archegonium; a multichambered jacket of sterile cells in which gametes are formed.
gametangium
A haploid cell such as an egg or sperm. Gametes unite during sexual reproduction to produce a diploid zygote.
gamete
The mature gamete-producing structure of a gametophyte body of a moss.
gametophore
The multicellular haploid form in organisms undergoing alternation of generations that mitotically produces haploid gametes that unite and grow into the sporophyte generation.
gametophyte
An amino acid that functions as a CNS neurotransmitter.
gamma aminobutyric acid (GABA)
A cluster (functional group) of nerve cell bodies in a centralized nervous system.
ganglion
Mutations in these genes cause "gaps" in Drosophila segmentation. The normal gene products map out the basic subdivisions along the anterior-posterior axis of the embryo.
gap genes
A type of intercellular junction in animal cells that allows the passage of material or current between cells; also known as a communicating junction.
gap junction
The uptake of molecular oxygen from the environment and the discharge of carbon dioxide to the environment.
gas exchange
The collection of fluids secreted by the epithelium lining the stomach.
gastric juice
A digestive hormone, secreted by the stomach, that stimulates the secretion of gastric juice.
gastrin
An extensive pouch that serves as the site of extracellular digestion and a passageway to disperse materials throughout most of an animal's body.
gastrovascular cavity
The two-layered, cup-shaped embryonic stage.
gastrula
The formation of a gastrula from a blastula.
gastrulation
A protein channel in a cell membrane that opens or closes in response to a particular stimulus.
gated channel
A gated channel for a specific ion. By opening and closing such channels, a cell alters its membrane potential.
gated ion channel
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.
gel electrophoresis
A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).
gene
The selective synthesis of DNA, which results in multiple copies of a single gene, thereby enhancing expression.
gene amplification
The production of multiple copies of a gene.
gene cloning
The loss or gain of alleles in a population due to the migration of fertile individuals or gametes between populations.
gene flow
The total aggregate of genes in a population at any one time.
gene pool
The alternation of the genes of a person afflicted with a genetic disease.
gene therapy
The random transfer of bacterial genes from one bacterium to another.
generalized transduction
Changes in the gene pool of a small population due to chance.
genetic drift
The direct manipulation of genes for practical purposes.
genetic engineering
An ordered list of genetic loci (genes or other genetic markers) along a chromosome.
genetic map
The general term for the production of offspring with new combinations of traits inherited from the two parents.
genetic recombination
An organism that has acquired one or more genes by artificial means; also known as a transgenic organism.
genetically modified (GM) organism
The scientific study of heredity and hereditary variation.
genetics
The complete complement of an organism's genes; an organism's genetic material.
genome
The parental effect on gene expression whereby identical alleles have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
genomic imprinting
A set of thousands of DNA segments from a genome, each carried by a plasmid, phage, or other cloning vector.
genomic library
The study of whole sets of genes and their interactions.
genomics
The genetic makeup of an organism.
genotype
A taxonomic category above the species level, designated by the first word of a species' binomial Latin name.
genus
The geographic area in which a population lives.
geographic range
Differences in genetic structure between populations.
geographic variation
A time scale established by geologists that reflects a consistent sequence of historical periods, grouped into four eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
geologic time scale
Compounds that have the same molecular formula but differ in the spatial arrangements of their atoms.
geometric isomers
A rapid J-shaped growth curve that typically occurs when members have access to abundant food and are free to reproduce at their physiological capacity.
geometric population growth
Three main layers that form the various tissues and organs of an animal body.
germ layers
Pregnancy; the state of carrying developing young within the female reproductive tract.
gestation
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.
gibberellins
A localized extension of the body surface of many aquatic animals, specialized for gas exchange.
gill
The flow of blood through gills.
gill circulation
A phylum of gymnosperms represented by a single extant species, Ginkgo biloba, characterized by fanlike leaves that turn gold and are deciduous in autumn.
Ginkgophyta
Epithelia that secrete chemical solutions.
glandular epithelia
The head end of the penis.
glans penis
Supporting cells that are essential for the structural integrity of the nervous system and for the normal functioning of neurons.
glia
A nonconducting cell of the nervous system that provides support, insulation, and protection for the neurons.
glial cell
A ball of capillaries surrounded by Bowman's capsule in the nephron and serving as the site of filtration in the vertebrate kidney.
glomerulus
A peptide hormone secreted by pancreatic endocrine cells that raises blood glucose levels; an antagonistic hormone to insulin.
glucagon
A corticosteroid hormone secreted by the adrenal cortex that influences glucose metabolism and immune function.
glucocorticoid
An amino acid that functions as a CNS neurotransmitter.
glutamate
The carbohydrate produced directly from the Calvin cycle.
glyceraldehyde-3-phosphate (G3P)
A three-carbon alcohol with a hydroxyl group on each carbon.
glycerol
An amino acid that functions as a CNS neurotransmitter.
glycine
A fuzzy coat on the outside of animal cells, made of sticky oligosaccharides.
glycocalyx
An extensively branched glucose storage polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.
glycogen
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.
glycolysis
A protein covalently attached to a carbohydrate.
glycoprotein
A covalent bond formed between two monosaccharides by a dehydration reaction.
glycosidic linkage
The vertebrate subgroup that possess jaws.
gnathostomes
A phylum of gymnosperms consisting of just three extant genera that are very different in appearance.
Gnetophyta
Typically unicellular, biflagellated, algae with yellow and brown carotene and xanthophyll accessory pigments.
golden algae
An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic reticulum.
Golgi apparatus
Hormones that stimulate the activities of the testes and ovaries; a collective term for follicle-stimulating and luteinizing hormones.
gonadotropins
The male and female sex organs; the gamete-producing organs in most animals.
gonads
A signal receptor protein in the plasma membrane that responds to the binding signal molecule by activating a G protein.
G-protein linked receptor
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.
graded potential
A view of Earth's history that attributes profound change to the cumulative product of slow but continuous processes.
gradualism
An attack against a patient's body cells by lymphocytes received in a bone marrow transplant.
graft versus host reaction
A staining method that distinguishes between two different kinds of bacterial cell walls.
Gram stain
The group of bacteria with a structurally more complex cell wall made of less peptidoglycan. Gram-negative bacteria are often more toxic than gram-positive bacteria.
gram-negative
The group of bacteria with simpler cell walls with a relatively large amount of peptidoglycan. Gram-positive bacteria are usually less toxic than gram-negative bacteria.
gram-positive
A stacked portion of the thylakoid membrane in the chloroplast. Grana function in the light reactions of photosynthesis.
granum
A response of a plant or animal in relation to gravity.
gravitropism
A light-gray region of cytoplasm located near the equator of the egg on the side opposite the sperm entry.
gray crescent
Regions of dendrites and clusters of nerve-cell bodies within the CNS.
gray matter
Photosynthetic protists that include unicellular, colonial, and multicellular species with grass green chloroplasts; closely related to true plants.
green algae
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.
greenhouse effect
The total primary production of an ecosystem.
gross primary production (GPP)
A primary meristem that gives rise to ground tissue in plants.
ground meristem
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.
ground tissue
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.
growth factor
A protein of about 200 amino acids that affects a wide variety of target tissues and has both direct effects and tropic effects.
growth hormone
A specialized epidermal plant cell that forms the boundaries of the stomata.
guard cell
Taste receptors.
gustatory receptors
The exudation of water droplets, caused by root pressure in certain plants.
guttation
A vascular plant that bears naked seeds—seeds not enclosed in specialized chambers.
gymnosperm
Places where organisms live; environmental situations in which organisms live.
habitats
A very simple type of learning that involves a loss of responsiveness to stimuli that convey little or no information.
habituation
A type of mechanoreceptor that detects sound waves and other forms of movement in air or water.
hair cell
The number of years it takes for 50% of an original sample of an isotope to decay.
half-life
The principle that for natural selection to favor an altruistic act, the benefit to the recipient, devalued by the coefficient of relatedness, must exceed the cost to the altruist.
Hamilton's Rule
A cell containing only one set of chromosomes (n).
haploid cell
The condition describing a non-evolving population (one that is in genetic equilibrium).
Hardy-Weinberg equilibrium
A formula for calculating the frequencies of genotypes in a gene pool from the frequencies of alleles, and vice versa.
Hardy-Weinberg formula
An axiom maintaining that the sexual shuffling of genes alone cannot alter the overall genetic makeup of a population.
Hardy-Weinberg theorem
In parasitic fungi, a nutrient-absorbing hyphal tip that penetrates the tissues of the host but remains outside the host cell membranes.
haustorium
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.
Haversian system
A muscular pump that uses metabolic energy to elevate hydrostatic pressure of the blood. Blood then flows down a pressure gradient through blood vessels that eventually return blood to the heart.
heart
The rate of heart contraction.
heart rate
The total amount of kinetic energy due to molecular motion in a body of matter. Heat is energy in its most random form.
heat
The quantity of heat a liquid must absorb for 1 gram of it to be converted from the liquid to the gaseous state.
heat of vaporization
A protein that helps protect other proteins during heat stress, found in plants, animals, and microorganisms.
heat-shock protein
Polypeptide chains that contribute to the structure of an antibody. Two identical heavy chains and two identical light chains, joined by disulfide brides, form a Y-shaped antibody molecule.
heavy chains
An enzyme that untwists the double helix of DNA at the replication forks.
helicase
Sun animals that live in fresh water. They have skeletons made of siliceous or chitinous unfused plates.
heliozoan
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.
helper T cell (TH)
A type of respiratory pigment that uses copper as its oxygen-binding component. Hemocyanin is found in the hemolymph of arthropods and many mollusks.
hemocyanin
An iron-containing protein in red blood cells that reversibly binds oxygen.
hemoglobin
In invertebrates with an open circulatory system, the body fluid that bathes tissues.
hemolymph
A human genetic disease caused by a sex-linked recessive allele, characterized by excessive bleeding following injury.
hemophilia
A large circulatory channel that conveys nutrient-laden blood from the small intestine to the liver, which regulates the blood's nutrient content.
hepatic portal vessel
The phylum of liverworts, small herbaceous (non-woody) plants.
Hepatophyta
A heterotrophic animal that eats plants.
herbivore
The consumption of plant material by an herbivore.
herbivory
The transmission of traits from one generation to the next.
heredity
An individual that functions as both male and female in sexual reproduction by producing both sperm and eggs.
hermaphrodite
A condition in which an individual has both female and male gonads and functions as both a male and female in sexual reproduction by producing both sperm and eggs.
hermaphroditism
Nontranscribed eukaryotic chromatin that is so highly compacted that it is visible with a light microscope during interphase.
heterochromatin
Evolutionary change in the timing or rate of development.
heterochrony
A specialized cell that engages in nitrogen fixation on some filamentous cyanobacteria.
heterocyst
A measurement of biological diversity considering richness and relative abundance.
heterogeneity
A mycelium formed by the fusion of two hyphae that have genetically different nuclei.
heterokaryon
Referring to a condition in the life cycle of all modern plants in which the sporophyte and gametophyte generations differ in morphology.
heteromorphic
Referring to plants in which the sporophyte produces two kinds of spores that develop into unisexual gametophytes, either female or male.
heterosporous
An organism that obtains organic food molecules by eating other organisms or their by-products.
heterotroph
Greater reproductive success of heterozygous individuals compared to homozygotes; tends to preserve variation in gene pools.
heterozygote advantage
Having two different alleles for a given genetic character.
heterozygous
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.
hibernation
A cholesterol-carrying particle in the blood, made up of cholesterol and other lipids surrounded by a single layer of phospholipids in which proteins are embedded. An HDL particle carries less cholesterol than a related lipoprotein, LDL, and may be correl
high-density lipoprotein (HDL)
One of three ancestral and embryonic regions of the vertebrate brain; develops into the medulla oblongata, pons, and cerebellum.
hindbrain
A substance released by injured cells that causes blood vessels to dilate during an inflammatory response.
histamine
A small protein with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in its chromatin structure.
histone
The attachment of acetyl groups to certain amino acids of histone proteins.
histone acetylation
The infectious agent that causes AIDS; HIV is an RNA retrovirus.
HIV (human immunodeficiency virus)
A rootlike structure that anchors a seaweed.
holdfast
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).
holoblastic cleavage
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.
homeobox
Evolutionary alteration in the placement of different body parts.
homeosis
The steady-state physiological condition of the body.
homeostasis
Any of the genes that control the overall body plan of animals by controlling the developmental fate of groups of cells.
homeotic gene
A term that refers to mammals that are more closely related to humans than to any other living species.
hominid
A term that refers to great apes and humans.
hominoid
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 chromosomes
Structures in different species that are similar because of common ancestry.
homologous structures
Similarity in characteristics resulting from a shared ancestry.
homology
Referring to plants in which a single type of spore develops into a bisexual gametophyte having both male and female sex organs.
homosporous
Having two identical alleles for a given trait.
homozygous
Neurons of the retina that help integrate information before it is sent to the brain.
horizontal cell
Any one of the many circulating chemical signals found 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.
hormone
Members of the phylum Anthocerophyta, they are small herbaceous (non-woody) plants.
hornworts
The larger participant in a symbiotic relationship, serving as home and feeding ground to the symbiont.
host
The limited range of host cells that each type of virus can infect and parasitize.
host range
A hormone secreted by the chorion that maintains the corpus leuteum of the ovary during the first three months of pregnancy.
human chorionic gonadotropin (HCG)
An international collaborative effort to map and sequence the DNA of the entire human genome.
Human Genome Project
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.
humoral immunity
Decomposing organic material found in topsoil.
humus
A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10–20 years after the onset of symptoms.
Huntington's disease
A region where two related populations that diverged after becoming geographically isolated make secondary contact and interbreed where their geographic ranges overlap.
hybrid zone
The mating, or crossing, of two varieties.
hybridization
The sphere of water molecules around each dissolved ion.
hydration shell
An organic molecule consisting only of carbon and hydrogen.
hydrocarbon
A type of weak chemical bond formed when the slightly positive hydrogen atom of a polar covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent bond in another molecule.
hydrogen bond
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+).
hydrogen ion
A chemical process that lyses, or splits, molecules by the addition of water; an essential process in digestion.
hydrolysis
Having an affinity for water.
hydrophilic
Having an aversion to water; tending to coalesce and form droplets in water.
hydrophobic
A type of weak chemical bond formed when molecules that do not mix with water coalesce to exclude the water.
hydrophobic interaction
A skeletal system composed of fluid held under pressure in a closed body compartment; the main skeleton of most cnidarians, flatworms, nematodes, and annelids.
hydrostatic skeleton
A water molecule that lost a proton.
hydroxide ion
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.
hydroxyl group
A thin membrane that partly covers the vaginal opening in the human female; ruptured by sexual intercourse or other vigorous activity.
hymen
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
hyperpolarization
In comparing two solutions, referring to the one with a greater solute concentration.
hypertonic
A filament that collectively makes up the body of a fungus.
hypha
The embryonic axis below the point at which the cotyledons are attached.
hypocotyl
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.
hypothalamus
In comparing two solutions, the one with a lower solute concentration.
hypotonic solution
The area near the edge of the sarcomere where there are only thin filaments.
I band
The last of three sections of the small intestine primarily involved in the absorption of nutrients and water.
ileum
An island of undifferentiated cells in an insect larva, which are committed (determined) to form a particular organ during metamorphosis to the adult.
imaginal disk
The collective action of antibodies, complement, and phagocytes. Microbes coated with antibodies and complement proteins adhere to blood vessel walls, making the pathogens easier prey for phagocytic cells circulating in the blood.
immune adherence
Also called vaccination, it is the exposure of an organism to agents that can no longer cause disease but retain the ability to act as antigens, thereby stimulating an immune response and immunological memory.
immunization
One of the class of proteins comprising the antibodies.
immunoglobulin (Ig)
Molds with no known sexual stages, also called deuteromycetes. This is an informal grouping without phylogenetic basis. If a mycologist discovers a sexual stage of an imperfect fungus, the species is reclassified into a particular phylum, depending upon t
imperfect fungi
A type of learned behavior with a significant innate component, acquired during a limited critical period.
imprinting
Fertilization of ova in laboratory containers followed by artificial implantation of the early embryo in the mother's uterus.
in vitro fertilization
A technique to discover the function of a gene by introducing specific changes into the sequence of a cloned gene, reinserting the mutated gene into a cell, and studying the phenotype of the mutant.
in vitro mutagenesis
The total effect an individual has on proliferating its genes by producing its own offspring and by providing aid that enables other close relatives to increase the production of their offspring.
inclusive fitness
A type of inheritance in which F1 hybrids have an appearance that is intermediate between the phenotypes of the parental varieties.
incomplete dominance
A flower lacking sepals, petals, stamens, or carpels.
incomplete flower
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
incomplete metamorphosis
The second of the three middle ear bones.
incus
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 cleavage
A type of growth characteristic of plants, in which the organism continues to grow as long as it lives.
indeterminate growth
The concept, put forth by H.A. Gleason, that a plant community is a chance assemblage of species found in the same area simply because they happen to have similar biotic requirements.
individualistic hypothesis
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.
induced fit
A specific small molecule that inactivates the repressor in an operon.
inducer
The ability of one group of embryonic cells to influence the development of another.
induction
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.
inflammatory response
A heterotrophic mode of nutrition in which other organisms or detritus are eaten whole or in pieces.
ingestion
In a cladistic study of evolutionary relationships among taxa of organisms, the group of taxa that is actually being analyzed.
ingroup
The reception of genetic qualities by transmission from parent to offspring.
inheritance
The negative effect of early species on the appearance of later species in ecological succession.
inhibit
A kind of hormone released from the hypothalamus that makes the anterior pituitary stop secreting hormone.
inhibiting hormone
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
inhibitory postsynaptic potential (IPSP)
A cluster of cells in a mammalian blastocyst that protrudes into one end of the cavity and subsequently develops into the embryo proper and some of the extraembryonic membranes.
inner cell mass
One of three main regions of the vertebrate ear; includes the cochlea, organ of Corti, and semicircular canals.
inner ear
A second messenger that functions as an intermediate between certain nonsteroid hormones and a third messenger, a rise in cytoplasmic Ca2+ concentration.
inositol trisphosphate (IP3)
The class of arthropods typically having six legs, two pairs of wings, and one pair of antennae. Insect species outnumber all other forms of life combined.
Insecta
An agent that destroys insects.
insecticide
A mutation involving the addition of one or more nucleotide pairs to a gene.
insertion
The simplest kind of transposon, consisting of inverted repeats of DNA flanking a gene for transposase, the enzyme that catalyzes transposition.
insertion sequence
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.
insight learning
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 call
insulin
A group of peptides produced by the liver, it circulates in blood plasma and directly stimulates bone and cartilage growth.
insulinlike growth factors
Typically transmembrane proteins with hydrophobic regions that completely span the hydrophobic interior of the membrane.
integral protein
The interpretation of sensory signals within neural processing centers of the central nervous system.
integration
Receptor proteins built into the plasma membrane that interconnect the extracellular matrix and the cytoskeleton.
integrin
Layers of sporophyte tissues that contribute to the structure of an ovule of a seed plant.
integuments
The concept, put forth by F.E. Clements, that a community is an assemblage of closely linked species, locked into association by mandatory biotic interactions that cause the community to function as an integrated unit, a sort of superorganism.
interactive hypothesis
Specialized junctions between cardiac muscle cells that provide direct electrical coupling among cells.
intercalated discs
A chemical messenger of the immune system, produced by virus-infected cells and capable of helping other cells resist the virus.
interferon
A cytokine secreted by a macrophage that is in the process of phagocytizing and presenting antigen. IL-1, in combination with the antigen, activates the helper T cell to produce IL-2 and other cytokines.
interleukin-1 (IL-1)
A cytokine that helps B cells that have contacted antigen differentiate into antibody-secreting plasma cells.
interleukin-2 (IL-2)
A component of the cytoskeleton that includes all filaments intermediate in size between microtubules and microfilaments.
intermediate filament
Reproduction in which sperm are typically deposited in or near the female reproductive tract and fertilization occurs within the tract.
internal fertilization
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.
interneuron
The segment of a plant stem between the points where leaves are attached.
internode
Sensory receptors that detect stimuli within the body, such as blood pressure and body position.
interoreceptor
The period in the cell cycle when the cell is not dividing. During interphase, cellular metabolic activity is high, chromosomes and organelles are duplicated, and cell size may increase. Interphase accounts for 90% of the cell cycle.
interphase
Individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex, also called mate choice.
intersexual selection
Competition for resources between plants, between animals, or between decomposers when resources are in short supply.
interspecific competition
Cells scattered among the seminiferous tubules of the vertebrate testis that secrete testosterone and other androgens, the male sex hormones.
interstitial cells
The internal environment of vertebrates, consisting of the fluid filling the spaces between cells.
interstitial fluid
The shallow zone of the ocean where land meets water.
intertidal zone
The joining of food vacuoles and lysosomes to allow chemical digestion to occur within the cytoplasm of a cell.
intracellular digestion
A direct competition among individuals of one sex (usually the males in vertebrates) for mates of the opposite sex.
intrasexual selection
The difference between the number of births and the number of deaths, symbolized as rmax; the maximum population growth rate.
intrinsic rate of increase
The transplantation of genes between species resulting from fertile hybrids mating successfully with one of the parent species.
introgression
A noncoding, intervening sequence within a eukaryotic gene.
intron
The infolding of cells.
invagination
An aberration in chromosome structure resulting from an error in meiosis or from mutagens; specifically, reattachment of a chromosomal fragment to the chromosome from which the fragment originated, but in a reverse orientation.
inversion
An animal without a backbone; invertebrates make up 95% of animal species.
invertebrate
Cells rolling over the edge of a lip into the interior.
involution
An atom that has gained or lost electrons, thus acquiring a charge.
ion
A chemical bond resulting from the attraction between oppositely charged ions.
ionic bond
Compounds resulting from the formation of ionic bonds, also called a salt.
ionic compound
Clusters of endocrine cells that secrete glucagon and insulin directly into the bloodstream.
islets of Langerhans
A condition in which male and female gametes are morphologically indistinguishable.
isogamy
One of several organic compounds with the same molecular formula but different structures and therefore different properties. The three types of isomers are structural isomers, geometric isomers, and enantiomers.
isomer
Referring to alternating generations in which the sporophytes and gametophytes look alike, although they differ in chromosome number.
isomorphic
One of the largest groups of crustaceans, primarily marine, but including pill bugs common under logs and moist vegetation next to the ground.
isopods
Having the same solute concentration as another solution.
isotonic
The middle section of the small intestine primarily involved in the absorption of nutrients and water.
jejunum
A unit of energy: 1 J=50.239 cal; 1 cal=54.184 J.
joule (J)
A hormone in arthropods, secreted by the corpora allata glands, that promotes the retention of larval characteristics.
juvenile hormone (JH)
A specialized tissue located near the afferent arteriole that supplies blood to the glomerulus.
juxtaglomerular apparatus
Nephrons with well-developed loops of Henle that extend deeply into the renal medulla.
juxtamedullary nephrons
The fusion of nuclei of two cells, as part of syngamy.
karyogamy
The whole process of mitosis.
karyokinesis
A method of organizing the chromosomes of a cell in relation to number, size, and type.
karyotype
An organic compound with a carbonyl group of which the carbon atom is bonded to two other carbons.
ketone
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 predator
Species that are not usually abundant in a community yet exert strong control on community structure by the nature of their ecological roles or niches.
keystone species
A thousand calories; the amount of heat energy required to raise the temperature of 1 kg of water by 1°C.
kilocalorie (kcal)
A phenomenon of inclusive fitness, used to explain altruistic behavior between related individuals.
kin selection
A change in activity or turning rate in response to a stimulus.
kinesis
The energy of motion, which is directly related to the speed of that motion. Moving matter does work by imparting motion to other matter.
kinetic energy
A specialized region on the centromere that links each sister chromatid to the mitotic spindle.
kinetochore
A group of protistans, including Trypanosoma, which have a single large mitochondrion associated with a kinetoplast that houses extranuclear DNA.
kinetoplastid
A taxonomic category, the second broadest after domain.
kingdom
The traditional embryophyte definition of the plant kingdom.
kingdom Plantae
The name given to the group that includes the traditional plant kingdom and the green algae most closely related to plants, the charophyceans and a few related groups.
kingdom Streptophyta
The broadest version of the plant kingdom that includes the members of the kingdom Streptophyta plus the chlorophytes (non-charophycean green algae).
kingdom Viridiplantae
A set of four criteria for determining whether a specific pathogen is the cause of a disease.
Koch's postulates
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.
Krebs cycle
The concept that in certain (K-selected) populations, life history is centered around producing relatively few offspring that have a good chance of survival.
K-selection
A pair of thick, fatty ridges that enclose and protect the labia minor and vestibule.
labia majora
A pair of slender skin folds that enclose and protect the vestibule.
labia minora
A series of strong, rhythmic contractions of the uterus that expel a baby out of the uterus and vagina during childbirth.
labor
The continued production of milk.
lactation
A tiny lymph vessel extending into the core of an intestinal villus and serving as the destination for absorbed chylomicrons.
lacteal
The conversion of pyruvate to lactate with no release of carbon dioxide.
lactic acid fermentation
Small cavities, pits, or discontinuities in an anatomical structure.
lacunae
A discontinuously synthesized DNA strand that elongates in a direction away from the replication fork.
lagging strand
One of a group of invertebrate chordates.
lancelet
A point of reference for orientation during navigation.
landmark
Several different primarily terrestrial ecosystems linked by exchanges of energy, materials, and organisms.
landscape
The application of ecological principles to the study of land-use patterns; the scientific study of the biodiversity of interacting ecosystems.
landscape ecology
A free-living, sexually immature form in some animal life cycles that may differ from the adult in morphology, nutrition, and habitat.
larva
The voicebox, containing the vocal cords.
larynx
The destination in the thalamus for most of the ganglion cell axons that form the optic nerves.
lateral geniculate nuclei
A process that sharpens the edges and enhances the contrast of a perceived image, by inhibiting receptors lateral to those that have responded to light.
lateral inhibition
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 line system
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.
lateral meristem
Roots that arise from the outermost layer of the pericycle of an established root.
lateral roots
Mendel's second law, stating that each allele pair segregates independently during gamete formation; applies when genes for two characteristics are located on different pairs of homologous chromosomes.
law of independent assortment
Mendel's first law, stating that allele pairs separate during gamete formation, and then randomly re-form as pairs during the fusion of gametes at fertilization.
law of segregation
The new continuous complementary DNA strand synthesized along the template strand in the mandatory 5' 3' direction.
leading strand
A behavioral change resulting from experience.
learning
The structure in an eye that focuses light rays onto the retina.
lens
The reptilian group that includes lizards, snakes, and two species of New Zealand animals called tuataras.
lepidosaurs
A white blood cell; typically functions in immunity, such as phagocytosis or antibody production.
leukocyte
Located between the seminiferous tubules of the testes, these cells produce testosterone and other androgens.
Leydig cell
The mutualistic collective formed by the symbiotic association between a fungus and a photosynthetic alga.
lichen
The generation-to-generation sequence of stages in the reproductive history of an organism.
life cycle
The series of events from birth through reproduction and death.
life history
A table of data summarizing mortality in a population.
life table
A type of fibrous connective tissue that joins bones together at joints.
ligament
A molecule that binds specifically to a receptor site of another molecule.
ligand
Protein pores in the plasma membrane that open or close in response to a chemical signal, allowing or blocking the flow of specific ions.
ligand-gated ion channel
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 concen
ligand-gated ion channel receptor
Polypeptide chains that contribute to the structure of an antibody. Two identical light chains and two identical heavy chains, joined by disulfide bridges, form a Y-shaped antibody molecule.
light chains
An optical instrument with lenses that refract (bend) visible light to magnify images of specimens.
light microscope (LM)
The steps in photosynthesis that occur on the thylakoid membranes of the chloroplast and that convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen in the process.
light reactions
A hard material embedded in the cellulose matrix of vascular plant cell walls that functions as an important adaptation for support in terrestrial species.
lignin
A group of nuclei (clusters of nerve cell bodies) in the lower part of the mammalian forebrain that interact with the cerebral cortex in determining emotions; includes the hippocampus and the amygdala.
limbic system
The well-lit, open surface waters of a lake farther from shore.
limnetic zone
A genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes. The greater the frequency of recombination between two genetic markers, the farther apart they are assumed to be. See also genetic map.
linkage map
Genes that are located on the same chromosome.
linked genes
An enzyme that hydrolyzes fat molecules in the intestinal lumen.
lipase
One of a family of compounds, including fats, phospholipids, and steroids, that are insoluble in water.
lipid
A protein bonded to a lipid; includes the low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) that transport fats and cholesterol in blood.
lipoprotein
The shallow, well-lit waters of a lake close to shore.
littoral zone
The liver performs diverse functions such as producing bile, preparing nitrogenous wastes for disposal, and detoxifying poisonous chemicals in the blood.
liver
Members of the phylum Hepatophyta, they are small herbaceous (non-woody) plants.
liverworts
The most fertile of all soils, loams are made up of roughly equal amounts of sand, silt, and clay.
loams
A chemical messenger that influences cells in the vicinity.
local regulator
Active movement from place to place.
locomotion
A particular place along the length of a certain chromosome where a given gene is located.
locus
A model describing population growth that levels off as population size approaches carrying capacity.
logistic population growth
A plant that flowers, usually in late spring or early summer, only when the light period is longer than a critical length.
long-day plant
A reduced responsiveness to an action potential (nerve signal) by a receiving neuron.
long-term depression (LTD)
The ability to hold, associate, and recall information over one's life.
long-term memory
An enhanced responsiveness to an action potential (nerve signal) by a receiving neuron.
long-term potentiation (LTP)
The long hairpin turn, with a descending and ascending limb, of the renal tubule in the vertebrate kidney; functions in water and salt reabsorption.
loop of Henle
The most widespread connective tissue in the vertebrate body. It binds epithelia to underlying tissues and functions as packing material, holding organs in place.
loose connective tissue
The group of animals that includes the Bryozoa, Phoronida, and Brachiopoda. Animals in these groups possess a lophophore.
lophophorate animals
A horseshoe-shaped or circular fold of the body wall bearing ciliated tentacles that surround the mouth.
lophophore
One of two distinct clades within the protostomes. It includes annelids and mollusks.
Lophotrochozoa
A cholesterol-carrying particle in the blood, made up of cholesterol and other lipids surrounded by a single layer of phospholipids in which proteins are embedded. An LDL particle carries the blood correlate with a tendency to develop blocked blood vessel
low-density lipoprotein (LDL)
The invaginated respiratory surfaces of terrestrial vertebrates, land snails, and spiders that connect to the atmosphere by narrow tubes.
lungs
That portion of the ovarian cycle during which endocrine cells of the corpus luteum secrete female hormones.
luteal phase
A protein hormone secreted by the anterior pituitary that stimulates ovulation in females and androgen production in males.
lutenizing hormone (LH)
The colorless fluid, derived from interstitial fluid, in the lymphatic system of vertebrate animals.
lymph
Organs located along lymph vessels. They filter lymph and help attack viruses and bacteria.
lymph node
A system of vessels and lymph nodes, separate from the circulatory system, that returns fluid and protein to the blood.
lymphatic system
A white blood cell. The lymphocytes that complete their development in the bone marrow are called B cells, and those that mature in the thymus are called T cells.
lymphocyte
Cancers of the tissues that form white blood cells.
lymphomas
A phage replication cycle in which the viral genome becomes incorporated into the bacterial host chromosome as a prophage and does not kill the host.
lysogenic cycle
A membrane-enclosed bag of hydrolytic enzymes found in the cytoplasm of eukaryotic cells.
lysosome
An enzyme in perspiration, tears, and saliva that attacks bacterial cell walls.
lysozyme
A type of viral replication cycle resulting in the release of new phages by death or lysis of the host cell.
lytic cycle
The mitotic phase of the cell cycle, which includes mitosis and cytokinesis.
M phase
Evolutionary change on a grand scale, encompassing the origin of new taxonomic groups, evolutionary trends, adaptive radiation, and mass extinction.
macroevolution
A giant molecule formed by the joining of smaller molecules, usually by a condensation reaction. Polysaccharides, proteins, and nucleic acids are macromolecules.
macromolecule
A chemical substance that an organism must obtain in relatively large amounts. See also micronutrient.
macronutrient
An amoeboid cell that moves through tissue fibers, engulfing bacteria and dead cells by phagocytosis.
macrophage
A large set of cell surface antigens encoded by a family of genes. Foreign MHC markers trigger T-cell responses that may lead to the rejection of transplanted tissues and organs.
major histocompatibility complex (MHC)
A cancerous tumor that is invasive enough to impair functions of one or more organs.
malignant tumor
The first of the three middle ear bones.
malleus
An animal whose diet is missing one or more essential nutrients.
malnourished
A unique excretory organ of insects that empties into the digestive tract, removes nitrogenous wastes from the blood, and functions in osmoregulation.
malpighian tubule
The class that includes endothermic vertebrates that possess mammary glands and hair.
Mammalia
Exocrine glands that secrete milk to nourish the young. These glands are characteristic of mammals.
mammary glands
Jawlike structures found in uniramians and crustaceans.
mandibles
A heavy fold of tissue in mollusks that drapes over the visceral mass and may secrete a shell.
mantle
A water-filled chamber that houses the gills, anus, and excretory pores of a mollusk.
mantle cavity
A measurement of the distance between genes; one map unit is equivalent to a 1 percent recombination frequency.
map units
A sampling techniue used to estimate wildlife populations.
mark-recapture method
A mammal, such as a koala, kangaroo, or opossum, whose young complete their embryonic development inside a maternal pouch called the marsupium.
marsupial
The sum of the number of protons and neutrons in an atom's nucleus.
mass number
A vertebrate body cell that produces histamine and other molecules that trigger the inflammatory response.
mast cell
A gene that, when mutant in the mother, results in a mutant phenotype in the offspring, regardless of the genotype.
maternal effect genes
The nonliving component of connective tissue, consisting of a web of fibers embedded in homogeneous ground substance that may be liquid, jellylike, or solid.
matrix
Anything that takes up space and has mass.
matter
A sensory receptor that detects physical deformations in the body's environment associated with pressure, touch, stretch, motion, and sound.
mechanoreceptor
Also called the medulla oblongata, it is the lowest part of the vertebrate brain; a swelling of the hindbrain dorsal to the anterior spinal cord that controls autonomic, homeostatic functions, including breathing, heart and blood vessel activity, swallowi
medulla
The floating, flattened, mouth-down version of the cnidarian body plan. The alternate form is the polyp.
medusa
A unit of pressure equivalent to 10 atmospheres of pressure.
megapascal (MPa)
The larger leaves of modern vascular plants served by a highly-branched vascular system.
megaphylls
A spore from a heterosporous plant that develops into a female gametophyte bearing archegonia.
megaspores
A two-stage type of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell.
meiosis
The first division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell.
meiosis I
The second division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell.
meiosis II
A hormone that regulates the activity of pigment-containing cells in the skin of some vertebrates.
melanocyte-stimulating hormone
A modified amino acid hormone secreted by the pineal gland.
melatonin
A molecular complex including complement proteins that generates a 7–10 nm diameter pore in a bacterial membrane, causing the cell to die.
membrane attack complex (MAC)
The charge difference between the cytoplasm and extracellular fluid in all cells, due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances.
membrane potential
A clone of long-lived lymphocytes, formed during the primary immune response, that remains in a lymph node until activated by exposure to the same antigen that triggered its formation. Activated memory cells mount the secondary immune response.
memory cell
The curved upper surface of a liquid column that is concave when the containing walls are wetted by the liquid and convex when not.
meniscus
A type of reproductive cycle in higher female primates, in which the nonpregnant endometrium is shed as a bloody discharge through the cervix into the vagina.
menstrual cycle
That portion of the menstrual cycle when menstrual bleeding occurs.
menstrual flow phase
The shedding of portions of the endometrium during a menstrual cycle.
menstruation
Plant tissue that remains embryonic as long as the plant lives, allowing for indeterminate growth.
meristem
Plant genes that promote the switch from vegetative growth to flowering.
meristem identity genes
A type of cleavage in which there is incomplete division of yolk-rich egg, characteristic of avian development.
meroblastic cleavage
A membrane that suspends many of the organs of vertebrates inside fluid-filled body cavities.
mesentery
The middle primary germ layer of an early embryo that develops into the notochord, the lining of the coelom, muscles, skeleton, gonads, kidneys, and most of the circulatory system.
mesoderm
A gelatinous region between the two layers of cells of a sponge.
mesohyl
The ground tissue of a leaf, sandwiched between the upper and lower epidermis and specialized for photosynthesis.
mesophyll
A loosely arranged photosynthetic cell located between the bundle sheath and the leaf surface.
mesophyll cell
Lakes with moderate amounts of nutrients and phytoplankton productivity intermediate to oligotrophic and eutrophic systems.
mesotrophic
A type of RNA, synthesized from DNA, that attaches to ribosomes in the cytoplasm and specifies the primary structure of a protein.
messenger RNA (mRNA)
The totality of an organism's chemical reactions, consisting of catabolic and anabolic pathways.
metabolism
The resurgence of development in an animal larva that transforms it into a sexually mature adult.
metamorphosis
In annelid worms, a type of excretory tubule with internal openings called nephrostomes that collect body fluids and external openings called nephridiopores.
metanephridium
The third subphase of mitosis, in which the spindle is complete and the chromosomes, attached to microtubules at their kinetochores, are all aligned at the metaphase plate.
metaphase
An imaginary plane during metaphase in which the centromeres of all the duplicated chromosomes are located midway between the two poles.
metaphase plate
A subdivided population of a single species.
metapopulation
The spread of cancer cells to locations distant from their original site.
metastasis
Microorganisms that obtain energy by using carbon dioxide to oxidize hydrogen, producing methane as a waste product.
methanogen
Very fine scale variations of climate, such as the specific climatic conditions underneath a log.
microclimate
A change in the gene pool of a population from generation to generation.
microevolution
A solid rod of actin protein in the cytoplasm of almost all eukaryotic cells, making up part of the cytoskeleton and acting alone or with myosin to cause cell contraction.
microfilament
An element that an organism needs in very small amounts and that functions as a component or cofactor of enzymes. See also macronutrient.
micronutrient
The small leaves of lycophytes that have only a single, unbranched vein.
microphylls
A spore from a heterosporous plant that develops into a male gametophyte with antheridia.
microspores
A hollow rod of tubulin protein in the cytoplasm of all eukaryotic cells and in cilia, flagella, and the cytoskeleton.
microtubule
One of many fine, fingerlike projections of the epithelial cells in the lumen of the small intestine that increase its surface area.
microvillus
One of three ancestral and embryonic regions of the vertebrate brain; develops into sensory integrating and relay centers that send sensory information to the cerebrum.
midbrain
One of three main regions of the vertebrate ear; a chamber containing three small bones (the hammer, anvil, and stirrup) that convey vibrations from the eardrum to the oval window.
middle ear
A thin layer of adhesive extracellular material, primarily pectins, found between the primary walls of adjacent young plant cells.
middle lamella
A phenomenon in which one species benefits by a superficial resemblance to an unrelated species. A predator or species of prey may gain a significant advantage through mimicry.
mimicry
In nutrition, a chemical element other than hydrogen, oxygen, or nitrogen that an organism requires for proper body functioning.
mineral
Essential chemical elements absorbed from the soil in the form of inorganic ions.
mineral nutrients
A corticosteroid hormone secreted by the adrenal cortex that regulates salt and water homeostasis.
mineralocorticoid
The amount of suitable habitat needed to sustain a viable population.
minimum dynamic area
The smallest number of individuals needed to perpetuate a population.
minimum viable population size (MVP)
The cellular process that uses special enzymes to fix incorrectly paired nucleotides.
mismatch repair
The most common type of mutation, a base-pair substitution in which the new codon makes sense in that it still codes for an amino acid.
missense mutation
The compartment of the mitochondrion enclosed by the inner membrane and containing enzymes and substrates for the Krebs cycle.
mitochondrial matrix
An organelle in eukaryotic cells that serves as the site of cellular respiration.
mitochondrion
A process of nuclear division in eukaryotic cells conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis conserves chromosome number by equally allocating replicated chromosomes to each of the daughter
mitosis
The phase of the cell cycle that includes mitosis and cytokinesis.
mitotic phase
An assemblage of microtubules and associated proteins that is involved in the movements of chromosomes during mitosis.
mitotic spindle
An organism chosen to study broad biological principles.
model organism
A comprehensive theory of evolution emphasizing natural selection, gradualism, and populations as the fundamental units of evolutionary change; also called neo-Darwinism.
modern synthesis
A common measure of solute concentration, referring to the number of moles of solute in 1 L of solution.
molarity
A rapidly growing, asexually reproducing fungus.
mold
The number of grams of a substance that equals its molecular weight in daltons and contains Avogadro's number of molecules.
mole
Evolutionary timing methods based on the observation that at least some regions of genomes evolve at constant rates.
molecular clocks
A type of molecular notation indicating only the quantity of the constituent atoms.
molecular formula
The sum of the weights of all the atoms in a molecule.
molecular weight
Two or more atoms held together by covalent bonds.
molecule
A process in arthropods in which the exoskeleton is shed at intervals to allow growth by the secretion of a larger exoskeleton.
molting
A defensive protein produced by cells descended from a single cell; an antibody that is secreted by a clone of cells and, consequently, is specific for a single antigenic determinant.
monoclonal antibody
A subdivision of flowering plants whose members possess one embryonic seed leaf, or cotyledon.
monocot
Cultivation of large land areas with a single plant variety.
monoculture
An agranular leukocyte that is able to migrate into tissues and transform into a macrophage.
monocyte
Referring to a plant species that has both staminate and carpellate flowers on the same individual.
monoecious
A type of relationship in which one male mates with just one female.
monogamous
An organism that is heterozygous with respect to a single gene of interest. A monohybrid results from a cross between parents homozygous for different alleles. For example, parents of genotypes AA and aa produce a monohybrid genotype of Aa.
monohybrid
The subunit that serves as the building block of a polymer.
monomer
Pertaining to a taxon derived from a single ancestral species that gave rise to no species in any other taxa.
monophyletic
The simplest carbohydrate, active alone or serving as a monomer for disaccharides and polysaccharides. Also known as simple sugars, the molecular formulas of monosaccharides are generally some multiple of CH2O.
monosaccharide
A chromosomal condition in which a particular cell has only one copy of a chromosome, instead of the normal two; the cell is said to be monosomic for that chromosome.
monosomic
An egg-laying mammal, represented by the platypus and echidna.
monotreme
A substance, such as bicoid protein, that provides positional information in the form of a concentration gradient along an embryonic axis.
morphogen
The development of body shape and organization during ontogeny.
morphogenesis
A substance that provides positional information in the form of a concentration gradient along an embryonic axis.
morphogens
The idea that species are defined by measurable anatomical criteria.
morphological species concept
A species defined by its anatomical features.
morphospecies
A solid ball of blastomeres formed by early cleavage.
morula
A pattern of development in which an organism consists of two sets of cells that differ according to which X chromosome is inactivated.
mosaic
A pattern of development, such as that of a mollusk, in which the early blastomeres each give rise to a specific part of the embryo. In some animals, the fate of the blastomeres is established in the zygote.
mosaic development
The evolution of different features of an organism at different rates.
mosaic evolution
The efferent neurons that convey information from the CNS to the effector cells.
motor division
A nerve cell that transmits signals from the brain or spinal cord to muscles or glands.
motor neuron
The conduction of signals from a processing center in a central nervous system to effector cells.
motor output
A single motor neuron and all the muscle fibers it controls.
motor unit
A series of small clumps or a narrow strip of quality habitat (usable by organisms) that connects otherwise isolated patches of quality habitat.
movement corridor
A protein complex required for a cell to progress from late interphase to mitosis; the active form consists of cyclin and cdc2, a protein kinase.
MPF (M-phase-promoting factor)
Smooth moist epithelium that lines the digestive tract and air tubes leading to the lungs.
mucous membrane
A mutual mimicry by two unpalatable species.
Müllerian mimicry
A type of phenotypic character influenced by genetic and environmental factors.
multifactorial
A collection of genes with similar or identical sequences, presumably of common origin.
multigene family
A fruit such as pineapple that develops from an inflorescence, a group of flowers tightly clustered together. When the walls of the many ovaries start to thicken, they fuse together and become incorporated into one fruit.
multiple fruit
The idea that modern humans evolved in each region of the Earth from local populations of Homo erectus.
multiregional hypothesis
An interoreceptor stimulated by mechanical distortion. Also called a stretch receptor.
muscle spindle
Tissue consisting of long muscle cells that are capable of contracting when stimulated by nerve impulses.
muscle tissue
A chemical or physical agent that interacts with DNA and causes a mutation.
mutagen
The creation of mutations.
mutagenesis
Traits that are alternatives to the wild type.
mutant phenotypes
A rare change in the DNA of a gene ultimately creating genetic diversity.
mutation
A symbiotic relationship in which both participants benefit.
mutualism
The densely branched network of hyphae in a fungus.
mycelium
A mutualistic association of plant root and fungus.
mycorrhiza
The general term for a fungal infection.
mycosis
In a neuron, an insulating coat of cell membrane from Schwann cells that is interrupted by nodes of Ranvier where saltatory conduction occurs.
myelin sheath
A fibril collectively arranged in longitudinal bundles in muscle cells (fibers); composed of thin filaments of actin and a regulatory protein and thick filaments of myosin.
myofibril
The thick and thin filaments that form the myofibrils.
myofilaments
An oxygen-storing, pigmented protein in muscle cells.
myoglobin
A type of protein filament that interacts with actin filaments to cause cell contraction.
myosin
Increased muscle tension.
myotonia
Nicotinamide adenine dinucleotide, a coenzyme present in all cells that helps enzymes transfer electrons during the redox reactions of metabolism.
NAD+
The reduced form of nicotinamide that is one of the products of glycolysis.
NADH
An acceptor that temporarily stores energized electrons produced during the light reactions.
NADP+
A form of contraception that relies upon refraining from sexual intercourse when conception is most likely to occur; also called the rhythm method.
natural family planning
A nonspecific defensive cell that attacks tumor cells and destroys infected body cells, especially those harboring viruses.
natural killer (NK) cell
Differential success in the reproduction of different phenotypes resulting from the interaction of organisms with their environment. Evolution occurs when natural selection causes changes in relative frequencies of alleles in the gene pool.
natural selection
A philosophy dedicated to discovering the Creator's plan by studying nature. Adaptations of organisms are viewed as evidence that the Creator had designed each and every species for a particular purpose.
natural theology
A primary mechanism of homeostasis, whereby a change in a physiological variable that is being monitored triggers a response that counteracts the initial fluctuation.
negative feedback
A breathing system in which air is pulled into the lungs.
negative pressure breathing
Stinging components of cnidocytes.
nematocysts
The tubular excretory unit of the vertebrate kidney.
nephron
The shallow regions of the ocean overlying the continental shelves.
neritic zone
A ropelike bundle of neuron fibers (axons and dendrites) tightly wrapped in connective tissue.
nerve
A weblike system of neurons, characteristic of radially symmetrical animals such as Hydra
nerve net
Tissue made up of neurons and supportive cells.
nervous tissue
The gross primary production of an ecosystem minus the energy used by the producers for respiration.
net primary production (NPP)
A band of cells along the border where the neural tube pinches off from the ectoderm; the cells migrate to various parts of the embryo and form the pigment cells in the skin, bones of the skull, the teeth, the adrenal glands, and parts of the peripheral n
neural crest
A tube of cells running along the dorsal axis of the body, just dorsal to the notochord. It will give rise to the central nervous system.
neural tube
Also called the posterior pituitary, it is an extension of the brain. The neurohypophysis stores and secretes oxytocin and antidiuretic hormone, both produced by neurosecretory cells in the hypothalamus.
neurohypophysis
A nerve cell; the fundamental unit of the nervous system, having structure and properties that allow it to conduct signals by taking advantage of the electrical charge across its cell membrane.
neuron
Relatively short chains of amino acids that serve as neurotransmitters.
neuropeptides
Hypothalamus cells that receive signals from other nerve cells, but instead of signaling to an adjacent nerve cell or muscle, they release hormones into the bloodstream.
neurosecretory cells
A chemical messenger released from the synaptic terminal of a neuron at a chemical synapse that diffuses across the synaptic cleft and binds to and stimulates the postsynaptic cell.
neurotransmitter
Genetic diversity that confers no apparent selective advantage.
neutral variation
An immune response in which an antibody binds to and blocks the activity of an antigen.
neutralization
An electrically neutral particle (a particle having no electrical charge), found in the nucleus of an atom.
neutron
The most abundant type of leukocyte. Neutrophils tend to self-destruct as they destroy foreign invaders, limiting their life span to but a few days.
neutrophil
The expected phenotypic ratio in the F2 generation for a cross of two characters that segregate independently.
9:3:3:1 ratio
A local regulator gas produced by many types of cells.
nitric oxide
The assimilation of atmospheric nitrogen by certain prokaryotes into nitrogenous compounds that can be directly used by plants.
nitrogen fixation
An enzyme complex, unique to certain prokaryotes, that reduces N2 to NH3.
nitrogenase
Microorganisms that restock nitrogenous minerals in the soil by converting nitrogen to ammonia.
nitrogen-fixing bacteria
A class of naked dendrites in the epidermis of the skin.
nociceptor
A point along the stem of a plant at which leaves are attached.
node
The small gaps in the myelin sheath between successive glial cells along the axon of a neuron; also, the site of high concentration of voltage-gated ion channels.
nodes of Ranvier
Swellings on the roots of legumes. Nodules are composed of plant cells that contain nitrogen-fixing bacteria of the genus Rhizobium.
nodules
A substance that reduces the activity of an enzyme by binding to a location remote from the active site, changing its conformation so that it no longer binds to the substrate.
noncompetitive inhibitor
A route of electron flow during the light reactions of photosynthesis that involves both photosystems and produces ATP, NADPH, and oxygen. The net electron flow is from water to NADP+.
noncyclic electron flow
The production of ATP by noncyclic electron flow.
noncyclic photophosphorylation
An accident of meiosis or mitosis, in which the members of a pair of homologous chromosomes or sister chromatids fail to move apart properly.
nondisjunction
The model of communities that emphasizes that they are not stable in time but constantly changing after being buffeted by disturbances.
nonequilibrium model
A type of covalent bond in which electrons are shared equally between two atoms of similar electronegativity.
nonpolar covalent bond
A mutation that changes an amino acid codon to one of the three stop codons, resulting in a shorter and usually nonfunctional protein.
nonsense mutation
The increased production of heat in some mammals by the action of certain hormones that cause mitochondria to increase their metabolic activity and produce heat instead of ATP.
nonshivering thermogenesis
A hormone produced in response to stress.
norepinephrine
The range of phenotypic possibilities for a single genotype, as influenced by the environment.
norm of reaction
A long flexible rod that runs along the dorsal axis of the body in the future position of the vertebral column.
notochord
The membrane in eukaryotes that encloses the nucleus, separating it from the cytoplasm.
nuclear envelope
A netlike array of protein filaments that maintains the shape of the nucleus.
nuclear lamina
A team of enzymes that hydrolyze DNA and RNA into their component nucleotides.
nuclease
A polymer (polynucleotide) consisting of many nucleotide monomers; serves as a blueprint for proteins and, through the actions of proteins, for all cellular activities. The two types are DNA and RNA.
nucleic acid
Base pairing between a gene and a complementary sequence on another nucleic acid molecule.
nucleic acid hybridization
In DNA technology, a labeled single-stranded nucleic acid molecule used to tag a specific nucleotide sequence in a nucleic acid sample. Molecules of the probe hydrogen-bond to the complementary sequence wherever it occurs; radioactive or other labeling of
nucleic acid probe
A dense region of DNA in a prokaryotic cell.
nucleoid
The region in a prokaryotic cell consisting of a concentrated mass of DNA.
nucleoid region
A specialized structure in the nucleus, formed from various chromosomes and active in the synthesis of ribosomes.
nucleolus
An organic molecule consisting of a nitrogenous base joined to a five-carbon sugar.
nucleoside
The basic, beadlike unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a protein core composed of two copies of each of four types of histone.
nucleosome
The building block of a nucleic acid, consisting of a five-carbon sugar covalently bonded to a nitrogenous base and a phosphate group.
nucleotide
The process of removing and then correctly replacing a damaged segment of DNA using the undamaged strand as a guide.
nucleotide excision repair
(1) An atom's central core, containing protons and neutrons. (2) The chromosome-containing organelle of a eukaryotic cell. (3) A cluster of neurons.
nucleus
An organism that requires oxygen for cellular respiration and cannot live without it.
obligate aerobe
An organism that cannot use oxygen and is poisoned by it.
obligate anaerobe
Most of the ocean's waters far from shore, constantly mixed by ocean currents.
oceanic pelagic biome
The region of water lying over deep areas beyond the continental shelf.
oceanic zone
Ancient forests that have never been seriously disturbed by humans. Trees dominating an old-growth forest may be thousands of years old.
old-growth
Smell receptors.
olfactory receptors
Glial cells that form insulating myelin sheaths around the axons of neurons in the central nervous system.
oligodendrocytes
A nutrient-poor, clear, deep lake with minimum phytoplankton.
oligotrophic lake
The facets of the compound eye of arthropods and some polychaete worms.
ommatidia
A heterotrophic animal that consumes both meat and plant material.
omnivore
A gene found in viruses or as part of the normal genome that is involved in triggering cancerous characteristics.
oncogene
The premise that a gene is a segment of DNA that codes for one polypeptide.
one gene-one polypeptide hypothesis
The embryonic development of an organism.
ontogeny
A condition in which male and female gametes differ, such that a small, flagellated sperm fertilizes a large, nonmotile egg.
oogamy
The process in the ovary that results in the production of female gametes.
oogenesis
An arrangement of internal transport in which blood bathes the organs directly and there is no distinction between blood and interstitial fluid.
open circulatory system
A type of associative learning in which an animal learns to associate one of its own behaviors with a reward or punishment and then tends to repeat or avoid that behavior. Also called trial-and-error learning.
operant conditioning
In prokaryotic DNA, a sequence of nucleotides near the start of an operon to which an active repressor can attach. The binding of the repressor prevents RNA polymerase from attaching to the promoter and transcribing the genes of the operon.
operator
A protective flap that covers the gills of fishes.
operculum
A unit of genetic function common in bacteria and phages, consisting of coordinately regulated clusters of genes with related functions.
operon
Microorganisms that are normal residents of a host but can cause illness when the host's defenses are weakened by such factors as poor nutrition or a recent bout with the flu.
opportunistic
An arrangement of the fingers such that the thumb can touch the ventral surface of the fingertips of all four fingers.
opposable thumb
A membrane protein bonded to a light-absorbing pigment molecule.
opsin
An immune response in which the binding of antibodies to the surface of a microbe facilitates phagocytosis of the microbe by a macrophage.
opsonization
The arrangement of the nerve tracts of the eye such that the visual sensations from the left visual field of both eyes are transmitted to the right side of the brain and the sensations from the right visual field of both eyes are transmitted to the left s
optic chiasm
The basis for analyzing behavior as a compromise of feeding costs versus feeding benefits.
optimal foraging theory
The mouth of an animal.
oral cavity
In classification, the taxonomic category above family.
order
A specialized center of body function composed of several different types of tissues.
organ
The actual hearing organ of the vertebrate ear, located in the floor of the cochlear canal in the inner ear; contains the receptor cells (hair cells) of the ear.
organ of Corti
A group of organs that work together in performing vital body functions.
organ system
One of several formed bodies with specialized functions, suspended in the cytoplasm of eukaryotic cells.
organelle
The study of carbon compounds (organic compounds).
organic chemistry
A plant gene in which a mutation causes a floral organ to develop in the wrong location.
organ identity gene
An individual living thing.
organism
The branch of ecology concerned with the morphological, physiological, and behavioral ways in which individual organisms meet the challenges posed by their biotic and abiotic environments.
organismal ecology
The development of organ rudiments from the three germ layers.
organogenesis
Rhythmic, involuntary contractions of certain reproductive structures in both sexes during the human sexual response cycle.
orgasm
Sites where the replication of a DNA molecule begins.
origins of replication
A large opening in a sponge that connects the spongocoel to the environment.
osculum
An animal that does not actively adjust its internal osmolarity because it is isotonic with its environment.
osmoconformer
Solute concentration expressed as molarity.
osmolarity
The control of water balance in organisms living in hypertonic, hypotonic, or terrestrial environments.
osmoregulation
An animal whose body fluids have a different osmolarity than the environment, and that must either discharge excess water if it lives in a hypotonic environment or take in water if it inhabits a hypertonic environment.
osmoregulator
The diffusion of water across a selectively permeable membrane.
osmosis
A measure of the tendency of a solution to take up water when separated from pure water by a selectively permeable membrane.
osmotic pressure
The vertebrate class of bony fishes, characterized by a skeleton reinforced by calcium phosphate; the most abundant and diverse vertebrates.
Osteichthyes
Bone-forming cells that deposit a matrix of collagen.
osteoblasts
The repeating organizational units forming the microscopic structure of hard mammalian bone.
osteons
An extinct agnathan; a fishlike creature encased in an armor of bony plates.
ostracoderm
One of three main regions of the ear in reptiles, birds , and mammals; made up of the auditory canal and, in many birds and mammals, the pinna.
outer ear
A species or group of species that is closely related to the group of species being studied, but clearly not as closely related as any study-group members are to each other.
outgroup
The idea that modern humans evolved from a second migration out of Africa that occurred about 100,000 years ago, replacing all the regional populations of hominids derived from the first migrations of Homo erectus out of Africa about 1.5 million years ago
"out of Africa" hypothesis
In the vertebrate ear, a membrane-covered gap in the skull bone, through which sound waves pass from the middle ear to the inner ear.
oval window
The cyclic recurrence of the follicular phase, ovulation, and the luteal phase in the mammalian ovary, regulated by hormones.
ovarian cycle
(1) In flowers, the portion of a carpel in which the egg-containing ovules develop. (2) In animals, the structure that produces female gametes and reproductive hormones.
ovary
A diet that is chronically excessive in calories.
overnourishment
A tube passing from the ovary to the vagina in invertebrates or to the uterus in vertebrates.
oviduct
Referring to a type of development in which young hatch from eggs laid outside the mother's body.
oviparous
Referring to a type of development in which young hatch from eggs that are retained in the mother's uterus.
ovoviviparous
The release of an egg from ovaries. In humans, an ovarian follicle releases an egg during each menstrual cycle.
ovulation
A structure that develops in the plant ovary and contains the female gametophyte.
ovule
The female gamete; the haploid, unfertilized egg, which is usually a relatively large, nonmotile cell.
ovum
The loss of electrons from a substance involved in a redox reaction.
oxidation
The production of ATP using energy derived from the redox reactions of an electron transport chain.
oxidative phosphorylation
The electron acceptor in a redox reaction.
oxidizing agent
A hormone produced by the hypothalamus and released by the posterior pituitary. It induces contractions of the uterine muscles and causes the mammary glands to eject milk during nursing.
oxytocin
The parent individuals from which offspring are derived in studies of inheritance; P stands for parental.
P generation
One of three binding sites for tRNA during translation, it holds the tRNA carrying the growing polypeptide chain; P stands for peptidyl-tRNA site).
P site
The "guardian angel of the genome," p53 is expressed when a cell's DNA is damaged. Its product, p53 protein, functions as a transcription factor for several genes.
p53 gene
A specialized region of the right atrium of the mammalian heart that sets the rate of contraction; also called the sinoatrial (SA) node.
pacemaker
The precocious development of sexual maturity in a larva.
paedogenesis
The retention in an adult organism of the juvenile features of its evolutionary ancestors.
paedomorphosis
A category of interoreceptors that detect pain.
pain receptor
Genes that define the modular patterns in terms of pairs of segments in Drosophila. Mutations in these genes result in embryos with half the normal segment number because every other segment fails to develop.
pair-rule genes
The study of human origins and evolution.
paleoanthropology
The scientific study of fossils.
paleontology
A gland with dual functions: The nonendocrine portion secretes digestive enzymes and an alkaline solution into the small intestine via a duct; the endocrine portion secretes the hormones insulin and glucagon into the blood.
pancreas
The supercontinent formed near the end of the Paleozoic era when plate movements brought all the landmasses of Earth together.
Pangaea
A group of protistans, including the trichomonads, that lacks mitochondria.
parabasalids
The sites of gas exchange in bird lungs. They allow air to flow past the respiratory surface in just one direction.
parabronchi
Pertaining to a taxon that excludes some members that share a common ancestor with members included in the taxon.
paraphyletic
An organism that absorbs nutrients from the body fluids of living hosts.
parasite
A symbiotic relationship in which the symbiont (parasite) benefits at the expense of the host by living either within the host (as an endoparasite) or outside the host (as an ectoparasite).
parasitism
One of two divisions of the autonomic nervous system; generally enhances body activities that gain and conserve energy, such as digestion and reduced heart rate.
parasympathetic division
Four endocrine glands, embedded in the surface of the thyroid gland, that secrete parathyroid hormone and raise blood calcium levels.
parathyroid glands
A peptide hormone secreted by the parathyroid glands that raise blood calcium level.
parathyroid hormone (PTH)
Members of the subkingdom of animals consisting of the sponges.
parazoa
A relatively unspecialized plant cell type that carries most of the metabolism, synthesizes and stores organic products, and develops into a more differentiated cell type.
parenchyma cell
The time and resources an individual must spend to produce and nurture offspring.
parental investment
Offspring with a phenotype that matches one of the parental phenotypes.
parental types
In scientific studies, the search for the least complex explanation for an observed phenomenon.
parsimony
A type of reproduction in which females produce offspring from unfertilized eggs.
parthenogenesis
The concentration of gases; a fraction of total pressure.
partial pressure
The expulsion of a baby from the mother, also called birth.
parturition
The order of perching birds.
passeriformes
Temporary immunity obtained by acquiring ready-made antibodies or immune cells; lasts only a few weeks or months because the immune system has not been stimulated by antigens.
passive immunity
The diffusion of a substance across a biological membrane.
passive transport
The ordering of cells into specific three-dimensional structures, an essential part of shaping an organism and its individual parts during development.
pattern formation
Extensive deposits of undecayed organic material formed primarily from the wetland moss Sphagnum.
peat
Extensive high latitude boreal wetlands occupied by Sphagnum.
peatlands
A family tree describing the occurrence of heritable characters in parents and offspring across as many generations as possible.
pedigree
The area of the ocean past the continental shelf, with areas of open water often reaching to very great depths.
pelagic zone
The copulatory structure of male mammals.
penis
An enzyme that adds carbon dioxide to phosphoenolpyruvate (PEP) to form oxaloacetate.
PEP carboxylase
An enzyme present in gastric juice that begins the hydrolysis of proteins.
pepsin
The inactive form of pepsin that is first secreted by specialized (chief) cells located in gastric pits of the stomach.
pepsinogen
The covalent bond between two amino acid units, formed by a dehydration reaction.
peptide bond
A type of polymer in bacterial cell walls consisting of modified sugars cross-linked by short polypeptides.
peptidoglycan
The interpretation of sensations by the brain.
perception
A plant that lives for many years.
perennial
A protein secreted by a cytotoxic T cell that lyses (ruptures) an infected cell by perforating its membrane.
perforin
The thickened wall of a fruit.
pericarp
A layer of cells just inside the endodermis of a root that may become meristematic and begin dividing again.
pericycle
The protective coat that replaces the epidermis in plants during secondary growth, formed of the cork and cork cambium.
periderm
The sensory and motor neurons that connect to the central nervous system.
peripheral nervous system (PNS)
Protein appendages loosely bound to the surface of the membrane and not embedded in the lipid bilayer.
peripheral protein
The impedance of blood flow by the arterioles.
peripheral resistance
Rhythmic waves of contraction of smooth muscle that push food along the digestive tract.
peristalsis
The upper part of the moss capsule (sporangium) often specialized for gradual spore discharge.
peristome
The network of tiny blood vessels that surrounds the proximal and distal tubules in the kidney.
peritubular capillaries
A permanently frozen stratum below the arctic tundra.
permafrost
A microbody containing enzymes that transfer hydrogen from various substrates to oxygen, producing and then degrading hydrogen peroxide.
peroxisome
A modified leaf of a flowering plant. Petals are the often colorful parts of a flower that advertise it to insects and other pollinators.
petal
The stalk of a leaf, which joins the leaf to a node of the stem.
petiole
A measure of hydrogen ion concentration equal to –log [H+] and ranging in value from 0 to 14.
pH
A virus that infects bacteria; also called a bacteriophage.
phage
A type of endocytosis involving large, particulate substances.
phagocytosis
An area in the vertebrate throat where air and food passages cross; in flatworms, the muscular tube that protrudes from the ventral side of the worm and ends in the mouth.
pharynx
A shift from one developmental phase to another.
phase change
An approach to taxonomy based entirely on measurable similarities and differences in phenotypic characters, without consideration of homology, analogy, or phylogeny.
phenetics
The physical and physiological traits of an organism.
phenotype
The ratio of phenotypes resulting from a genetic cross.
phenotypic ratio
A small, volatile chemical signal that functions in communication between animals and acts much like a hormone in influencing physiology and behavior.
pheromone
The portion of the vascular system in plants consisting of living cells arranged into elongated tubes that transport sugar and other organic nutrients throughout the plant.
phloem
Tube-dwelling marine worms ranging from 1 mm to 50 cm in length.
phoronids
A functional group important in energy transfer.
phosphate group
A molecule that is a constituent of the inner bilayer of biological membranes, having a polar, hydrophilic head and a nonpolar, hydrophobic tail.
phospholipid
A molecule that has been the recipient of a phosphate group.
phosphorylated
The narrow top slice of the ocean, where light permeates sufficiently for photosynthesis to occur.
photic zone
An organism that harnesses light energy to drive the synthesis of organic compounds from carbon dioxide.
photoautotroph
An organism that uses light to generate ATP but that must obtain carbon in organic form.
photoheterotroph
A quantum, or discrete amount, of light energy.
photon
A physiological response to day length, such as flowering in plants.
photoperiodism
The process of generating ATP from ADP and phosphate by means of a proton-motive force generated by the thylakoid membrane of the chloroplast during the light reactions of photosynthesis.
photophosphorylation
One of a family of visual pigments in the cones of the vertebrate eye that absorb bright, colored light.
photopsin
Receptors of light.
photoreceptor
A metabolic pathway that consumes oxygen, releases carbon dioxide, generates no ATP, and decreases photosynthetic output; generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide
photorespiration
The conversion of light energy to chemical energy that is stored in glucose or other organic compounds; occurs in plants, algae, and certain prokaryotes.
photosynthesis
The light-harvesting unit in photosynthesis, located on the thylakoid membrane of the chloroplast and consisting of the antenna complex, the reaction-center chlorophyll a, and the primary electron acceptor. There are two types of photosystems, I and II; t
photosystem
One of two light-harvesting units of a chloroplast's thylakoid membrane; it uses the P700 reaction-center chlorophyll.
photosystem I
One of two light-harvesting units of a chloroplast's thylakoid membrane; it uses the P680 reaction-center chlorophyll.
photosystem II
Growth of a plant shoot toward or away from light.
phototropism
An alignment of cytoskeletal elements and Golgi-derived vesicles across the mid-line of a dividing plant cell.
phragmoplast
The evolutionary history of a species or group of related species.
phylogeny
A taxonomic category. Phyla are divided into classes.
phylum
The group of hornworts, small herbaceous (non-woody) plants.
phylum Anthocerophyta
A formal group of mosses. Note that the term "bryophyte " refers instead to the informal group of mosses, liverworts, and hornworts, nonvascular plants that inhabit the land but lack many of the terrestrial adaptations of vascular plants.
phylum Bryophyta
The animal group that includes horseshoe crabs, scorpions, ticks, spiders, and an extinct group called the eurypterids.
phylum Chelicerata
The animal group that includes mostly aquatic animals such as crabs, lobsters, crayfish, and shrimp.
phylum Crustacea
The group of liverworts, small herbaceous (non-woody) plants.
phylum Hepatophyta
An extinct group of arthropods with pronounced segmentation.
phylum Trilobita
The animal group that includes centipedes, millipedes, and insects.
phylum Uniramia
The study of the functions of an organism.
physiology
An antibiotic, produced by plants, that destroys microorganisms or inhibits their growth.
phytoalexin
A pigment involved in many responses of plants to light.
phytochrome
Algae and photosynthetic bacteria that drift passively in the pelagic zone of an aquatic environment.
phytoplankton
An emerging, nondestructive technology that seeks to cheaply reclaim contaminated areas by taking advantage of the remarkable ability of some plant species to extract heavy metals and other pollutants from the soil and to concentrate them in easily harves
phytoremediation
Coloring matter in animals and plants, especially in a cell or tissue.
pigments
A surface appendage in certain bacteria that functions in adherence and the transfer of DNA during conjugation.
pilus
A small endocrine gland on the dorsal surface of the vertebrate forebrain; secretes the hormone melatonin, which regulates body functions related to seasonal day length.
pineal gland
A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.
pinocytosis
A function of a sound wave's frequency, or number of vibrations per second, expressed in hertz.
pitch
The core of the central vascular cylinder of monocot roots, consisting of parenchyma cells, which are ringed by vascular tissue; ground tissue interior to vascular bundles in dicot stems.
pith
Thinner regions in the walls of tracheids and vessels where only primary walls are present.
pits
An endocrine gland at the base of the hypothalamus; consists of a posterior lobe (neurohypophysis), which stores and releases two hormones produced by the hypothalamus, and an anterior lobe (adenohypophysis), which produces and secretes many hormones that
pituitary gland
A structure in the pregnant uterus for nourishing a viviparous fetus with the mother's blood supply; formed from the uterine lining and embryonic membranes.
placenta
A member of a group of mammals, including humans, whose young complete their embryonic development in the uterus, joined to the mother by a placenta.
placental mammal
Plant cells that enhance the transfer of nutrients from parent to embryo.
placental transfer cells
A member of an extinct class of fishlike vertebrates that had jaws and were enclosed in a tough, outer armor.
placoderm
Carnivores that prey on smaller animals or feed on dead animals.
planarians
Mostly microscopic organisms that drift passively or swim weakly near the surface of oceans, ponds, and lakes.
plankton
The kingdom that contains the plants.
plantae
The liquid matrix of blood in which the cells are suspended.
plasma
A derivative of B cells that secretes antibodies.
plasma cell
The membrane at the boundary of every cell that acts as a selective barrier, thereby regulating the cell's chemical composition.
plasma membrane
A small ring of DNA that carries accessory genes separate from those of a bacterial chromosome; also found in some eukaryotes, such as yeast.
plasmid
An open channel in the cell wall of plants through which strands of cytosol connect from adjacent cells.
plasmodesma
A type of protist that has amoeboid cells, flagellated cells, and an amoeboid plasmodial feeding stage in its life cycle.
plasmodial slime mold
The fusion of the cytoplasm of cells from two individuals; occurs as one stage of syngamy.
plasmogamy
A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall when the cell loses water to a hypertonic environment.
plasmolysis
The shrinkage of a cell due to water loss.
plasmolyze
One of a family of closely related plant organelles, including chloroplasts, chromoplasts, and amyloplasts (leucoplasts).
plastid
A small enucleated blood cell important in blood clotting; derived from large cells in the bone marrow.
platelet
Behavior with no apparent external goal but involves movements closely associated with goal-directed behaviors.
play
One form of the secondary structure of proteins in which the polypeptide chain folds back and forth, or where two regions of the chain lie parallel to each other and are held together by hydrogen bonds.
pleated sheet
The ability of a single gene to have multiple effects.
pleiotropy
A primitive phenotypic character possessed by a remote ancestor.
plesiomorphic character