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2261 Cards in this Set

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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 nervous inputs resulting from stress.
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 by a chitinous exoskeleton, molting, jointed appendages, and a body formed of distinct groups of segments.
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 synthases provide a port through which hydrogen ions diffuse into the matrix of a mitrochondrion.
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 kilocalorie.
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 the Calvin cycle during the day, when stomata are closed.
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 harmful allele to offspring.
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 the secondary cell wall is stronger and more rigid and is the primary constituent of wood.
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 specialized cell types.
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 effector cells specific for the stimulating antigen.
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 gene cloning.
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 microorganisms or other foreign cells.
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 more concentrated to where they are less concentrated.
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 functions.
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 concentration, and temperature.
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 of 270 mV in the direction of zero voltage.
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 block the passage of sperm.
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 of an organism, are tested for hybridization with various samples of cDNA molecules.
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 all the waste it generates.
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. A scanning electron microscope (SEM) is used to study the fine details of cell surfaces.
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 regulation and maintain homeostasis.
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 maintenance of the female reproductive system and secondary sex characteristics.
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 potential.
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