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Glossary
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Glossary

B
B cell
A type of lymphocyte that develops in the bone marrow and later produces antibodies, which mediate humoral immunity.
Bacteria
One of two prokaryotic domains, the other being the Archaea.
bacteriophage
[L. bacterium + Gk. phagein, to eat]
A virus that parasitizes a bacterial cell.
bacterium pl. bacteria
A prokaryotic microorganism in Domain Bacteria.
balanced polymorphism
A type of polymorphism in which the frequencies of the coexisting forms do not change noticeably over many generations.
bark
All tissues external to the vascular cambium in a plant growing in thickness, consisting of phloem, phelloderm, cork cambium, and cork.
Barr body
A dense object lying along the inside of the nuclear envelope in female mammalian cells, representing an inactivated X chromosome.
basal body
[Gk. basis, foundation]
A eukaryotic cell organelle consisting of a 9 + 0 arrangement of microtubule triplets; may organize the microtubule assembly of a cilium or flagellum; structurally identical to a centriole.
basal metabolic rate (BMR)
[Gk. basis, foundation + metabole, change]
The minimal number of kilocalories a resting animal requires to fuel itself for a given time.
base
A substance that reduces the hydrogen ion concentration in a solution. See Alkaline.
basement membrane
The floor of an epithelial membrane on which the basal cells rest.
base-pair substitution
A point mutation; the replacement of one nucleotide and its partner from the complementary DNA strand by another pair of nucleotides.
base-pairing principle
In the formation of nucleic acids, the requirement that adenine must always pair with thymine (or uracil) and guanine with cytosine.
basidium pl. basidia
A reproductive appendage that produces sexual spores on the gills of mushrooms. The fungal division Basidiomycota is named for this structure.
Batesian mimicry
(baytz-ee-un mim-ih-kree)
A type of mimicry in which a harmless species looks like a different species that is poisonous or otherwise harmful to predators.
behavior
All of the acts an organism performs, as in, for example, seeking a suitable habitat, obtaining food, avoiding predators, and seeking a mate and reproducing.
behavioral ecology
A heuristic approach based on the expectation that Darwinian fitness (reproductive success) is improved by optimal behavior.
benthic zone
The bottom surfaces of aquatic environments.
biennial
(by-en-ee-ul) [L. biennium, a space of two years; bi, twice + annus, year]
A plant that requires two years to complete its life cycle.
bilateral symmetry
[L. bi, twice, two + lateris, side; Gk. summetros, symmetry]
Characterizing a body form with a central longitudinal plane that divides the body into two equal but opposite halves.
bilateria
(by-leh-teer-ee-uh) [L. bi, twice, two + lateris, side]
Members of the branch of eumetazoans possessing bilateral symmetry.
bile
A yellow secretion of the vertebrate liver, temporarily stored in the gallbladder and composed of organic salts that emulsify fats in the small intestine.
binary fission
The type of cell division by which prokaryotes reproduce; each dividing daughter cell receives a copy of the single parental chromosome.
binomial
[L. bi, twice, two + Gk. nomos, usage, law]
The two-part Latinized name of a species, consisting of genus and specific epithet.
biochemical pathway
[Gk. bios, life + chemeia, alchemy]
An ordered series of chemical reactions in a living cell, in which each step is catalyzed by a specific enzyme; different biochemical pathways serve different functions in the life of the cell.
biodiversity hotspot
A relatively small area with an exceptional concentration of species.
bioenergetics
The study of how organisms manage their energy resources.
biogeochemical cycles
[Gk. bios, life + geo, earth + chemeia, alchemy; kyklos, circle, wheel]
The various nutrient circuits, which involve both biotic and abiotic components of ecosystems.
biogeography
The study of the past and present distribution of species.
biological clock
[Gk. bios, life + logos, discourse]
Proposed internal factor(s) in organisms that governs functions that occur rhythmically in the absence of external stimuli.
biological magnification
A trophic process in which retained substances become more concentrated with each link in the food chain.
biological species
A population or group of populations whose members have the potential to interbreed.
biomass
[Gk. bios, life]
The dry weight of organic matter comprising a group of organisms in a particular habitat.
biome
(by-ome)
One of the world's major communities, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment.
biosphere
(by-oh-sfeer) [Gk. bios, life + sphaira, globe]
The entire portion of Earth that is inhabited by life; the sum of all the planet's communities and ecosystems.
biosynthesis
[Gk. bios, life + synthesis, a putting together]
Formation by living organisms of organic compounds from elements or simple compounds.
biotechnology
The industrial use of living organisms or their components to improve human health and food production.
biotic
(by-ot-ik) [Gk. bios, life]
Pertaining to the living organisms in the environment.
bipedal
[L. bi, twice, two + pes, foot]
Walking upright on two feet.
blade
(1) The broad, expanded part of a leaf. (2) The broad, expanded photosynthetic part of the thallus of a multicellular alga or a simple plant.
blastocoel
(blas-toh-seel) [Gk. blastos, sprout + koilos, a hollow]
The fluid-filled cavity that forms in the center of the blastula embryo.
blastocyst
[Gk. blastos, sprout + kystis, sac]
An embryonic stage in mammals; a hollow ball of cells produced one week after fertilization in humans.
blastodisc
[Gk. blastos, sprout + discos, a round plate]
Disklike area on the surface of a large, yolky egg that undergoes cleavage and gives rise to the embryo.
blastopore
(blas-toh-por) [Gk. blastos, sprout + poros, a way, means, path]
The opening of the archenteron in the gastrula that develops into the mouth in protostomes and the anus in deuterostomes.
blastula
(blas-tyoo-la) [Gk. blastos, sprout]
The hollow ball of cells marking the end stage of cleavage during early embryonic development.
blood
A type of connective tissue with a fluid matrix called plasma in which blood cells are suspended.
blood-brain barrier
A specialized capillary arrangement in the brain that restricts the passage of most substances into the brain, thereby preventing dramatic fluctuations in the brain's environment.
blood pressure
The hydrostatic force that blood exerts against the wall of a vessel.
bond energy
The quantity of energy that must be absorbed to break a particular kind of chemical bond; equal to the quantity of energy the bond releases when it forms.
bond strength
The strength with which a chemical bond holds two atoms together; conventionally measured in terms of the amount of energy, in kilocalories per mole, required to break the bond.
book lungs
Organs of gas exchange in spiders, consisting of stacked plates contained in an internal chamber.
botany
[Gk. botanikos, of herbs]
The study of plants.
bottleneck effect
Genetic drift resulting from the reduction of a population, typically by a natural disaster, such that the surviving population is no longer genetically representative of the original population.
Bowman's capsule
A cup-shaped receptacle in the vertebrate kidney that is the initial, expanded segment of the nephron where filtrate enters from the blood.
box
A small portion of a gene or protein that appears in many genes or proteins that are related in structure; the box usually has some specific function, sometimes called a "motif", like binding DNA or interacting with specific proteins or other molecules.
brain
The master control center in an animal; in vertebrates, the brain and spinal cord make up the central nervous system.
brainstem
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.
bryophyte
(bry-oh-fites)
The mosses, liverworts, and hornworts; a group of nonvascular plants that inhabit the land but lack many of the terrestrial adaptations of vascular plants.
bronchus pl. bronchi
(bronk-us, bronk- eye) [Gk. bronchos, windpipe]
One of a pair of respiratory tubes branching into either lung at the lower end of the trachea; it subdivides into progressively finer passageways, the bronchioles, culminating in the alveoli.
bud
(1) In plants, an embryonic shoot, including rudimentary leaves, often protected by special bud scales. (2) In animals, an asexually produced outgrowth that develops into a new individual.
budding
An asexual means of propagation in which outgrowths from the parent form and pinch off to live independently or else remain attached to eventually form extensive colonies.
buffer
A substance that consists of acid and base forms in solution and that minimizes changes in pH when extraneous acids or bases are added to the solution.
bulb
A modified bud with thickened leaves adapted for underground food storage.
bulbourethral gland
(bul-bo-you-ree-thral)
One of a pair of glands near the base of the penis in the human male that secrete fluid that lubricates and neutralizes acids in the urethra during sexual arousal.
bulk flow
The movement of water due to a difference in pressure between two locations.
bundle of His
In the vertebrate heart, a group of muscle fibers that carry impulses from the atrioventricular node to the walls of the ventricles; the only electrical bridge between the atria and the ventricles.