glossary G-3Barr body In the cells of females, a condensed X chromosome
that was inactivated during early embryonic development.
basal metabolic rate (BMR) Amount of energy required to
sustain body functions when a person is resting, awake, and
has not eaten for 12–18 hours.
base A substance that accepts H+ when it dissolves in water.
basement membrane Noncellular layer of mostly proteins
and polysaccharides that is sandwiched between an epithe-
lium and underlying connective tissue.
base pair A pair of hydrogen-bonded nucleotide bases in
two strands of nucleic acids. In a DNA double helix, adenine
pairs with thymine, and guanine with cytosine. When an
mRNA strand forms on a DNA strand during transcription,
uracil (U) pairs with the DNA’s adenine.
base-pair substitution A mutation occurring in a replicating
DNA molecule (a chromosome) when one base is wrongly
substituted for another in a base pair.
basophil Fast-acting white blood cell that secretes histamine
and other substances during inflammation.
B cell B lymphocyte; the only white blood cell that produces
antibodies, then positions them at the cell surface or secretes
them as weapons in immune responses.
B cell receptor Antigen receptor on a B cell.
bicarbonate–carbon dioxide buffer system A system used
to restore the body’s normal pH level by neutralizing excess
H+ and allowing for the exhalation of carbon dioxide formed
during the reaction. It does not eliminate the excess H+ and
therefore has only a temporary effect.
bile A yellowish fluid made in the liver, stored in the gall-
bladder, and released into the upper small intestine where it
aids in the digestion and absorption of fats.
binge eating Eating an abnormally large quantity of food in
a few hours at least twice a week for 6 months or more.
biofuel An alternative, renewable fuel made from plants and
organic wastes.
biogeochemical cycle The movement of an element such as
carbon or nitrogen from the environment to organisms, then
back to the environment.
biogeography [Gk. bios, life, and geographein, to describe the
surface of Earth] The study of major land regions, each hav-
ing distinguishing types of plants and animals.
biological clock Internal time-measuring mechanism that
has a role in adjusting an organism’s daily activities, seasonal
activities, or both in response to environmental cues.
biological magnification The increasing concentration of
a nondegradable or slowly degradable substance in body
tissues as it is passed along food chains.
biological molecule A molecule that contains carbon and
that is formed in a living organism.
biomass The combined weight of all the organisms at a par-
ticular feeding (trophic) level in an ecosystem.to smooth out pulsations in blood pressure caused by heart
contractions.
atom The smallest unit of matter that is unique to a particu-
lar element.
atomic number The number of protons in the nucleus of
each atom of an element; it differs for each element.
ATP Adenosine triphosphate (ah-den-uh-seen try-foss-
fate) A nucleotide composed of adenine, ribose, and three
phosphate groups. As the main energy carrier in cells, it
directly or indirectly delivers energy to or picks up energy
from nearly all metabolic pathways.
ATP/ADP cycle In cells, a mechanism of ATP renewal. When
ATP donates a phosphate group to other molecules (and so
energizes them), it reverts to ADP, then forms again by phos-
phorylation of ADP.
atrioventricular (AV) node In the septum dividing the heart
atria, a site that contains bundles of conducting fibers. Stimuli
arriving at the AV node from the cardiac pacemaker (sino-
atrial node) pass along the bundles and continue on via Pur-
kinje fibers to contractile muscle cells in the ventricles.
atrioventricular valve One-way flow valve between the
atrium and ventricle in each half of the heart.
atrium (ay-tree-um) Upper chamber in each half of the heart;
the right atrium receives deoxygenated blood (from tissues)
entering the pulmonary circuit of blood flow, and the left
atrium receives oxygenated blood from pulmonary veins.
autoimmunity Misdirected immune response in which lym-
phocytes mount an attack against normal body cells.
autonomic nerves (ah-toe-nom-ik) Those nerves leading
from the central nervous system to the smooth muscle, car-
diac muscle, and glands of internal organs and structures—
that is, to the visceral portion of the body.
autosomal dominant Condition caused by a dominant allele
on an autosome (not a sex chromosome). The allele is always
expressed to some extent, even in heterozygotes.
autosomal recessive Condition caused by a recessive allele
on an autosome (not a sex chromosome). Only recessive
homozygotes show the resulting phenotype.
autosome Any chromosome that is not a sex (gender-
determining) chromosome.
autotroph (ah-toe-trofe) [Gk. autos, self, and trophos,
feeder] An organism able to build its own large organic mol-
ecules by using carbon dioxide and energy from the physical
environment. Compare heterotroph.
axial skeleton (ax-ee-uhl) The skull, backbone, ribs, and
breastbone (sternum).
axon Of a neuron, a long, cylindrical extension from the
cell body, with finely branched endings. Action potentials
move rapidly, without alteration, along an axon; their arrival
at axon endings may trigger the release of neurotransmitter
molecules that influence an adjacent cell.
baroreceptor reflex The short-term control over arterial
pressure. It keeps blood pressure within normal limits in the
face of sudden changes in blood pressure.
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