BIBLIOGRAPHY
❮ 315abiotic components The nonliving players in an
ecosystem, such as climate and nutrients.
abscisic acid Plant hormone that inhibits cell growth,
prevents premature germination, and stimulates
closing of the stomata.
achondroplasia Autosomal dominant form of
dwarfism seen in one out of 10,000 people.
ACTH Seeadrenocorticotropic hormone.
active sitePart of the enzyme that interacts with the
substrate in an enzyme–substrate complex.
active transport The movement of a particle across
a selectively permeable membrane againstits con-
centration gradient. This movement requires the
input of energy, which is why it is termed “active”
transport.
adaptationA trait that, if altered, affects the fitness
of the organism. Adaptations are the result of nat-
ural selection and can include not only physical
traits such as eyes and fingernails but also the intan-
gible traits of organisms, such as lifespan.
adaptive radiationA rapid series of speciation events
that occur when one or more ancestral species
invades a new environment.
ADH Seeantidiuretic hormone.
adrenocorticotropic hormone (ACTH)A hormone
that stimulates the secretion of adrenal cortical
hormones, which work to maintain electrolytic
homeostasis in the body.
aerobic respiration Energy-producing reactions in
animals that involve three stages: glycolysis, the
Krebs cycle, and oxidative phosphorylation. Requires
oxygen.
age structure Statistic that compares the relative
number of individuals in the population from each
age group.
agonistic behaviorBehavior that results from a con-
flict of interest between individuals; often involves
intimidation and submission.
alcohol Organic compound that contains a hydroxyl
(—OH) functional group.
alcohol fermentation Occurs in fungi, yeast, and
bacteria. Pyruvate is converted in two steps to
ethanol, regenerating two molecules of NAD+.
aldehyde Carbonyl group in which one R is a hydro-
gen and the other is a carbon chain. Hydrophilic
and polar.
aldosteroneReleased from the adrenal gland, this
hormone acts on the distal tubules to cause the
reabsorption of more Na+and water. This increases
blood volume and pressure.
allantois Transports waste products in mammals to
the placenta. Later it is incorporated into the
umbilical cord.
allele A variant of a gene for a particular character.
allopatric speciationInterbreeding ceases because
some sort of barrier separates a single population
into two (an area with no food, a mountain, etc.).
The two populations evolve independently, and if
they change enough, then, even if the barrier is
removed, they cannot interbreed.
alternation of generations Plant life cycle, so
named because during the cycle, plants sometimes
exist as a diploid organism and at other times as a
haploid organism.
altruistic behaviorBehavior pattern that reduces the
overall fitness of one organism while increasing the
fitness of another.
alveoliFunctional unit of the lung where gas exchange
occurs.
amines Compounds containing amino groups.
amino acidA compound with a carbon center sur-
rounded by an amino group, a carboxyl group, a
hydrogen, and an R group that provides an amino
acid’s unique chemical characteristics.
aminoacyl tRNA synthetase Enzyme that makes
sure that each tRNA molecule picks up the appro-
priate amino acid for its anticodon.
amino groupA functional group that contains
—NH 2 and that acts as a base; an example is an
amino acid.
amnion Structure formed from epiblast that encloses
the fluid-filled cavity that helps cushion the devel-
oping embryo.
amygdalaThe portion of the human brain that
controls impulsive emotions and anger.Glossary
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