Glossary • MHR 569abiotic describes all the non-living physical and
chemical factors that influence an organism’s
survival, such as temperature, light, water, and
nutrients. (13.1)
accommodation adjustment the ciliary body makes
to the shape of the lens in order to focus on
objects at varying distances. (5.3)
acetylcholine the primary neurotransmitter of
both the somatic nervous system and the
parasympathetic nervous system. (5.2)
acetyl-CoA molecule produced during the transition
reaction for use in the Krebs cycle. (3.1)
acid substance that donates H+ions when it
dissolves or dissociates in water. (1.3)
acid precipitation rain or snow that has a pH of
about 4 to 5, as compared to normal
precipitation, which has a pH of about 6.5. (15.3)
acromegaly condition in which excess HGH
production during adulthood causes thickening
of bone tissue, leading to spinal deformities and
abnormal growth of the head, hands, and feet.
The condition is often caused by a tumour in the
pituitary gland. (6.1)
action potential in an axon, the change in charge
that occurs when the gates of the K+channels
close and the gates of the Na+channels open after
a wave of depolarization is triggered. (5.2)
activating enzyme enzyme that binds the correct
amino acid to the tRNA molecule having the
correct anticodon. Each activating enzyme has
two binding sites: one that binds to the anticodon
on the tRNA molecule, and one that binds to the
amino acid corresponding to that anticodon. (8.3)
activation energy input of energy required to start a
chemical reaction. (2.1)
activator molecule that promotes the action of
enzymes by binding to the allosteric site. (2.2).
Also a catabolite activator protein or CAP, that
stimulates gene expression in E. coliby binding
first to a cAMP molecule and then to a site near
the Placpromoter. This action makes it easier for
RNA polymerase to bind to the promoter, which
in turn speeds up the rate at which lacgenes are
transcribed. (8.4)
active site site on an enzyme where substrates fit
and where catalysis occurs. (2.2)
active transport process that moves particles across
the cell membrane against a concentration
gradient. The process requires the help of special
proteins (often called pumps) and energy. (2.3)
adaptation the process by which a receptor
responds to changes in the environment. If a
stimulus continues for a long time, the brain
becomes unaware of it. (5.3) Also, in evolution,
a particular structure, physiology, or behaviour
that helps an organism survive and reproduce
in a particular environment. (10.1, 12.1)
adaptive radiation diversification of a common
ancestral species into a variety of species, all of
which are differently adapted. (12.3)
adenine (A) one of four nitrogen-containing bases in
a nucleotide that make up DNA and RNA. (7.1)
adrenal cortex outer part of adrenal gland, which
produces two types of steroid hormones:
glucocorticoids (cortisol) and mineralcorticoids
(aldosterone). (6.2)
adrenal gland organ composed of two layers: an
outer cortex and an inner medulla. Each layer
produces different hormones and functions as an
independent organ. (6.2)
adrenal medulla inner part of adrenal gland, which
secretes adrenaline and noradrenaline. (6.2)
adrenocorticotropic hormone (ACTH) hormone that
regulates the production of cortisol and
aldosterone. (6.2)
aerobic in an organism, environment, or cellular
process, requiring oxygen. (3.1)
aerobic cellular respiration process composed of
metabolic pathways that contribute to the
production of ATP in the presence of oxygen.
(3.1)
age pyramid age structure of a population
represented in a way that shows the proportion
of individuals in the population in each age
class. (14.4)
age structure distribution of individuals among the
age classes. (14.4)
Agricultural Revolution time period during which
humans started to plant and harvest crops and
domesticate animals, resulting in a stable food
supply and increasing human population growth.
(15.1)
alleles alternative forms of a gene. (11.1)
allopatric speciation formation of a new species
that occurs when a population is split into two or
more isolated groups by a geographical barrier.
(12.3)
all-or-none principle principle that governs the
response of an axon to a stimulus. If a neuron
is stimulated sufficiently, an impulse will travel
the length of an axon, if the stimulus is not
sufficient, no impulse will travel down the axon.A
Glossary