Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Glossary
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α cell a cell in the islets of Langerhans in the pancreas that
senses when blood glucose levels are low and secretes
glucagon in response
α-helix a helical structure formed by a polypeptide chain,
held in place by hydrogen bonds; an α-helix is an example
of secondary structure in a protein
abiotic factor a physical characteristic of a habitat, such as
temperature, light intensity and soil pH
abscisic acid (ABA) an inhibitory plant growth regulator that
causes closure of stomata in dry conditions
absorption spectrum a graph of the absorbance of
different wavelengths of light by a compound such as a
photosynthetic pigment
accessory pigment a pigment that is not essential to
photosynthesis but which absorbs light of different
wavelengths and passes the energy to chlorophyll a
acetylcholine (ACh) a transmitter substance found, for
example, in the presynaptic neurone at neuromuscular
junctions
acetylcholinesterase an enzyme that rapidly breaks down
acetylcholine at synapses
actin the protein that makes up the thin filaments in striated
muscle
action potential a brief change in the potential difference
across cell surface membranes of neurones and muscle cells
caused by the inward movement of sodium ions followed
by the outward movement of potassium ions; it rapidly
travels along the length of the neurone
action spectrum a graph showing the effect of different
wavelengths of light on a process, e.g. on the rate of
photosynthesis
activation energy the energy that must be provided to make
a reaction take place; enzymes reduce the activation energy
required for a substrate to change into a product
active immunity immunity gained when an antigen enters
the body, an immune response occurs and antibodies are
produced by plasma cells
active site an area on an enzyme molecule where the
substrate can bind
active transport the movement of molecules or ions through
transport proteins across a cell membrane, against their
concentration gradient, using energy from ATP
adenine (A) nitrogen-containing purine base found in DNA
and RNA
ADP adenosine diphosphate
adrenaline a hormone secreted by the adrenal glands in times
of stress or excitement
aerenchyma plant tissue containing air spaces
afferent leading towards; e.g. the afferent blood vessel leads
towards a glomerulus
agarose gel a jelly made from agar, used in electrophoresis
alcoholic fermentation anaerobic respiration in which
glucose is converted to ethanol
aleurone layer a layer of tissue around the endosperm in a
cereal seed that synthesises amylase during germination
alien species a species that has been moved into a new
ecosystem where it was previously unknown; also known as
invasive species
allele a particular variety of a gene
allopatric speciation speciation that takes place as a result
of two populations living in different places and having no
contact with each other
allopolyploid possessing more than two sets of
chromosomes, where the chromosomes come from two
different species
all-or-nothing law a law that states neurones and muscle cells
only transmit impulses if the initial stimulus is sufficient
to increase the membrane potential above a threshold
potential
amniocentesis taking a small sample of amniotic fluid during
the 15th or 16th week of pregnancy, so that the genotype of
the fetus can be determined
amylopectin a polymer of α-glucose monomers linked by
both 1, 4 and 1, 6 linkages, forming a branched chain;
amylopectin is a constituent of starch
amylose a polymer of α-glucose monomers linked by 1, 4
linkages, forming a curving chain; amylose is a constituent
of starch
Animalia one of the four kingdoms of the domain Eukarya;
eukaryotic organisms which are multicellular and
heterotrophic and have a nervous system
antibiotic a substance produced by a living organism
that is capable of killing or inhibiting the growth of a
microorganism
antibody a glycoprotein (immunoglobulin) made by plasma
cells derived from B-lymphocytes, secreted in response to
an antigen; the variable region of the antibody molecule is
complementary in shape to its specific antigen
anticodon sequence of three unpaired bases on a tRNA
molecule that binds with a codon on mRNA
antidiuretic hormone (ADH) a hormone secreted from the
pituitary gland that increases water reabsorption in the
kidneys and therefore reduces water loss in urine
antigen a substance that is foreign to the body and stimulates
an immune response
apoplastic pathway the non-living system of interconnected
cell walls extending throughout a plant, used as a transport
pathway for the movement of water and mineral ions
Archaea a domain of prokaryotic organisms that resemble
bacteria but share some features with eukaryotes
arteriole small blood vessel that carries blood from arteries
to capillaries; arterioles determine how much blood flows
through capillaries