Cambridge International A Level Biology Revision Guide

Glossary
creatinine a nitrogenous excretory substance produced from
the breakdown of creatine
crista (plural: cristae) a fold of the inner membrane of
the mitochondrial envelope on which are found stalked
particles of ATP synthase and electron transport chains
associated with aerobic respiration
crossing over an event that occurs during meiosis I, when
chromatids of two homologous chromosomes break and
rejoin so that a part of one chromatid swaps places with the
same part of the other
cystic fibrosis a genetic disease caused by a recessive allele of the
CFTR (cystic fibrosis transmembrane regulator) gene
cytokinins plant growth regulators that stimulate cell division
cytoplasm the contents of a cell, excluding the nucleus
cytosine (C) nitrogen-containing pyrimidine base found in
DNA and RNA
deamination the breakdown of excess amino acids in the
liver, by the removal of the amine group; ammonia and
eventually urea are formed from the amine group
dendrite a short cytoplasmic process of a neurone that
receives nerve impulses from other neurones
deoxyribose a pentose (5C) sugar found in DNA
dependent variable in an experiment, the variable that
changes as a result of changing the independent variable
depolarisation the reversal of the resting potential across the
cell surface membrane of a neurone or muscle cell, so that
the inside becomes positively charged compared with the
outside
diabetes an illness in which the pancreas does not
make sufficient insulin, or where cells do not respond
appropriately to insulin
diffusion the net movement of molecules or ions from
a region of higher concentration to a region of lower
concentration down a gradient, as a result of the random
movements of particles
dihybrid cross a genetic cross in which two different genes
are considered
diploid cell one that possesses two complete sets of
chromosomes; the abbreviation for diploid is 2n
directional selection a type of selection in which the most
common varieties of an organism are selected against,
resulting in a change in the features of the population
disaccharide a sugar molecule consisting of two
monosaccharides joined together by a glycosidic bond
disruptive selection natural selection that favours the
survival of individuals at two different points within the
range of variation, resulting in two different phenotypes
DNA ligase an enzyme that catalyses the linkage of
nucleotides during the formation of a DNA molecule
domain one of the three major groups into which all
organisms are classified
dominant an allele is said to be dominant when its effect on
the phenotype of a heterozygote is identical to its effect in a
homozygote
dormancy a state of ‘suspended animation’, in which
metabolism is slowed right down, enabling survival in
adverse conditions
double circulation a circulatory system in which the blood
travels twice through the heart on one complete circuit of
the body; the pathway from heart to lungs and back to the
heart is known as the pulmonary circulation, and that from
heart to the rest of the body and back to the heart as the
systemic circulation
ecosystem all the living organisms of all species (the biotic
component) and all the non-living components (the abiotic
component) that are found together in a defined area and
that interact with one another
effector an organ or tissue that carries out an action in
response to a stimulus; muscles and glands are effectors
efferent leading away from
electrochemical gradient a gradient across a cell surface
membrane that involves both a difference in concentrations
of ions and a potential difference e.g. the entry of sodium
ions into neurones
electron transport chain chain of adjacently arranged carrier
molecules in the inner mitochondrial membrane, along
which electrons pass by redox reactions
embryo transfer embryos are removed from a female
mammal and transferred to one or more surrogate mothers
to complete development until birth
emphysema a disease in which alveoli are destroyed, giving
large air spaces and decreased surface area for gaseous
exchange; it is often associated with chronic bronchitis
endemic of disease, a disease that is always in a population;
of species, a species that is only found in a certain area and
nowhere else
endocrine gland a gland that secretes its products, which are
always hormones, directly into the blood
endocytosis the bulk movement of liquids (pinocytosis) or
solids (phagocytosis) into a cell, by the infolding of the
cell surface membrane to form vesicles containing the
substance; endocytosis is an active process requiring ATP
endoplasmic reticulum (ER) a network of flattened sacs
running through the cytoplasm of eukaryotic cells;
molecules, particularly proteins, can be transported
through the cell inside the sacs separate from the rest of the
cytoplasm; ER is continuous with the outer membrane of
the nuclear envelope
endosperm a tissue that stores food in some seeds, such as
cereal grains
endothelium a tissue that lines the inside of a structure, such
as the inner surface of a blood vessel
enzyme a protein produced by a living organism that acts
as a catalyst in a specific reaction by reducing activation
energy
epithelium a tissue that covers the outside of a structure
Eukarya domain that contains all eukaryotic organisms:
protoctists, fungi, plants and animals
eukaryotic cell a cell containing a nucleus and other
membrane-bound organelles
evolution changes in a population that may lead to speciation
or extinction
excretion the removal of toxic or waste products of
metabolism from the body
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