Cambridge International A Level Biology Revision Guide

Glossary
haemoglobin the red pigment found in red blood cells,
whose molecules contain four iron atoms within a globular
protein made up of four polypeptides, and that combines
reversibly with oxygen
haemophilia a genetic disease in which there is an insufficient
amount of a clotting factor such as factor VIII in the blood
haploid cell one that possesses one complete set of
chromosomes; the abbreviation for haploid is n
heterotroph an organism needing a supply of organic
molecules as its carbon source
heterozygous having two different alleles of a gene
homeostasis maintaining a relatively constant environment
for the cells within the body
homologous chromosomes a pair of chromosomes in a
diploid cell that have the same structure as each other,
with the same genes (but not necessarily the same alleles of
those genes) at the same loci, and that pair together to form
a bivalent during the first division of meiosis
homozygous having two identical alleles of a gene
hormone a substance secreted by an endocrine gland, that is
carried in blood plasma to another part of the body where
it has an effect (plant hormone – see plant
growth regulator)
hybridoma a cell formed by the fusion of a plasma cell and
a cancer cell; it can both secrete antibodies and divide to
form other cells like itself
hydrogen bond a relatively weak bond formed by the
attraction between a group with a small positive charge
on a hydrogen atom and another group carrying a small
negative charge, e.g. between two –O δ– H δ+ groups
hydrolysis a reaction in which a complex molecule is broken
down to simpler ones, involving the addition of water
hypha (plural: hyphae) a long threadlike structure
surrounded by a cell surface membrane and cell wall that
makes up part of the body of a fungus; hyphae may or may
not have cross-walls
IAA a type of auxin
immobilised enzymes enzymes that have been fixed to a
surface or within a bead of agar gel
immune response the complex series of responses of the
body to the entry of a foreign antigen; it involves the
activity of lymphocytes and phagocytes
immune system the body’s defence system
immunity protection against infectious diseases, gained
either actively or passively
immunoglobulin an antibody
immunological memory the ability of the immune system
to respond quickly to antigens that it recognises as having
entered the body before
in vitro fertilisation (IVF) the addition of sperm to eggs in
a Petri dish or other container, in which fertilisation takes
place
inbreeding depression increase in the proportion of
debilitated offspring as a result of breeding between closely
related organisms of the same species
independent assortment the way in which different alleles
of genes on different chromosomes may end up in any
combination in gametes, resulting from the random
alignment of bivalents on the equator during meiosis I
independent variable the variable (factor) that is deliberately
changed in an experiment
induced fit hypothesis a model for enzyme action; the
substrate is a complementary shape to the active site of the
enzyme, but not an exact fit – the enzyme or sometimes
substrate can change shape slightly to ensure a perfect fit
infectious disease a disease caused by an organism such as a
bacterium or virus
insulin a small peptide hormone secreted by the β cells in
the islets of Langerhans in the pancreas that reduces blood
glucose levels
interquartile range the range of values into which the middle
50% of a set of data falls
islet of Langerhans a group of cells in the pancreas which
secrete insulin and glucagon
isotopes atoms of the same element with different atomic
mass numbers
K m
see Michaelis–Menten constant
karyogram a photograph or diagram of a set of chromosomes
from an individual; the chromosomes are normally
arranged in their homologous pairs in order of size; sex
chromosomes may be shown separately
kinetochore a protein structure found at the centromere of a
chromatid to which microtubules attach during
cell division
Krebs cycle a cycle of reactions in aerobic respiration in the
matrix of a mitochondrion in which hydrogens pass to
hydrogen carriers for subsequent ATP synthesis and some
ATP is synthesised directly
lactate (or lactic acid) the end-product of lactic fermentation,
often produced by muscles during exercise
lactic fermentation respiration in which, in the absence of
oxygen, glucose is converted to lactic acid (lactate)
leukaemia cancer of the stem cells that give rise to white
blood cells
light dependent reactions reactions in photosynthesis for
which light energy is needed
light independent reactions reactions in photosynthesis for
which light energy is not needed
limiting factor the one factor, of many affecting a process,
that is nearest its lowest value and hence is rate-limiting
linkage the presence of two genes on the same chromosome,
so that they tend to be inherited together and do not assort
independently
link reaction decarboxylation and dehydrogenation of
pyruvate, resulting in the formation of acetyl coenzyme A,
linking glycolysis with the Krebs cycle
lock and key hypothesis a model for enzyme action; the
substrate is a complementary shape to the active site of the
enzyme, and fits exactly into the site
locus the position at which a particular gene is found on a
particular chromosome; the same gene is always found at
the same locus
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