Cambridge International A Level Biology Revision Guide

Cambridge International AS and A Level Biology
520
loop of Henle the part of the nephron between the proximal
and distal convoluted tubules; in humans, some of the
loops of Henle are long and reach down into the medulla of
the kidney
luteinising hormone (LH) a glycoprotein hormone that
is secreted by the anterior pituitary gland to stimulate
ovulation and the development of the corpus luteum
lymph an almost colourless fluid, very similar in composition
to blood plasma but with fewer plasma proteins, that is
present in lymph vessels
lymphocyte a type of white blood cell that is involved in the
immune response; unlike phagocytes they become active
only in the presence of a particular antigen that ‘matches’
their specific receptors or antibodies
lysosome a spherical organelle found in eukaryotic cells; a
lysosome contains digestive (hydrolytic) enzymes and has a
variety of destructive functions, such as removal of old cell
organelles
macromolecule (see also polymer) a large biological
molecule such as a protein, polysaccharide or nucleic acid
macrophage phagocytic cell found in tissues throughout the
body; they act as antigen-presenting cells (APCs)
magnification the number of times greater that an image is
than the actual object; magnification = image size ÷ actual
(real) size of the object
mark–release–recapture a method of estimating
the numbers of individuals in a population of
mobile animals
median the middle value of all the values in a set of data
showing a normal distribution
meiosis the type of nuclear division that results in a halving
of chromosome number and a reshuffling of alleles; in
humans, it occurs in the formation of gametes
memory cells lymphocytes which develop during an immune
response and retain the ability to respond quickly when
an antigen enters the body on a second or any subsequent
occasion
menstrual cycle the changes that occur in the ovary and the
uterus approximately every 28 days involving ovulation
and the breakdown and loss of the lining of the uterus
(menstruation)
mesophyll the internal tissue of a leaf blade with chloroplasts
for photosynthesis and consisting of an upper layer of
palisade mesophyll (the main photosynthetic tissue) and
a lower layer of spongy mesophyll with large air spaces for
gas exchange
messenger RNA (mRNA) a single-stranded RNA molecule
that carries the genetic code from DNA to
a ribosome
Michaelis–Menten constant (K m
) the substrate concentration
at which an enzyme works at half its maximum rate
(½V max
), used as a measure of the efficiency of an enzyme;
the lower the value of K m
, the more efficient the enzyme
microtubules tiny tubes made of a protein called tubulin
and found in most eukaryotic cells; microtubules have
a large variety of functions, including cell support
and determining cell shape; the ‘spindle’ on which
chromosomes separate during nuclear division is made of
microtubules
mitochondrion ( plural: mitochondria) the organelle in
eukaryotes in which aerobic respiration takes place
mitosis the division of a nucleus into two so that the two
daughter cells have exactly the same number and type of
chromosomes as the parent cell
mode (modal class) the value that occurs most frequently in
a set of data showing a normal distribution
monoclonal antibodies many identical antibodies, made by
hybridoma cells formed by the fusion of a plasma cell and a
cancer cell
monohybrid cross a cross in which the inheritance of one
gene is considered
monomer a relatively simple molecule which is used as a
basic building block for the synthesis of a polymer; many
monomers are joined together to make the polymer, usually
by condensation reactions; common examples of molecules
used as monomers are monosaccharides, amino acids and
nucleotides
monosaccharide a molecule consisting of a single sugar unit
with the general formula (CH 2
O) n
motor end plate the ending of an axon of a motor neurone,
where it forms a synapse with a muscle
motor neurone a neurone whose cell body is in the brain
or spinal cord, and that transmits action potentials to an
effector such as a muscle or gland
multiple alleles the existence of three or more alleles of a
gene, as, for example, in the determination of A,B,O blood
groups
multiple resistance possession by a bacterium of a plasmid
carrying resistance genes for several antibiotics or
other drugs
murein see peptidoglycan
mutagen a substance that can cause mutation
mutation an unpredictable change in the structure of DNA,
or in the structure and number
of chromosomes
myelin a substance that surrounds many axons, made up of
many layers of the cell surface membranes of Schwann cells
myofibril one of many cylindrical bundles of thick and thin
filaments inside a muscle fibre
myosin the contractile protein that makes up the thick
filaments in striated muscle
natural immunity immunity gained by being infected
(active) or by receiving antibodies from the mother across
the placenta or in breast milk (passive)
natural selection the way in which individuals with
particular characteristics have a greater chance of survival
than individuals without those characteristics, and are
therefore more likely to breed and pass on the genes for
these characteristics to their offspring