Cambridge International A Level Biology Revision Guide

Cambridge International AS and A Level Biology
524
sieve element or sieve tube element a cell found in phloem
tissue, with non-thickened cellulose walls, very little
cytoplasm, no nucleus and end walls perforated to form
sieve plates, through which sap containing sucrose is
transported
sieve tube tubes formed by sieve elements lined up end
to end
silent mutation a mutation in which the change in the DNA
has no discernible effect on an organism
Simpson’s index of diversity a method to assess the
biodiversity of an ecosystem
sinoatrial node a patch of muscle in the wall of the right
atrium of the heart, whose intrinsic rate of rhythmic
contraction is faster than that of the rest of the cardiac
muscle, and from which waves of excitation spread to
the rest of the heart to initiate its contraction during the
cardiac cycle
smooth muscle type of muscle tissue found in walls of
blood vessels (except capillaries), trachea, bronchi and
bronchioles, alimentary canal and ureter; the muscle cells
are not striated
sodium–potassium pump a membrane protein (or proteins)
that moves sodium ions out of a cell and potassium ions
into it, using ATP
somatic cells cells that are not involved in gamete formation
Spearman’s rank correlation a statistical test to determine
whether two variables are correlated
speciation the production of new species
species a group of organisms with similar morphology and
physiology, which can breed together to produce fertile
offspring and which is reproductively isolated from other
species
species diversity all the species in an ecosystem
spongy mesophyll see mesophyll
stabilising selection a type of natural selection in which the
status quo is maintained because the organisms are already
well adapted to their environment
standard deviation a measure of how widely a set of data is
spread out on either side of the mean
standard error a measure of how likely it is that a mean
calculated from a sample represents the true mean for the
whole population
standardised variables variables (factors) that are kept
constant in an experiment; only the independent variable
should be changed
stem cell a relatively unspecialised cell that retains the ability
to divide an unlimited number of times
stimulus a change in the environment that is detected by a
receptor, and which may cause a response
stoma (plural: stomata) a pore in the epidermis of a leaf,
bounded by two guard cells and needed for efficient
gas exchange
striated muscle type of muscle tissue in skeletal muscles; the
muscle fibres have regular striations that can be seen under
the light microscope
stroke damage to the brain caused by bursting or blockage of
an artery
stroma the matrix of a chloroplast in which the light
independent reactions of photosynthesis occur
surrogacy see surrogate
surrogate female mammal which receives an embryo from
another female; the embryo develops inside this surrogate
mother which is not its biological parent
sympatric speciation the emergence of a new species
from another species where the two are living in the same
place; it can happen, for example, as a result
of polyploidy
symplast the living system of interconnected protoplasts
extending throughout a plant, used as a transport pathway
for the movement of water and solutes; individual
protoplasts are connected via plasmodesmata
synapse a point at which two neurones meet but do not
touch; the synapse is made up of the end of the presynaptic
neurone, the synaptic cleft and the end of the postsynaptic
neurone
synaptic cleft a very small gap between two neurones at
a synapse
T-lymphocyte a lymphocyte that does not secrete antibodies;
T helper lymphocytes stimulate the immune system to
respond during an infection, and killer T lymphocytes
destroy human cells that are infected with pathogens such
as bacteria and viruses
taxon (plural taxa) one of the groups used in the hierarchical
classification system for organisms,
e.g. species, genus, family, order, class, phylum,
kingdom and domain
telomeres repetitive sequences of DNA at the end of
chromosomes that protect genes from the chromosome
shortening that happens at each cell division
tertiary structure the compact structure of a protein
molecule resulting from the three-dimensional coiling of
the already-folded chain of amino acids
test cross a genetic cross in which an organism showing a
characteristic caused by a dominant allele is crossed with
an organism that is homozygous recessive; the phenotypes
of the offspring can be a guide to whether the first organism
is homozygous or heterozygous
tetraploid possessing four complete sets of chromosomes
thermoregulation the control of core body temperature
thylakoid a flattened, membrane-bound, fluid-filled sac,
which is the site of the light-dependent reactions of
photosynthesis in a chloroplast
thymine (T) nitrogen-containing pyrimidine base found in
DNA
tissue fluid the almost colourless fluid that fills the spaces
between body cells; tissue fluid forms from the fluid that
leaks from blood capillaries, and most of it eventually
collects into lymph vessels where it forms lymph
tonoplast the partially permeable membrane that surrounds
plant vacuoles
transcription production of an mRNA molecule on DNA
transect a line along which samples are taken, either by
noting the species at equal distances (line transect) or
placing quadrats at regular intervals (belt transect)