Cambridge International A Level Biology Revision Guide

Glossary
transfer RNA (tRNA) a folded, single-stranded RNA
molecule that carries an amino acid to a ribosome for
protein synthesis
transformed describes an organism that has taken up DNA
introduced from a different species of organism
translation production of a polypeptide at a ribosome from
the code on mRNA
translocation the transport of assimilates such as sucrose
through a plant, in phloem tissue; translocation requires
the input of metabolic energy; the term is sometimes used
more generally to include transport in the xylem
transmission the transfer of a pathogen from one person to
another
transmitter substance a chemical that is released from a
presynaptic neurone when an action potential arrives, and
that then diffuses across the synaptic cleft and may initiate
an action potential in the postsynaptic neurone
transpiration the loss of water vapour from a plant to its
environment, by diffusion down a water potential gradient;
most transpiration takes place through the stomata in the
leaves
transverse system tubule (also known as T-system tubule or
T-tubule) infolding of the sarcolemma that goes deep into a
muscle fibre
triglyceride a lipid whose molecules are made up of a glycerol
molecule and three fatty acids
triploid possessing three complete sets of chromosomes
tropomyosin a fibrous protein that is part of the thin
filaments in myofibrils in striated muscle
troponin a calcium-binding protein that is part of the thin
filaments in myofibrils in striated muscle
t-test a statistical procedure used to determine whether the
means of two samples differ significantly
ultrafiltration filtration on a molecular scale e.g. the filtration
that occurs as blood flows through capillaries, especially
those in glomeruli in the kidney
uracil (U) nitrogen-containing pyrimidine base found
in RNA
urea a nitrogenous excretory product produced in the liver
from the deamination of amino acids
ureter a tube that carries urine from a kidney to the bladder
urethra a tube that carries urine from the bladder to
the outside
uric acid a nitrogenous excretory product, made by the
breakdown of purines
V max
the theoretical maximum rate of an enzyme-controlled
reaction, obtained when all the active sites
are occupied
vaccination giving a vaccine containing antigens for a
disease, either by injection or by mouth; vaccination
confers artificial active immunity
vacuole an organelle found in eukaryotic cells; a large,
permanent central vacuole is a typical feature of plant
cells, where it has a variety of functions, including storage
of biochemicals such as salts, sugars and waste products;
temporary vacuoles, such as phagocytic vacuoles (also
known as phagocytic vesicles), may form in animal cells
(see endocytosis)
variable number tandem repeat (VNTR) a length of DNA
that contains different numbers of repeated base sequences
in different individuals in a species
vector a means of delivering genes into a cell, used in
gene technology; e.g. plasmids and viruses; see also disease
vector
vein a blood vessel that carries blood back towards the heart;
it has relatively thin walls and contains valves
vessel element see xylem vessel element
virus very small (20–300 nm) infectious particle which can
replicate only inside living cells and consists essentially of
a simple basic structure of a genetic code of DNA or RNA
surrounded by a protein coat
VNTR see variable number tandem repeat
voltage-gated channel a channel protein through a cell
membrane that opens or closes in response to changes in
electrical potential across the membrane
water potential a measure of the tendency of water to move
from one place to another; water moves from a solution
with higher water potential to one with lower water
potential; water potential is decreased by the addition of
solute, and increased by the application of pressure; symbol
is ψ or ψ w
xerophyte a plant adapted to survive in conditions where
water is in short supply
xylem tissue containing xylem vessel elements and other
types of cells, responsible for support and the transport of
water through a plant
xylem vessel a dead, empty tube with lignified walls, through
which water is transported in plants; it is formed by xylem
vessel elements lined up end to end
xylem vessel element a dead, lignified cell found in
xylem specialised for transporting water and support;
the ends of the cells break down and join with
neighbouring elements to form long tubes called
xylem vessels
zygote a cell formed by the fusion of two gametes; normally
the gametes are haploid and the zygote is diploid
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