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AS Biology Definitions
Active site the specifically shaped receptor site which messenger molecules with a complimentary
shape bind to, enabling the message to be passed across
Active transport the active movement of a molecule from a low concentration to a high
concentration, requiring energy from ATP, and a carrier protein
Affinity for oxygen the tendency for Haemoglobin to combine with oxygen
Apoplastic pathway a route that water takes when entering a plant's roots, through the cell walls
of the plant cells
Atria the top chambers of the heart, separated from the ventricles by atrioventricular valves.
Atrioventricular Node (AVN) conducts electrical activity/ impulses and directs them to the Bundle of
Bohr Shift the effect that a higher CO2 concentration has on the Haemoglobin dissociation curve,
shifting it down and right
Cardiac Cycle the events that occur in one whole beat of the heart
Carrier protein proteins that transport a specific substance across the cell membrane by either
facilitated diffusion or active transport
Casparian strip a waxy waterproof layer in the endodermis of a plant tissue, which blocks water in
the Apoplastic pathway and forces it to take the Symplastic pathway
Cell cycle the process that all animal cells use to grow and divide
Cell signalling the way that cells can communicate with each other or recognise each other (when a
messenger molecule is released, transported and then binds to the receptor site of the target cell
complimentary shape) to provoke a response.
Channel protein a protein that allows a specific substance to diffuse through the channel and
therefore across the cell membrane by facilitated diffusion. If gated, a messenger molecule/
hormone is required.
Closed Circulatory System a Circulatory System in which the blood is contained and transported in
vessels; namely capillaries, veins and arteries. The blood has a much higher pressure.
Cytokinesis the splitting of cytoplasm once cells have divided, often described as the final "pinching
Cytoskeleton a structure inside the cell mainly made up of fibres, which provide mechanical
strength to cells, aid transport within cells and enable cell movement
Diastole relaxation of muscles (e.g. atrial diastole)
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Differentiation the process by which a cell becomes specialised for a particular function, by
changing shape, size, the number or types of organelles, or contents.
Diffusion the passive process of particles moving from a high concentration to a low concentration,
through a partially permeable membrane
Diploid cell cell with normal number of chromosomes; normal body cell
Eukaryotic cells animal and plant cells; they contain a "true" nucleus (double membraned) and also
double membraned organelles, e.g. mitochondria.…read more
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Prokaryotic a bacterial cell identified easily as they do not have a nucleus, but the DNA is free as a
loop in the cytoplasm. The ribosomes are smaller (18nm) and they don't contain any double
membraned organelles (e.g. mitochondria). They often have flagella to propel the cell.
Resolution the ability to distinguish between two very close together points. It is the smallest
distance for which this can be done.
Stem Cells unspecialised cells which can develop into any type of cell.…read more