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Biology Dictionary for GCSE Biology and A Level Biology
Capillary: Narrow blood vessel with single celled endothelium (wall) to allow exchange of materials. Walls are permeable, which allows the formation of tissue fluid.
Cardiovascular Centre: centre in the medulla (lower part of brain) which can modify the rate of heartbeat.
Carotid Body: patch of chemoreceptors located in wall of carotid artery. Senses change in blood CO2 and H+ levels. Sends sensory impulses to the medulla. See also; Aortic body.
Casparian Strip: In plant roots, a waxy waterproof strip, made from suberin, which surrounds endodermal cells. Its function is to block the apoplast pathway, so that all water and ions enter by the symplast route and so the plant has some control over the water/ions entering the xylem.
Centriole: Bundles of protein fibres, found near the nucleus. Centrioles migrate to opposite poles of the cell during mitosis and form the spindle.
Centromere: The point of attachment of two chromatids on a double chromosome.
Chemoreceptor: cells sensitive to chemical change. e.g. in the aortic and carotid bodies there are chemoreceptors which are sensitive to CO2/pH levels.
Chiasmata: The point at which crossing over occurs. i.e. sites where homologous chromosomes break / rejoin.
Chromatid: One half of a double chromosome (present at the start of cell division)
Chromatin: ‘Spread out’ DNA in the nucleus of a non-dividing cell i.e. not coiled up into chromosomes
Chromosome: Condensed mass of DNA which appears just before cell division. Each chromosome is one super-coiled DNA molecule containing 1000s of genes.
Clone: A identical genetic copy. You can clone DNA strands, cells or whole organisms
Codon: A sequence of three bases coding for a particular amino acid e.g GUG = valine.
Cohesion Tension: The force that draws water up the xylem of a plant. Water evaporating from the upper surfaces of a leaf creates a negative pressure in the xylem which draws water up through the plant from roots to leaves. Water is cohesive (sticky) so it can form long, continuous unbroken columns all the way up a plant.
Companion cells: a cell in the phloem that is connected to a sieve-tube member by numerous plasmodesmata (strands of cytoplasm). They control the activity of the sieve tubes.