AS Key Words

• Absorbtion


  The passage of molecules and ions through the gut wall into the capillaries or lacteals
• Accuracy


  The closeness of a reading to the true value
• Activation Energy


  The minimum energy that must be put into a chemical system for a reaction to occur
• Active


  Requiring energy from ATP produced by the cell's respiration
• Active site


  The specific three-dimensional site on an enzyme molecule to which the substrate binds by weak chemical bonds
• Active transport


  The movement of molecules or ions across a membrane against a concentration gradient, using energy from the hydrolysis of ATP made by the cell in respiration
• Adenosine triphosphate (ATP)


  A nucleotide in all living cells; its hydrolysis makes energy available and it is formed when chemical reactions release energy
• Adhesion


  Attraction between water molecules and hydrophilic molecules in cell walls of the xylem
• Affinity


  The degree to which two molecules are attracted to each other
• Analogous structures


  Have a corresponding function and similar shape, but have different developmental origin
• Anticodon


  Group of three bases on a tRNA molecule, correlated with the specific amino acid carried by that tRNA
• Antiparallel


  Running parallel but facing in opposite directions
• Apoplast pathway


  Pathway of water through non-living spaces between cells and in cell walls outside the cell membrane
• Atrio-ventricular node (AVN)


  The only conducting area of tissue in the wall of the heart between the atria and ventricles, through which electrical excitation passes from the atria to conducting tissue in the walls of the ventricles
• Autotroph


  An organism that synthesises its own complex organic molecules from simpler molecules using either light or chemical energy
• Binomial system 


  The system of giving organisms a unique name with two parts, the genus and species
• Biodiversity


  The number of species and the number of individuals in each species in a specified region
• Biosenor


  A device that combines a biomolecules, such as an enzyme, with a transducer, to produce and electrical signal which measures the concentration of a chemical
• Bivalent


  The association of two chromosomes of a homologous pair at prophase I of meiosis
• Bohr effect


  The movement of the oxygen dissociation curve to the right at a higher partial pressure of carbon dioxide, because at a given oxygen partial pressure haemoglobin has a lower affinity for oxygen
• Capillarity


  The movement of water up narrow tubes by capillary action
• Carnassials


  The last upper premolar and first lower molar teeth of a carnivore, which have a shearing action as the premolar slices over the molar when the jaws close
• Casparian strip


  The impermeable band of suberin in the cell walls of endodermal cells, blocking the movement of water in the apoplast, so it moves into the cytoplasm
• Catalyst


  An atom of molecule that alters the rate of a chemical reaction without taking part in the reaction or being changed by it
• Cell cycle


  The sequence of events that takes place between one cell division and the next
• Centromere


  Specialised region of a chromosomes where two chromatids join and to which the microtubules of the spindle attach at cell division
• Chemoautotroph


  An organism that uses chemical energy to make complex organic molecules
• Chiasma (Chiasmata)


  The site, as seen in a microscope, at which chromosomes exchange DNA in genetic crossing over
• Chloride shift


  The diffusion of chloride ions from the plasma into the red blood cell, preserving electrical neutrality
• Chromatid


  One of the two identical copies of a chromosome joined at the centromere prior to cell division
• Chromosome


  A long, thin structure of DNA and protein, in the nucleus of eukaryotic cells, carrying the genes
• Classification


  Putting things into group
• Codon


  Triplet of bases in mRNA that codes for a particular amino acid, or a punctuation signal
• Cohesion


  Attraction of water molecules for each other, seen as hydrgoen bonds, resulting from the dipole structure of the water molecule
• Cohesion-tension theory


  The theory of the mechanism by which the water moves up the xylem, as a result of the cohesion and adhesion of water molecules and the tension in the water column, all resulting from water's dipole structure
• Competative inhibition


  Reduction of the rate of an ezyme-controlled reaction by a molecule or ion that has a complementary shape to the active site, similar to the substrate, and binds to the active site, preventing the substrate from binding
• Condensation reaction


  Chemical process in which two molecules combine to form a more complex molecule, with the elimination of a molecule of water
• Controlled variable


  Factor that is kept constant throughout an experiment, to avoid affecting the dependent variable
• Convergent evolution


  The development of similar features in unrelated organisms over long periods of time, related to natural selection of similar features in a common environment
• Cooperative binding


  The increasing ease with which haemoglobin binds its second and third oxygen molecules, as the conformation of the haemoglobin molecule changes