AQA Biology A level

• abiotic


  Ecological factor that makes up part of the non-biological environment of an organism, e.g. temperature, pH, rainfall and humidity
• acetylcholine


  One of a group of chemicals, called neurotransmitters, released by neurones. It diffuses across the gap (synapse) between adjacent neurones and so passes an impulse from one neurone to the next
• action potential


  Change that occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes
• actin


  Filamentous protein which is involved in contraction within cells, especially muscle cells
• active transport


  Movement of a substance from a region where it is in a low concentration to a region where it is in a high concentration. The process requires the expenditure of metabolic energy
• adrenaline


  A hormone produced by the adrenal glands in times of stress that prepares the body for an emergency
• aerobic


  Connected with the presence of free oxygen. Aerobic respiration requires free oxygen to release energy from glucose
• anaerobic


  Connected with the absence of oxygen. Anaerobic respiration releases energy from glucose or other foods without the presence of oxygen
• antidiuretic hormone (ADH)


  A hormone produced by the hypothalamus that passes to the posterior pituitary gland from where it is secreted. ADH reduces the volume of water in urine by increasing water reabsorption in the kidneys
• antioxidant


  Chemical which reduces or prevents oxidation. Often used as an additive to prolong the shelf-life of certain foods
• apoplastic pathway


  Route through the cell walls and intercellular spaces of plants by which water and dissolved substances are transported
• artificial selection


  Breeding of organisms by human selection of parents/gametes in order to perpetuate certain characteristics and/or to eliminate others
• atheroma


  Fatty deposits in the walls of arteries, often associated with high cholesterol levels in the blood
• autonomic nervous system


  Part of the nervous system, controlling the muscles and glands, that is not under voluntary control
• autosome


  A chromosome which is not a sex chromosome
• axon


  A process extending from a neurone that conducts action potentials away from the cell body
• Benedict's test


  A simple biochemical reaction to detect the presence of reducing sugars
• biodiversity


  The range and variety of genes, species and habitats within a particular region
• biomass


  The total mass of living material, normally measured in a specific area over a given period of time
• biotic


  An ecological factor that makes up part of the living environment of an organism. Examples include food availability, competition and predation
• biosensor


  A device that uses biological molecules to measure the level of certain chemicals
• body mass index (BMI)


  A person's body mass in kilograms divided by the square of their height in metres
• Calvin cycle


  A biochemical pathway that forms part of the light-independent reaction of photosynthesis, during which carbon dioxide is reduced to form carbohydrate
• Casparian strip


  A distinctive band of suberin around the endodermal cells of a plant root that prevents water passing into xylem via the cell walls. The water is forced through the living part (protoplast) of the endodermal cells
• centrifugation


  Process of separating out particles of different sizes and densities by spinning them at high speed in a centrifuge
• cholinesterase


  Enzyme that breaks down and therefore inactivates the neurotransmitter, acetylcholine, in the synapse
• cholesterol


  Lipid that is an important component of cell-surface membranes. Excess in the blood can lead to atheroma
• chromatid


  One of the two strands of a chromosome that are joined together by a single centromere prior to cell division
• chromatin


  The material that makes up chromosomes. It consists of DNA and the protein histone
• chromosome


  A thread-like structure made of protein and DNA by which hereditary information is physically passed from one generation to the next
• climax community


  The organisms that make up the final stage of ecological succession
• co-dominance


  Condition in which both alleles for one gene in a heterozygous organism contribute to the phenotype
• cohesion


  Attraction between molecules of the same type. It is important in the movement of water up a plant
• collagen


  Fibrous protein that is the main constituent of connective tissues such as tendons, cartilage and bone
• community


  All the living organisms present in an ecosystem at a given time
• condensation


  Chemical process in which two molecules combine to form a more complex one with the elimination of a simple substance, usually water. Many biological polymers, such as polysaccharides and polypeptides, are formed by condensation
• conservation


  Method of maintaining ecosystems and the living organisms that occupy them. It requires planning and organisation to make best use of resources while preserving the natural landscape and wildlife
• consumer


  Any organism that obtains energy by 'eating' another. Organisms feeding on plants are known as primary consumers and organisms feeding on primary consumers are known as secondary consumers
• continuous variation


  Variation in which organisms do not fall into distinct categories but show gradations from one extreme to the other
• correlation


  When a change in one variable is reflected by a change in the second variable