AQA Biology A-Level ALL DEFINITIONS

abiotic

an 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.

activation energy

energy required to bring about a reaction. The activation energy is lowered by the presence of enzymes.

active immunity

resistance to disease resulting from the activities of an individual's own immune system whereby an antigen induces plasma cells to produce antibodies.

active site

a group of amino acids that makes up the region of an enzyme into which the substrate fits in order to catalyse a reaction.

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.

adenosine triphosphate (ATP)

an activated nucleotide found in all living cells that acts as an energy carrier. The hydrolysis of ATP leads to the formation of adenosine diphosphate (ADP) and inorganic phosphate, with the release of 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.

allele

one of a number of alternative forms of a gene. For example, the gene for the shape of pea seeds has two alleles: one for 'round' and one for 'wrinkled'.

allele frequency

the number of times an allele occurs within the gene pool.

allergen

a normally harmless substance that causes the immune system to produce an immune response.

allergy

the response of the immune system to an allergen. Examples include hay fever and asthma.

anaerobic

connected with the absence of oxygen. Anaerobic respiration releases energy from glucose or other foods without the presence of oxygen.

antibiotic

a substance produced by living organisms that can destroy or inhibit the growth of microorganisms.

antibiotic resistance

the development in microorganisms of mechanisms that prevent antibiotics from killing them.

antibody

a protein produced by lymphocytes in response to the presence of the appropriate antigen.

anticodon

a sequence of three adjacent nucleotides on a molecule of transfer RNA that is complementary to a particular codon on a messenger RNA molecule.

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.

antigen

a molecule that triggers an immune response by lymphocytes.

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.

asthma

a chronic illness in which there is resistance to air flow to the alveoli of the lungs as a result of the airways becoming inflamed due to an allergic response to an allergen.

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.

B cell (B lymphocyte)

type of white blood cell that is produced and matures within the bone marrow. B lymphocytes produce antibodies as part of their role in immunity.

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.

Bluret test

a simple biochemical reaction to detect the presence of protein.

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.

cancer

a disease, resulting from cells that break away from an original tumour to form secondary tumours elsewhere in the body.

carcinogen

a chemical, a form of radiation, or other agent that causes cancer.

cardiac cycle

a continuous series of events which make up a single heart beat.

cardiac muscle

type of muscle found only in the heart. It has fewer striations than skeletal muscle and can contract continuously throughout life without stimulation by nerve impulses.

cardiac output

the total volume of blood that the heart can pump each minute. It is calculated as the volume of blood pumped at each beat (stroke volume) multiplied by the number of heart beats per minute (heart rate).

carrier molecule (carrier protein)

a protein on the surface of a cell that helps to transport molecules and ions across plasma membranes.

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.

clone

a group of genetically identical cells or organisms formed from a single parent as the result of asexual reproduction or by artificial means.

co-dominance

condition in which both alleles for one gene in a heterozygous organism contribute to the phenotype.

codon

a sequence of three adjacent nucleotides in mRNA that codes for one amino acid.

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.

complementary DNA

DNA that is made from messenger RNA in a process that is the reverse of normal transcription.

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.

primary consumers

organisms feeding on plants.

secondary consumers

organisms feeding on primary consumers.

continuous variation

variation in which organisms do not fall into distinct categories but show gradations from one extreme to the other.

coronary arteries

arteries that supply blood to the cardiac muscle of the heart.

coronary heart disease (CHD)

any condition, for example, atheroma and thrombosis, affecting the coronary arteries that supply heart muscle.

correlation

when a change in one variable is reflected by a change in the second variable.

co-transport

the transport of one substance coupled with the transport of another substance across a plasma membrane in the same direction through the same protein carrier.

countercurrent system

a mechanism by which the efficiency of exchange between two substances is increased by having them flowing in opposite directions.

covalent bond

type of chemical bond in which two atoms share a pair of electrons, one from each atom.

crossing over

the process whereby a chromatid breaks during meiosis and rejoins to the chromatid of its homologous chromosome so that their alleles are exchanged.

cuticle

exposed non-cellular outer layer of certain animals and the leaves of plants. It is waxy and impermeable to water. It therefore helps to reduce water loss.

deciduous

term applied to plants that shed all their leaves together at one season.

denaturation

permanent changes due to the unravelling of the three-dimensional structure of a protein as a result of factors such as changes in temperature or pH.

dendrite

a process, usually branched, extending from the cell body of a neurone, which conducts impulses towards the cell body.

denitrifying bacteria

bacteria that convert nitrates to nitrogen gas as part of the nitrogen cycle.

depolarisation

temporary reversal of charges on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted.

diabetes

a metabolic disorder in which the body is unable to regulate the level of blood glucose. There are two forms of the disorder - Type I and Type II diabetes.

diastole

the stage in the cardiac cycle when the heart muscle relaxes.

differentiation

the process by which cells become specialised for different functions.

diffusion

the movement of molecules or ions from a region where they are in high concentration to one where their concentration is lower.

diploid

a term applied to cells in which the nucleus contains two sets of chromosomes.

dipolar

having a pair of equal and opposite electrical charges.

directional selection

selection that operates towards one extreme in a range of variation.

discontinuous variation

variation shown when the characteristics of organisms fall into distinct categories, e.g. blood groups in humans.

DNA helicase

enzyme that acts on a specific region of the DNA molecule to break the hydrogen bonds between the bases causing the two strands to separate and expose the nucleotide bases in that region.

DNA replication

the process in which the double helix of a DNA molecule unwinds and each strand acts as a template on which a new strand is constructed.

dominant allele

a term applied to an allele that is always expressed in the phenotype of an organism.

ecological niche

describes how an organism fits into its environment. It describes what a species is like, where it occurs, how it behaves, its interactions with other species and how it responds to its environment.

ecosystem

all the living and nonliving components of a particular area.

ectothermic

an animal that uses the environment to regulate its body temperature.

effector

an organ that responds to stimulation by a nerve impulse resulting in a change or response.

electron

negatively charged subatomic particle that orbits the positively charged nucleus of all atoms.

electron carrier molecules

a chain of carrier molecules along which electrons pass, releasing energy in the form of ATP as they do so.

emphysema

a disease in which the walls of the alveoli break down, reducing the surface area for gaseous exchange, thereby causing breathlessness in the patient.

endocytosis

the inward transport of large molecules through the cell-surface membrane.

endotherm

an animal maintaining its body temperature by physiological mechanisms.

enzyme

a protein or RNA that acts as a catalyst and so alters the speed of a biochemical reaction.