AQA Glossary of Terms

Definitions of all the key terms of A2 AQA Biology

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An ecological factor that makes up part of the non-biological environment of an organism.
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One of the group of chemicals, released by neurones. It diffuses across the synapse between adjacent neurones and so passes the impulse from one neurone to the next.
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Action Potential
Change that occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes.
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Filamentous protein which is involved in contraction within cells, especially muscle cells.
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Activation Energy
Energy required to bring about a chemical reaction. The activation energy is lowered by the presence of enzymes.
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Active Transport
Movement of a substance across a membrane from a region where it is in a low concentration to a region where it is in high concentration. The process requires the use of ATP.
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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 ADP and inorganic phosphate, with the release of energy.
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A hormone produced by the adrenal glands in times of stress that prepares the body for an emergency.
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Connected with the presence of oxygen. Aerobic respiration requires oxygen to release energy from glucose and other foods.
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One/different form of a gene.
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Allele Frequency
The number of times an allele occurs within the gene pool.
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A normally harmless substance that causes the immune system to produce an immune response.
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Connected with the absence of oxygen. Anaerobic respiration releases energy from glucose or other foods without the presence of oxygen.
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Antibiotic Resistance
The development, in populations of microorganisms, of mechanisms that prevent antibiotics from killing them.
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A protein produced by lymphocytes in response to the presence of a specific antigen.
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A sequence of three adjacent nucleotides on a molecule of transfer RNA that is complementary to a particular codon on a messenger RNA molecule.
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A molecule that triggers an immune response by causing lymphocytes to produce specific antibodies which bind to it
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Autonomic Nervous System
Part of the nervous system, controlling the muscles and glands, that is not under voluntary control.
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A process extending from a neurone that conducts action potentials away from the cell body.
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The range and variety of living organisms within a particular area.
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The total mass of living material in a specific area at a given time. It is usually measured as dry mass because the amount of water in an organism is very variable.
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An ecological factor that makes up part of the living environment of an organism. Examples include food availability, competition and predation.
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Calvin Cycle
A biochemical pathway that forms part of the light-independent reaction of photosynthesis, during which carbon dioxide is reduced to form carbohydrate.
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Cardiac Muscle
Type of muscle found only in the heart. It has fewer striations than skeletal muscle and can contract continuously throughout life without simulation by nerve impulses.
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A group of genetically identical organisms formed from a single parent as a result of asexual reproduction or by artificial means.
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Enzyme that breaks down and therefore inactivates the neurotransmitter, acetylcholine, in the synapse.
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Climax Community
The organisms that make up the final stage of ecological succession.
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Condition in which both alleles for one gene in a heterozygous organism contribute to the phenotype.
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A sequence of three adjacent nucleotides in mRNA that codes for one amino acid.
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The organisms of all species that live in the same area.
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Condensation Reaction
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 are formed by condensation reactions.
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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.
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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.
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Continuous Variation
Variation in which organisms do not fall into distinct categories, but rather there are gradations from one extreme to the other e.g. height in humans.
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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.
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Cystic Fibrosis
Inherited disease in which the body produces abnormally thick mucus that obstructs the breathing passages and prevents the secretion of pancreatic enzymes.
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Term applied to plants that shed all their leaves together at one season.
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Permanent changes to the structure, and hence the three-dimensional shape, of a protein due to factors such as changes in temperature or pH.
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A process, usually branched, extending from the cell body of a neurone, which conducts impulses towards the cell body.
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Denitrifying Bacteria
Bacteria that convert nitrates to nitrogen gas as part of the nitrogen cycle.
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Temporary reversal of changes on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted.
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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 II
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(2n) A term applied to cells in which the nucleus contains two sets of chromosomes.
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Directional Selection
Selection that operates towards one extreme in a range of variation.
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Discontinuous Variation
Variation shown when the characteristics of organisms fall into distinct categories e.g. blood groups in humans.
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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.
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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.
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Dominant Allele
A term applied to an allele that is always expressed in the phenotype of an organism.
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Ecological Niche
All conditions and resources required for an organism to survive, reproduce and maintain a viable population.
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More or less self-contained functional unit in ecology made up of all the interacting biotic and abiotic factors in a specific area.
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An animal that uses the environment to regulate its body temperature.
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An organ that responds to stimulation by a nerve impulse resulting in a change or response.
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Negatively charged sub-atomic particle that orbits the positively charged nucleus of all atoms.
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Electron Carrier Molecules
A chain of carrier molecules along which electrons pass, releasing energy in the form of ATP as they do so.
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An animal maintaining its body temperature by physiological mechanisms.
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Eukaryotic Cell
A cell with a membrane-bound nucleus that contains chromosomes. The cells also possess a variety of other membranous organelles, e.g. mitochondria and endoplasmic reticulum.
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Consequence of an increase in nutrients, especially nitrates and phosphates, in fresh-water lakes and rivers, that often leads to a decrease in biodiversity.
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Reproductive cell that fuses with another gamete during fertilisation.
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Gel Electrophoresis
A technique used to separate DNA fragments of different lengths by placing them on agar gel and passing a voltage across them.
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Length of DNA on a chromosome normally coding for a particular polypeptide.
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Gene Pool
Total number of alleles in a particular population at a specific time.
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Gene Marker
A section of DNA that is used to indicate the location of a gene or other section of DNA.
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Gene Therapy
A mechanism by which genetic diseases, e.g. cystic fibrosis, may be cured by masking the effect of the defective gene by inserting a functional gene.
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Generator Potential
Depolaristation of the membrane of a receptor cell as a result of a stimulus.
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Genetically Modified Organism
Organism that has had its DNA altered as a result of recombinant DNA technology.
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The genetic composition of an organism.
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A hormone produced by alpha cells of the islets of Langerhans in the pancreas that increases blood glucose levels by initiating the breakdown of glycogen to glucose.
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The conversion of non-carbohydrate molecules to glucose.
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The conversion of glucose to glycogen.
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The conversion of glycogen to glucose.
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First part of cellular respiration in which glucose is broken down anaerobically in the cytoplasm to two molecules of pyruvate.
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Substance made up of a carbohydrate molecule and a protein molecule. Parts of cell-surface membrane and certain hormones and glycoproteins.
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A stack of thylakoids in a chloroplast that resembles a pile of coins. This is the site of the light-dependent reaction of photosynthesis.
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Greenhouse Gases
Gases such as methane and carbon dioxide which in the atmosphere cause more heat energy to be trapped, so raising the temperature at the Earth's surface.
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Guard Cell
One of a pair of cells that surround a stoma in plant leaves and control its opening and closing.
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The place where an organism normally lives, which is characterised by physical conditions and the species of other organisms present.
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(n) Term referring to cells that contain only a single copy of each chromosome, e.g. the sex cells or gametes.
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Condition in which the alleles of a particular gene are different.
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An inactive, dormant state, accompanied by a very low body temperature, that certain animals go into during periods of prolonged cold.
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Homologous Chromosomes
A pair of chromosomes that have the same gene loci and therefore determine the same features. Not necessarily identical, as individual alleles of the same genes may vary.
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Condition in which the alleles of a particular gene are identical.
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Human Genome Project
International scientific project to map the entire sequence of all the base pairs of the genes in a single human cell.
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Hydrogen Bonds
Chemical bonds formed between the positive charge on a hydrogen atom and the negative charge on another atom of an adjacent molecule, e.g. between the hydrogen atom of one water molecule and the oxygen atom of an adjacent water molecule.
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The breaking down of large molecules into smaller ones by the addition of water molecules.
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A condition that results from the core body temperature rising above normal.
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Region of the brain adjoining the pituitary gland that acts as the control centre for the autonomic nervous system and regulates body temperature and fluid balance.
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A condition that results from the core body temperature falling below normal.
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A hormone, produced by the beta cells of the islet of Langerhans in the pancreas, which decreases blood glucose levels by, amongst other things, increasing the rate of conversion of glucose glycogen.
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Interspecific Competition
Competition between organisms of different species.
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Intraspecific Competition
Competition between organisms of the same species.
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Intrinsic Proteins
Proteins of the cell-surface membrane that completely span the phospholipid bilayer from one side to the other
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Portions of DNA within a gene that do not code for a polypeptide. The introns are removed fro pre-messenger RNA after transcription.
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An atom or group of atoms that has lost or gained one or more electrons. Ions therefore have either a positive or negative charge.
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Islets of Langerhans
Groups of cells in the pancreas comprising large alpha cells, which produce the hormone glucagon, and small beta cells, which produce the hormone insulin.
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Variations of a chemical element which have the same number of protons and electrons but a different number of neutrons. Similar chemical properties, different mass.
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in vitro
Refers to experiments carried out outside the living body.
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in vivo
Refers to experiments that are carried out within living bodies.
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Kinetic Energy
Energy that an object possesses due to its motion.
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Krebs Cycle
Series of aerobic biochemical reactions in the matrix of mitochondria of most eukaryotic cells by which energy is obtained through the oxidation of acetylcoenzyme A produced from the breakdown of glucose.
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A tough, fibrous connective tissue, rich in collagen, that joins bone to bone.
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Light-Dependent Reaction
Stage of photosynthesis in which light energy is required to produce ATP and reduced NADP.
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Light-Independent Reaction
Stage of photosynthesis that does not require light energy directly but does need the products of the light-dependent reaction to reduce carbon dioxide and so form carbohydrate.
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Limiting Factor
A variable that limits the rate of a chemical reaction.
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Link Reaction
The process linking glycolysis with the Krebs cycle in which hydrogen and carbon dioxide are removed from pyruvate to form acetylcoenzyme A in the matrix of mitochondria.
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Tissue found between the two layers of epidermis in a plant lead, comprising an upper layer of palisade cells and a lower layer of spongy cells.
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The temporary movement of a population of organisms from one locality to another.
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Term used to describe a large area of land in which only one type of crop is grown.
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Motor Neurone
Neurone that transmits action potentials from the central nervous system to an effector, e.g. a muscle or gland.
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Multiple Alleles
Term used to describe a gene that has more than two possible alleles.
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Any agent that induces a mutation.
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A change in the sequence of bases in DNA.
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A fatty substance that surrounds axons and dendrites in certain neurones.
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The thick filamentous protein found in skeletal muscle.
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A molecule that carries electrons and hydrogen ions during aerobic respiration.
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A molecule that carries electrons produced in the light-dependent reaction of photosynthesis.
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Negative Feedback
A series of changes, important in homeostasis that result in a substance being restored to its normal level.
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A nerve cell, comprising a cell body, axon and dendrites, which is adapted to conduct action potentials.
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Neuromuscular Junction
A synapse that occurs between a neurone and a muscle.
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One of a number of chemicals that are involved in communication between adjacent neurones or between nerve cells and muscles. Two important examples are acetylcholine and noradrenaline.
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Nitrifying Bacteria
Microorganisms that convert ammonium compounds to nitrites and nitrates.
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Nitrogen Fixation
Incorporation of atmospheric nitrogen gas into organic nitrogen-containing compounds. It can be brought about by lightning, industrial processes and by both free-living and mutualistic bacteria
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Node of Ranvier
A gap in the myelin sheath that surrounds the axon of a neurone.
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Normal Distribution
A bell-shaped curve produced when a certain distribution is plotted on a graph.
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Complex chemicals made up of an organic base, a sugar and a phosphate. They are the basic units of which the nucleic acids DNA and RNA are made.
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The period in the oestrous cycle immediately after ovulation when the female is most fertile.
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Mutated versions of proto-oncogenes that result in increased cell division leading to the growth of a tumour.
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The net passage of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane, as a result of the random motion of water molecules.
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Chemical reaction involving the loss of electrons.
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A chemical reaction in which electrons are transferred from one substance to another substance. The substance losing electrons is oxidised and the substance gaining the electrons is reduced.
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Oxidative Phosphorylation
The formation of ATP in the electron transport system of aerobic respiration
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An organism that lives on or in a host organism. The parasite gains a nutritional advantage and the host is harmed in some way.
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Any microorganism that causes disease
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Pentose Sugar
A sugar that possesses five carbon atoms. Two examples are ribose and deoxyribose.
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Peptide Bond
The chemical bond formed between two amino acids during a condensation reaction.
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Mechanism by which cells transport large particles across the cell-surface membrane.
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The characteristics of an organism, both visible, resulting from both its genotype and the effects of the environment.
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Plant tissue that transports the products of photosynthesis from leaves to the rest of the plant.
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Lipid molecules in which one of the three fatty acid molecules is replaced by a phosphate molecule. They are important in the structure and functioning of all plasma membranes.
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Splitting of a water molecule by light such as occurs during the light-dependent reaction of photosynthesis.
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Pioneer Species
A species that can colonise bar rock or ground.
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Small circular piece of DNA found in bacterial cells and used as a vector in recombinant DNA technology.
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Group of genes that are responsible for controlling a characteristic.
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Large molecule made up of repeating smaller molecules.
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Polymerase Chain Reaction
Process of making many copies of a specific sequence of DNA or part of a gene. It is used extensively in gene technology and genetic fingerprinting.
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A group of individuals of the same species that occupy the same habitat at the same time.
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Positive Feedback
Process which results in a substance that departs from its normal level becoming further from its norm.
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Primary Succession
The progressive colonisation of bare rock or other barren terrain by living organisms.
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An organism that synthesises organic molecules from simple inorganic ones such as carbon dioxide and water. Most producers are photosynthetic and form the first trophic level in a food chain.
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Prokaryotic Cell
A cell, belonging to the kingdom Prokaryotae, which is characterised by being less than 5microm in diameter and which lacks a nucleus and membrane-bond organelles.
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Positively charged sub-atomic particle found in the nucleus of an atom.
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A cell adapted to detect changes in the environment.
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Recessive Allele
The condition in which the effect of an allele is apparent in the phenotype of a diploid organism only in the presence of another identical allele.
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Recognition Site
A nucleotide sequence, usually of 4, 6 or 8 nucleotides, that is recognised by a restriction endonuclease and to which it attaches.
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Recombinant DNA Technology
General term that covers the processes by which genes are manipulated, altered or transferred from organism to organism.
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Reflex Arc
The nerve pathway in the body taken by an action potential that leads to a rapid, involuntary response to a stimulus.
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Refractory Period
Period during which the membrane of the axon of a neurone cannot be depolarised and no new action potential can be initiated.
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Return to the resting potential in the axon of a neurone after an action potential.
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Resting Potential
The difference in electrical charge maintained across the membrane of the axon of a neurone when not stimulated.
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Restriction Endonucleases
A group of enzymes that cut DNA molecules at a specific sequence of bases called a recognition sequence.
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RNA Polymerase
Enzyme that joins together nucleotides to form messenger RNA during transcription.
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Saltatory Conduction
Propagation of a nerve impulse along a myelinated dendron or axon in which the action potential jumps from one node of Ranvier to another.
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Saprobiotic Microorganism
Also known as a saprophyte, this is an organism that obtains its food from the dead or decaying remains of other organisms.
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A section of myofibril between two Z-lines that forms the basic structural unit of skeletal muscle.
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Schwann Cell
Cell around a neurone whose cell-surface membrane wraps around the dendron or axon to form the myelin sheath.
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Secondary Succession
The recolonisation of an area after an early community has been removed or destroyed.
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Process that results in the best-adapted individuals in population surviving to breed and so pass their favourable alleles to the next generation.
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Selection Pressure
The environmental force altering the frequency of alleles in a population.
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Selective Breeding
Breeding of organisms by human selection of parents/gametes in order to perpetuate certain characteristics and/or eliminate others.
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Sensory Neurone
A neurone that transmits an action potential from a sensory receptor to the central nervous system.
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Skeletal Muscle
The muscle that makes up the bulk of the body and which works under conscious control. Also known as voluntary muscle.
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Sickle-Cell Anaemia
Inherited blood disorder in which abnormal haemoglobin leads to red cells becoming sickle-shaped and less able to carry oxygen.
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Area of heart muscle in the right atrium that controls and coordinated the contraction of the heart. Sometimes known as the pacemaker.
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Smooth Muscle
Also known as involuntary or unstriated muscle, smooth muscle is found in the alimentary canal and the walls of blood vessels. Its contraction is not under conscious control.
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Sodium-Potassium Pump
Protein channels across cell-surface membranes that use ATP to move sodium ions out of the cell in exchange for potassium ions that move in.
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The evolution of two or more species from existing species.
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A group of similar organisms that can breed together to produce fertile offspring.
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Species Diversity
The number of different species and the number of individuals of each species within any one community.
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Stabilising Selection
Selection that tends to eliminate the extremes of the phenotype range within a population. It arises when environmental conditions are constant.
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Stem Cells
Undifferentiated dividing cells that occur in embryos and in adult animal tissues that require constant replacement e.g. bone marrow.
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A detectable alteration in the internal or external environment of an organism that produces some change in that organism.
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A pore, surrounded by two guard cells, mostly in the lower epidermis of a leaf, through which gases diffuse in and out of the leaf.
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Matrix of chloroplast where the light-independent reaction of photosynthesis takes place.
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Substrate-Level Phosphorylation
The formation of ATP by the direct transfer of a phosphate group from a reactive intermediate to ADP.
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A junction between neurones in which they do not touch but have a narrow gap, the synaptic cleft, across which a neurotransmitter can pass.
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Tough, flexible, but inelastic, connective tissue that joins muscle to bone.
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Threshold Level/Value
The minimum intensity that stimulus must reach in order to trigger an action potential in a neurone.
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Series of flattened membranous sacs in a chloroplast that contain chlorophyll and the associated molecules needed for the light-dependent reaction of photosynthesis.
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Tissue Fluid
Fluid that surrounds the cells of the body. Its composition is similar to that of blood plasma except that it lacks some of the larger proteins, in particular those that cause the blood to clot. Supplies nutrients to the cells and removes waste.
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Formation of messenger RNA molecules from the DNA that makes up a particular gene. It is the first stage of protein synthesis.
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Transducer Cells
Cells that convert a non-electrical signal, such as light or sound, into an electrical (nervous) signal and vice versa.
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The process by which one form of energy is converted into another. In microbiology, the natural process by which genetic material is transferred between one host cell and another by a virus.
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Evaporation of water from a plant.
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Trophic Level
The position of an organism in a food chain.
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Rumour Suppressor Gene
A gene that maintains normal rates of cell division and so prevents the development of tumours.
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Narrowing of the internal diameter of blood vessels.
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Widening of the internal diameter of blood vessels.
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A carrier. The term may refer to something such as a plasmid, which carries DNA into a cell, or to an organism that carries a parasite to its host.
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Voltage-Gated Channel
Protein channel across a cell-surface membrane that opens and closes according to changes in the electrical potential across the membrane.
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Water Potential
Measure of the extent to which a solution gains or loses water. The greater the number of water molecules present, the higher (less negative) the water potential. Pure water has a water potential of zero.
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A plant adapted to living in dry conditions.
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Xylem Vessels
Dead, hollow, elongated tubes with lignified side walls and no end walls, that transport water in most plants.
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Card 2


One of the group of chemicals, released by neurones. It diffuses across the synapse between adjacent neurones and so passes the impulse from one neurone to the next.



Card 3


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


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Card 4


Filamentous protein which is involved in contraction within cells, especially muscle cells.


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Card 5


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


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