AQA A-Level Biology Definitions

Accurate result

An accurate result is really close to the true answer.

Activation Energy

The minimum amount of kinetic energy that particles need to have in order to react when they collide.

Active Site

The part of an enzyme where a substrate molecule binds.

Active Transport

Active movement of molecules and ions across plasma membranes, usually against a concentration gradient.

Adaptation

A characteristic that increases an organism's chances of survival, e.g. antibiotic resistance.

Affinity for Oxygen

The tendency a molecule has to bind with oxygen.

Agglutination

The clumping together of cells.

AIDS

A condition caused by HIV, in which the immune system deteriorates and eventually fails.

Allele

An alternative form of a gene.

Alveolus

A microscopic air sac in the lungs where gas exchange occurs.

Amino Acid

A monomer of proteins.

Anomalous Result

A result that doesn't fit in with the pattern of the other results in a set of data.

Antibiotic

A medicine that is designed to kill or inhibit the growth of bacteria (or sometimes fungi).

Antibiotic Resistance

When bacteria are able to survive in the presence of antibiotics.

Antibody

A protein produced by ß-cells in response to the presence of a pathogen.

Antigen

A molecule (usually a protein) that can trigger an immune response.

Antigenic Variation

Where pathogens change their antigens.

Antigen

Presenting Cell - An immune system cell that processes and presents antigens on its surface to activate other immune system cells.

Antimicrobial Substance

A substance designed to kill microorganisms.

Artefact (microscope)

Something you can see on a microscope slide that isn't part of the specimen you're looking at.

Arteriole

A blood vessel that branches off an artery.

Aseptic Technique

A technique used to prevent the unwanted growth or transfer of microorganisms.

Atheroma

A fibrous plaque caused by the build up and hardening of white blood cells, lipids and connective tissue.

ATP

A molecule made up of adenine, a ribose sugar and three phosphate groups. It is the immediate source of energy in a cell.

ATP Hydrolase

An enzyme which catalyses the hydrolysis of ATP into ADP and P.

ATP Synthase

An enzyme which catalyses the synthesis of ATP from ADP and P.

Atrioventricular Valve

A valve in the heart linking the atria to the ventricles.

Attachment Protein

A protein on the surface of a virus cling onto a suitable host cell.

Autoradiography

A technique that reveals the location of radioactive tracers.

B

cell - A type of white blood cell involved in the immune response. It produces antibodies.

Base

A nitrogen-containing molecule that forms part of a DNA nucleotide.

Benedict's Test

A biochemical test for the presence of sugars

Bile Salt

A type of salt produced by the liver to aid the digestion of lipids.

Binary Fission

The process by which prokaryotic cells replicate.

Binomial System

The system used in classification for naming organisms using a two-part Latin name.

Biodiversity

The variety of living organisms in an area.

Biuret Test

A biochemical test for the presence of polypeptides and proteins.

Bohr Effect

An effect by which an increase of carbon dioxide in the blood results in a reduction of haemoglobin's affinity for oxygen.

Cancer

A tumour that invades surrounding tissue.

Capillary bed

A network of capillaries.

Capsid

The protein coat surrounding a virus' genetic material.

Capsule (cell)

A layer of secreted slime surrounding some prokaryotic cells.

Cardiac Cycle

An ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating around the body.

Cardiac Output

The volume of blood pumped by the heart per minute (cm3 min-1).

Cardiovascular Disease

Any disease associated with the heart and blood vessels.

Carrier Protein

A protein in a cell membrane that allows the facilitated diffusion of large molecules.

Catalyst

A chemical that speeds up a chemical reaction without being used up itself.

Causal Relationship

Where a change in one variable causes a change in the other.

Cell Cycle

The process that all body cells from multicellular organisms use to grow and divide.

Cell Fractionation

A method that separates the organelles in a cell.

Cell

Surface Membrane - The membrane found on the surface of animal cells (and just inside the cell wall of other cells). Regulates the movement of substances into and out of the cell.

Cellular Immune Response

The immune response that involves T-cells and the other immune system cells they interact with, e.g. phagocytes.

Cellulose

A polysaccharide made of long, unbranched chains of ß-glucose.

Cell Wall

The outermost cell layer found in plant, algal, and fungal cells.

Centromere

The point at which two strands of a chromosome are joined together.

Channel Protein

A protein that forms a pore in a cell membrane and allows the facilitated diffusion of charged particles.

Chlorophyll

A green substance found in chloroplast.

Chloroplast

An organelle present in plant and algal cells where photosynthesis occurs.

Cholesterol

A type of lipid present in cell membrane (except bacterial cell membranes).

Chromatid

One 'arm' of a double stranded chromosome.

Chromosome

A threadlike structure made up of one long DNA molecule.

Chromosome Nondisjunction

Failure of the chromosomes to separate properly during meiosis or mitosis.

Classification

The act of arranging organisms into groups.

Community

All the populations of different species in a habitat.

Competitive Inhibitor

A molecule that has a similar shape to a substrate and blocks an enzyme's active site.

Complementary Base Pairing

Hydrogen bonding between specific pairs of bases on opposing polynucleotide strands.

Continuous Data

Data that can take any value in a range.

Control Group

A group in a study that is treated in exactly the same way as the experimental group, apart from the factor you're investigating.

Control Variable

A variable you keep constant throughout an experiment.

Coronary Artery

An artery supplying the heart muscle with blood.

Coronary Heart Disease

When the coronary arteries have lots of atheromas in the, which restricts blood flow to the heart.

Correlation

A relationship between two variables.

Co

transporter - A type of carrier protein that binds two molecules at the same time.

Counter

Current System - The system in which blood flows in one direction and water flows in the opposite direction across the gills of a fish.

Courtship Behaviour

Behaviour carried out by organisms to attract a mate of the right species.

Crossing Over

When chromatids twist around each other and bits of them swap over during meiosis.

Cytokinesis

The division of the cytoplasm during eukaryotic cell division.

Cytoplasm

A gel-like substance where most of the chemical reactions in a cell happen.

Cytotoxic T

Cell - A T-cell that kills abnormal or foreign cells.

Denatured

The point at which an enzyme no longer functions as a catalyst.

Deoxyribose

A pentose sugar in DNA.

Dependent Variable

The variable you measure in an experiment.

Dicotyledonous Plant

A type of flowering plant, e.g. non-woody plants, bushes and trees.

Differential Reproductive Success

The fact that in any population, some individuals are more likely to survive and reproduce than others.

Diffusion (simple)

Net movement of particles from an area of higher concentration to an area of lower concentration.

Digestion

The process of breaking down food into substances that can be used by the body.

Dipeptidase

An endopeptidase enzyme that hydrolyses peptide bonds within a protein.

Dipeptide

A molecule formed from two amino acids.

Diploid

When a cell contains two copies of each chromosome.

Directional Selection

When individuals with alleles for characteristics of an extreme type are more likely to survive, reproduce and pass on their alleles.

Disaccharide

A molecule formed from two monosaccharides.

Discrete data

Numerical data that can only take certain values in a range.

DNA (Deoxyribonucleic Acid)

The molecule in cells that stores genetic information.

DNA Helicase

An enzyme that breaks the hydrogen bonds between two polynucleotide DNA strands during DNA replication.

DNA Polymerase

An enzyme that joins together the nucleotides on a new strand of DNA during DNA replication.

DNA Sequencing

The process of determining the base order of a section of DNA.

Double

Helix - The structure of a DNA molecule - two separate strands wound together in a spiral.

Emulsion Test

A biochemical test for the presence of lipids.

Endopeptidase

An enzyme that hydrolyses peptide bonds within a protein.

Endoplasmic Reticulum

A system of membranes enclosing a fluid-filled space. Involved with lipid and protein processing.