LGCSE Biology (0180) key terms

A thin, flexible layer of lipid and protein surrounding every cell. It controls which substances enter and leave, allowing small molecules through while restricting others.

An organelle with a folded inner membrane where aerobic respiration takes place. It releases energy from glucose, so cells that need lots of energy contain many of them.

A green organelle found in plant cells that contains chlorophyll and carries out photosynthesis, trapping light energy to make glucose from carbon dioxide and water.

The net movement of particles from a region of higher concentration to a region of lower concentration, caused by random motion and requiring no energy from the cell.

The movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated one, down a water potential gradient.

A protein that speeds up a specific reaction by binding its substrate at an active site. Enzymes are biological catalysts and are affected by temperature and pH.

The small region of an enzyme where the substrate binds. Its shape is complementary to the substrate, so each enzyme catalyses only particular reactions.

Loss of the precise three-dimensional shape of a protein such as an enzyme, usually caused by high temperature or extreme pH, so the active site no longer fits the substrate.

The breakdown of glucose using oxygen to release energy, producing carbon dioxide and water. It yields far more energy per glucose molecule than anaerobic respiration.

Release of energy from glucose without oxygen. In human muscle it produces lactic acid; in yeast it produces ethanol and carbon dioxide, and less energy is released overall.

The process in which plants use light energy trapped by chlorophyll to convert carbon dioxide and water into glucose, releasing oxygen as a by-product.

The environmental condition in shortest supply that holds back the rate of a process such as photosynthesis, for example light intensity, carbon dioxide concentration or temperature.

A measure comparing an organism's or structure's outer surface with its internal volume. A high ratio speeds exchange; large organisms need specialised exchange surfaces and transport systems.

One of many tiny air sacs in the lungs. Their huge combined surface area, thin walls and rich blood supply make them efficient sites of gas exchange.

A finger-like projection of the small intestine lining. Villi increase surface area for absorption of digested food and contain capillaries and a lacteal.

A green fluid made in the liver and stored in the gall bladder. It emulsifies fats into small droplets, increasing their surface area for enzyme action, and neutralises stomach acid.

Loss of water vapour from the leaves of a plant, mainly through stomata. It draws water up the xylem and is affected by light, temperature, humidity and wind.

Plant tissue of hollow, dead, lignified cells that transports water and dissolved minerals upward from the roots to the leaves and helps support the plant.

A small pore in the leaf epidermis, surrounded by two guard cells, through which gases and water vapour pass. Guard cells open and close it depending on conditions.

The maintenance of a stable internal environment within narrow limits, for example controlling blood glucose, body temperature and water balance despite external change.

A control mechanism in which a change away from a set point triggers a response that reverses the change, restoring the original level. It keeps conditions such as temperature steady.

A hormone secreted by the pancreas when blood glucose rises. It causes liver and muscle cells to take up glucose and store it as glycogen, lowering blood glucose.

A rapid, automatic response to a stimulus that does not involve conscious thought, using a pathway called a reflex arc from receptor to effector.

A tiny gap between two neurones, or between a neurone and an effector. A chemical neurotransmitter diffuses across it to carry the impulse onward.

A short section of DNA that codes for a particular protein or characteristic. Genes occur in pairs, one inherited from each parent, and can exist as different alleles.

One of the alternative forms of a gene, such as the tall or short form for plant height. Alleles may be dominant or recessive and determine the phenotype shown.

The combination of alleles an organism carries for a gene, for example homozygous dominant, heterozygous or homozygous recessive. It is written with paired letters.

The observable characteristic of an organism, such as eye colour or seed shape, resulting from its genotype interacting with the environment.

A random change in the base sequence of DNA. Mutations may have no effect, alter the protein produced, or create new alleles on which natural selection can act.

The process by which individuals with inherited features better suited to their environment survive and reproduce more, so those alleles become more common over generations.

An inherited feature of a species that improves its chance of survival and reproduction in a particular habitat, such as a thick coat or a streamlined body.

A diagram showing the direction of energy transfer from one organism to the next, beginning with a producer and using arrows to show what eats what.

An organism, usually a green plant or alga, that makes its own food by photosynthesis and so forms the starting point of every food chain.

A feeding position in a food chain, such as producer, primary consumer or secondary consumer. Energy and biomass generally decrease at each successive level.

The transfer of pollen from the anther of a flower to the stigma, either within the same flower or between flowers, bringing male gametes to the female part.

The fusion of a male and female gamete to form a zygote containing the full chromosome number, restoring the diploid number after the halving that occurs in meiosis.

A specialised sex cell, such as a sperm, egg or pollen grain, that carries half the normal chromosome number and joins with another gamete at fertilisation.

A factor that is deliberately kept the same throughout an investigation so that it cannot affect the outcome, making the results fair and comparable.

The extent to which repeated measurements give consistent results. It is improved by repeating readings, taking a larger sample and identifying and removing anomalous values.

An experiment in which only the independent variable is changed, all other relevant factors are controlled, and the dependent variable is measured accurately.