MATSEC SEC Biology key terms

The thin, partially permeable layer around a cell that controls which substances enter and leave. It is made mainly of phospholipids and proteins and is also called the plasma membrane.

The net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It needs no energy from the cell and continues until concentrations are even.

The movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution. It is a special case of diffusion and needs no energy from the cell.

The movement of substances across a membrane from lower to higher concentration, using carrier proteins and energy released by respiration. Cells use it to take in ions and nutrients against a concentration gradient.

The difference in concentration of a substance between two regions. A steep gradient makes diffusion faster; the gradient is the driving force for diffusion and osmosis.

A protein that speeds up a chemical reaction in living things without being used up. Each enzyme has an active site shaped to fit a specific substrate, so it usually catalyses only one type of reaction.

The small region of an enzyme molecule where the substrate binds and the reaction takes place. Its shape is complementary to the substrate, and changes to it stop the enzyme working.

The loss of an enzyme's normal shape, and therefore its activity, caused by high temperature or extremes of pH. The active site is distorted so the substrate can no longer fit.

The substance on which an enzyme acts. It binds to the enzyme's active site and is converted into products during the reaction.

The process by which green plants make glucose from carbon dioxide and water, using light energy trapped by chlorophyll. Oxygen is released as a by-product and the glucose stores chemical energy.

The green pigment found in chloroplasts that absorbs light energy for photosynthesis. It absorbs mainly red and blue light and reflects green light, which is why leaves look green.

The green organelle in plant cells where photosynthesis takes place. It contains chlorophyll and stacks of membranes that capture light energy.

The factor in shortest supply that holds back the rate of a process such as photosynthesis. Increasing light, carbon dioxide or temperature only raises the rate until another factor becomes limiting.

The chemical reactions in cells that release energy from glucose for activities such as movement, growth and active transport. Aerobic respiration uses oxygen; anaerobic respiration does not.

Respiration that uses oxygen to break down glucose completely, releasing carbon dioxide and water and a large amount of energy. It takes place mainly in the mitochondria.

Respiration that releases energy from glucose without oxygen, producing less energy per glucose molecule. In human muscle it forms lactic acid; in yeast it produces ethanol and carbon dioxide.

A rod-shaped organelle in which aerobic respiration takes place. Cells that need lots of energy, such as muscle cells, contain many mitochondria.

The firmness of a plant cell caused by water pushing the cell contents against the cell wall. Turgid cells support soft plant parts; loss of turgor makes a plant wilt.

The shrinking of plant cell contents away from the cell wall when the cell loses water by osmosis in a strong solution. The cell becomes flaccid and the plant may wilt.

The combination of alleles an organism carries for a particular gene, for example homozygous dominant or heterozygous. It is the genetic make-up that, together with the environment, determines the phenotype.

The observable characteristics of an organism, such as eye colour or flower colour. It results from the genotype interacting with environmental factors.

One of the alternative forms of a gene, such as the tall and dwarf alleles for plant height. An organism inherits one allele from each parent for each gene.

An allele that shows its effect in the phenotype even when only one copy is present. Its effect masks that of a recessive allele in a heterozygote.

An allele whose effect on the phenotype appears only when two copies are present. A single recessive allele is hidden by a dominant allele in a heterozygote.

Having two identical alleles for a particular gene, for example both dominant or both recessive. A homozygous organism breeds true for that characteristic.

Having two different alleles for a particular gene, one dominant and one recessive. A heterozygous organism shows the dominant phenotype but can pass either allele to its offspring.

The process by which organisms best adapted to their environment survive and reproduce, passing on their favourable alleles. Over many generations this changes the characteristics of a population.

A feature of an organism that increases its chance of survival and reproduction in its habitat. Examples include camouflage, streamlined bodies and thick fur.

A random change in the DNA base sequence of a gene or chromosome. Mutations are the source of new alleles and may be harmful, neutral or occasionally beneficial.

A sequence showing the transfer of energy from one organism to the next, beginning with a producer. Each step is a trophic level, and the arrows show the direction of energy flow.

An organism that makes its own food by photosynthesis, usually a green plant or alga. Producers form the first trophic level of a food chain and supply energy to all other organisms.

An organism that obtains its energy by eating other organisms. Primary consumers eat producers, while secondary and tertiary consumers eat other animals.

An organism such as a bacterium or fungus that breaks down dead organisms and waste, releasing nutrients back into the soil. Decomposers recycle materials in an ecosystem.

The place where an organism lives, such as a pond, woodland or rocky shore. A habitat provides the food, water, shelter and space an organism needs.

The maintenance of a stable internal environment in the body, such as constant temperature and blood glucose. It involves receptors detecting changes and effectors responding to correct them.

A control mechanism in which a change from the normal level triggers a response that reverses the change. It keeps conditions such as body temperature and blood glucose steady.

A chemical messenger produced by a gland and carried in the blood to a target organ, where it brings about a response. Hormones control processes such as growth, blood glucose and the menstrual cycle.

The introduction of a weakened or harmless form of a pathogen to stimulate the body to make antibodies and memory cells. It gives immunity against that disease without causing illness.

A protein produced by white blood cells that binds specifically to an antigen on a pathogen and helps destroy it. Antibodies give the body immunity to particular diseases.

A microorganism that causes disease, such as certain bacteria, viruses, fungi or protozoa. Pathogens may be spread by air, water, food, contact or vectors.