IGCSE Biology - Key Terms and Definitions

Absorption

Movement of digested food molecules through the wall of the intestine into the blood or lymph.

Accomodation

The adjustment of the shape of the lens so as to focus light onto the retina.

Active site

Part of the surface of an enzyme molecule into which the substrate fits.

Active Transport

The movement of molecules or ions in or out of a cell through the cell membrane from a region of their lower concentration to a region of their higher concentration against a concentration gradient, using energy released from respiration.

Adaptation

A feature that helps an organism to survive in its environment.

Addiction

When a person has taken a drug so regularly that they cannot do without it.

Adrenaline

A hormone produced by the adrenal glands that prepares the body for emergencies and stress, for example by increasing the glucose concentration of the blood.

Aerobic respiration

The process that happens in cells to release a relatively large amount of energy from food substances, such as glucose, in the presence of oxygen.

Allele

Any of two or more alternative forms of a gene

Alveolus

A tiny air sac in the lungs where exchange of gases between the air and blood occurs.

Amnion

The membrane that surrounds the developing fetus in the uterus.

Amniotic fluid

The liquid that is contained within the amnion and which protects the fetus from mechanical damage.

Anaerobic respiration

The process that happens in cells to release a relatively small amount of energy from glucose in the absence of oxygen.

Anaemia

A disorder of haemoglobin in the red blood cells which means that the blood cannot carry sufficient oxygen.

Antagonistic muscles

A pair of muscles which brings about movement at a joint.

Anther

The part of a flower where pollen grains are produced.

Antibiotic

A drug that is taken to kill or stop the growth of bacteria.

Antibody

A protein released by lymphocytes to protect against pathogens.

Antitoxin

A chemical released by lymphocytes that neutralises the poisonous waste products produced by bacteria.

Asexual reproduction

Reproduction without the formation of gametes. New individuals are genetically identical (clones) of the parent.

Assimilation

The movement of digested food molecules into the cells of the body where they are used, becoming part of the cells.

Artery

A blood vessel through which blood travels away from the heart.

Atrium

A chamber of the heart that receives blood from veins and pumps it to a ventricle.

Artificial selection

The selection of plants and animals for breeding because of their useful characteristics.

Auxin

A plant growth hormone which controls cell elongation.

Balanced diet

A diet that provides sufficient energy for a person's needs and all the food nutrients in the correct proportions.

Binary fission

A type of asexual reproduction in which once cell divides into two.

Binomial system

A method for naming organisms in which each is given two names: a genus name and a trivial name, e.g. Homo sapiens for humans.

Biomass

The mass of living material in a particular area.

Biodegradable

Something that can be broken down by biological processes.

Bladder

A muscular sac that stores urine and passes it out through the urethra.

Brain

The part of a central nervous system that coordinates most activities of the body.

Bronchiole

A small branch of a bronchus which ends in alveoli (air sacs)

Bronchus

One of the two tubes that branch off the trachea and pass into the lungs.

Capillary

The smallest blood vessel with walls only one cell thick. Substances are exchanged through capillary walls between blood and tissue fluid.

Cancer

A disease resulting from the uncontrolled division of cells in one or more parts of the body.

Carbohydrase

An enzyme that digests carbohydrates to simple sugars.

Carbohydrate

A class of food substance that provides energy. Composed of carbon, hydrogen and oxygen.

Carbon cycle

The flow of carbon compounds through plants, animals, decomposers and their environment.

Carbon dioxide

The gas produced as a waste product during respiration and absorbed by plants and used during photosynthesis to make simple sugars.

Carnivore

An animal that eats other animals - a meat-eater.

Catalyst

A substance that speeds up a chemical reaction and is not changed by a reaction.

Cell membrane

The boundary of the cell which controls the material that pass into and out of it.

Cell wall

The outer layer of a plant cell made of cellulose, which supports the cell and gives it shape.

Cellulose

A complex carbohydrate that makes up the cells walls of plant cells.

Chlorophyll

The green pigment found in many plant cells that absorbs light for photosynthesis.

Chloroplast

Small structure containing chlorophyll that is found in plant cells that absorbs light for photosynthesis.

Chromosome

A thread-like structure made up of genes. They are found inside the nucleus and are only visible when a cell is dividing.

Ciliary muscle

A muscle in the eye that controls the shape of the lens during focussing.

Circulatory system

The organ system made up of blood vessels and the heart that transports blood.

Codominance

The existence of two alleles for a particular characteristic. where neither is dominant over the other and both are expressed in heterozygous individiuals.

Colon

The part of the alimentary canal between the small intestine and the rectum where the absorption of water occurs.

Community

All the animals, plants and microorganisms that are found in a particular habitat.

Cone

A sensory cell in the retina of the eye that responds to light of high intensity and detects colour.

Concentration gradient

The difference in concentration of a substance between two places.

Consumer

An organism that gains its energy by feeding on other organisms.

Continuous variation

Variation in a feature that shows a range of phenotypes between two extremes with many intermediates.

Contraceptive

Any device or substance that prevents fertilisation.

Coronary arteries

Arteries that branch from the aorta to supply oxygenated blood to heart muscle.

Cornea

The transparent layer at the front of the eye which helps to refract light rays onto the retina.

Coronary heart disease

Heart disease caused by blockage of coronary arteries that supply heart muscle with blood.

Cotyledon

Part of the embryo of a flowering plant - a seed leaf.

Cuticle

The waxy covering of the epidermis in plant stems and leaves that reduces the loss of water by transpiration.

Cystic fibrosis

An inherited disease affecting the lungs and the digestive system, caused by a faulty, recessive allele.

Cytoplasm

Jelly-like contents of the cell not including the nucleus/

Deamination

The process, which takes place in the liver, where the nitrogen-containing part of the amino acids is removed to form ammonia.

Decay

The breakdown of dead organisms and waste material by decomposers.

Decomposers

Microorganisms, mainly bacteria and fungi that gain their energy by breaking down dead organisms and waste material.

Development

An increase in complexity as an embryo grows and gains new tissues, organs and organ systems.

Diabetes

A medical condition in which the blood glucose concentration is not controlled.

Diaphragm

Sheet of muscular and fibrous tissue that separates the thorax from the abdomen. Its movements cause air to flow in and out of the lungs.

Diocotyledon

A type of flowering plant with an embryo that has two cotyledons and a net-like arrangement of veins in its leave.

Diffusion

The net movement of molecules or ions, from a place with a high concentration to a place with a lower concentration down a concentration gradient as a result of random movement.

Digestion

The breakdown of large, insoluble food molecules into small, water-soluble molecules using mechanical and chemical processes.

Digestive system

The organ system that breaks down food and absorbs it into the blood.

Diploid nucleus

A nucleus containing two sets of chromosomes.

Discontinuous variation

Limited number of phenotypes for a feature with no intermediates.

Dominant

An allele that is expressed if it is present (e.g. T or G).

Drug

Any substance taken into the body that modifies or affects chemical reactions in the body.

Duodenum

The first part of the small intestine.

Ecosystem

All the living organisms (the community) in a place and the interactions between them and their physical environment.

Egestion

The removal of undigested food.

Embryo

The early stage of an animal or plant as it develops form a fertilised egg.

Emulsification

The breakdown of large fat globules into many tiny globules.

Enzymes

Biological catalysts that speed up the rate of chemical reactions in the body.

Eutrophication

The enrichment of waters with plant nutrients that can stimulate growth of algae and plants.

Evolution

The process in which inherited features change in populations of organisms over time.

Excretion

The removal from organisms of toxic materials, the waste products of metabolism and substances in excess of requirements.

Fertilisation

The fusion of the male and female sex cells that grows into a new individual.

Fertilisers

Chemicals that provide plant nutrients and are put on the land to increase the growth of a crop and produce a higher yield.

Food chain

This shows the feeding relationships in a community beginning with a producer. Each organism is fed on by the next organism in the chain.

Food web

A network of interconnected food chains showing the energy flow through an ecosystem.

Gall bladder

A sac in the liver which stores bile before it is released down the bile duct into the small intestine.

Gametes

Sex cells with the haploid number of chromosomes.

Gene

A length of DNA, found on a chromosome, that codes for a particular characteristic.

Genetic engineering

Taking a gene from one species and placing it into the DNA of another species.

Genotype

The genetic make up of an organism in terms of the alleles it possesses (e.g. Tt or GG).

Geotropism

A growth response by a plant to the stimulus of gravity.

Glucagon

A hormone produced by the pancreas that stimulates the liver to convert glycogen to glucose and so increase the concentration of glucose in the blood.

Glycogen

A complex carbohydrate found as an energy store in the liver and muscles.