Term describing behaviour that is freely emitted by an animal and can be reinforced by the experimenter.

A variety of instrumental conditioning in which an animal emits a behaviour at a time it decides and that behaviour is then reinforced by its consequences. For example, an animal might press a lever in a Skinner box to earn a pellet of food. Not all instrumental conditioning can be described as operant. For instance, where an experimenter puts a rat in a maze and then removes it after its choice, is not described as operant since the timing of behaviour is not under the control of the rat.

Learning based upon consequences. Certain classes of behaviour are said to be emitted and, if the consequences are favourable, there is an increase in the tendency to repeat the behaviour.

A class of drug that includes opium, morphine and heroin.

A distinct structure in the body serving a particular function, e.g. brain or kidney.

Membrane-bound structure in the cytosol, e.g. mitochondrion, lysosome, chloroplast (in plant cells only).

Term used to describe the majority of compounds that contain carbon. (Some carboncontaining compounds are classified as inorganic, e.g. carbon dioxide, carbonates.)

A group of organs that all work together, e.g. the cardiovascular system, the urinary system, the digestive system. Also sometimes called body system.

The movement of water across a membrane that is freely permeable to water but not to solutes (dissolved substances), from a region of low solute concentration to one where the solute concentration is higher.

A disease of bones. It is brought about when old bone is lost faster than new bone is gained in the remodelling process.