The expression of different sets of genes in different types of cell or at different stages of a cell’s life cycle.

Term describing cells that have become specialised into particular types.

The passive (i.e. non-energy-requiring) movement of a substance from a region where it is at a high concentration to a region where it is at a lower concentration.

A two-ring sugar, e.g. sucrose, lactose.

The ability to distinguish between two stimuli. It is measured by rewarding a response in the presence of one stimulus but not in the presence of the other and seeing whether behaviour shows a discrimination.

A stimulus that is paired with either reward (i.e. reinforcer) availability (+) or its lack (−). An animal’s operant behaviour can be brought under the control of a discriminative stimulus such that it responds only in the presence of the reinforcer. In other words, discrimination training involves the association of a discriminative stimulus with a reinforcer.

This theory suggests that older people voluntarily withdraw from society in preparation for death, and in order that society can continue to function.

A theory of the evolution of ageing and death, which suggests that organisms derive little benefit from investing resources in maintaining and repairing body tissues and thereby increasing their lifespan beyond a certain point. To do so would be inefficient as death is inevitable and resources that could have been put into reproduction would have been wasted.

An –S–S– covalent bond between two cysteine (Cys) units in a polypeptide chain, or between Cys units in different chains in the same protein.

Endogenous rhythmic changes, with a periodicity of approximately 24 hours, in the behaviour or physiology of animals and plants, such as the sleep/activity cycle in animals or growth movement in plants.