differential gene expression
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.
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.
disposable soma theory
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.
diurnal (circadian) rhythm