The cell components that detect stimuli are called receptors. A receptor consists of one or more proteins and is usually located on the outer face of the surface membrane. In the case of a chemical signal, the signal molecule binds tightly and specifically to the receptor. The general term for a molecule that binds specifically to a receptor is a ligand. The receptor undergoes a subtle change of shape when it binds a ligand or is activated by a physical stimulus such as light.
This change of shape alters the receptor's interactions with molecules inside the cell, which are also changed as a result. These intracellular molecules constitute a signalling pathway. In effect, the receptor transduces the extracellular signal (the ligand or physical stimulus) to an intracellular one, which is conducted via the signalling pathway components.
The intracellular signal modifies the cell's structure and function. It might change the cell's ability to respond to further external stimuli; it might alter the internal state; or it might alter the pattern of gene expression. These alternatives are not mutually exclusive. A single intracellular signal might modify the cell in all three of these ways.
If the receptor is symbolised by R, the ligand by s, the subtly altered receptor as R* and the intracellular signalling system by M, we can write this chain of events16 as
M* represents the activated intracellular signal - usually a succession of chemical processes rather than a single changed molecule. M* directly or indirectly alters the cell's behaviour. The double arrows in the scheme indicate that each step is reversible; a ligand-receptor (Rs) complex, for example, can split apart into its components, R and s.
As a broad generalisation, therefore, stimulus is connected to response by a three-stage process: receptor activation (e.g. ligand binding); formation of an intracellular signal; and modification of cell behaviour by the intracellular signal.
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