Cell signalling enables cell-cell recognition and communication to occur; it means cells are triggered to work together and the actions are co-ordinated to trigger a responseinside cells
Cells must be able to respond to signals from their environment, in order to do this they must have receptors (often protein or glycoproteins) in their plasma membranes (called target cells)
Receptor has a specific shape complementary to the signalling molecule (often chemicals in the tissue fluid surrounding the cells); when they bind it triggers a response in the target cell
Location of the receptors
Some signalling molecules (e.g. hydrophilic proteins) only bind cell surface receptors and do not enter (e.g. insulin)
Others (e.g. hydrophobicsteroids) diffuse in to bind to intracellular receptors, often on DNA (e.g. oestrogen)
Signalling molecules may trigger an immediate change in cell metabolism or electricalcharge across plasma membrane (e.g. actionpotentials), or trigger a change of geneexpression in nucleus (transcription)
