4.1.4—Cell signalling process

Cell signalling begins when a signalling molecule binds to a complementary receptor on the target cell surface. The receptor links recognition outside the cell to a change inside the cell, producing a response only in cells with the appropriate receptor.

1. Signal outside: A hormone, neurotransmitter or other signalling molecule reaches the cell surface; it remains outside the cell at this membrane-receptor step.
2. Specific binding: The signal binds to a receptor whose binding site is complementary to it. This specificity helps determine which cells are target cells.
3. Receptor change: Binding changes the receptor’s state or arrangement at the membrane, which alters the receptor’s interactions with components on the inside of the cell.
4. Internal signal: The receptor change initiates a cell-internal signalling change that carries the information onward, without requiring the original signalling molecule to enter.
5. Target-cell response: The cell changes an appropriate activity, such as enzyme activity, membrane transport or gene expression, according to the signalling pathway and cell type.

The receptor is both a recognition site and a transducer: complementary binding selects the signal, then the receptor’s altered state changes intracellular activity. This explains why the same extracellular signal can affect a target cell while a neighbouring cell lacking the matching receptor does not respond.

A receptor detects and relays information; it is not automatically a channel or carrier that transports the signalling molecule across the bilayer. The signalling molecule does not need to cross the membrane for this surface-receptor pathway, and the exact internal relay is not specified here.