This involves movement of protein complexes along a signaling pathway that ultimately triggers a biochemical event in a cell.
Signal transduction occurs when an extra cellular signalling molecule activates a specific receptor located on the cell surface or inside of he cell. This receptor triggers a biochemical chain of events inside the cell, that eventually elicits a response. This is called as the signalling cascade.
Depending upon th cell the response may alter the cells metabolism, shape, gene expression, or ability to divide.
The signal can be amplified at any step, thus , one signaling molecule can generate a response involving hundreds to millions of molecules.