Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response.
There are two stages in this process:
1. A signaling molecule activates a specific receptor protein on the cell membrane.
2. A second messenger transmits the signal into the cell, eliciting a physiological response. In either step, the signal can be amplified. Thus, one signalling molecule can cause many responses.