Three stages, and why the middle one exists
Signal transduction has reception, transduction and response. Reception is a signalling molecule binding a receptor with high specificity — a water-soluble ligand binds a receptor in the plasma membrane, while a small hydrophobic signal such as a steroid passes through the membrane and binds an intracellular receptor that then acts as a transcription factor. Transduction is a relay of usually several steps, and students often ask why a cell bothers. Two reasons: each step is an opportunity for regulation by a different input, and each step amplifies, so one hormone molecule can end in millions of product molecules. The response is the specific cellular outcome, which may be enzymatic or transcriptional.