Signal Transduction

Three steps, always

1. Reception — a signal molecule (ligand) binds a receptor. Water-soluble signals bind receptors on the membrane; small hydrophobic signals such as steroid hormones pass through and bind receptors inside the cell.
2. Transduction — a cascade of relay proteins, often involving phosphorylation and second messengers such as cyclic AMP or $\text{Ca}^{2+}$.
3. Response — a change in gene expression or in enzyme activity.

Why a cascade rather than a direct link

Each step can activate many molecules at the next step, so the cascade gives enormous amplification — one hormone molecule can trigger millions of product molecules. It also provides many points at which the signal can be regulated or switched off.

Because receptors are specific, only cells with the right receptor respond — which is how one hormone in the bloodstream affects some tissues and not others.