4.3 Signal Transduction

Relaying the signal

Once received, a signal is relayed through the cell in a signal transduction cascade — a series of molecules passing the message along. A common relay is a phosphorylation cascade, where enzymes called kinases add phosphate groups to the next protein, switching it on.

Second messengers and amplification

Small intracellular molecules called second messengers (like cAMP or Ca²⁺) spread the signal quickly. A big advantage of cascades is amplification: one signal molecule can activate many downstream molecules, producing a large response from a small input. G-protein-coupled receptors are common starting points.

Key terms

Signal transduction cascade

A chain of molecules that relays and converts a signal through the cell.

Kinase

An enzyme that adds phosphate groups to proteins, often activating them.

Phosphorylation

Adding a phosphate group to a molecule, commonly used to switch proteins on.

Second messenger

A small intracellular molecule (e.g., cAMP, Ca²⁺) that spreads a signal.

Signal amplification

One signal activating many downstream molecules for a large response.

Try an AP-style question

In a phosphorylation cascade, one activated kinase activates many copies of the next kinase. What is the main advantage?

1. A small amount of signal produces a large response
2. It removes the need for a receptor protein
3. It makes the response permanent after one signal
4. It prevents any response until many signals arrive