The Messenger Is Not the Worker

You'll learnwhy a signal that arrives at a cell does nothing until it is received, relayed, and answered.

A signal molecule arrives at a cell and stops there. It does not walk in and throw the switch itself — it binds a receptor, and the receptor starts a relay that ends in something the cell does. Eleven short steps; press Next for each one.

AP BIOLOGY · 11 STEPS · 6 QUICK CHECKS · RECEPTION → TRANSDUCTION → RESPONSE · v5

ligand · receptor · relay · response

Before you start

What you're looking at

One cell seen edge-on: a signal molecule waits outside the grey membrane wall, and a notched receptor protein sits ready to meet it.

The question

What has to happen between a signal arriving and the cell actually responding?

Step 1 / 11