The Messenger Never Gets In

You'll learnhow a receptor relays a signal inward through a protein cascade to a final cellular response.

A hormone reaches your liver cell, binds the outside of it, and stays there — yet seconds later the cell dumps a million molecules of glucose into your blood. Everything in between is a relay of proteins passing a shape change hand to hand, multiplying it at every tier. Ten short steps; press Next for each one.

AP BIOLOGY · 10 STEPS · 6 QUICK CHECKS · RECEPTION → TRANSDUCTION → RESPONSE · THE LIGAND STAYS OUTSIDE · v6

ligand · receptor · relay · response

Before you start

What you're looking at

A target cell in cross-section: outside above, cytosol below, a membrane between, and a signal molecule approaching one receptor.

The question

How does a message cross the membrane when the messenger itself never enters the cell?

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