The Protein Is a Door, Not a Pump

You'll learnhow a channel or carrier ferries a solute down its gradient with no ATP spent.

A glucose molecule cannot swim through the oily middle of a membrane, so a protein hands it a route across — and the moment students see that protein, they reach for ATP. Watch the whole trip and the account never moves: the glucose is still rolling downhill, and the protein only opened the door. Twelve short steps; press Next for each one.

AP BIOLOGY · 12 STEPS · 6 QUICK CHECKS · PASSIVE · ZERO ATP · DOWN THE GRADIENT · v5

glucose stuck outside · a protein opens a door · ATP stays at zero

Before you start

What you're looking at

A membrane in cross-section — extracellular fluid above, cytosol below — with glucose, K⁺, O₂ and CO₂ arriving from outside.

The question

Which molecules get across, and does the cell pay ATP for any of it?

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