Not every transport protein costs ATP
- The idea
- Predict
- See it
- Why
- Check
The idea
“If a protein moves something across the membrane, the cell is spending ATP.”
A protein looks like a little machine, so it's easy to assume it runs on ATP. Many transport proteins are really just doors.
The short version
The mistake: If a protein moves something across the membrane, the cell is spending ATP.
What's actually true: Moving down a gradient through a channel or carrier costs no ATP. Moving against one, or in vesicles, takes energy.
- Molecules spread from more to less on their own. In facilitated diffusion, a channel or carrier gives polar or charged ones a path, and the gradient still does the work.
- Moving against a gradient takes energy. That's active transport, done by pumps like the sodium-potassium pump, which spends ATP.
- Vesicles cost energy too. Endocytosis and exocytosis use it to move large loads, whichever way the gradient runs.
On the exam: Ask which way the gradient runs. Down it is free, even through a protein. Up it takes energy, usually ATP.