Not every carrier moves two things at once
- The idea
- Predict
- See it
- Why
- Check
The idea
“A transport protein always moves two substances together, one riding along with the other.”
Cotransport is a memorable picture, with two molecules riding through one protein together. It's easy to stretch that picture to every carrier.
The short version
The mistake: A transport protein always moves two substances together, one riding along with the other.
What's actually true: Channels and uniporters let a substance cross on its own. Cotransporters are one kind of carrier, built to couple two.
- A uniporter is a carrier that moves one substance down its gradient. GLUT carriers move glucose this way, with no partner and no ATP.
- Channels don't pair substances either. Aquaporins pass water, and an open potassium channel passes K+. Neither needs a second substance to work.
- A cotransporter couples two substances. One flows down its gradient and drives the other, often up its own. A symporter moves both one way, an antiporter opposite ways.
On the exam: Call it cotransport only when a second substance's gradient drives the move. One substance can cross alone.