Forgetting the ion that crosses with it
- The idea
- Predict
- See it
- Why
- Check
The idea
“The sugar gets in by itself, through its own transport protein.”
The question names glucose, so you watch glucose. The ion doing the pushing is easy to miss.
The short version
The mistake: The sugar gets in by itself, through its own transport protein.
What's actually true: In cotransport, a solute crosses only when a partner ion crosses too, and the ion's gradient powers the move.
- Diffusion alone would carry glucose out, since inside has more. Something else has to pay for the uphill move.
- The Na+ gradient pays. The sodium-potassium pump spends ATP to keep Na+ high outside. Using that gradient to move glucose is secondary active transport.
- A cotransporter carries both. In a symporter they cross the same way. In an antiporter, one goes in as the other goes out.
On the exam: A solute moving uphill with no ATP of its own? Find the ion crossing with it.