Pumps split ATP. Cotransporters run on a gradient.
- The idea
- Predict
- See it
- Why
- Check
The idea
“Anything that moves a solute uphill splits ATP, and a pump can run on an ion gradient.”
Both proteins move things uphill, and both count as active transport. Only one of them splits ATP.
The short version
The mistake: Anything that moves a solute uphill splits ATP, and a pump can run on an ion gradient.
What's actually true: A primary pump splits ATP to build an ion gradient. A secondary cotransporter spends that gradient to move another solute uphill.
- In primary active transport, the pump itself splits ATP to move ions uphill. The Na+/K+ pump spends one ATP to move 3 Na+ out and 2 K+ in.
- The Na+ gradient it leaves stores energy. Na+ flows back in wherever a protein lets it.
- In secondary active transport, a cotransporter lets Na+ or H+ flow downhill and uses that energy to move another solute uphill.
On the exam: For an uphill move, ask what flows downhill through the protein. An ion means secondary. None means primary.