The sodium-potassium pump sends sodium out
- The idea
- Predict
- See it
- Why
- Check
The idea
“The pump brings sodium in and sends potassium out, or it moves 2 sodium for every 3 potassium.”
Sodium does rush into cells and potassium does flow out, through channels. So it's easy to give the pump that job, and to swap the 3 and the 2.
The short version
The mistake: The pump brings sodium in and sends potassium out, or it moves 2 sodium for every 3 potassium.
What's actually true: Each cycle, the pump uses one ATP to move 3 Na+ out of the cell and 2 K+ in, each against its concentration gradient.
- The pump is an ATPase, a membrane protein that splits ATP. The energy comes straight from ATP, so this is primary active transport. Each ion moves against its gradient.
- Channels let Na+ leak in and K+ leak out, down their gradients. The pump's job is to undo that leak.
- Three positive charges leave and two enter, so each cycle moves one net positive charge out. That makes the pump electrogenic. It helps keep the inside negative.
On the exam: Sodium out, potassium in, 3 for 2 per ATP.