Osmosis doesn't cost the cell any ATP
- The idea
- Predict
- See it
- Why
- Check
The idea
“Cells spend ATP, or run a pump, to move water in and out.”
Cells swell and shrink fast, and they really do run pumps, so it seems fair to credit them for moving water. The water moves by itself.
The short version
The mistake: Cells spend ATP, or run a pump, to move water in and out.
What's actually true: Water moves by osmosis on its own, down its water potential gradient. The cell spends no energy on it.
- Osmosis is passive transport, movement down a gradient with no energy input. Water moves from higher water potential to lower.
- Cells do spend ATP on pumps. The sodium-potassium pump moves Na+ and K+ ions. Water then follows those ions by osmosis.
- Aquaporins are channel proteins that let water cross much faster. They are passive too. The gradient still sets the direction.
On the exam: Water moving means a water potential gradient. Osmosis is passive, so it never costs ATP.