Channels and carriers don't pick a direction
- The idea
- Predict
- See it
- Why
- Check
The idea
“A glucose carrier moves glucose into the cell. That's the way it always goes.”
Most examples show these proteins bringing things in. Pumps really do have a set direction, so it seems every transport protein should.
The short version
The mistake: A glucose carrier moves glucose into the cell. That's the way it always goes.
What's actually true: In passive transport, a channel or carrier just opens a path. Net flow goes down the gradient. For ions, charge counts too.
- In facilitated diffusion, a channel or carrier lets a polar molecule or ion cross the membrane. It adds no energy, so it can't push anything uphill.
- Molecules cross both ways. More cross from the crowded side, because more of them reach the protein there. Reverse the gradient and the net flow reverses.
- For ions, charge matters too, so the electrochemical gradient sets the direction. Pumps are different. They spend ATP to move a solute one way, even uphill.
On the exam: Find the gradient. Net flow runs down it.