A channel doesn't make a membrane leaky
- The idea
- Predict
- See it
- Why
- Check
The idea
“Adding or opening a channel makes the membrane leakier, so more of everything gets through.”
A channel looks like a hole in the bilayer, so it sounds like a leak. It works more like a door with a lock, built to let only certain particles through.
The short version
The mistake: Adding or opening a channel makes the membrane leakier, so more of everything gets through.
What's actually true: A channel raises permeability only to the particles it's built to pass. Every other solute crosses the same as before.
- A channel is a protein with a narrow pore. The pore's width and the charged or polar groups lining it decide what fits. This is called selectivity.
- A K+ channel passes K+ about a thousand times more easily than Na+, though a bare Na+ ion is smaller. Aquaporins pass water but not ions.
- So an open channel raises permeability only to what it passes. Other solutes keep their old routes, so their permeability doesn't change.
On the exam: Ask what a channel carries. Only the permeability to that changes.