Simple or facilitated diffusion? Check for the protein
- The idea
- Predict
- See it
- Why
- Check
The idea
“If it moves from more to less with no ATP, it's simple diffusion.”
Downhill and no ATP is the first check for passive transport, so it's natural to stop there. Both kinds pass it.
The short version
The mistake: If it moves from more to less with no ATP, it's simple diffusion.
What's actually true: Both kinds run downhill for free. Facilitated diffusion needs a membrane protein, and simple diffusion doesn't.
- Both kinds of diffusion are passive. They run from more to less and cost no ATP.
- Simple diffusion goes straight through the lipid bilayer. Small nonpolar molecules like O2 and CO2 do this, and so do steroids.
- Facilitated diffusion uses a channel or carrier protein. Ions and large polar molecules like glucose need one to get past the nonpolar core.
On the exam: Ask if it needs a channel or carrier. Yes means facilitated diffusion. No means simple.