Some molecules cross a membrane without a protein
- The idea
- Predict
- See it
- Why
- Check
The idea
“Nothing gets through a membrane unless a channel or carrier protein lets it through.”
Membrane diagrams are full of channels, carriers and pumps, so they look like the only way in. Some molecules don't need them at all.
The short version
The mistake: Nothing gets through a membrane unless a channel or carrier protein lets it through.
What's actually true: Small nonpolar molecules like O2 and CO2 diffuse right through the lipid bilayer. Ions and large polar molecules need a protein.
- The middle of the bilayer is fatty acid tails, which are nonpolar, like oil. Small nonpolar molecules like O2 and CO2 dissolve in and cross by simple diffusion.
- Steroid hormones are lipids, so they cross the same way. Water is polar and leaks across slowly. Many cells have aquaporins to speed it up.
- Ions like Na+ and large polar molecules like glucose can't get through the nonpolar core. They need a channel or carrier protein.
On the exam: O2, CO2 and steroids are nonpolar and cross the bare lipids. Ions and large polar molecules need a protein.