What really keeps a membrane protein from flipping
- The idea
- Predict
- See it
- Why
- Check
The idea
“A membrane protein can't flip because something holds it, like an anchor, its size, or its sugars.”
A protein that never flips looks stuck, so you look for something holding it. Yet the same protein drifts around the membrane freely.
The short version
The mistake: A membrane protein can't flip because something holds it, like an anchor, its size, or its sugars.
What's actually true: A flip would drag the protein's polar parts through the oily core of the bilayer, and that costs too much energy.
- The middle of the bilayer is nonpolar fatty acid tails. Polar and charged groups stay out of it, which is also why ions need proteins to cross.
- Drifting sideways or spinning in place keeps each part in the same surroundings. Large proteins do both, so size isn't what stops a flip.
- Sugars don't lock a protein. They're polar too, so they would only make a flip cost more.
On the exam: Sliding and spinning are easy. A flip drags polar parts through the oily core, so it almost never happens.