Membrane molecules drift sideways but rarely flip
- The idea
- Predict
- See it
- Why
- Check
The idea
“The membrane is fluid, so proteins and lipids can flip from one face to the other whenever they like.”
You learned that everything in the membrane moves. It's easy to picture that motion going every way, end over end included.
The short version
The mistake: The membrane is fluid, so proteins and lipids can flip from one face to the other whenever they like.
What's actually true: Membrane molecules drift sideways easily, but rarely cross to the other face, because that means passing through the oily core.
- The middle of the bilayer is fatty acid tails, so it's hydrophobic. Polar heads, sugars and charged amino acids stay out of it.
- Sliding sideways keeps every polar part in water, so it's easy. That's the fluid in the fluid mosaic model.
- Flipping would drag those polar parts through the core. Proteins don't do it. Phospholipids do it rarely, so the two layers can hold different lipids.
- A protein's orientation is set as it's made in the rough ER, and it keeps that orientation for good.
On the exam: Fluid means sideways. A membrane protein's two ends stay on the faces they started on.