Fluid mosaic model
The membrane is a double layer of phospholipids with proteins floating in it. The parts can move sideways, so the layer is fluid, and the mix of proteins makes it a mosaic.
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Analogy
Beach balls (proteins) drifting in a sea of tiny swimmers (lipids).
In practice
Cholesterol sits between phospholipids in animal cells and keeps the membrane from getting too stiff or too runny as temperature changes.
Practice
Flashcards
- What does cholesterol do in an animal cell membrane?It helps keep the membrane fluid but not too runny, so it works across temperature changes.
Interview questions
- Explain how a cell moves water and ions across its membrane, including one example that needs energy.
- Water follows solute by osmosis
- Small gradients allow diffusion through channels
- Active transport needs ATP
- Sodium-potassium pump as example
Questions people ask
Why can membrane proteins move around?
Phospholipids are held together by weak forces rather than strong bonds, so they slide past each other like people in a crowd. Proteins embedded among them drift too. Some are anchored to the cytoskeleton, which keeps them in place where a cell needs them fixed.
What are the heads and tails of a phospholipid?
The head contains a phosphate group and likes water, so it faces outward toward the watery inside and outside of the cell. The two fatty acid tails hate water and tuck together in the middle. This arrangement forms the two-layer membrane on its own.