fluid mosaic model
Summary
The fluid mosaic model pictures the cell membrane as a phospholipid bilayer with proteins embedded in it, and both lipids and proteins can drift sideways within the layer. Most testable point: unsaturated (kinked) tails and higher temperature raise fluidity, and cholesterol buffers it.
Detail
Meaning: The membrane is a bilayer of amphipathic phospholipids: polar heads face the water on both sides, hydrophobic tails face inward. "Mosaic" refers to the proteins scattered through the bilayer. Integral proteins are embedded in or span it; peripheral proteins attach to the surface. "Fluid" means lipids and many proteins diffuse laterally within their own leaflet, while flipping between leaflets is rare without enzyme help. The leaflets differ in composition, and carbohydrates (glycolipids, glycoproteins) face outside.
On the MCAT: Cis double bonds kink the tails, so they pack loosely and fluidity rises. Longer, saturated tails pack tightly and fluidity falls. Higher temperature increases fluidity. Cholesterol is a buffer. Passages may describe cell-fusion or photobleaching experiments showing labeled proteins diffusing, or mention lipid rafts. Questions also test which molecules cross freely (small, nonpolar) and which need transport proteins (polar, charged).
Common confusion: Students assume cholesterol simply makes a membrane more rigid (less fluid). It moderates fluidity in both directions: it decreases fluidity at high temperature and increases it at low temperature by preventing tight packing. Also, "fluid" does not mean unrestricted: proteins can be anchored or confined to rafts, and the core still blocks ions.
Sources
- Lehninger Principles of Biochemistry
- Campbell Biology
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.