A Mosaic That Flows
The fluid mosaic model describes the membrane as a two-dimensional fluid: a phospholipid bilayer in which proteins float, diffuse laterally, and are held in place only when the cell anchors them. Lipids exchange places with their neighbours millions of times a second; flipping between the two leaflets, by contrast, is rare and requires an enzyme, which is what keeps the two faces chemically different.
Membrane fluidity is tuned by three things:
| Factor | More fluid when |
|---|---|
| Temperature | Higher |
| Fatty-acid saturation | More unsaturated |
| Cholesterol | Buffers both ways |
Unsaturated fatty acids carry a double bond that puts a kink in the tail, so neighbouring lipids pack less tightly. This is why organisms living in the cold have more unsaturated membrane lipids, and why cooking oils from cold-adapted sources stay liquid in a refrigerator.
Cholesterol is the interesting case: it is a fluidity buffer, not a simple increaser or decreaser. At high temperature it restrains movement and reduces fluidity; at low temperature it wedges between tails and prevents the tight packing that would otherwise solidify the membrane.
Membrane proteins fall into two classes by how they sit. Integral proteins span the bilayer, held by a hydrophobic belt of amino acids; extracting one requires disrupting the membrane. Peripheral proteins sit on a surface, attached to lipid heads or to integral proteins, and can be removed by changing the salt concentration.
Their functions cover almost everything a membrane does: transport, enzymatic activity, signal reception, cell recognition, intercellular junction and anchorage to the cytoskeleton.
Membranes are asymmetric: the two leaflets differ in lipid composition, and the carbohydrates of the glycocalyx face outward without exception. That outward-facing sugar coat is what makes cell recognition possible, including the ABO blood groups.
Common pitfall: treating cholesterol as simply "making membranes less fluid". It works in both directions depending on temperature, which is why it is described as a buffer. A membrane without it would be far more sensitive to temperature change than one with it.