Question

Cholesterol is a rigid, planar molecule yet – when incorporated into a lipid membrane at low temperature – the fluidity of that membrane is enhanced, preventing transition to a gel-like state. Briefly explain this phenomenon

Answer 1

The plasma membrane is a phospholipid bilayer, made up of outer leaflet and in a leaflet of phospholipids. It is selectively permeable in nature and controls the movement of molecules across cell surface.

Cholesterol is an important molecule present inside plasma membrane. It controls the fluidity of plasma membrane by allowing lipid reordering.

Lipid reordering is a phenomena in which lipids are able to move from one leaflet of the plasma membrane to the other leaflet. This movement is called as flip flop movement and is catalyzed by enzymes called as flippases and floppases. Cholesterol also allows lateral movement of phospholipids within the same leaflet.

Whenever the external temperature is low, plasma membrane tends take up a gel like state and the fluidity of the membrane decreases. In order to prevent this, cholesterol allows flip flop movement of lipids across the two leaflets of the membrane. This help in increasing the fluidity of the membrane and therefore restores back the normal fluidity level.

Similarly, when the external temperature is high, plasma membrane tend to lose up and the fluidity of the membrane increases. In order to restore back the normal fluidity level, cholesterol prevent lipid reordering by preventing their movement from one leaflet to the other.

Therefore cholesterol is an important molecule in controlling and maintaining the fluidity of plasma membrane with respect to external temperature.

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