Question

Lipid bilayers can exist in a gel-like state or a more fluid-like state, which is a function of both temperature and lipid composition. Which of these states is more common in a biological membrane and why? Specifically address the advantages this may provide for associated membrane proteins

Answer 1

The fluidity of a cell membrane in an organism is regulated in a perfect manner, it is an importnat property of membranes that they tend to maintain their fluidity levels intact. The fluidity of a biological membranes depends upon two things i.e. temperature and its composition. When a membrane contains shorter hydrocarbon chains of phospholipids having more double bonds then it becomes difficult to freeze quickly and the membrane remains in fluid state at lower temperatures rather than in gel-like state. It has been found through experimentation that if a membrane contains a single phospholipid then there will be change in its fluidity i.e. change from fluidity state into rigid or gel state at lower temperatures. This is known as phase transition, however in majority of cell membranes the fluidity is retained as this is essential for exchange of biomolecules across the membranes under different temperatures. If the membranes freeze at lower temperatures, then the exchange of molecules will not take place, resulting in the membrane imbalance.

Depending upon the composition of the cell membranes, the fluidity varies e.g. In bacterial membranes, usually one single type of phospholipid will be present without cholesterol and once the temperature outside changes, they adjust themselves by sunthesizing fatty acids that contain more cis-double bonds so as to maintain the fluidity. However,in majority of eukaryotic cell membranes they contain not only phospholipids but also cholesterol and glycolipids which further help it to retain the fluidity state of membranes by preventing hydrocarbon chains from coming together or crystallizing.

In biological membranes, there are small specialized areas known as lipid rafts, these rafts contain few proteins and other lipids. These proteins help in the transport of small vesicles and membrane signaling cascade.