Question

Describe the fluid mosaic model of the phospholipid bilayer.

Answer 1

All living cells possess a thin (7-10nm) outer layer called biological membrane which separates the cell from the external environment. The selective permeability property of membrane, promote the cell to regulate the movement of molecules across the membrane. In 1895 first time E. Overton revealed that biological membranes are made up of lipids, later in 1935 two scientists H. Davson & J. Danielli proposed tri-laminar model/ sandwich model to explain the structure of the biological membrane. According to them, a lipid layer is sandwiched between the two protein layers. The J. D. Robertson in 1957 by examining the RBC membrane with the electron microscope proposed unit membrane model. According to his model, a lipid bilayer is surrounded by the protein monolayers on both sides. In 1972 fluid mosaic model is S.J. Singer & G.L. Nicolson to explain the structure of the membrane. According to this the lipids in the phospholipid bilayer move continuously from one position to another position within the lipid bilayer and the proteins are embedded within the Phospholipid bilayer which forms mosaic patches in the membrane.

The plasma membrane is composed of phospholipids, proteins, and cholesterol. The Phospholipids are most abundant amphipathic molecules form the cell membranes. Each phospholipid molecule made up of a glycerol molecule, two fatty acids, and a phosphate group which is modified by an alcohol (e.g. choline). A large number of phospholipids molecules combine to form the lipid bilayers and the phosphate group of phospholipid is negatively charged polar head & hydrophilic faced towards the outer side of the of lipid bilayer but the fatty acid chains of phospholipid are uncharged, nonpolar, which are hydrophobic in nature and they always avoid contact with water and face inside of lipid bilayer. Two different types of proteins present in the membranes. Intrinsic proteins are attached tightly to the membranes and completely embedded in the lipid bilayer but extrinsic proteins partly associated with the lipid bilayer. The integrity & permeability of the membrane is maintained by the cholesterol.