Describe the fluid mosaic model ?

By the late 1960s, evidence was accumulating that the unit membrane hypothesis was inadequate to account for the dynamic nature of membrane proteins, although it was clearly in harmony with the known distribution of membrane lipids. In 1972, a new hypothesis of membrane structure was developed by Singer and Nicholson. Known as the fluid mosaic model, it views the membrane as a double layer of fluid phospholipid into which proteins are inserted in various ways (a mosaic) rather than maintained as a continuous layer. The proteins that are associated with the exterior or interior face of the lipid mosaic are called extrinsic proteins. In different membranes, they are highly variable and may even be absent. The proteins found within the lipid bilayer are known as intrinsic proteins. They may be confined entirely to the lipid bilayer, or they may project through either the interior or exterior surface. In some cases, a large intrinsic protein may extend through both surfaces. The proteins found within the lipid matrix tend to be rich in hydrophobic amino acids, which permit a maximum amount of interaction between these proteins and the surrounding medium; the extrinsic proteins, on the other hand, tend to be rich in hydrophilic groups, which promote interaction with the surrounding water and the ions it may contain. In many cases, single proteins associated with the membrane twist and fold so that their hydrophobic portions remain embedded in the lipid matrix, while their charged, or hydrophilic, regions tend to project past the surface into the surrounding aqueous medium.

Separate proteins in the membrane may interact with one another to form a complex unit such as a channel or pore. The linking of proteins within the membrane may also provide some measure of stability for the protein arrangement, a necessary condition to assure the functional continuity of the membrane. The fluid phospholipid layer is relatively free to move in a direction parallel to the plane of the membrane itself, since the lipid molecules are generally held together by weak forces rather than by covalent bonding. However, both cholesterol and intrinsic proteins may inhibit such movement within the membrane and impose a modest degree of rigidity.

Although recent studies with the electron microscope tend to confirm the predictions of the fluid mosaic model, the unit membrane hypothesis has been of great value in guiding membrane studies and should not be viewed as a false step in our gradually increasing understanding of membrane structure. The fluid mosaic model itself may well be supplanted by a more useful concept in the future.