Fluid mosaic model of plasma membrane
Singer and Nicolson (1972) put forward the “fluid mosaic model” of membrane structure. It is the latest and most widely accepted model. According to this model,
the cell membrane consists of a highly viscous fluid matrix of two layers of phospholipids molecules.
These serve as a relatively impermeable barrier to the passage of most
water soluble molecules.
Protein molecules on their complexes occur in the membrane, but not in continuous layer; instead, these occur as separate particles asymmetrically arranged in a mosaic pattern.
Some of these (peripheral or extrinsic proteins) are loosely bound at the polar surfaces of lipid layers. Others (called integral or intrinsic proteins), penetrate deeply into the lipid layer. Some of the integral proteins penetrate through the phospholipids layers and project on both the surfaces.These are called Trans membranes or tunnel proteins.
The plasma membrane contains lipids (32%), proteins (42%), carbohydrates (6%) and water (20%) although variations are always there.
The carbohydrates occur only at the outer surface of the membrane. Their molecules
are covalently linked to (i) the polar heads of some lipid molecules (forming glycolipids) and (ii) most of the proteins exposed at outer surface (forming
glycoproteins). The carbohydrates so bound to membrane components constitute
the glycocalyx of cell surface.
The sugar portions of glycolipids and glycoproteins are involved in recognition mechanisms:-
(a) Sugar recognition sites of two neighbouring cells may bind each other causing cell-to-cell adhesion. This enables cells to orient themselves and to form tissues.
(b) Through glycoproteins, bacteria recognise each other (female bacteria are
recognized by male bacteria; Paramoecia of different mating types recognize each other).
(c) These provide the basic of immune response and various control systems, where glycoproteins act as antigens.
Lipids and integral proteins are amphipathic in nature (i.e. have both hydrophobic and hydrophilic groups). The hydrophobic ends are situated inside the bilayer while the hydrophilic groups are directed outwards. Thus, the membrane is held together primarily by hydrophobic attraction.
However, the lipids have links in their fatty acid tails. These links prevent close packing of molecules and make the membrane structure more fluid. The fluidity increases with decreasing length of fatty acid tails.
Thousand of different types of proteins can occur in cell membranes. These may be
purely structural (provide elasticity and mechanical support) or have additional
functions as :
(a) Carriers, for transporting specific molecules into or out of the cell.
(b) Receptors, for immediate flow of information into the cells.
Gases like O2 and CO2 diffuse rapidly in solution through membranes.
Comments
Post a Comment