Discuss about the Molecular Organization of Cell Membrane.
Molecular Organization of Cell Membrane.
The cell membrane is composed or proteins (20-70), lipids (20-79 percent) carbohydrates (oligosaccharides, 1.5 percent) and water (20 percent), lipids and proteins are held together by no covalent bonds, while oligosaccharides are covalently bound to some of proteins and lipids. Various protein molecules are distributed on the surface of the membrane as well as embedded in the lipid bilayer forming a mosaic arrangement. The chemical composition of cell membrane is variable while it have the same structure and function.
The main lipid components of the cell membrane are phospholipid, glycolipids and cholesterol with their relative proportion varying in different cell membranes. Membrane associated lipids are asymmetric with polar and non-polar ends.
They contain both hydrophilic and hydrophobic region, so these are also called amphipathic, the lipid form a bilayer, which outer surface is hydrophilic and inner surface in hydrophobic, sterol (such as cholesterol) molecules are more rigid than the phospholipids and therefore, presence of sterol confers stability on eukaryotic membranes. Prokaryotic membranes lack sterol. The proteins in the cell-membrane can be found in a variety of forms.
These may be trans-membrane proteins extending through the bilipid layer as a single helix. The extrinsic proteins on the other hand, may be present on the cytosolic face or toward the external face. These extrinsic protein may be covalently attached to fatty acid chains or non-covalently attached to other trans-membrane proteins.
Some of these proteins cannot be easily released in comparison to extrinsic protein and so are called intrinsic proteins. Carbohydrates in cell membrane occur, in the form of glycoproteins and glycolipids, which bound to protein and lipid molecules. A single glycoprotein may have many oligosaccharides side chains whereas a glycolipids molecule has only one. Carbohydrate chains of all the cell membrane are located exclusively on non-cytoplasmic surface, i.e. outside of the cell.