To study the chemical composition of the cell membrane, the preferred source is RBC, because they lack cell organelles and thus no contamination of other cellular organelle membranes. The membranes of the RBCs devoid of cytosol are called as ‘ghosts’.
Four major constituents are present in the cell membrane. They are (i) lipids (28 – 79%) (ii) proteins (20 – 70%). (iii) oligosaccharides (only 1 – 5%) and (iv) water (20%).
Depending upon the tissue from which the cell membrane is isolated, the composition also differs. Nearly 80% of the myelin sheath is made up of lipids, while in liver, it constitutes only 28%.
The main lipid components of the membranes are phospholipids, cholesterol and glycolipids. The major phospholipids present are phophatidyl choline (lecithin), phophatidyl ethanolamine, phophatidyl serine and phophatidyl inositol.
Membrane lipids are amphipathic in nature and they have a head portion, which is hydrophilic and a tail portion which is hydrophobic. As the membranes are exposed to the hydrophilic environments, the lipids arrange themselves to form a bilayer in which the hydrophobic core is buried inside the membrane.
All the major functions of the plasma membrane are executed by the proteins present in the membrane. Proteins account for about 20 – 70% of the membrane depending on the type of the cell. They can be classified into two types. Integral membrane proteins and peripheral membrane proteins.
Some of the membrane proteins are tightly embedded in the membrane and they cannot be isolated unless, the membrane is disintegrated. They are called as Integral or Intrinsic membrane proteins. They are again classified into two. (a). Transmembrane proteins, which traverse (pass through) or span the membrane. These proteins will have domains on either side of the membrane. Many cell surface receptors belong to this class. (b). Lipid anchored proteins that are present either on the cytosolic side or on the extracytosolic side. They insert themselves in the membrane by a lipid (acyl chain) attached to the N terminal end.
Transmembrane proteins are of two types. Single pass transmembrane proteins that traverse the membrane only once.Multipass transmembrane proteins that traverse the membrane more than once.
Those proteins that are present on the surface of the membrane are called as peripheral proteins. They can be easily isolated from the membrane. eg. spectrin present in the RBC membrane.