The below mentioned article provides an explanatory note on the structure of Phospholipids.
Phospholipids are a very important class of lipids containing phosphorous bound in a diester linkage. The phospholipids are very important structural components of all cell membranes including those of the cell organelles. Some phospholipids like lecithin’s may also play an important role in transport of ions across the membranes.
Unlike neutral fats which are not oriented properly in cells and occur as oil-droplets, the phospholipids are believed to have a definite orientation in the membranes in relation to each other and also with proteins associated with them.
Phospholipids are of different kinds but all of them contain a glyceryl residue to which are attached in ester linkage two long chain fatty acids and a phosphorylated component. The fatty acids may be saturated or unsaturated. In higher plants phospholipids commonly contain palmitic acid, linoleic acid and linolenic acid as their fatty acid components in different proportions.
Depending upon the type of phosphorylated component of the phospholipids, the latter are classified under following categories:
1. Phosphatidylcholine (Lecithin). This phospholipid has nitrogen containing choline in its phosphorylated component.
2. Phosphatidylethanolamine (Cephalin). The phosphorylated component contains ethanolamine here.
3. Phosphatidylinositol. This phospholipid contains hexahydric alcohol called inositol in its phosphorylated component.
4. Phosphatidyl glycerol. This has glycerol in its phosphorylated component.
5. Diphosphatidylglycerol (Cardiolipin). It consists of two glycerol residues with their usual esterified fatty acids chains. Both these residues in turn are linked to a common glycerol molecule which is phosphorylated on both of its sides.
Structures of various phospholipids described above are given in Fig. 14.4.
(Phospholipids are sometimes grouped under ‘polar lipids’ because they are readily soluble in polar solvents such as ethanol. They are also called as amphipathic compounds because of the presence of both hydrophilic (polar) and hydrophobic (non-polar) regions in their molecule).
The phospholipids are synthesized involving complex sequences of reactions which are understood more clearly in animals than in plants. On the other hand, the phospholipids are broken down to their residues by the action of the enzymes called phosphatidases which hydrolyse their fatty acids and phosphoric acid ester bonds specifically.