Lipids (Molecular Biology)

Lipids are water-insoluble organic compounds that serve a number of essential and diverse roles including intracellular storage of metabolic fuel as triglycerides, structural elements of cell membranes as phospholipids and cholesterol, protective waxes as fatty acid esters of monohydroxylic alcohols, and substances with intense biochemical activity as the various water- and fat-soluble vitamins. They are usually classified as simple or complex lipids based on whether hydrolysis of the compound yields one or two products, in the former, or more than two, in the latter. Among the simple lipids are cholesterol and its fatty acid esters, triglycerides and fatty acids. There are three major subdivisions of complex lipids: (i) Glycerophospholipids, which produce on hydrolysis glycerol, fatty acids, inorganic phosphate, and an organic base or polyhydroxy compound. This group includes the major phospholipids of cell membranes: phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositides, phosphatidylserine, and phosphatidylglycerol. (ii) Glycoglycerolipids, which yield glycerol, fatty acids, and carbohydrates. These lipids are found primarily in plants and bacteria, as well as in small amounts in brain and nervous tissue of some mammals. (iii) Sphingolipids, which contain a long-chain base, fatty acids and inorganic phosphate, and carbohydrates. They are present in the membranes of both plants and animals, particularly in brain and nerve tissue. Sphingomyelin is the most abundant member of this group.