Lipids – classification, fatty acids. Simple lipids. Fats – structure and properties.
Lipids - defined as any fat-soluble (lipophilic), naturally occurring molecule, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoacylglycerols, diacylglycerols, phospholipids, and others. They can be separated from polar (water soluble) naturally occurring molecules using their solubility in nonpolar solvents such as ether, chloroform, benzene, etc.
Functions of lipids. The main biological functions of lipids include energy storage, acting as structural components of cell membranes (as phospholipids) and participating as important signaling molecules. Some lipids function as protective coating of cell walls of bacteria, the leaves of higher plants, the cuticle of insects and the skin of vertebrates; some other lipids are starting materials for the synthesis of vitamins, prostaglandins, and hormones.
Triacylglycerols (fats and oils). The most abundant lipids are neutral fats, called triacylglycerols. . The triacylglycerols are esters of glycerol (a triprotic alcohol) with three FA which can be saturated (giving solids, fats) or unsaturated (giving liquids, oils). The FAs can be identical - then the lipids are called simple triacylglycerols, or they can be different - then the lipids are mixed triacylglycerols. These compounds are the main form of fat storage in plants and in adipose tissue cells (fat cells, adipocytes) of animals. Average man’s body is 21% fat, enough to supply energy for 2-3 months.
Fatty acids. A fatty acid is carboxylic acid often with a long unbranched aliphatic chain (tail), which is either saturated or unsaturated. Short carboxylic acids such as butyric acid (4 carbon atoms) are considered to be fatty acids, whereas fatty acids derived from natural fats and oils may be assumed to have at least 8 carbon atoms, e.g., caprylic acid (octanoic acid). Most of the natural fatty acids have an even number of carbon atoms because their biosynthesis involves acetyl-CoA, a coenzyme carrying two carbon atom groups.
Common unsaturated fatty acids found in liquid oils are:
oleic acid: cis CH3(CH2)7CH=CH(CH2)7COOH, symbol 18:1 -9
linoleic acid: cis,cis CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH, symbol 18:2 -6
linolenic acid: cis, cis, cis CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH, symbol 18:3 -3.
Fat stored in animal adipose tissue (beneath the skin and around internal organs; the human fat cells are called adipocytes) serves also a structural purpose, providing insulation against cold and padding to protect the internal organs. The higher melting saturated fats of animals are better suited to these purposes than liquid oils.