Lipids are a heterogeneous group of chemicals found in the cell. The simplest ones contain only carbon, hydrogen and oxygen and by hydrolysis they yield glycerol and fatty acids. Most fatty acids are long straight chains in which the carbon atoms contain either the maximum number of hydrogen atoms (saturated) or fewer number of hydrogen atoms (unsaturated). The fatty acids may also have branched chains. Saturated fatty acids do not have any double bonds between the carbons of the molecular chain, e.g. palmitic acid (16C) stearic acid (18C). Unsaturated fatty acids have one or more double bonds between carbons of the chain, e.g. oleic acid (one double bonds linolenic acid (two double bonds); linolenic acid (three double bonds) all these fatty acids contain 18 carbon atoms. All fatty acids end in a carboxyl (acid) group.
Fats (fatty oils, such as olive oil and cod liver oil) contain a mixture of what are called triglycerides, which are links between glycerol and fatty acids. The reaction of an acid with an alcohol forms an ester. The carboxyl group of the fatty acid reacts with the hydroxyl (OH) group of glycerol to form a triglyceride.
In contrast to the simple lipids are the waxes and related substances in which the glycerol is replaced by a longer-chain alcohol. Bees wax, for instance, is an ester of palmitic acid (a 16-carbon saturated fatty acid) and myricyl alcohol (a 30-carbon-chain saturated alcohol). In the simple lipid, one of the fatty acids may be replaced by compounds containing phosphorus and nitrogen to form the phospholipids or phosphatides, lecithin and cephlin which frequently represent the major portion of cellular lipids. Phospholipid molecules carry both hydrophilic (water-attracting) polar groups and hydrophobic (water repellent) nonpolar groups. The hydrocarbon chains of fatty acids are the non-polar tails of the molecule. The phosphate and the nitrogenous or non-nitrogenous groups form polar head-group of the molecule. These compounds are soluble in both water and fats and therefore serve a vital role in the cell by binding water soluble compounds (i.e. proteins) and lipid-soluble compounds together. Lecithin is a key structural material in the cell membrane, because it can maintain continuity between the aqueous and lipid phases of the inside and outside of the cell. Many phospholipid molecules may arrange themselves in a double-layered membrane (lipid bilayer) in aqueous media. Such lipid bilayers are the basic components of the cell membrane. Glycolipids are another class of lipids which contain one or more simple sugars.
A third class of lipids is the sterols which are not straight chain compounds. Sterols consists of fused hydrocarbon rings and a long hydrocarbon side chain. Cholesterol is an example of sterols found in animals. Diosgenin is produced by the Yam plant (Dioscorea).
Fats are storage products in plants as well as animals. In oil seeds (groundnut, mustard, coconut and castor) the fat stored serves to provide nourishment for the embryo. Oil extracted from these seeds serves as a cooking medium for man. In animals it is the adiposities which store fats. Fats are highly concentrated sources of energy. They are oxidized in the cell to produce energy when necessary. Fats stored beneath the skin and around internal organs serve as effective insulators preventing loss of heat. They also serve as efficient shock absorbers.
Phospholipids, glycolipids and sterols serve mainly as structural lipids, forming important constituents of biomembranes. Sterols such as cholesterol are precursors for the production of steroid hormones, vitamin D, and bile salts.
These are complex compounds commonly found in cell membranes and animal hormones. The best known of these is the sterol called cholestrol, which reinforces the structure of the cell membrane in animal cells and in an unusual group of cell wall deficient bacteria called mycoplasmas. The cell membrane of fungi also contains a sterol called ergosterol. Prostaglandins are fatty acid derivatives found in trace amounts that function in inflammatory and allergic reactions, blood clotting and smooth muscle contraction.
Waxes are esters formed between a long chain alcohol and saturated fatty acids. This material is typically pliable and soft when warm but hard and water resistant when cold. Fur, feathers, fruits, leaves, human skin and insect exoskeleton are naturally waterproofed with a coating of wax. Bacteria that cause tuberculosis and leprosy produce a wax that contributes to their pathogenicity.