The term “lipid” covers a miscellaneous collection of compounds having a predominantly hydrocarbon nature. They are consequently insoluble in water and readily soluble in most organic solvents. Apart from solubility considerations, inclusion of a compound in the group usually depends on its content of long chain fatty acids. Lipids are subdivided into simple lipids, compound lipids, and steroids. Lipids are widely distributed throughout both plant and animal kingdoms and are essential constituents of cell membranes; the phospholipids and cholesterol are particularly significant in this respect. Quantitatively, the triglycerides are the most abundant of the lipids and form the chief energy reserve of animals. They also protect the body from heat loss and mechanical injury. In plants, triglycerides occur in quantity only in seeds. The waxes have a protective function. In animals, they keep feathers, skin, and hair soft, pliable, and water repellant, while the cuticle waxes of plants protect them both from dehydration and from invasion by harmful organisms.
The term ‘lipid’ covers a miscellaneous collection of compounds having a predominantly hydrocarbon nature. They are consequently insoluble in water and readily soluble in most organic solvents. Apart from solubility considerations, inclusion of a compound in the group usually depends on its content of long chain fatty acids.
3. Steroids. Although they do not often contain fatty acids the steroids are frequently classed as lipids on account of their occurrence in natural fats and their solubility characteristics. They include cholesterol and the sex and adrenocortical hormones.
Lipids are widely distributed throughout both plant and animal kingdoms and are essential constituents of cell membranes; the phospholipids and cholesterol are particularly significant in this respect. Quantitatively the triglycerides are the most abundant of the lipids and form the chief energy reserve of animals. They also protect the body from heat loss and mechanical injury. In plants triglycerides occur in quantity only in seeds.
The waxes have a protective function. In animals they keep feathers, skin and hair soft, pliable and water repellant, while the cuticle waxes of plants protect them both from dehydration and from invasion by harmful organisms.
These are mainly straight chain aliphatic monocarboxylic acids containing an even number of carbon atoms. The series ranges from acetic acid (C2) to members containing twenty or more carbon atoms which may be either saturated or unsaturated. The saturated fatty acids containing up to eight carbon atoms are liquid at room temperature but those with longer chains are solids. The introduction of a double bond renders even the long chain acids liquid, thus the 18-carbon saturated stearic acid has a melting point of 70°C whereas oleic acid, which contains 18 carbon atoms and one double bond, has a melting point of only 14°C.
The fatty acids were originally given Greek names indicating their source but their systematic names based on Greek numbers indicate not only the length of the chain but also the degree of saturation. The suffix -anoic indicates that the acid is saturated and -enoic that it is unsaturated. If there are two or three double bonds the suffix becomes -dienoic and -trienoic respectively. The carbon atoms are numbered along the chain, beginning with C-1, the carboxyl group. Alternatively the carbon atom adjacent to the -COOH group is known as the α-carbon atom and the one furthest from the -COOH group as the ω-carbon atom. The position of the double bond may be indicated by inserting the lower of the numbers of the two carbon atoms involved in the bond. Thus the systematic name for oleic acid which has 18 carbon atoms and a double bond between carbons 9 and 10 is 9-octadecenoic acid while that of linoleic acid which contains 18 carbon atoms and double bonds between carbon atoms 9 and 10 and between 12 and 13 is 9,12-octadecadienoic acid (Tables 8.1 and 8.2).
|No. of C atoms||Common name||Systematic name|
|No. of C atoms||No. of double bonds||Common name||Systematic name|
The general type of acid may be more simply represented by stating merely the number of C atoms and double bonds which it contains. Thus the type specifications for palmitic, oleic and linoleic acids are 16:0, 18:1 and 18:2 respectively.
Free fatty acids dissociate in water and in most cases the pKa lies in the range 4·7–5·0. However, because of their negligible solubility in water, the acid properties of the higher members of the series cannot readily be measured.
The presence of both the non-polar hydrophobic hydrocarbon chain and the polar hydrophilic -COOH group is reflected in their physical characteristics. Thus fatty acids may be spread on the surface of water as monomolecular films or monolayers in which the molecules are orientated so that the polar -COOH group penetrates the surface and the hydrocarbon chain projects away from it. This property is even more pronounced in the case of the phospholipids.
The fatty acids that occur in greatest amount in natural fats are the saturated 16:0 palmitic acid, the 18:0 stearic acid and the unsaturated 18:1 oleic acid. Branched chain fatty acids and fatty acids containing an uneven number of C atoms are rarely found.
Oleic acid which contains a single double bond in the middle of a C18 chain is one of the most abundant fatty acids. Other members of the series are found in smaller amounts as are a number of polyunsaturated fatty acids containing more than one double bond (Table 8.2). These include linoleic acid and arachidonic acid which, although present in relatively small amounts, are physiologically important. They cannot be synthesized in the body and must therefore be supplied in the diet (page 123). Hence they are known as essential fatty acids.
The hydrocarbon chains of saturated fatty acids (and unsaturated fatty acids in which the configuration is all trans) assume a zigzag configuration with the C-C bonds forming a bond angle of 109°; if there is a cis double bond present, the zigzag alters course at this point. Since the naturally occurring unsaturated fatty acids usually have a cis configuration about their double bonds their chains tend to be less compact than those of saturated acids, which can fit into smaller spaces (Figure 8.1). This is a significant consideration in membrane structure (page 194) for, whereas saturated fatty acid chains are flexible, the unsaturated ones are kinked and relatively rigid.
As a result of the different relationships and distances between their component groups, geometrical isomers such as oleic and elaidic acids and maleic and fumaric acids, unlike optical isomers, have different properties and reactions: