Lipids (from the Greek lipos - fat) are compounds of vegetable or animal origin that are characterized by their solubility properties1. They are practically insoluble in water but highly soluble in nonpolar organic solvents. Lipids can be extracted from cells and tissues by organic solvents, such as chloroform, ether, or hydrocarbons.
Lipids vary considerably in their chemical structure. In general they are considered as derivatives of long-chain carboxylic acids. The distinguishing feature of lipids lies in their biphilic properties, resulting from the presence in their molecules polar (hydrophilic) and non-polar (hydrophobic) regions. Thus, lipids have an affinity both to water and to non-aqueous phase.
12.1. CLASSIFICATION
Lipids are classified as either simple (two-component) or complex ones; the latter consist of three or more components (Table 12.1). Simple lipids represent esters in which carboxylic acids acylate trihydroxylic alcohol glycerol (as in fats and oils) or long-chain alcohols (as in waxes). Lipids of this group give only alcohols and carboxylic acids on hydrolysis.
Complex lipids may contain other components such as a substituted phosphate group (as in phosphatides) or carbohydrate units (as in glycolipids). Moreover, the amino alcohol sphingosine (instead of glycerol) is a constituent of the sphingolipid
group.
Table 12.1. Classification of lipids
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* Unstated R's are saturated or unsaturated long carbon chains.
1 Note that this definition is based on a physical property (solubility) differing from that used for other classes. The latter were defined on the basis of their structures.
12.2. STRUCTURAL COMPONENTS OF LIPIDS
There are two obligatory components in all groups of lipids: long-chain carboxylic acids and alcohols.