Lipids

Although lipids can be large molecules, they are not regarded as macromolecules because unlike proteins, polysaccharides and nucleic acids, they are not polymers of a basic subunit. Moreover, lipids do not share any single structural characteristic; they are a diverse group structurally, but have in common the fact that they are insoluble in water, but soluble in a range of organic solvents. This non-polar nature is due to the predominance of covalent bonding, mainly between atoms of carbon and hydrogen.

Fats are simple lipids, whose structure is based on fatty acids (see Box 2.5). Fatty acids are long hydrocarbon chains ending in a carboxyl (-COOH) group. They have the

Figure 2.24 Adenine pairs only with thymine, and guanine with cytosine, thus if the sequence of bases in one strand of a DNA molecule is known, that of the other can be predicted. This critical feature of Watson and Crick's model offers an explanation for how DNA is able to replicate itself. Note that GC pairs are held together by three hydrogen bonds, while AT pairs only have two

Figure 2.24 Adenine pairs only with thymine, and guanine with cytosine, thus if the sequence of bases in one strand of a DNA molecule is known, that of the other can be predicted. This critical feature of Watson and Crick's model offers an explanation for how DNA is able to replicate itself. Note that GC pairs are held together by three hydrogen bonds, while AT pairs only have two general formula:

where n is usually an even number. They combine with glycerol according to the basic reaction:

Alcohol + Acid ^ Ester

The bond so formed is called an ester linkage, and the result is an acylglycerol (Figure 2.25). One, two or all three of the -OH groups may be esterified with a fatty acid, to give respectively mono-, di- and triacylglycerols. Natural fats generally contain a mixture of two or three different fatty acids substituted at the three positions; consequently, a considerable diversity is possible among fats. Fats serve as energy stores; a higher proportion of C—C and C—H bonds in comparison with proteins or carbohydrates results in a higher energy-storing capacity.

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