Lipids

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Chapter 10 Lipids

Chapter contents

Characteristics of fatty acids 73

General information on fatty acids 76

Significance of phospholipids 78

Waxes 79

Glycolipids 80

These are generally water-insoluble organic substances found in cells. Lipids form the basis of a diverse range of compounds. Some examples are:

These are ultimately derived from fatty acids.

Lipids have several functions. They are:

• structural components of membranes

• intracellular storage for metabolic fuel

• a transport mechanism for metabolic fuel

• precursors of many metabolic processes and hormones.

Characteristics of Fatty Acids

• All posses a long hydrocarbon chain (a compound consisting entirely of carbons and hydrogens) and a terminal carboxyl group.

• The chain can be saturated (without double bonds) or unsaturated (contains one or more double bonds) (Figure 10.1).

• They differ in chain length and in the position and number of their bonds.

• Nearly all of them have an even number of carbon atoms; those with 16 to 18 carbons are the most common.

• Unsaturated fatty acids have lower melting points than the saturated fatty acids (this becomes significant when considering whether they are solid or fluid at room temperature).

• The double bonds of nearly all the naturally occurring unsaturated fatty acids are in the cis formation (Figure 10.2).

• The most abundant unsaturated fatty acids in higher organisms are oleic, linoleic, linolenic, and arachidonic acids (see Figure 10.6).

Figure 10.1 A saturated compound (top; containing only single bonds) and an unsaturated compound (below; containing some double bonds).

Figure 10.2 Demonstrating the difference between cis (E) and trans (z) fatty acids.

Figure 10.6 Various structures of fatty acids.

Unsaturated Fatty Acids

The cis conformations (see Figure 10.2) of the double bonds of unsaturated fatty acids have a bend in their structure of about 30 degrees. This creates significant structural features, particularly for membranes, and as components of membranes the fatty acids tend to have phosphate groups added to them to make phospholipids.

Cis fatty acids can be turned into trans forms by an industrial process. This creates fats with a much higher melting point, which are solid at room temperature and can be made into a spread (e.g. margarines). The natural formation of the bonds has been changed.