1.5 Lipids
1.5.1 Introduction
The term lipid is the collective name given to a wide variety of classes of naturally
occurring compounds isolated from plant and animal material by extraction
with nonpolar solvents. This section discusses a selection of the classes of
compound that are classified as lipids. However, not all classes of compound
obtained by extraction with nonpolar solvents are classified as lipids.
1.5.2 Fatty acids
This is the most abundant group of compounds that are classified as lipids. They
occur as isolated molecules and are more commonly found as residues in other
lipid structures. The fatty acids and residues that are commonly found are
normally referred to by trivial names (Table 1.4). They usually have ‘straight
chain structures’ with even numbers of between 14 and 22 carbon atoms inclu-
sive. Both saturated and unsaturated residues are found. In the latter case both
cisandtransisomers are known but thecisisomers are more common. A few
residues have structures that have side chains and/or other functional groups.
Table 1.4 Fatty acids that are commonly found in lipids. Ricinoleic acid is optically active
because its number 12 carbon atom is chiral
Trivial name Systematic name Structure
Palmitic acid Hexadecanoic acid CH 3 (CH 2 ) 14 COOH
Palmitoleic acid cis-Hexadecenoic acid CH 3 (CH 2 ) 5 CH¼CH(CH 2 ) 7 COOH
Stearic acid Octadecanoic acid CH 3 (CH 2 ) 16 COOH
Oleic acid cis-9-Octadecanoic acid CH 3 (CH 2 ) 7 CH¼CH(CH 2 ) 7 COOH
Linoleic acid cis-9-cis-12-
Octadecadienoic acidCH 3 (CH 2 ) 3 (CH 2 CH¼CH) 2 (CH 2 ) 7 COOHLinolenic acid cis-9-cis-12-cis-15-
Octadecatrienoic acidCH 3 (CH 2 CH¼CH) 3 (CH 2 ) 7 COOHRicinoleic acid 12-hydroxy-cis-9-
OctadecanoicacidCH 3 (CH 2 ) 5 CHOHCH 2 CH¼CH(CH 2 ) 7 COOHArachidonic acid cis-5-cis-8-cis-11-cis-14-
Eicosatetraenoic acidCH 3 (CH 2 ) 3 (CH 2 CH¼CH) 4 (CH 2 ) 3 COOH20 BIOLOGICAL MOLECULES