Lipids refer to a loose category of compounds with the common property that they have fairly low solubility in water or are extracted from biological materials by solvents having polarity much less than water, such as ethanol or chloroform. There are five major types, of which the first four are described here and the fifth in a later section.
• Fatty acids, long-chain aliphatic carboxylic acids
• Fats, esters of fatty acids with glycerol
• Phospholipids, esters of phosphate and fatty acids with glycerol
• Lipids not containing glycerol, including waxes and steroids
• Hybrid lipids, such as those combined with carbohydrates or proteins
Fatty acids are simply straight-chain hydrocarbons with a carboxylic acid functional group at one end (Figure 3.7). They usually, but not always, have an even number of carbon atoms. It is the hydrocarbon chain that imparts hydrophobicity, since the carboxyclic acid group is water soluble. The larger the chain is, the more hydrophobic the molecule. The simplest fatty acid is formic acid, where a simple hydrogen makes up the variable R-group. The most familiar is acetic acid, formed with a methyl group. Vinegar is about 5% acetic acid. Both of these are quite water soluble. The melting point of fatty acids tends to increase with chain length.
Fatty acids usually do not accumulate in nature. Systems may be engineered to produce them, as is done in fermentation processes. An important environmental application is anaerobic digestion, in which fatty acids consisting mainly of acetic acid, but also
Stearic fatty acids:
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