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Fatty acids are found in biological systems either as free molecules or as components of more-complex lipids. They are derived from dietary sources or produced by metabolism, as described below.

The most chemically reactive portion of fatty acids is the acidic carboxyl group (COOH). It reacts with alcohols (R′OH) to form products known as esters (RCOOR′) and releases water in the process. This ester bond is the principal covalent bond linking fatty acid moieties to other groups in the more-complex lipids discussed in other sections of this article. A second chemical bond, occurring…

Pure fatty acids form crystals that consist of stacked layers of molecules, with each layer the thickness of two extended molecules. The molecules in a layer are arranged so that the hydrophobic (water-fearing) hydrocarbon chains form the interior of the layer and the hydrophilic (water-loving) carboxylic acid groups form the two faces. For a specific fatty acid…

In addition to the very common fatty acids with straight saturated or unsaturated acyl chains, many fatty acids are chemically modified by substituents on the hydrocarbon chain. For example, the preening gland of ducks secretes a fatty acid 10 carbons long with methyl (CH3) groups substituted for one of the hydrogens on carbons 2, 4, 6, and 8. Some…

Unsaturated fatty acids have one or more carbon-carbon double bonds. The term unsaturated indicates that fewer than the maximum possible number of hydrogen atoms are bonded to each carbon in the molecule. The number of double bonds is indicated by the generic name—monounsaturated for molecules with one double bond or polyunsaturated for molecules with two or more double bonds. Oleic…

The simplest fatty acids are unbranched, linear chains of CH2 groups linked by carbon-carbon single bonds with one terminal carboxylic acid group. The term saturated indicates that the maximum possible number of hydrogen atoms are bonded to each carbon in the molecule. Many saturated fatty acids have a trivial or common name as well as a chemically descriptive systematic name. The systematic names are…

Biological fatty acids, members of the class of compounds known as carboxylic acids, are composed of a hydrocarbon chain with one terminal carboxyl group (COOH). The fragment of a carboxylic acid not including the hydroxyl (OH) group is called an acyl group. Under physiological conditions in water, this acidic group usually has lost a hydrogen ion (H+) to form a negatively charged…

Fatty acids rarely occur as free molecules in nature but are usually found as components of many complex lipid molecules such as fats (energy-storage compounds) and phospholipids (the primary lipid components of cellular membranes). This section describes the structure and physical and chemical properties of fatty acids. It also explains how living organisms obtain fatty acids,…

Lipid, any of a diverse group of organic compounds including fats, oils, hormones, and certain components of membranes that are grouped together because they do not interact appreciably with water. One type of lipid, the triglycerides, is sequestered as fat in adipose cells, which serve as the energy-storage depot for organisms and also provide thermal insulation. Some lipids such as steroid hormones serve as chemical messengers between cells, tissues, and organs, and…

Lipid rafts are possible areas of the cell membrane that contain high concentrations of cholesterol and glycosphingolipids. The existence of lipid rafts has not been conclusively established, though many researchers suspect such rafts do indeed exist and may play a role in membrane fluidity, cell-to-cell communication, and infection by viruses.