A long-chain aliphatic carboxylic acid. Fatty acids are to a large extent responsible for the physical properties of complex *lipids
. They may be saturated, monounsaturated (monoenoic), or polyunsaturated (polyenoic). Some of the less common ones may also contain polar or cyclic groups. The nature of the hydrocarbon chain greatly alters the solubility and reactivity of the fatty acid.
The majority of fatty acids have an even number of carbon atoms, although some odd-carbon fatty acids are found. The commonest fatty acids in plants are *oleic acid
, monounsaturated) and *palmitic acid
, saturated), but in specialized tissues, such as the chloroplast, *linolenic acid
, triun-saturated) is predominant. Other important fatty acids are *stearic
, myristic, lauric, and palmitoleic acids. Linoleic and linolenic acid can be synthesized by plants but not by animals. As precursors of prostaglandins they are essential in animal diets and are termed essential fatty acids. See also triacylglycerol