Some polyunsaturated fatty acids cannot be synthesized by animals and are therefore to be supplied though food and they are called essential fatty acids. Linoleic acid and linolenic acid are the two fatty acids known to be essential for the complete nutrition of many species of animals. The third essential fatty acid arachidonic acid can be formed from linoleic acid in most mammals. Double bonds can be introduced at the ∆⁴ , ∆⁵ , ∆⁶ , ∆⁹ , positions (counting from the carboxy 1 terminal) but never beyond the ∆⁹, position in animals. In contrast, plants are able to introduce additional double bonds at ∆¹², and ∆¹⁵ positions and can herefore synthesize the nutritionally essential fatty acids.

The nonessential monounsaturated fatty acids are synthesized from smaller saturated fatty acids by several tissues including the liver.

Functions

The essential fatty acids in high concentrations, along with other lipids constitute the structural element of tissues. They are needed for the synthesis of eicosanoids which have important functions.The lipids of gonads also contain a high concentration of polyunsaturated fatty acids which suggest the importance in reproduction. They retard atherosclerosis being esterified and emulsified with cholesterol and are incorporated into lipoproteins for transport to the liver for further oxidation. They cure skin lesions. They effect prolongation of clotting time and increase the fibrinolytic activity.