Fatty acid
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually not found in organisms, but instead as three main classes of esters: triglycerides, phospholipids, and cholesterol esters. In any of these forms, fatty acids are both important dietary sources of fuel for animals and they are important structural components for cells.
Essential fatty acid
Mammals lack the ability to introduce double bonds in fatty acids beyond carbon 9 and 10, hence ω-6 linoleic acid (18:2,9,12), abbreviated LA (18:2n-6), and the ω-3 linolenic acid (18:3,9,12,15), abbreviated ALA (18:3n-3), are essential for humans in the diet. In humans, arachidonic acid (20:4,5,8,11,14, abbreviated 20:4n-6) can be synthesized from LA by alternative desaturation and chain elongation.