Each fatty acid consists of a hydrocarbon chain with a carboxyl group at one end. The glycerol molecule has three hydroxyl groups , each able to interact with the carboxyl group of a fatty acid. Removal of a water molecule at each of the three positions forms a triglyceride . They may contain as few as 4 carbon atoms or as many as 24. Because fatty acids are synthesized from fragments containing two carbon atoms, the number of carbon atoms in the chain is almost always an even number.
Some fatty acids have one or more double bonds between their carbon atoms.
They are called unsaturated because they could hold more hydrogen atoms than they do. Monounsaturated fats have a single double bond in their fatty acids. Polyunsaturated fats, such as trilinolein shown here, have two or more. Double bonds are rigid and those in natural fats introduce a kink in the molecule. This prevents the fatty acids from packing close together and as a result, unsaturated fats have a lower melting point than do saturated fats.