A fat is formed of two kinds of smaller molecules, i.e., glycerol and fatty acids. In making a fat three fatty acid molecules each join to glycerol by an ester linkage. A fatty acid has a long carbon skeleton, usually 16 or 18 carbon atoms in length. It there are no double bonds between carbon atoms composing the chain, then as many hydrogen atoms as possible are bonded to the carbon skeleton. This is called saturated fatty acid. Unsaturated fatty acid has one or more double bonds. Phosphoglycerides are esters of two fatty acids, phosphoric acid and a trifunctional alcohal glycerol. The fatty acids are attached to the glycerol at the 1 and 2 position on glycerol through ester bonds.