Fatty Acids and Glycerol

Triglycerides and phospholipids are built from two kinds of smaller molecule: glycerol and fatty acids.Glycerol (propane-1,2,3-triol, C₃H₈O₃) is a small alcohol with three carbon atoms, each carrying an –OH (hydroxyl) group. These three –OH groups are where fatty acids attach.A fatty acid has a carboxyl group (–COOH) at one end attached to a long hydrocarbon chain, usually 14 to 22 carbon atoms long. It is often written as R–COOH, where R is the hydrocarbon tail. The carboxyl group is what reacts with glycerol; the tail is non-polar and hydrophobic.Fatty acids differ in the length of the chain and in whether it contains double bonds:• Saturated fatty acids have no C=C double bonds: every carbon holds as many hydrogen atoms as possible. The chains are straight, pack closely together and have higher melting points, so lipids rich in them (animal fats such as butter) are solid at room temperature.
• Unsaturated fatty acids have one or more C=C double bonds. Monounsaturated have one; polyunsaturated have two or more. Each double bond puts a kink in the chain, so the molecules cannot pack closely and the melting point is lower. Lipids rich in them (plant oils such as olive oil) are liquid at room temperature.Worked example: a saturated fatty acid with 16 carbon atoms has the formula C₁₅H₃₁COOH. If it had one C=C double bond it would have two fewer hydrogens: C₁₅H₂₉COOH.