1.5—Triglycerides and saturated or unsaturated lipids

A triglyceride forms when glycerol reacts with three fatty acids in condensation reactions, creating three ester bonds and releasing three water molecules. Its hydrocarbon tails store substantial chemical energy.

A saturated fatty acid has no carbon–carbon double bond; an unsaturated fatty acid has one or more. Double bonds introduce bends, so unsaturated tails usually pack less tightly and have lower melting points.

Long reduced hydrocarbon chains contain many bonds that can be oxidised, giving lipids a high energy value per gram. Their insolubility also makes them compact stores with little osmotic effect.

A fat rich in saturated tails tends to be solid at room temperature, whereas an oil rich in cis-unsaturated tails is more likely to remain liquid. The physical state reflects tail packing, not simply the word ‘lipid’.

One double bond does not make a lipid ‘healthy’ or determine every biological effect. Separate molecular structure, physical state and health evidence.