International A Level Biology / Chemistry for biologists
Lipids: triglycerides, saturated and unsaturated
Edexcel International A Level Biology (YBI11), chapter 1: Chemistry for biologists, specification point 1.5. Free lesson.
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Triglycerides form when glycerol joins to three fatty acids. The bonds in the fatty-acid chains affect how closely lipid molecules pack, which helps explain why fats are usually solid and oils usually liquid.
What glycerol and fatty acids contain
A triglyceride is made from glycerol and three fatty acids. Glycerol has three carbon atoms in a row, with one hydroxyl group (OH) on each carbon. A fatty acid has a carboxyl group (COOH) at one end and a long hydrocarbon chain at the other. The chain is often represented by R, and it is where fatty acids can differ. The OH groups on glycerol can each react with one fatty acid. The COOH group is the reactive end of a fatty acid.
How a triglyceride forms
A hydroxyl group on glycerol reacts with the carboxyl group of a fatty acid in a condensation reaction. This removes the OH from the acid and an H from glycerol, which join to form water. The remaining parts join by an ester bond. Glycerol has three OH groups, so the reaction happens three times. The overall reaction is: 1 glycerol + 3 fatty acids → 1 triglyceride + 3 water molecules. A triglyceride has one glycerol joined to three fatty acids by three ester bonds. The three fatty acids do not all have to be the same.
How saturated and unsaturated fatty acids differ
The difference is in the hydrocarbon chain, not the carboxyl group. A saturated fatty acid has only single bonds between carbon atoms in its chain. Each carbon holds as many hydrogen atoms as it can. An unsaturated fatty acid has one or more carbon-to-carbon double bonds, written C=C. These bonds make the chain bend, forming a kink. One C=C bond makes a fatty acid monounsaturated. Two or more C=C bonds make it polyunsaturated.
Why fats are solid and oils are liquid
Straight saturated chains can pack closely together. Kinked unsaturated chains cannot pack as closely, so the molecules sit further apart. Closely packed lipids are usually solid at room temperature and are called fats. Loosely packed lipids are usually liquid and are called oils. Fats high in saturated fatty acids mostly come from animals, such as butter. Oils high in unsaturated fatty acids mostly come from plants, such as olive oil. A diet high in saturated fat is linked to raised blood cholesterol and a higher risk of cardiovascular disease (CVD).
How to describe triglyceride formation for marks
Include how many of each molecule join, the reaction type, the bond formed and the water released. Be precise when describing saturated fatty acids: they still have a C=O double bond in the carboxyl group. Do not say that saturated fatty acids have no double bonds at all. They have no carbon-to-carbon (C=C) double bonds.