Triglycerides are formed by the condensation of one molecule of glycerol…

The idea

A triglyceride is made from one molecule of glycerol and three molecules of fatty acid. Glycerol carries three hydroxyl groups; each fatty acid carries a carboxyl group, written RCOOH. Each hydroxyl group reacts with one carboxyl group in a condensation reaction, so three condensations occur, three ester bonds form and three molecules of water are released for every triglyceride assembled. The three fatty acids need not be the same, and it is their R groups - the hydrocarbon tails - that determine the properties of the finished molecule. Because all three tails are hydrocarbon, the completed triglyceride is non-polar and insoluble in water, so it can be stored as large droplets in a cell without lowering the water potential or drawing water in by osmosis.

What earns the marks

• One glycerol joined to three fatty acids.
• Stating that the bonds are formed by condensation reactions.
• Naming the ester bond and stating that three are formed.
• Three molecules of water being released, one per ester bond.
• Stating that lipase hydrolyses the ester bonds, using water, to release glycerol and fatty acids.

Common mistakes and how to put them right

• Saying one molecule of water is released when a triglyceride forms, instead of three.

• Reversing the components and writing three glycerols with one fatty acid.

• Describing the bond as a glycosidic or peptide bond.

• Stating that the fatty acids join to each other rather than each to the glycerol.

• Claiming a triglyceride is a polymer of three fatty acid monomers.

Exam tips

• Draw the glycerol as a vertical backbone with three attachment points; it makes the count of bonds and water molecules obvious.
• Write 'three condensation reactions' explicitly rather than 'condensation', because the number is often the mark.
• Mention that the three fatty acids may differ - it sets up the saturated and unsaturated question that usually follows.