Triglycerides and ester bonds

Molecules, Transport and Health · Water, carbohydrates and lipids · note 5 of 6

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Triglycerides and ester bondsSpec 1.5(i)

In short

A triglyceride is formed when glycerol joins to three fatty acids. Each of the three condensation reactions links an –OH group of glycerol to the carboxyl group (–COOH) of a fatty acid, forming an ester bond and releasing a molecule of water. Hydrolysis of the ester bonds releases glycerol and fatty acids.

Lipids contain carbon, hydrogen and oxygen, but far less oxygen than carbohydrates. They are hydrophobic and do not dissolve in water. Triglycerides are the most common lipids in the diet and in fat stores.

Glycerol is a small molecule with three carbon atoms, each carrying an –OH (hydroxyl) group.
A fatty acid has a carboxyl group (–COOH) at one end and a long hydrocarbon chain (R) of typically 14 to 20 carbon atoms.

In each condensation reaction, the –OH of glycerol and the –OH of the carboxyl group of a fatty acid react. A molecule of water is removed and an ester bond (–COO–) forms between them. This happens three times, once at each –OH group of glycerol, so three water molecules are released. The three fatty acids in a triglyceride can be the same or different.

glycerol + 3 fatty acids → triglyceride + 3 H₂O

Glycerol with three –OH groups reacting with three fatty acids; three water molecules are removed and the triglyceride has three ester bonds, shown boxed. — Three condensation reactions release three water molecules and form three ester bonds.

Hydrolysis of the ester bonds (catalysed by lipase) uses three water molecules and releases glycerol and three fatty acids.

Exam tip:

The bond is an ester bond, not a glycosidic bond. Quote the numbers: three fatty acids, three ester bonds, three molecules of water.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

Why is water a good solvent?

Water is a good solvent because its molecules are dipoles, with a slightly negative oxygen end and slightly positive hydrogen ends. These charged ends are attracted to ions and polar molecules, surround them and keep them apart, so they dissolve. This lets blood plasma and sap carry substances.

What is the difference between starch and glycogen?

Both are polysaccharides made of α-glucose, but starch is the energy store in plants and glycogen is the store in animals. Glycogen is more highly branched, with α-1,6 branches every 8 to 12 units, so it has more free ends and releases glucose faster.

What is a condensation reaction?

A condensation reaction joins two molecules together and releases a molecule of water. When monosaccharides join, a glycosidic bond forms. When glycerol and fatty acids join, ester bonds form. Hydrolysis is the reverse, as water is added to break the bond.

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