Triglycerides form when one glycerol molecule bonds with three fatty acid molecules through three condensation reactions. Three ester bonds are formed and three water molecules are released. This content applies to both SL and HL IB Biology.
The Formation Mechanism
A glycerol molecule has three hydroxyl groups, while each fatty acid has a carboxyl group. Each fatty acid reacts with one of glycerol’s hydroxyl groups.
During each reaction, anOH and anH are removed, producing one molecule of water. The remaining parts of the glycerol and fatty acid become covalently linked by an ester bond. Because glycerol has three hydroxyl groups, this process occurs three times:
glycerol + 3 fatty acids → triglyceride + 3 water molecules
The completed triglyceride therefore consists of a glycerol backbone joined to three fatty acid chains. The fatty acids may be identical or different and may be saturated or unsaturated.
Triglycerides are largely non-polar and therefore hydrophobic and insoluble in water. This property helps make them suitable for long-term energy storage because they do not affect the osmotic concentration of cells. In animals, triglycerides stored in adipose tissue also provide thermal insulation.
The reverse process is hydrolysis. During hydrolysis, three water molecules are used to break the ester bonds, producing glycerol and three fatty acids.
A common misconception is that the three fatty acids bond directly to one another. They do not: each fatty acid forms a separate ester bond with the central glycerol molecule.
Exam Technique
For an IB question using “outline” or “explain,” state the reactants, identify condensation, give the products, and include the numbers: one glycerol + three fatty acids → one triglyceride + three water molecules. Do not confuse condensation, which releases water, with hydrolysis, which uses water.