2.2.7—Triglycerides

A triglyceride is one glycerol molecule joined to three fatty acids. Each fatty acid is connected to glycerol by an ester bond, and the three fatty-acid tails make the molecule non-polar and hydrophobic.

• Formation: three condensation reactions join the three fatty acids to glycerol by three ester bonds, releasing three water molecules.
• Fatty-acid variation: saturated tails have no carbon–carbon double bonds; unsaturated tails contain one or more double bonds, which can introduce bends and affect packing.
• Hydrophobic storage: a triglyceride does not dissolve in water, so many molecules can be stored together without the same osmotic effect as an equivalent store of free glucose.
• Energy: the molecule contains many energy-rich carbon–hydrogen bonds and little oxygen, so oxidation releases a large amount of energy per gram.

The structure therefore supports a compact, long-term fuel reserve. Triglycerides are stored in organisms as energy reserves; in specialised tissues, their hydrophobicity and fatty-acid composition can also contribute to insulation or buoyancy, but those are separate consequences from the fuel value.

Do not say a triglyceride contains three glycerol molecules or three ester bonds per fatty acid. It stores chemical energy rather than ATP, and “more energy per gram” is not caused by containing more ATP; energy is released when the molecule is oxidised.