Triglycerides

Lipids are organic compounds made of carbon, hydrogen and oxygen. All lipids contain a high proportion of CH2​ groups. Phospholipids also contain phosphorus.

All lipids have a low solubility in water but a high solubility in organic solvents (e.g., ethanol, tetrachloromethane).

Triglycerides are molecules that form fats or oils depending on the size of the molecule. The more carbon atoms, the higher the melting point because the intermolecular forces are stronger and more energy is required to overcome them.

Each triglyceride is made from glycerol combined with three fatty acids through a condensation reaction with the release of three molecules of water.

The fatty acids bind to the glycerol by means of ester bonds.

Triglycerides are broken down in a hydrolysis reaction with the chemical insertion of three molecules of water.

The diagram shows how triglycerides are formed and are broken down.

The carbon chain in fatty acids is often represented by the letter R, a variable group containing a chain of between 4 and 24 carbon atoms.

Lipids with long hydrocarbon chain fatty acids are more likely to be solid at room temperature – these are fats.

Those with short hydrocarbon chains form oils which are liquid at room temperature.

This is because there are weak forces of attraction between the fatty acid chains – the longer the carbon chain, the greater the force of attraction, so the higher the melting point. Some examples of different fatty acids and their melting points are shown below.

Functions of triglycerides

Triglycerides are efficient energy storage molecules. They are more efficient than carbohydrates:

• 1 g fat provides about 38 kJ energy

• 1 g carbohydrate provides about 17 kJ energy.

Because of this, fats and oils are the preferred energy storage molecule in animals and many seeds (lipids store twice as much energy per gram than carbohydrates).

Triglycerides are also good thermal insulators and provide mechanical protection for delicate organs.

Because fats are less dense than water, they are used to provide buoyancy for many aquatic animals.

Some animals spread oil onto their fur or feathers because this makes them waterproof. This is because fats are hydrophobic and repel water.