LIpids - 1

Cell membranes are made of phospholipids, which are made of lipids. Lipids are made up of the elements carbon, hydrogen and oxygen but in different proportions to carbohydrates. The most common type of lipid is the triglyceride.

• Lipids can exist as fats, oils and waxes. Fats and oils are very similar in structure (triglycerides).
• At room temperature, fats are solids and oils are liquids. Fats are of animal origin, while oils tend to be found in plants.
• Waxes have a different structure (esters of fatty acids with long chain alcohols) and can be found in both animals and plants.

Triglycerides:

These are made up of 3 fatty acid chains attached to a glycerol molecule.

• Fatty acids are chains of carbon atoms, the terminal one having an OOH group attached making a carboxylic group (COOH).
• The length of the chain is usually between 14 and 22 carbons long (most commonly 16-18).

Three of these chains become attached to a glycerol molecule which has 3 OH groups attached to its 3 carbons.

This is called a condensation reaction:

• 3 water molecules are formed from 3 OH groups from the fatty acids chains.
• 3 H atoms from the glycerol.

The bond between the fatty acid chain and the glycerol is called an ester linkage.

In the fatty acid chains, the carbon atoms may have single bonds between them making the lipid saturated.

• These are usually solid at room temperature and are called fats.

If one or more bonds between the carbon atoms are double bonds, the lipid is unsaturated.

• These are usually liquid at room temperature and are called oils.