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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.
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 because 3 water molecules are formed from 3 OH groups from the fatty acids chains and 3 H atoms from the glycerol. The bond between the fatty acid chain and the glycerol is called an ester linkage.
The 3 fatty acids may be identical or they may have different structures.
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.
- Storage - lipids are non-polar and so are insoluble in water.
- High-energy store - they have a high proportion of H atoms relative to O atoms and so yield more energy than the same mass of carbohydrate.
- Production of metabolic water - some water is produced as a final result of respiration.
- Thermal insulation - fat conducts heat very slowly so having a layer under the skin keeps metabolic heat in.
- Electrical insulation - the myelin sheath around axons prevents ion leakage.
- Waterproofing - waxy cuticles are useful, for example, to prevent excess evaporation from the surface of a leaf.
- Hormone production - steroid hormones. Oestrogen requires lipids for its formation, as do other substances such as plant growth hormones.
- Buoyancy - as lipids float on water, they can have a role in maintaining buoyancy in organisms.
A phosphate-base group replaces one fatty acid chain. It makes this part of the molecule (the head) soluble in water whilst the fatty acid chains remain insoluble in water.
Due to this arrangement, phospholipids form bilayers (the main component of cell and organelle membranes).