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Biology A-level: Biological Molecules and Enzymes

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

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.

Functions of lipids

   1. Storage - lipids are non-polar and so are insoluble in water.
   2. 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.
   3. Production of metabolic water - some water is produced as a final
      result of respiration.
   4. Thermal insulation - fat conducts heat very slowly so having a layer
      under the skin keeps metabolic heat in.
   5. Electrical insulation - the myelin sheath around axons prevents ion
   6. Waterproofing - waxy cuticles are useful, for example, to prevent
      excess evaporation from the surface of a leaf.
   7. Hormone production - steroid hormones. Oestrogen requires lipids for
      its formation, as do other substances such as plant growth hormones.
   8. 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).