Chemistry for Biologists

Lipids

Lipid is the collective name for fats, oils, waxes and fat-like molecules (such as steroids) found in the body. Their roles include:

  • components of cell membranes (phospholipids and cholesterol)
  • energy stores
  • chemical messengers (steroid 'hormones')
  • protection, waterproofing, insulation and buoyancy agents.

The basic unit of lipids is a triglyceride, synthesised from glycerol (propane-1,2,3-triol) and fatty acids.

Glycerol is a type of alcohol. Alcohols are organic compounds. Their molecules are characterised by hydroxyl groups, -OH. Glycerol is a trihydric alcohol, which means each molecule has three hydroxyl groups.

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Glycerol
Glycerol - click on image to open

Fatty acids

Fatty acids are also organic compounds. Each fatty acid has a carboxylic acid (-COOH) group. Some fatty acids are saturated. These means all carbon-carbon bonds are single covalent bonds. Other fatty acids are unsaturated. This means that while most carbon-carbon bonds are single covalent bonds, some are double bonds.

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Saturated fatty acid
Saturated fatty acid - click on image to open

Unsaturated fatty acid
Unsaturated fatty acid - click on image to open

Esters

Alcohols react with carboxylic acids to form esters

R1OH + R2COOH  -->  R2COOR1 + H2O

As with the formation of glycosidic and peptide bonds, this is a condensation reaction. A water molecule splits out. The glycerol molecule has three hydroxyl groups. When one reacts with a fatty acid a monoglyceride forms.

When two react with two fatty acid molecules a digylceride forms. And when three react with three fatty acid molecules a trigylceride forms.

ppt logo Representation of a triglyceride

ppt logo Formation of a triglyceride

Triglycerides formed between glycerol and unsaturated fatty acids are usually oils. Those formed with saturated acids are usually fats.

Phospholipids

Phosphoric acid is an inorganic compound. Its molecular formula is H3PO4. Like fatty acids, it can react with alcohols to form esters (in this case, usually called phosphate esters). In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate group.

ppt logo Representation of a phospholipid

Phospholipids have a key role in the structure and properties of cell membranes.

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