Lipids

Lipids are insoluble in water but soluble in organic solvents like ethanol. 

Common lipids are: triglycerides, waxes, steroids and cholesterol. 

Lipids contain carbon, hydrogen and oxygen. However there is a bigger proportion of hydrogen and a lesser proportion of oxygen. 

Roles of Lipids: 

• Contribute to the flexibility of cell membranes. 

• Source of energy 

• Waterproofing 

• Provide insulation 

• Provide protection 

Triglycerides are made up of 3 fatty acids + 1 glycerol which is formed by a condensation reaction.  

How are triglycerides formed? - A glycerol and 3 fatty acid molecules go through 3 condensation reactions, thus releasing 3 molecules of water – producing a triglyceride. 

A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond. 

Reasons why triglycerides are good storage molecules: 

• Have high ratio of energy storing carbon-hydrogen bonds to carbon atoms. 

• Have a low mass to energy ratio, meaning more energy stored in a smaller volume. 

Why are triglycerides insoluble? 

They are insoluble because they are non-polar and are large molecules. As a results their storage doesn't affect osmosis or the water potential of them.  

Why are triglycerides an important source of water? 

As they have a high ratio of hydrogen to oxygen atoms, triglycerides release water when oxidised.  

Emulsion test for lipids: 

Method: 

• Add 5cm³ lipid sample to 5cm³ of  ethanol in a test tube 

• Shake the test tube with the solution for about a minute. 

• Then pour the solution into another test tube that is filled with 10 cm³ of water 

• Any lipid present will show a milky emulsion. 

Emulsion – taking something insoluble and making it dissolve with water 

E.g. by adding ethanol.