Biochemical Tests: Sugars & Starch (AQA A Level Biology): Revision Note

Biochemical tests: sugars & starch

• Several tests can be carried out to determine if a sample contains a certain type of sugar

• The following tests are qualitative - they do not give a quantitative value as to how much of each type of molecule may be present in a sample

• Sugars can be classified as reducing or non-reducing; this classification is dependent on their ability to donate electrons

Test for reducing sugars

• Benedict’s reagent is a blue solution that contains copper (II) sulfate ions (CuSO₄ ); in the presence of a reducing sugar, copper (I) oxide forms

  • Copper (I) oxide is not soluble in water, so it forms a precipitate

Benedict's test method

1. Add Benedict's reagent to a sample solution in a test tube

   • An excess of Benedict’s solution must be used so that there is more than enough copper (II) sulfate present to react with any sugar present

2. Heat the test tube in a water bath or a beaker of water that has been brought to a boil for a few minutes

3. If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced to copper (I) oxide

   • A positive test result is a colour change somewhere along a colour scale from blue (no reducing sugar), through green, yellow and orange (low to medium concentration of reducing sugar) to brown/brick-red (a high concentration of reducing sugar)

     • This test is semi-quantitative as the degree of the colour change can indicate the concentration of reducing sugar present

Test for non-reducing sugars

• Non-reducing sugars are tested differently from reducing sugars because they lack the chemical groups necessary for reduction reactions

• This means they cannot directly react with oxidising agents like those in Benedict's test

  • The addition of an acid will hydrolyse any glycosidic bonds present in any carbohydrate molecules

  • The resulting monosaccharides left will have an aldehyde or ketone functional group that can donate electrons to copper (II) sulfate (reducing the copper), allowing a precipitate to form

Method

1. Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil

2. Neutralise the solution with sodium hydrogencarbonate

   • Use a suitable indicator (such as red litmus paper) to identify when the solution has been neutralised, and then add a little more sodium hydrogencarbonate as the conditions need to be slightly alkaline for Benedict’s test to work

3. Then carry out the Benedict’s test as normal (see above)

Test for starch

• To test for the presence of starch in a sample, add a few drops of orange/brown iodine in potassium iodide solution to the sample

  • The iodine is in potassium iodide solution, as iodine is insoluble in water

• If starch is present, iodide ions in the solution interact with the centre of starch molecules, producing a complex with a distinctive blue-black colour

• This test is useful in experiments to show that starch, in a sample, has been digested by enzymes