OCR AS Biology

Revision Notes

2.2.6 Biochemical Tests: Reducing Sugars & Starch

Biochemical Tests: Reducing Sugars & Starch

  • There are a number of tests that can be carried out quickly and easily in a lab 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 (a reducing sugar that is able to donate electrons is itself oxidised)
    • OILRIG in Chemistry

The Benedict’s 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

Method

  • Add Benedict’s reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube
  • Heat the test tube in a water bath or beaker of water that has been brought to a boil for a few minutes
  • If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water
  • It is important that an excess of Benedict’s solution is used so that there is more than enough copper (II) sulfate present to react with any sugar present
  • 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 give an indication of how much (the concentration of) reducing sugar present

The Benedict's test for glucose, IGCSE & GCSE Biology revision notes

Benedict’s test for reducing sugars produces a colour change from blue towards red if a reducing sugar is present.

Reducing & Non-reducing Sugars Table

Reducing and Non-reducing sugars, downloadable AS & A Level Biology revision notes

To test for non-reducing sugars:

  • Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil
  • 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 the Benedict’s test to work
  • Then carry out Benedict’s test as normal
    • Add Benedict’s reagent to the sample and heat in a water bath that has been boiled – if a colour change occurs, a reducing sugar is present

Explanation

  • The addition of 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

The iodine 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 for showing that starch in a sample has been digested by enzymes

The iodine test for starch, IGCSE & GCSE Biology revision notes
Iodine will change colour in the presence of starch.

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