AQA A Level Biology Biochemical Tests 

What are reducing sugars?

• ALL MONOSACCHARIDES + some disaccharides like MALTOSE AND LACTOSE.
• Reducing sugars can donate electrons (the carbonyl group becomes oxidised), so the sugars become the reducing agent.
• So reducing sugars can be detected using Benedict’s test as they reduce the soluble copper sulphate into insoluble brick-red copper oxide.

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Describe the Benedict’s test for reducing sugar?

1. Add the Benedict’s reagent (which is blue as it contains copper (II) sulfate ions) to the sample in a test tube.
2. Heat the sample with excess benedict’s reagent in a gently boiling water bath for 5 minutes.
3. If it stays blue, there is no reducing sugar present. If it’s brick red, an insoluble precipitate forms then a reducing sugar is present.

Note- Benedict’s reagent is copper (II) sulphate; in the presence of a reducing sugar, copper (II) sulphate is reduced to copper (I) oxide.

In increasing amount of reducing sugar the colour goes

blue>green>yellow>orange>red.

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What are non-reducing sugars?

• Some types of disaccharides for example sucrose but NO monosaccharides.
• Non-reducing sugars cannot donate electrons, therefore they cannot be oxidised.
• To be detected non-reducing sugars must first be hydrolysed to break the disaccharide into its two monosaccharides before a Benedict’s test can be carried out.

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• Describe the Benedict’s test for non-reducing sugars?

  1. Add a few drops of dilute hydrochloric acid (hydrolyse sugar into its constituent reducing sugar) into the sample.
  2. Heat the sample in a boiling water bath
  3. Neutralise the solution with sodium bicarbonate.
  4. Add Benedict’s reagent and heat again
  5. If a non-reducing sugar is present a green/yellow/orange or red predicate will form.

  The reason is

  • 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.

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Describe a test for starch?

1. Add iodine dissolved in potassium iodide solution to the solution and shake/stir

2. If starch is present → colour change from orange/brown to blue/black.

   \n How do you use chromatography to detect monosaccharides?

• Mixtures containing coloured molecules, such as ink or chlorophyll, do not have to be stained as they are already coloured.
• Mixtures of colourless molecules, such as a mixture of monosaccharides, have to be stained first.
• A spot of the stained monosaccharide sample mixture is placed on a line at the bottom of the chromatography paper.
• Spots of known standard solutions of different monosaccharides are then placed on the line beside the sample spot.
• The chromatography paper is then suspended in a solvent.
• As the solvent travels up through the chromatography paper, the different monosaccharides within the mixture separate out at different distances from the line.
• The unknown monosaccharides can then be identified by comparing  and matching them with the chromatograms of the known standard solutions of different monosaccharides.
• If a spot from the monosaccharide sample mixture is at the same distance from the line as a spot from one of the known standard solutions, then the mixture must contain this monosaccharide.

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