Biochemical Tests for Molecules

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  • Created by: yott33
  • Created on: 17-02-16 11:05

Benedict's Test for Reducing Sugars

Reducing sugars include all monosaccharides and some disaccharides.

1. Add Benedict's reagent (blue) to a sample and heat it in a water bath that's been brought to the boil.

2. If the test is positive = coloured precipitate

3. The higher the concentration of reducing sugar, the further the colour change goes. You can use this to compare the amount of reducing sugar in different solutions. A more accurate way to do this is to filter the solution and weigh the precipitate.

The colour changes from blue - green - yellow - orange - brick red  (BGYOB)

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Benedict's Test for Non-Reducing Sugars

If the result of the reducing sugars is negative, there could still be a non-reducing sugar present. To test for non-reducing sugars, you have to break them down into monosaccharides.

1. Get a new sample of the test solution and add dilute hydrochloric acid (HCl). Carefully heat it in a water bath that's been brought to the boil.

2. Neutralise it with sodium hydrogencarbonate. Add Benedict's reagent and heat the sample again.

3. If the test is positive = coloured precipitate

4. If the test is negative = stay blue  -  which means it doesn't contain any sugar.

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Testing for Glucose

Glucose can be tested for by using test strips coated in a reagent.

Dip the strips in a test solution and they will change colour if glucose is present. The colour change can be compared to a chart to give an indication of concentration.

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Iodine Test for Starch

To test for starch, add iodine dissolved in potassium iodide solution to the test sample.

  • If starch is present = blue-black colour
  • If there is no starch = browny-orange colour
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The Biuret Test for Proteins

1. The test solution must be alkaline so add a few drops of sodium hydroxide solution

2. Then you add some copper II sulfate solution.

If protein is present = solution turns purple

If there's no protein = solution will stay blue

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Emulsion Test for Lipids

1. Shake the test substance with ethanol for a minute

2. Pour the solution into water.

If lipid is present = milky solution

The more lipid there is, the more noticeable the milky colour will be

If no lipid = clear solution

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Colorimetry to Determine Concentration of Glucose

You can use Benedict's reagent and a colorimeter to get a quantitative estimate of how much glucose/other reducing sugar there is in a solution.

A colorimeter = device that measures strength of a coloured solution by seeing how much light passes through it.

A colorimeter measures absorbance. The more concentrated the colour of the solution, the higher the absorbance.

It's easiest to measure the concentration of the blue Benedict's solution that's left after the test. The paler the solution, the more glucose there was.

Therefore, the higher the glucose concentration, the lower the absorbance of the solution.

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Chromatography is a method used to separate and identify chemicals in a misture that relies on the movement of a gas or liquid through a medium. The liquid or gas is known as the mobile phase. The medium that doesn't move is the stationary phase.

In paper chromatography, the stationary phase is water trapped between the fibres of the paper. The mobile phase is a solvent and is moving through or over the stationary phase. 

Different solvents are used to identify different biological molecules e.g. amino acids of proteins

Components of a mixture spend different amounts of time in the mobile and stationary phase. Those that spend longer in the mobile phase travel faster/further.

Time spent in different phases separates out the components.

Amino acids are colourless so ninhydrin is used and turns them purple to make them visible. Ninhydrin and solvent is a volative combination so a fume cupboard must be used.

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