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Testing for Biological Molecules

Biology: AQA GCSE Higher, topic 2.2.11

Testing for Carbohydrates, Lipids and Proteins

Reagents can help us test what food substances are present. The first step is to grind up the food and add distilled water to dissolve some of the food. You can then test for the food substances:

Starch

Starch

  • Add iodine solution to test for starch.
  • It will turn blue-black if starch is present.
Sugar

Sugar

  • Add Benedict’s reagent and heat for about two minutes to test for sugar.
  • It will turn any of green, yellow or red if sugar is present.
  • The colour depends on the concentration.
Proteins

Proteins

  • Add Biuret solution to test for proteins.
  • It will turn mauve or purple if proteins are present.
Lipids

Lipids

  • To test for lipids, add Sudan III (a fat-soluble dye).
  • If lipids are present, a red-stained oil layer will float on the water surface.
  • The presence of lipids can also be tested by adding ethanol.
  • The solution is added to water and shaken. If the solution turns cloudy, lipids are present.

Test yourself on Testing for Biological Molecules

  • Select the correct reagent and substance pairing in each row

  • What colour does iodine solution turn when starch is present?

  • What colour does Biuret solution turn when protein is present?

  • Which colour is not produced when Benedict's reagent gives a positive result for sugar?

  • What happens when Sudan III is added to a sample that contains lipids?

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