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Biological Molecules

Biochemical Tests

OxfordAQA International AS and A-level Biology

Tests for biological molecules

Test forMethodPositive resultNegative result
Reducing sugarAdd Benedict's solution and heatRed precipitateBlue
StarchAdd iodine in potassium iodide solutionBlue-blackBrown
ProteinAdd biuret reagentPurpleBlue
LipidAdd ethanol, shake, then add to waterWhite milky emulsionSolution stays clear

Reducing sugars

  • Glucose, fructose and maltose are reducing sugars.
  • Add Benedict's solution and heat: boil the mixture.
  • Reducing sugar present: red precipitate. Reducing sugar absent: the solution stays blue.
  • The reagent is Benedict's solution alone. Never write add HCl in the reducing sugar test.

Non-reducing sugars

Method: testing for a non-reducing sugar
  1. Heat with Benedict's solution: it stays blue, so the sugar is not a reducing sugar.
  2. Boil with an acid, for example HCl.
  3. Neutralise with an alkali, for example NaHCO3.
  4. Heat with Benedict's solution again.
  5. A red precipitate shows a non-reducing sugar.
Heat in the second Benedict's test

The final Benedict's test needs its own heating step: heat must be stated again. Never write using residual heat from boiling with acid, because the mixture has cooled while being neutralised.

Starch

  • Add iodine in potassium iodide solution.
  • Starch present: blue-black. Starch absent: brown.

Protein

  • Add biuret reagent: sodium hydroxide and copper sulphate solutions.
  • Protein present: purple.
  • The mixture is not heated. Never write heating in the biuret test.
  • Never write blue or pink for the biuret test.

Lipids

Method: the emulsion test
  1. Mix the sample with ethanol and shake.
  2. Add the mixture to water.
  3. A white milky emulsion shows a lipid.
Reagent and result of the emulsion test

The sequence is fixed: ethanol first, then water. The reagent is ethanol, so never write ethanal in the emulsion test. Nothing is heated: never write heated emulsion test. The result is an emulsion, droplets suspended in water, so never write precipitate in the emulsion test.

Judging a colour

  • A colour change is subjective.
  • Use a colour standard.

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