Biological Molecules
Biochemical Tests
AQA A-level Biology
Tests for biological molecules
| Test for | Method | Positive result |
|---|---|---|
| Reducing sugar | Heat with Benedict's solution in a boiling water bath | Red precipitate |
| Starch | Add iodine/potassium iodide solution | Blue-black |
| Lipid | Add ethanol, then add water and shake | White emulsion |
| Protein | Add biuret solution; do not heat | Purple |
Reducing sugars
- Glucose, fructose and maltose are reducing sugars. Sucrose is a non-reducing sugar.
- With no reducing sugar, Benedict's solution stays blue.
- The reducing sugar test is Benedict's solution and heat alone. Never write heating with acid in the reducing sugar test, and never write sucrose detected by the reducing sugar test.
- To show a solution contains amylase: add starch, leave for a time, then test for reducing sugar.
Proteins
- The reagent is biuret. Never write burette test for protein or Beirut test for protein.
- Never write heating the biuret test.
Lipids
The lipid dissolves in ethanol and comes out of solution as tiny droplets only when water is added afterwards, so the order is fixed: add ethanol, then add water. The reagent is ethanol: never write ethanal in the emulsion test. Nothing is heated, so never write heating the emulsion test. The positive result is a white emulsion, droplets suspended in water, and never a precipitate in the emulsion test.
Non-reducing sugars
- Do Benedict's test: heat with Benedict's solution in a boiling water bath. It stays blue.
- Boil with hydrochloric acid.
- Neutralise with sodium hydrogencarbonate.
- Heat with Benedict's solution in a boiling water bath again.
- A red precipitate shows a non-reducing sugar.
Heat again in step 4. Never write using the heat left from boiling with acid.
Measuring the amount of reducing sugar
- Semi-quantitative: filter and dry the precipitate, then find its mass. More precipitate means more reducing sugar.
- Judging a colour change by eye is subjective; a colorimeter is quantitative.
- Keep constant: Benedict's solution volume, Benedict's solution concentration, sample volume, water bath temperature and duration of heating.
- Make solutions of known concentrations of the sugar.
- Do the Benedict's test on each and read each in a colorimeter.
- Plot a calibration curve: concentration on the x-axis, colorimeter reading on the y-axis.
- Test the unknown sample the same way and find its concentration from the calibration curve.
Higher absorbance means more sugar.
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