9700 · 2.1
Testing for biological molecules
In Cambridge International Biology, understanding the **composition of biological samples** is fundamental. From analysing food to studying cellular contents, the ability to **identify key biological molecules** is crucial. This lesson will equip you with the knowledge of precise biochemical tests for **carbohydrates**, **lipids**, and **proteins**, covering the essential reagents, procedures, and observable changes that will ace your Paper 1 and Paper 2 questions. Key points align with **A-Level Notes** 9700 Biology chapter **2** (topic **2.1**). Notes and diagrams from **A-Level Notes** chapter **2**, topic **2.1** (1 pages).
Need to know
What you need to know
- **Reducing sugars** (glucose, fructose, maltose): add an equal volume of Benedict's solution and heat in a water bath at 80–100 °C. Blue → green → yellow → orange → brick-red precipitate of copper(I) oxide; the final colour gives a semi-quantitative estimate of concentration.
- **Non-reducing sugars** (sucrose): a negative Benedict's test first. Then hydrolyse with dilute hydrochloric acid and heat, neutralise with sodium hydrogencarbonate, and repeat the Benedict's test — a brick-red precipitate now shows the sugar was present.
- **Starch:** add iodine in potassium iodide solution at room temperature. Yellow-brown → blue-black.
- **Lipids:** the emulsion test. Dissolve the sample in ethanol, then add the solution to water. A cloudy white emulsion shows lipid is present.
- **Proteins:** the Biuret test. Make the sample alkaline with potassium (or sodium) hydroxide solution, then add a few drops of dilute copper(II) sulfate solution, at room temperature. Blue → lilac/purple when peptide bonds are present.
Explanation
Testing for biological molecules
- **Reducing sugars** (glucose, fructose, maltose): add an equal volume of Benedict's solution and heat in a water bath at 80–100 °C. Blue → green → yellow → orange → brick-red precipitate of copper(I) oxide; the final colour gives a semi-quantitative estimate of concentration.
- **Non-reducing sugars** (sucrose): a negative Benedict's test first. Then hydrolyse with dilute hydrochloric acid and heat, neutralise with sodium hydrogencarbonate, and repeat the Benedict's test — a brick-red precipitate now shows the sugar was present.
- **Starch:** add iodine in potassium iodide solution at room temperature. Yellow-brown → blue-black.
- **Lipids:** the emulsion test. Dissolve the sample in ethanol, then add the solution to water. A cloudy white emulsion shows lipid is present.