Nutrition & Digestion
Structures and Functions in Living Organisms · 9 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 16% of your exam marks.
Enzymes in digestion and the role of digestive structures appear regularly across both papers.
Four standard food tests detect the four major nutrient groups. Each works by adding a reagent to a food sample and watching for a specific colour change.
Preparing a food sample
For solid food, before testing:
- Grind the food into a paste using a pestle and mortar
- Add a small volume of distilled water and stir
- Filter through filter paper to give a clear solution to test
The four tests
| Test for | Reagent | Method | Positive result |
|---|---|---|---|
| Glucose (reducing sugar) | Benedict's solution | Add a few drops; heat in a water bath for 5 minutes | Blue → brick-red (orange/yellow if less glucose) |
| Starch | Iodine solution | Add a few drops at room temperature | Yellow-brown → blue-black |
| Protein | Biuret solution | Add a few drops at room temperature | Blue → violet (lilac/purple) |
| Lipids | Ethanol (then water) | Mix the sample with ethanol, then add an equal volume of cold water | Clear → cloudy white emulsion |
Notes for each test
- The Benedict's test needs heat. Use a water bath rather than a Bunsen burner directly because the test tubes are easy to break and the Bunsen flame is hard to control. Switch the burner off once the water is hot.
- The iodine test is quick and simple, with no heating needed.
- Biuret solution contains copper sulfate and sodium hydroxide, both of which are irritants. Always wear goggles.
- Ethanol is highly flammable. Keep it well away from any naked flame.
Food tests: starch (iodine) and reducing sugar (Benedict's)
What comes up: "Describe how to test a sample for starch" or "Describe how to test for reducing sugar" — each is a standard 2-mark question.
Write (two marks each): Starch test: (1) add iodine solution; (2) a positive result turns the solution blue-black.
Reducing sugar test: (1) add Benedict's solution and heat (in a water bath); (2) a positive result produces a red, orange, or yellow colour.
Watch out: The mark scheme credits black or dark blue for the iodine test and explicitly ignores "purple" — writing "purple" alone will not earn the colour mark. For Benedict's, any colour in the red-orange-yellow range is accepted (the deeper brick-red indicates more sugar), but the starting blue colour is not a credit point, so you don't need to state it.
Food tests: protein (Biuret) and lipid (ethanol emulsion)
What comes up: "Describe how to test a sample for protein" or "Describe how to test for lipid" — both come up as 2-mark describe questions.
Write (two marks each): Protein (Biuret) test: (1) add Biuret solution (or sodium hydroxide and copper sulfate); (2) a positive result changes the solution to lilac, purple, or pink.
Lipid (emulsion) test: (1) mix the sample with ethanol, then add an equal volume of cold water; (2) a positive result produces a cloudy white emulsion (milky appearance).
Watch out: The Biuret reagent starts blue — you do not need to state the starting colour. The credited end colour is lilac/purple/pink; "blue" or "green" as a final colour will not gain the mark. For the lipid test, it is the addition of water to the ethanol extract that produces the cloudiness — leaving out the water step loses the method mark.