MSH | B4 Biological molecules

B4.1

Biological molecules

Success criteria: identify the elements in carbohydrates, fats and proteins; link large molecules to their building blocks; carry out and interpret four food tests.

Carbohydrates and fats contain carbon, hydrogen and oxygen. Proteins contain carbon, hydrogen, oxygen and nitrogen.

Molecule builder

Select the correct smaller molecule(s) used to make each large molecule.

Large moleculeBuilding block

Choose a pair.

Watch: food tests

If school filtering blocks the player, open the video on YouTube.

Interactive practical

Food-test simulator

Choose a sample and test. Predict the positive result before revealing the observation.

Food sampleTest

Awaiting test…

Four tests to know

Nutrient	Positive result
Starch	iodine: orange-brown → blue-black
Reducing sugar	Benedict's + heat: blue → green/yellow/orange/brick-red
Protein	biuret: blue → lilac/purple
Fat/oil	ethanol emulsion: cloudy white emulsion

Paper 6 focus: state reagent, key step (including heating for Benedict's) and the observation.

Original Cambridge-style practice

Paper 2, Paper 4 and Paper 6 practice

[1]1. Which molecules join to form a protein?

[3]2. Describe the test for reducing sugar and the positive result.

Add Benedict's solution; heat in a hot water bath; positive result changes from blue to green/yellow/orange/brick-red depending on concentration.

[4]3. A food gives a blue-black colour with iodine and a lilac colour with biuret. State the nutrients present and explain the evidence.

Starch present because iodine became blue-black; protein present because biuret became lilac/purple.

[4]4. State two variables to control when comparing reducing sugar concentration in fruit juices using Benedict's test, and explain how results could be compared.

Any two sensible controls: volumes of sample/reagent, heating time, water-bath temperature. Compare final colour using a standard colour scale, or measure absorbance with suitable equipment; repeat and calculate a mean.

Sources and teacher note

Aligned to the Cambridge IGCSE Co-ordinated Sciences (Double Award) 0654 syllabus for examinations in 2025–2027. The 0973 course has the same subject content. Embedded video links were checked during creation on 14 September 2026.

Video: Cognito, “Food Tests Practicals”.

This independent learning resource is not endorsed by Cambridge International. Cambridge-style questions are original and are not copied from past papers.