IGCSE Biology: Biological Molecules Practice Questions
Carbohydrates, fats and proteins are the three main biological molecules. Carbohydrates and fats contain carbon, hydrogen and oxygen, while proteins also contain nitrogen. Each has a standard chemical test with a specific colour change.
Topic 4 of Cambridge IGCSE Biology 0610 is largely about food tests, and food test questions are marked strictly. You need the reagent, the procedure including any heating, and the colour change stated from and to. Vague answers such as it turns positive score nothing. The questions below use the exact format Cambridge uses.
What you need to know for Biological Molecules
- Elements presentCarbohydrates contain carbon, hydrogen and oxygen. Fats and oils contain carbon, hydrogen and oxygen. Proteins contain carbon, hydrogen, oxygen and nitrogen, and sometimes sulfur.
- Building blocksStarch, glycogen and cellulose are built from glucose. Proteins are built from amino acids. Fats and oils are built from fatty acids and glycerol.
- Starch testAdd iodine solution. A positive result changes the orange brown solution to blue black.
- Reducing sugar testAdd Benedict's solution and heat in a water bath. The blue solution changes through green and yellow to orange or brick red if reducing sugar is present.
- Protein testAdd biuret solution. The blue solution changes to purple or lilac if protein is present. No heating is required.
- Fat testThe ethanol emulsion test. Dissolve the sample in ethanol, then add an equal volume of water. A cloudy white emulsion indicates fat.
- DNA structureTwo strands coiled together to form a double helix, held by cross links formed by pairs of bases. Adenine always pairs with thymine, and cytosine always pairs with guanine.
IGCSE Biology Biological Molecules questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Describe how you would test a food sample for reducing sugar, and state the result if reducing sugar is present.
Show the worked answer
- Grind the food and mix it with distilled water to make a solution, then filter it.
- Add an equal volume of Benedict's solution to a sample in a test tube.
- Heat the tube in a water bath at about 80 degrees Celsius for a few minutes. Heating is essential, since the reaction does not occur at room temperature.
- A positive result changes the blue solution through green and yellow to orange or brick red. If no reducing sugar is present it stays blue.
A student has one sample that contains starch and protein but no fat. Describe the two tests she would carry out and the results she would expect for each.
Show the worked answer
- Test 1: add a few drops of iodine solution to the sample. It changes from orange brown to blue black, showing starch is present.
- Test 2: add biuret solution to a fresh sample. It changes from blue to purple or lilac, showing protein is present. No heating is needed.
- If she also tested for fat with the ethanol emulsion test, the mixture would remain clear rather than turning cloudy white.
- Each test must be carried out on a separate sample so that the reagents do not interfere with one another.
DNA consists of two strands coiled together. Explain how the two strands are held together and state the base pairing rule.
Show the worked answer
- The two strands are coiled around each other to form a double helix.
- They are held together by cross links formed between bases on opposite strands.
- Adenine always pairs with thymine.
- Cytosine always pairs with guanine. This means the sequence on one strand determines the sequence on the other.
Name the elements present in a protein molecule and explain how this differs from a carbohydrate.
Show the worked answer
- Proteins contain carbon, hydrogen, oxygen and nitrogen. Some proteins also contain sulfur.
- Carbohydrates contain only carbon, hydrogen and oxygen.
- The difference is therefore the presence of nitrogen in proteins.
- This is why nitrate ions are needed by plants to make amino acids, and therefore proteins.
Common mistakes in this topic
- Forgetting to heat the Benedict's test. Without heating there is no colour change and the answer scores nothing.
- Saying a solution turns clear instead of colourless, or stating only the final colour rather than the change.
- Confusing biuret with Benedict's. Biuret tests protein and needs no heat. Benedict's tests reducing sugar and needs heat.
- Writing that fats contain nitrogen.
- Giving glucose as the building block of protein. Proteins are built from amino acids.
Exam tips
- Learn the four food tests as a grid: reagent, procedure, negative colour, positive colour. That grid answers every food test question.
- Always state the colour change as from X to Y. One colour alone rarely scores full marks.
- For the ethanol emulsion test, the order matters. Ethanol first, then water.
- If a question gives a table of results, read across the row before deciding. A sample can contain more than one food group.
- Link nitrogen to protein whenever plant mineral requirements come up. Nitrate ions are needed to make amino acids.
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Biological Molecules FAQs
What are the four food tests for IGCSE Biology?
Iodine solution tests for starch, turning from orange brown to blue black. Benedict's solution with heating tests for reducing sugar, turning from blue to orange or brick red. Biuret solution tests for protein, turning from blue to purple. The ethanol emulsion test detects fat, producing a cloudy white emulsion.
Why does the Benedict's test need heating?
The reaction between Benedict's solution and a reducing sugar only proceeds at a raised temperature. Without heating in a water bath the solution stays blue even when reducing sugar is present, giving a false negative. Cambridge awards a specific mark for stating that the tube is heated.
What elements are in carbohydrates, fats and proteins?
Carbohydrates contain carbon, hydrogen and oxygen. Fats and oils contain the same three elements but in different proportions. Proteins contain carbon, hydrogen, oxygen and nitrogen, and some also contain sulfur. Nitrogen is the element that distinguishes protein from the other two.
What are the DNA base pairing rules?
Adenine always pairs with thymine, and cytosine always pairs with guanine. The two strands of the double helix are held together by cross links between these base pairs, which means the base sequence on one strand determines the sequence on the other.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.