O Level / IGCSE Biology chapter 4: Biological molecules MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
11 exam-style questions on Biological molecules, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. A food is shaken with ethanol and the liquid is then poured into water. A cloudy white emulsion forms. Which nutrient is present?
A
lipid
B
protein
C
starch
D
glucose
Show answer
Answer: A. Lipids dissolve in ethanol; when mixed with water they form tiny droplets that make a milky emulsion.
2. What colour shows a positive result for the biuret test?
A
blue-black
B
cloudy white
C
brick red
D
purple
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Answer: D. Biuret reagent changes from blue to purple (lilac) when protein is present. Blue-black is the starch test.
3. From which smaller molecules are lipids made?
A
fatty acids and glycerol
B
glucose
C
amino acids
D
nucleotides
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Answer: A. Lipids (fats and oils) are built from fatty acids and glycerol, which are also the products of their digestion by lipase.
4. Which pair correctly matches a large molecule with the smaller molecules it is made from?
A
protein – glucose
B
DNA – fatty acids
C
cellulose – amino acids
D
starch – glucose
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Answer: D. Starch is a chain of glucose. Proteins are made from amino acids and DNA from nucleotides.
5. Which element is present in all proteins but not in carbohydrates?
A
nitrogen
B
carbon
C
oxygen
D
hydrogen
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Answer: A. Carbohydrates contain only carbon, hydrogen and oxygen. Proteins also contain nitrogen (and sometimes sulfur).
6. Four foods are tested. Results: P, iodine blue-black, biuret blue; Q, iodine orange-brown, biuret blue, Benedict's brick red; R, iodine orange-brown, biuret purple; S, iodine blue-black, biuret blue, Benedict's blue. Which food contains protein?
A
food S
B
food P
C
food R
D
food Q
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Answer: C. A purple colour with biuret solution shows protein, so only food R contains protein. Blue-black with iodine shows starch, and brick red with Benedict's solution shows a reducing sugar.
7. A food sample is tested. Iodine solution stays orange-brown, Benedict's solution stays blue after heating, and the biuret test turns purple. Which food could it be?
A
boiled rice
B
glucose solution
C
egg white
D
cooking oil
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Answer: C. Only the protein test is positive. Egg white is mostly protein; rice would give blue-black with iodine and glucose would change Benedict's colour.
8. Four glucose solutions are tested with Benedict's solution in the same way. The colours are blue, green, orange and brick red. Which colour came from the most concentrated solution?
A
green with a little precipitate
B
brick-red precipitate
C
orange precipitate
D
blue, with no precipitate
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Answer: B. The more reducing sugar there is, the further the colour goes from blue through green and orange to brick red. Brick red therefore shows the most concentrated glucose solution.
9. Which food test must be heated in a water bath?
A
Benedict's test for reducing sugars
B
the biuret test for protein in a food sample
C
the emulsion test for lipids using ethanol and water
D
the iodine test for starch in a food sample
Show answer
Answer: A. Benedict's solution must be heated with the food in a water bath to form a coloured precipitate. The iodine, biuret and emulsion tests are done at room temperature.
10. Glycogen is made from many molecules of
A
amino acids
B
fatty acids and glycerol
C
glucose
D
nucleotides
Show answer
Answer: C. Starch, cellulose and glycogen are all made from glucose. Amino acids make proteins.
11, each with the answer and a short explanation, across 1 topics: Biological molecules. Every chapter test draws ten of them, so you can take it many times.
Is the O Level / IGCSE Biology chapter 4 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
