IGCSE Biology Topic 4: Biological Molecules MCQs – 30 Practice Questions with Answers (Cambridge 0610)

Free practice for Cambridge IGCSE Biology 0610 on biological molecules, with every question followed by a short worked explanation to help you check your understanding.

Building Blocks and Elements

Q1. Which elements are found in a molecule of carbohydrate?

  • (A) Carbon, hydrogen, oxygen and nitrogen
  • (B) Carbon, hydrogen and nitrogen
  • (C) Carbon, hydrogen and oxygen
  • (D) Carbon, oxygen and nitrogen
Show Answer

Answer: C – Carbohydrates contain carbon, hydrogen and oxygen only, with no nitrogen in the molecule.

Q2. Which element is present in proteins but is not present in carbohydrates or fats?

  • (A) Nitrogen
  • (B) Carbon
  • (C) Hydrogen
  • (D) Oxygen
Show Answer

Answer: A – Nitrogen is found in proteins, in their amino acid units, but not in carbohydrates or fats.

Q3. Which large molecule is made entirely from glucose units?

  • (A) A protein
  • (B) A fat
  • (C) DNA
  • (D) Starch
Show Answer

Answer: D – Starch is built by joining many glucose molecules together into a large carbohydrate.

Q4. Which statement best describes how large biological molecules are formed?

  • (A) Small molecules are joined together
  • (B) Small molecules are broken apart at random
  • (C) Large molecules dissolve into elements
  • (D) Large molecules form only in water
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Answer: A – Large molecules are made from smaller molecules joined together, such as glucose units forming starch.

Q5. Which pair of molecules are both built from glucose units?

  • (A) Starch and protein
  • (B) Starch and glycogen
  • (C) Glycogen and fat
  • (D) Cellulose and protein
Show Answer

Answer: B – Both starch and glycogen are large carbohydrates made from glucose units.

Q6. In which organism is glycogen the main storage carbohydrate?

  • (A) An animal
  • (B) A flowering plant
  • (C) A fungus only
  • (D) A bacterium only
Show Answer

Answer: A – Glycogen is the storage carbohydrate in animals, while plants store carbohydrate mainly as starch.

Q7. Which molecule forms the strong framework of plant cell walls?

  • (A) Cellulose
  • (B) Glycogen
  • (C) A fatty acid
  • (D) Glycerol
Show Answer

Answer: A – Cellulose, made from glucose units, builds plant cell walls and gives them strength.

Q8. What are the smaller units from which proteins are made?

  • (A) Glucose units
  • (B) Amino acids
  • (C) Fatty acids
  • (D) Glycerol units
Show Answer

Answer: B – Proteins are large molecules formed by joining many amino acid units together.

Q9. A molecule of fat is formed when glycerol joins with which smaller units?

  • (A) Three glucose units
  • (B) Three amino acids
  • (C) Three fatty acids
  • (D) Three starch units
Show Answer

Answer: C – Fats and oils are formed from glycerol joined to fatty acids, usually three fatty acids per glycerol.

Q10. Which molecule is not built from glucose units?

  • (A) Starch
  • (B) A protein
  • (C) Cellulose
  • (D) Glycogen
Show Answer

Answer: B – Proteins are built from amino acids, whereas starch, cellulose and glycogen are all built from glucose.

Food Tests

Q11. A food sample turns blue-black when iodine solution is added. What does this result show?

  • (A) A reducing sugar is present
  • (B) Starch is present
  • (C) Protein is present
  • (D) Fat is present
Show Answer

Answer: B – Iodine solution changes from brown-orange to blue-black in the presence of starch.

Q12. A sample is heated with Benedict’s solution and a brick-red precipitate forms. What is present in the sample?

  • (A) (‘A’, ‘Starch only’)
  • (B) (‘B’, ‘Fat only’)
  • (C) Protein only in the system described
  • (D) A reducing sugar
Show Answer

Answer: D – Benedict’s solution turns from blue to brick red on heating when a reducing sugar is present.

Q13. Which colour change shows a positive biuret test for protein?

  • (A) Blue to brick red
  • (B) Brown to blue-black
  • (C) Blue to colourless
  • (D) Blue to purple
Show Answer

Answer: D – The biuret test changes from blue to purple or lilac when protein is present.

Q14. In the ethanol emulsion test for fat, what is observed when water is added to the ethanol extract?

  • (A) A blue-black colour
  • (B) A purple colour
  • (C) A milky white emulsion
  • (D) A brick-red precipitate
Show Answer

Answer: C – Fat dissolved in ethanol forms a milky white emulsion when water is added.

Q15. A few drops of blue DCPIP solution lose their colour when a fruit juice is added drop by drop. What does this suggest?

  • (A) The juice contains vitamin C
  • (B) (‘A’, ‘The juice contains starch’)
  • (C) (‘B’, ‘The juice contains fat’)
  • (D) The juice contains protein when the reaction takes place
Show Answer

Answer: A – DCPIP is decolourised by vitamin C, so loss of the blue colour indicates vitamin C is present.


Q16. Which food test requires the sample to be heated?

  • (A) (‘A’, ‘The iodine test for starch’)
  • (B) The Benedict’s test for reducing sugars
  • (C) (‘B’, ‘The biuret test for protein’)
  • (D) The ethanol emulsion test for fat for the organism involved
Show Answer

Answer: B – The Benedict’s test needs heating for the blue solution to change colour in the presence of a reducing sugar.

Q17. A sample gives a negative iodine test but a positive Benedict’s test. What can be concluded?

  • (A) It contains starch but no reducing sugar in most ordinary conditions
  • (B) (‘C’, ‘It contains protein but no sugar’)
  • (C) It contains a reducing sugar but no starch
  • (D) (‘D’, ‘It contains fat but no starch’)
Show Answer

Answer: C – A negative iodine result rules out starch, while a positive Benedict’s result shows a reducing sugar is present.

Q18. Which row correctly matches each test with its positive result?

  • (A) Iodine: purple; biuret: blue-black
  • (B) Iodine: brick red; biuret: milky
  • (C) Iodine: blue-black; biuret: purple
  • (D) Iodine: milky; biuret: brick red
Show Answer

Answer: C – Iodine gives blue-black with starch and biuret gives purple with protein.

Q19. Why is heating needed in the Benedict’s test?

  • (A) (‘A’, ‘To dissolve starch in the sample’)
  • (B) To change protein into amino acids during the process described
  • (C) (‘D’, ‘To evaporate water from the sample’)
  • (D) To provide energy for the reaction with a reducing sugar
Show Answer

Answer: D – Heating supplies energy so the reducing sugar can react with Benedict’s solution and produce the colour change.

Q20. A clear liquid remains after a food sample is shaken with ethanol and water is added, with no milky appearance. What does this show?

  • (A) Fat is present
  • (B) Starch is present
  • (C) Protein is present
  • (D) Fat is absent
Show Answer

Answer: D – A milky emulsion would show fat, so its absence means no fat was detected by this test.

DNA Structure

Q21. How is a DNA molecule best described?

  • (A) (‘A’, ‘A single straight strand’)
  • (B) Two strands coiled into a double helix
  • (C) (‘C’, ‘Three strands twisted together’)
  • (D) A ring of bases with no strands in a typical living cell
Show Answer

Answer: B – DNA consists of two strands coiled together to form a double helix.

Q22. In DNA, which base always pairs with adenine (A)?

  • (A) Thymine (T)
  • (B) Cytosine (C)
  • (C) Guanine (G)
  • (D) Adenine (A)
Show Answer

Answer: A – Base pairing in DNA is always the same: adenine pairs with thymine.

Q23. In DNA, which base always pairs with cytosine (C)?

  • (A) Guanine (G)
  • (B) Adenine (A)
  • (C) Thymine (T)
  • (D) Cytosine (C)
Show Answer

Answer: A – Cytosine always pairs with guanine, just as adenine always pairs with thymine.

Q24. What holds the two strands of a DNA molecule together?

  • (A) Bonds between two sugars only
  • (B) Links between whole cells
  • (C) Bonds between paired bases
  • (D) Bonds within a single base
Show Answer

Answer: C – Bonds between pairs of bases hold the two strands of DNA together.

Q25. Each strand of DNA contains chemicals called what?

  • (A) Amino acids
  • (B) Fatty acids
  • (C) Bases
  • (D) Enzymes
Show Answer

Answer: C – Each DNA strand contains chemicals called bases, which pair up across the two strands.

Q26. One DNA strand has the base sequence A C G T. What is the sequence of the matching strand?

  • (A) A C G T
  • (B) T G C A
  • (C) C A T G
  • (D) G T A C
Show Answer

Answer: B – Each base pairs with its partner, so A pairs with T, C with G, G with C and T with A.

Q27. Which statement about DNA base pairing is correct?

  • (A) A pairs with C and G pairs with T
  • (B) A pairs with G and C pairs with T
  • (C) Pairing changes from strand to strand
  • (D) A pairs with T and C pairs with G
Show Answer

Answer: D – The pairing rule never changes: A with T and C with G.

Q28. A DNA sample is found to contain 30 percent adenine. What percentage of thymine would be expected?

  • (A) 15 percent
  • (B) 60 percent
  • (C) 70 percent
  • (D) 30 percent
Show Answer

Answer: D – Because adenine always pairs with thymine, the two bases occur in equal amounts.

Q29. Why does the double helix stay together as a stable structure?

  • (A) Bonds between paired bases link the strands
  • (B) (‘A’, ‘The strands are glued by water’)
  • (C) Each strand is a single long base under the conditions described
  • (D) (‘D’, ‘The bases repel each other evenly’)
Show Answer

Answer: A – Bonds between the paired bases join the two strands and keep the double helix together.

Q30. Which feature of DNA allows one strand to act as a pattern for its partner strand?

  • (A) The strands are identical in sequence
  • (B) The helix unwinds into single bases
  • (C) The bases pair in a fixed way
  • (D) The strands contain amino acids
Show Answer

Answer: C – Because bases pair in a fixed way, the sequence of one strand determines the sequence of the other.

Work through any questions you missed again, then revise those areas before moving on to the next topic.