A-Level Biology Topic 2: Biological Molecules MCQs – 30 Practice Questions with Answers (Cambridge 9700)

Practice for Cambridge International AS and A Level Biology 9700, Topic 2, covering food tests, carbohydrates, lipids, proteins and water, with a short explanation after every question.

Food Tests and Carbohydrates

Q1. A solution gives a brick-red precipitate on heating with Benedict’s solution. What does this show?

  • (A) Starch is present
  • (B) A reducing sugar is present
  • (C) A lipid is present
  • (D) A protein is present in the solution tested
Show Answer

Answer: B – Benedict’s solution turns from blue to a brick-red precipitate when heated with a reducing sugar.

Q2. Which result is positive for starch in the iodine test?

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

Answer: D – Iodine solution turns blue-black in the presence of starch.

Q3. Which sugar is a non-reducing sugar?

  • (A) Sucrose
  • (B) Glucose
  • (C) Fructose
  • (D) Maltose
Show Answer

Answer: A – Glucose, fructose and maltose are reducing sugars, while sucrose is a non-reducing sugar.

Q4. How is a non-reducing sugar identified?

  • (A) By the emulsion test alone, without heating or hydrolysis
  • (B) By the biuret test alone
  • (C) By acid hydrolysis followed by a Benedict’s test
  • (D) By the iodine test alone
Show Answer

Answer: C – Acid hydrolysis breaks the non-reducing sugar into reducing sugars, which then give a positive Benedict’s result.

Q5. In a semi-quantitative Benedict’s test, how can the concentration of a reducing sugar be estimated?

  • (A) By the mass of the test tube before and after heating it
  • (B) By the time to first colour change or colour standards
  • (C) By the smell of the solution
  • (D) By the volume of water added
Show Answer

Answer: B – Standardising the test allows the time to the first colour change, or matching against colour standards, to estimate concentration.

Q6. What type of reaction forms a glycosidic bond?

  • (A) Condensation, with water removed
  • (B) Hydrolysis, with water added
  • (C) Neutralisation, with a salt formed
  • (D) Oxidation, with oxygen gained
Show Answer

Answer: A – A glycosidic bond forms by condensation between monosaccharides, removing a molecule of water.

Q7. Which bond joins two glucose molecules to form maltose?

  • (A) A peptide bond between amino acids
  • (B) An ester bond
  • (C) An ionic bond
  • (D) A glycosidic bond
Show Answer

Answer: D – Maltose is a disaccharide formed when two glucose molecules join by a glycosidic bond.

Q8. Which statement describes α-glucose and β-glucose?

  • (A) They are lipids with different fatty acids
  • (B) They are amino acids with different side chains attached to a central carbon atom
  • (C) They are ring forms of glucose differing in the position of one hydroxyl group
  • (D) They are proteins with different quaternary structures
Show Answer

Answer: C – The α and β ring forms of glucose differ in the orientation of the hydroxyl group on carbon 1.

Q9. Which polysaccharide is the main energy store in animal cells?

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

Answer: B – Glycogen is the storage polysaccharide in animals, while starch serves the same role in plants.

Q10. Why is starch well suited as a storage molecule in plants?

  • (A) It is compact, insoluble and easily hydrolysed to glucose when needed
  • (B) It dissolves quickly and raises the water potential of cells a great deal
  • (C) It forms strong fibres in cell walls
  • (D) It carries oxygen in the blood
Show Answer

Answer: A – Starch is compact and insoluble, so it stores glucose without affecting water potential, and it can be hydrolysed when glucose is needed.

Lipids and Water

Q11. Which molecules make up a triglyceride?

  • (A) Three amino acids and glycerol
  • (B) Three glucose molecules
  • (C) Glycerol and three fatty acids
  • (D) Glycerol and one phosphate group
Show Answer

Answer: C – A triglyceride consists of one glycerol molecule joined to three fatty acid molecules.

Q12. What type of bond joins fatty acids to glycerol in a triglyceride?

  • (A) A glycosidic bond
  • (B) An ester bond
  • (C) A peptide bond
  • (D) A hydrogen bond
Show Answer

Answer: B – Fatty acids join to glycerol by ester bonds formed in condensation reactions.

Q13. What is the difference between a saturated and an unsaturated fatty acid?

  • (A) Saturated fatty acids have no double bonds; unsaturated fatty acids have one or more double bonds
  • (B) Saturated fatty acids contain nitrogen in their chains, while unsaturated fatty acids contain none
  • (C) Saturated fatty acids are water-soluble; unsaturated fatty acids are not
  • (D) Saturated fatty acids have phosphate groups; unsaturated fatty acids do not
Show Answer

Answer: A – A saturated fatty acid has no carbon-carbon double bonds, while an unsaturated fatty acid has at least one.

Q14. Which property of triglycerides suits them for energy storage?

  • (D) They act as biological catalysts
  • (B) They dissolve easily in cytoplasm
  • (C) They form rigid cell walls
  • (A) They are large, non-polar and hydrophobic, storing much energy per gram
Show Answer

Answer: D – Triglycerides are non-polar, hydrophobic molecules that store a large amount of energy and do not disturb the water balance of cells.

Q15. In the emulsion test, what shows that a lipid is present?

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

Answer: C – When a lipid is shaken with ethanol and water is added, a milky white emulsion forms.


Q16. Which description of a phospholipid is correct?

  • (A) A non-polar head and a polar tail
  • (B) Two glycerol molecules and one fatty acid joined by ester bonds
  • (C) A phosphate group and three fatty acids
  • (D) A hydrophilic phosphate head and hydrophobic fatty acid tails
Show Answer

Answer: D – A phospholipid has a hydrophilic, polar phosphate head and two hydrophobic, non-polar fatty acid tails.

Q17. Why is cellulose strong enough to support plant cell walls?

  • (A) Its β-glucose chains form hydrogen bonds in bundles
  • (B) Its chains coil into compact helices for storage in the cytoplasm
  • (C) Its molecules dissolve in water to form a gel
  • (D) Its chains are joined by peptide bonds
Show Answer

Answer: A – Cellulose chains of β-glucose form many hydrogen bonds between them and group into strong microfibrils.

Q18. Which property of water helps it act as a solvent in cells?

  • (A) Its molecules are non-polar
  • (B) Its molecules are polar, so they surround charged particles
  • (C) Its molecules are large and insoluble
  • (D) Its molecules form covalent bonds with dissolved solutes in the cytoplasm
Show Answer

Answer: B – Water is polar, so its molecules can surround ions and other polar substances and keep them dissolved.

Q19. Why is the high specific heat capacity of water useful to living organisms?

  • (A) It lets body temperature change rapidly
  • (B) It prevents water from evaporating
  • (C) It resists changes in temperature, keeping conditions stable
  • (D) It makes water a good electrical insulator in living organisms
Show Answer

Answer: C – A high specific heat capacity means much heat is needed to change the temperature of water, so organisms stay at a steadier temperature.

Q20. How does the latent heat of vaporisation of water help organisms cool down?

  • (D) Freezing absorbs heat from the surroundings
  • (B) Evaporation releases heat into the body
  • (C) Condensation cools the skin directly
  • (A) Evaporation absorbs a large amount of heat from the surface
Show Answer

Answer: D – Evaporation requires a large amount of heat to break hydrogen bonds between water molecules, so heat is taken from the surface and cooling results.

Proteins

Q21. What is the general structure of an amino acid?

  • (A) An amino group, a carboxyl group, a hydrogen atom and an R group on one carbon
  • (B) A phosphate group, a sugar and a nitrogenous base joined together in one molecule
  • (C) Glycerol joined to three fatty acids
  • (D) Two glucose molecules joined by a glycosidic bond
Show Answer

Answer: A – Every amino acid has an amino group, a carboxyl group, a hydrogen atom and an R group attached to a central carbon atom.

Q22. What bond joins two amino acids together?

  • (A) A glycosidic bond
  • (B) An ester bond
  • (C) A peptide bond
  • (D) A phosphodiester bond
Show Answer

Answer: C – Amino acids join by peptide bonds formed in condensation reactions between the carboxyl group of one and the amino group of the next.

Q23. What does the primary structure of a protein describe?

  • (A) The folding of a polypeptide into an alpha helix
  • (B) The sequence of amino acids in a polypeptide
  • (C) The arrangement of several polypeptides together
  • (D) The overall three-dimensional shape of a globular protein
Show Answer

Answer: B – Primary structure is the order, or sequence, of amino acids in a polypeptide chain.

Q24. Which interactions hold a protein in its three-dimensional shape?

  • (A) Only peptide bonds
  • (B) Only glycosidic bonds joining the units of the polypeptide chain
  • (C) Only ionic bonds
  • (D) Hydrogen, ionic and hydrophobic bonds, plus disulfide bonds
Show Answer

Answer: D – Protein shape is held by hydrogen bonding, ionic bonding, hydrophobic interactions and covalent disulfide bonds, in addition to the peptide bonds of the chain.

Q25. Which statement about globular and fibrous proteins is correct?

  • (A) Globular proteins are soluble with physiological roles; fibrous proteins are insoluble and structural
  • (B) Globular proteins are always insoluble; fibrous proteins are always soluble
  • (C) Globular proteins form fibres; fibrous proteins form compact balls
  • (D) Globular proteins lack tertiary structure; fibrous proteins lack primary structure in the cell altogether
Show Answer

Answer: A – Globular proteins are usually soluble and carry out physiological roles, while fibrous proteins are usually insoluble and structural.

Q26. What is the quaternary structure of haemoglobin made from?

  • (A) One polypeptide chain only
  • (B) Four haem groups only
  • (C) Two alpha chains, two beta chains and a haem group in each chain
  • (D) Three collagen chains wound together into a strong triple helix itself
Show Answer

Answer: C – Haemoglobin has a quaternary structure of two alpha globin chains and two beta globin chains, each with a haem group.

Q27. Why is iron important in haemoglobin?

  • (A) It gives collagen its strength
  • (B) It binds oxygen in the haem group
  • (C) It forms peptide bonds
  • (D) It carries carbon dioxide in plasma
Show Answer

Answer: B – Each haem group contains an iron ion that can bind one oxygen molecule, allowing haemoglobin to transport oxygen.

Q28. In the biuret test, what colour shows that protein is present?

  • (A) Blue-black
  • (B) Brick-red
  • (C) Milky white
  • (D) Purple
Show Answer

Answer: D – Biuret solution turns purple in the presence of protein, because it reacts with peptide bonds.

Q29. Which protein is an example of a fibrous protein?

  • (A) Haemoglobin
  • (B) Collagen
  • (C) Amylase
  • (D) Insulin
Show Answer

Answer: B – Collagen is a fibrous protein, while haemoglobin is a globular protein.

Q30. Why is collagen well suited to its structural role in tendons?

  • (A) Its molecules form strong fibres that resist pulling forces
  • (B) Its molecules dissolve easily in tissue fluid around the tendon
  • (C) Its molecules act as enzymes
  • (D) Its molecules transport oxygen
Show Answer

Answer: A – Collagen molecules are arranged into strong fibres, giving tendons high tensile strength.

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