O Level Biology Cells Biomolecules Quiz

O-Level Biology Quiz - Cells Biomolecules - ANSWER KEY

Total Marks: 40

Section A: Short Answer (10 marks)

1. State the chemical elements present in carbohydrates. [1]

• Answer: Carbon, hydrogen, and oxygen (C, H, O). [1]
• Marking note: All three elements must be stated. Accept chemical symbols.

2. Name the reagent used to test for the presence of reducing sugars and state the positive result observed. [2]

• Answer: Benedict's solution / Benedict's reagent [1]; brick-red precipitate / orange-red precipitate forms (on heating) [1].
• Marking note: Accept "Benedict's solution" or "Benedict's reagent". Do NOT accept just "red" without "precipitate" or "brick-red". Heating must be implied or stated for full credit.

3. Explain what is meant by the term "enzyme specificity". [2]

• Answer: An enzyme can only catalyse one specific reaction / act on one specific substrate [1] because the shape of its active site is complementary to the shape of the substrate molecule [1].
• Marking note: Award [1] for "only one substrate/reaction" and [1] for linking to complementary shape of active site. Accept reference to lock-and-key hypothesis.

4. A student observes a plant cell under a light microscope and identifies a large, central sap-filled vacuole. State one function of this vacuole. [1]

• Answer: Any one of:
  • Stores water and dissolved substances (e.g., sugars, salts)
  • Maintains turgor pressure / keeps the cell turgid
  • Provides support to the plant
• Marking note: Award [1] for any one valid function.

5. State two features of a bacterial cell that are different from an animal cell. [2]

• Answer: Any two of:
  • Bacterial cells have a cell wall (made of peptidoglycan); animal cells have no cell wall [1]
  • Bacterial cells have plasmids; animal cells do not [1]
  • Bacterial cells have circular DNA / no true nucleus (nucleoid region); animal cells have a nucleus [1]
  • Bacterial cells have 70S ribosomes; animal cells have 80S ribosomes [1]
  • Bacterial cells may have flagella; animal cells do not [1]
• Marking note: Award [1] for each correct feature with clear structural difference stated. Must contrast bacterial vs. animal cell.

Section B: Structured Questions (10 marks)

6. Name the organelle responsible for synthesising proteins in a cell. [1]

• Answer: Ribosomes [1].
• Marking note: Accept "ribosome" (singular or plural).

7. State one function of the Golgi body in a cell. [1]

• Answer: Any one of:
  • Modifies and packages proteins for secretion / transport out of the cell
  • Forms lysosomes
  • Transports lipids
• Marking note: Award [1] for any one valid function.

8. Enzyme-catalysed reaction.

(a) Name the hypothesis that describes how enzymes bind to their substrates. [1]

• Answer: Lock-and-key hypothesis [1].
• Marking note: Accept "lock and key hypothesis/model".

(b) Explain why the enzyme remains unchanged at the end of the reaction. [2]

• Answer: The enzyme is not used up / not consumed in the reaction [1]; it is released after the products are formed and can be reused to catalyse another reaction [1].
• Marking note: Award [1] for "not used up/consumed" and [1] for "can be reused/recycled".

(c) State the effect of increasing temperature beyond the optimum on enzyme activity. Explain your answer. [3]

• Answer: Enzyme activity decreases / the rate of reaction decreases [1]. Explanation: High temperature causes the enzyme to denature [1]; the active site loses its specific shape / complementary shape, so the substrate can no longer bind / fit into the active site [1].
• Marking note: Award [1] for stating decrease, [1] for "denature", and [1] for explaining loss of active site shape and inability to bind substrate.

9. Food test results.

(a) Identify the food substances present in the sample based on the results. [2]

• Answer:
  • Test A (Iodine solution → blue-black): Starch [1]
  • Test B (Biuret solution → purple): Protein [1]
  • Test C (Ethanol + water → white emulsion): Fats / lipids [1]
• Marking note: Award [1] for each correct identification. Total [2] as marks allocated.

Section C: Structured Questions (10 marks)

10. Explain why Test C requires the addition of water after ethanol. [2]

• Answer: Ethanol dissolves / extracts the fats from the sample [1]; when water is added, a white emulsion forms because fats are insoluble in water and come out of solution / form a suspension [1].
• Marking note: Award [1] for ethanol dissolving fats and [1] for emulsion formation upon water addition due to insolubility.

11. Electron microscope organelles.

(a) Which of the above organelles would be found ONLY in the plant cell? [1]

• Answer: Chloroplasts [1].

(b) Describe the function of the rough endoplasmic reticulum and explain how its structure is adapted for this function. [3]

• Answer: Function: Synthesises and transports proteins [1]. Structural adaptation: It has ribosomes attached to its surface [1]; the ribosomes synthesise proteins which enter the RER for transport [1].
• Marking note: Award [1] for function (protein synthesis/transport), [1] for ribosomes on surface, and [1] for linking ribosomes to protein synthesis and transport role.

(c) Explain why muscle cells contain many mitochondria. [2]

• Answer: Muscle cells require a lot of energy / ATP for contraction [1]; mitochondria are the site of aerobic respiration where ATP / energy is produced [1].
• Marking note: Award [1] for linking to high energy demand and [1] for mitochondria producing ATP/energy via aerobic respiration.

12. pH and enzyme activity.

(a) State the optimum pH for this enzyme. [1]

• Answer: pH 7 [1].

(b) Explain why the rate of reaction decreases at pH 12. [2]

• Answer: The enzyme has denatured [1]; the extreme pH (alkaline) alters the shape of the active site so the substrate can no longer bind / fit [1].
• Marking note: Award [1] for denaturation and [1] for explaining loss of active site shape and inability to bind substrate.

Section D: Data-Based and Extended Response (10 marks)

13. Substrate concentration graph.

(a) Explain why the rate of reaction increases rapidly at low substrate concentrations. [2]

• Answer: At low substrate concentrations, many enzyme active sites are available / not occupied [1]; as substrate concentration increases, more enzyme-substrate complexes form, increasing the rate of reaction [1].
• Marking note: Award [1] for availability of active sites and [1] for more enzyme-substrate complexes forming.

(b) Explain why the graph levels off (plateaus) at high substrate concentrations. [2]

• Answer: All enzyme active sites are occupied / saturated with substrate [1]; the enzyme concentration becomes the limiting factor, so increasing substrate concentration further has no effect on the rate [1].
• Marking note: Award [1] for saturation of active sites and [1] for enzyme concentration as limiting factor.

(c) Suggest one way, other than increasing substrate concentration, to increase the rate of reaction once the graph has plateaued. Explain your answer. [2]

• Answer: Increase the enzyme concentration [1]; this provides more active sites available for substrate binding, allowing more enzyme-substrate complexes to form per unit time [1].
• Marking note: Accept "increase temperature to optimum" with explanation that more kinetic energy leads to more frequent successful collisions. Award [1] for valid suggestion and [1] for correct explanation.

14. Describe the roles of enzymes in human digestion. Give examples in your answer. [4]

• Answer: Enzymes act as biological catalysts that speed up the rate of digestion without being used up [1]. They break down large, insoluble food molecules into small, soluble molecules that can be absorbed [1].
  • Example 1: Amylase breaks down starch into maltose [1].
  • Example 2: Proteases (e.g., pepsin) break down proteins into amino acids [1].
  • Example 3: Lipases break down fats into fatty acids and glycerol [1].
• Marking note: Award [1] for catalyst role, [1] for breaking down large to small molecules, and up to [2] for valid examples. Maximum [4].

15. Compare the structure of a typical plant cell and a typical animal cell. [4]

• Answer: Similarities: Both have a nucleus, cytoplasm, cell membrane, mitochondria, ribosomes [1]. Differences: Plant cells have a cellulose cell wall; animal cells do not [1]. Plant cells have chloroplasts; animal cells do not [1]. Plant cells have a large central vacuole; animal cells have small temporary vacuoles [1].
• Marking note: Award [1] for similarities and [1] for each valid difference. Maximum [4].

16. Explain the importance of carbohydrates in living organisms. [3]

• Answer: Carbohydrates provide energy for cellular activities (e.g., glucose is used in respiration) [1]. They serve as energy storage (e.g., starch in plants, glycogen in animals) [1]. They form structural components (e.g., cellulose in plant cell walls) [1].
• Marking note: Award [1] for each valid point.

17. Describe the process of protein synthesis in a cell. [4]

• Answer: Transcription: DNA unwinds and mRNA is synthesised using DNA as a template [1]. mRNA moves from the nucleus to the ribosome [1]. Translation: Ribosome reads the mRNA codons [1]. tRNA brings specific amino acids to the ribosome, and amino acids are linked to form a polypeptide chain [1].
• Marking note: Award [1] for each key step. Maximum [4].

18. Explain how enzymes are affected by changes in temperature. [3]

• Answer: At low temperatures, enzyme activity is low because molecules have less kinetic energy, leading to fewer collisions [1]. As temperature increases to the optimum, activity increases due to more frequent successful collisions [1]. Beyond the optimum, the enzyme denatures; the active site loses its shape, and the substrate cannot bind [1].
• Marking note: Award [1] for each phase of temperature effect.

19. Discuss the importance of water as a solvent in living organisms. [3]

• Answer: Water dissolves many substances (e.g., salts, sugars, gases) allowing them to be transported in organisms [1]. It acts as a medium for metabolic reactions in cells [1]. It helps in excretion of waste products (e.g., urea dissolved in urine) [1].
• Marking note: Award [1] for each valid point.

20. Explain how the structure of a red blood cell is adapted to its function. [3]

• Answer: It has a biconcave shape, which increases surface area for oxygen diffusion [1]. It contains haemoglobin, which binds to oxygen for transport [1]. It lacks a nucleus, providing more space for haemoglobin [1].
• Marking note: Award [1] for each adaptation linked to function.

END OF ANSWER KEY