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O Level Biology Practice Paper 3
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TuitionGoWhere Practice Paper - Biology O-Level (Answer Key)
Version 3 of 5
Subject: Biology (6093)
Level: O-Level
Total Marks: 50
Section A: Multiple Choice & Short Structured Questions
1. D
Reasoning: Proteins always contain C, H, O, N. Some contain S (e.g., in cysteine/methionine). P is found in nucleic acids/phospholipids, not typically standard proteins.
2. C
Reasoning: Benedict's + Heat = Brick-red indicates reducing sugar. Iodine staying orange-brown means no starch. Biuret staying blue means no protein.
3. B
Reasoning: Muscle cells require high energy (many mitochondria) and synthesize contractile proteins (many ribosomes/RER). RBCs have no organelles. Root hair cells have large vacuoles. Palisade cells have chloroplasts.
4. D
Reasoning: Bacteria are prokaryotes with cell walls made of peptidoglycan. Human cells are animal cells (eukaryotes) and lack cell walls. Both have membranes, cytoplasm, and ribosomes (though ribosome sizes differ, the presence is common).
5. B
Reasoning: The lock and key hypothesis states that the substrate has a specific shape complementary to the enzyme's active site.
6. [2 marks]
- Bag X will increase more in mass. [1]
- Because the concentration gradient between the 10% sucrose in Bag X and the distilled water (0%) is steeper than that of Bag Y (5%), resulting in a faster/higher net movement of water into Bag X by osmosis. [1]
7. [2 marks]
- The Golgi body modifies, sorts, and packages proteins (and lipids) received from the RER. [1]
- It transports these materials in vesicles to the cell membrane for secretion or to other parts of the cell. [1]
8. [3 marks]
- Mineral ions are often present in lower concentrations in the soil than inside the root hair cells. [1]
- Therefore, they must move against the concentration gradient. [1]
- Diffusion only occurs down a concentration gradient; active transport requires energy (ATP) to pump ions into the cell against the gradient. [1]
9. (a) pH 7 [1] (b) [2 marks]
- At pH 3, the high acidity causes the enzyme to denature. [1]
- The shape of the active site changes, so the substrate can no longer fit/bind to it. [1]
10. [3 marks] (i) Glucose (ii) Amino acids (iii) Glycerol
Section B: Structured Response Questions
11. (a) [3 marks] A: Cell Wall B: Chloroplast C: Vacuole (or Large Central Vacuole / Sap-filled Vacuole)
(b) [1 mark]
- Contains genetic material (DNA) / Controls cell activities / Regulates cell division.
(c) [2 marks]
- Animal cells need to be flexible to change shape (e.g., muscle contraction, white blood cell movement). [1]
- A rigid cell wall would prevent this flexibility and movement. [1]
12. (a) [1 mark]
- Protease
(b) [3 marks]
- High temperatures (above optimum) cause the enzyme to denature. [1]
- The heat breaks the bonds holding the enzyme's tertiary structure, changing the shape of the active site. [1]
- The substrate can no longer fit into the active site, so the reaction stops. [1]
(c) [2 marks]
- Stains are composed of different biological molecules (e.g., proteins, fats, carbohydrates). [1]
- Different enzymes are specific to different substrates (e.g., protease for protein, lipase for fat), so a mixture ensures various stain types are removed. [1]
13. (a) [2 marks]
- Osmosis is the net movement of water molecules. [1]
- From a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane. [1]
(b) [3 marks]
- The salt solution has a lower water potential than the potato cell sap. [1]
- Water moves out of the potato cells into the salt solution by osmosis. [1]
- The loss of water causes the vacuole to shrink and the cytoplasm to pull away from the cell wall (plasmolysis), making the tissue soft/flaccid. [1]
14. (a) [2 marks] (Any two)
- Large surface area due to finger-like projection.
- Thin epithelium (one cell thick) for short diffusion distance.
- Rich blood supply (capillaries) to maintain concentration gradient.
(b) [3 marks]
- Glucose is absorbed by active transport. [1]
- This requires energy (ATP) produced by respiration in the epithelial cells. [1]
- Carrier proteins in the cell membrane pump glucose from the lumen into the blood capillary against the concentration gradient. [1]
15. (a) [3 marks]
- Add Biuret solution (or Sodium Hydroxide and Copper Sulphate) to the food sample. [1]
- Shake/mix gently. [1]
- A positive result is indicated by a colour change from blue to purple/violet. [1]
(b) [2 marks]
- Enzymes have a specific active site shape. [1]
- Only the substrate with a complementary shape can fit into this active site to form an enzyme-substrate complex. [1]
Section C: Free Response / Data Analysis
16. (a) [2 marks]
- As temperature increases from 0°C to 40°C, the rate of reaction increases. [1]
- This is because kinetic energy increases, leading to more frequent and successful collisions between enzyme and substrate molecules. [1]
(b) [3 marks]
- Above 40°C, the rate of reaction decreases rapidly. [1]
- The high temperature causes the enzyme (catalase) to denature. [1]
- The active site changes shape, preventing the substrate (hydrogen peroxide) from binding, so no oxygen is produced. [1]
(c) [2 marks]
- The rate of reaction would remain zero (or very low). [1]
- Denaturation is irreversible; cooling the enzyme does not restore the original shape of the active site. [1]
17. (a) [4 marks] (i) Absent (or No nucleus) (ii) Long hair-like extension (or projection) (iii) Absorption of water and mineral ions (iv) Large surface area for absorption
(b) [2 marks]
- To provide more space for haemoglobin. [1]
- This maximizes the oxygen-carrying capacity of the cell. [1]
18. (a) [1 mark]
- Starch is used for energy storage, while cellulose is used for structural support (in cell walls).
(b) [2 marks]
- Fats contain more carbon-hydrogen bonds per gram than carbohydrates. [1]
- Therefore, they release more energy (approx. 2x) upon oxidation/breakdown. [1]
19. (a) [3 marks]
- The membrane consists of a phospholipid bilayer. [1]
- Protein molecules are embedded within or span across the bilayer (mosaic). [1]
- The phospholipids and proteins can move laterally, making the membrane fluid. [1]
(b) [2 marks]
- It allows small, non-polar molecules (like oxygen, carbon dioxide) to pass through freely. [1]
- It restricts or controls the passage of large, polar, or charged molecules (like ions, glucose) which require transport proteins. [1]
20. (a) [1 mark]
- 0.4 mol/dm³
(b) [2 marks]
- At this concentration, the water potential of the sucrose solution is equal to the water potential of the potato cell sap. [1]
- There is no net movement of water into or out of the cells (dynamic equilibrium). [1]
(c) [2 marks]
- Percentage change is -10%. [1]
- Calculation: . [1]
- (Award 1 mark for correct working even if final answer is wrong due to arithmetic error)