O Level Biology Practice Paper 5

TuitionGoWhere Practice Paper - Biology O-Level (Answer Key)

Version: 5 of 5
Subject: Biology (6093)
Topic: Cells and Biomolecules

Section A: Multiple Choice and Short Structured Questions

1. A [1]
Reasoning: Plant cells have cell walls; animal cells do not. B is incorrect because animal cells lack chloroplasts. C is incorrect because animal cells have small/temporary vacuoles. D is incorrect because plants store starch, animals store glycogen.

2. C [1]
Reasoning: White blood cells are animal cells (no cell wall, have nucleus/mitochondria). Bacterial cells have no nucleus. Palisade and root hair cells are plant cells (have cell walls).

3. [2]
Organelle: Golgi apparatus (or Golgi body) [1]
Function: Modifies, sorts, and packages proteins/lipids for secretion or transport within the cell. [1]

4. [2]

• To provide more space for haemoglobin. [1]
• This maximises the oxygen-carrying capacity of the cell. [1]
  (Accept: Biconcave shape increases surface area, but the question asks about the nucleus specifically).

5. [4]
(a) Osmosis [1]
(b)

• The sugar solution has a lower water potential (higher solute concentration) than the potato cell sap. [1]
• Water moves out of the potato cells by osmosis. [1]
• Water moves from a region of higher water potential (inside cell) to lower water potential (outside) across the partially permeable membrane. [1]
  (Note: Do not accept "sugar moves into potato").

6. [4]
(a)

• As temperature increases, the kinetic energy of enzyme and substrate molecules increases. [1]
• This leads to more frequent successful collisions between enzyme and substrate, forming more enzyme-substrate complexes. [1]
  (b)
• High temperature breaks the hydrogen bonds holding the enzyme's tertiary structure. [1]
• The active site changes shape (denaturation), so the substrate no longer fits/complementary shape is lost. [1]

7. B [1]
Reasoning: Proteins contain Nitrogen (always) and often Sulphur. Carbohydrates only contain C, H, O.

8. Reducing Sugar (e.g., Glucose/Maltose) [1]
Reasoning: Benedict's positive = reducing sugar. Iodine negative = no starch. Biuret negative = no protein.

9. [3]

• The enzyme has a specific active site with a unique shape. [1]
• The substrate has a complementary shape that fits into the active site. [1]
• They bind to form an enzyme-substrate complex, lowering the activation energy for the reaction. [1]

10. [2]
(a) Glucose [1]
(b) Amino acids [1]

Section B: Structured Response Questions

11. [6]
(a) Palisade mesophyll [1]
(b)

• Contains many chloroplasts to maximise light absorption for photosynthesis. [1]
• Arranged in vertical columns/packed tightly to absorb maximum light near the upper surface. [1]
  (c)
• Guard cells take up water by osmosis. [1]
• They become turgid/swollen. [1]
• The inner wall is thicker than the outer wall, causing the cells to curve apart, opening the stoma. [1]

12. [6]
(a) The movement of substances/ions against a concentration gradient (from low to high concentration). [1]
Requires energy (ATP). [1]
(b)

• The concentration of ions is higher in the root hair cell than in the soil. [1]
• Diffusion would cause ions to move out of the cell. [1]
• Active transport allows the plant to absorb ions from the dilute soil solution against the gradient to ensure sufficient mineral uptake. [1]
  (c) ATP (from respiration) [1]

13. [7]
(a)

• Stains often contain proteins (e.g., blood, egg). [1]
• Protease breaks down proteins into amino acids/peptides which are soluble and can be washed away. [1]
  (b)
  (i) The rate of cleaning/stain removal will be slower or zero. [1]
  (ii)
• At 60°C, the protease enzyme may be denatured. [1]
• The active site changes shape, so it cannot bind to the protein stain. [1]
  (Note: If the enzyme is thermostable, the rate might just decrease due to other factors, but standard O-Level assumption is denaturation at high temps unless specified).

14. [9]
(a) Immediate energy source / Energy storage (glycogen) [1]
(b) C, H, O [1] (Note: Fats do not contain N or P typically in simple triglycerides)
(c) Growth and repair / Enzymes / Hormones / Antibodies [1]
(d)

• Add Biuret solution (or Sodium Hydroxide and Copper Sulphate) to the milk sample. [1]
• Shake/mix gently. [1]
• A purple/violet colour indicates the presence of protein. [1]

15. [6]
Ciliated Epithelial Cell:

• Structure: Has cilia (hair-like projections) on the surface. [1]
• Function Link: Cilia beat in a coordinated way to move mucus/dust/trapped particles out of the airways. [1]

Sperm Cell:

• Structure: Has a long tail (flagellum) and many mitochondria. [1]
• Function Link: Tail allows it to swim to the egg; mitochondria provide energy for movement. [1]
  (Award marks for clear structure-function links for both. Max 3 marks per cell type).

Section C: Free Response Questions

16. [8]
(a) Any 5 structures with functions:

1. Cell Wall: Made of cellulose; provides support and maintains shape. [1]
2. Cell Membrane: Controls movement of substances in/out. [1]
3. Cytoplasm: Site of chemical reactions. [1]
4. Nucleus: Contains genetic material/DNA; controls cell activities. [1]
5. Vacuole: Contains cell sap; maintains turgor pressure. [1]
6. Chloroplasts: Contains chlorophyll; site of photosynthesis. [1]
   (Max 5 marks).
   (b)

• Permeability: Cell wall is fully permeable; Cell membrane is partially permeable. [1]
• Composition: Cell wall is made of cellulose; Cell membrane is made of phospholipids and proteins. [1]
• Function: Cell wall provides structural support; Cell membrane controls transport. [1]

17. [8]
(a) The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. [2]
(b)
Solution A (Distilled Water):

• Description: The red blood cells will swell and burst (haemolysis). [1]
• Explanation: Distilled water has a higher water potential than the cytoplasm. Water enters the cells by osmosis. Animal cells lack a cell wall to withstand the pressure. [2]

Solution C (5.0% Salt):

• Description: The red blood cells will shrink/shrivel (crenation). [1]
• Explanation: The salt solution has a lower water potential than the cytoplasm. Water leaves the cells by osmosis. [2]

18. [8]
(a)

• A substance produced by living organisms. [1]
• It speeds up the rate of chemical reactions without being used up/changed itself. [1]
  (b) Experimental Design:
• Independent Variable: pH of the solution (use buffer solutions of different pH). [1]
• Dependent Variable: Rate of reaction (measured by time taken for starch to disappear or amount of maltose produced). [1]
• Control Variables: Temperature (keep constant at optimum, e.g., 37°C), concentration of enzyme, concentration of substrate. [1]
• Method: Mix amylase and starch at specific pH. Sample at intervals. Test with iodine. [1]
• Measurement: Record time taken for iodine to remain orange-brown (starch gone). [1]
• Conclusion: The pH with the shortest time/highest rate is the optimum. [1]

19. [7]
(a)

• Add Benedict’s solution to the food sample. [1]
• Heat the mixture in a water bath (boil) for a few minutes. [1]
• A colour change from blue to green/yellow/orange/brick-red indicates the presence of reducing sugar. [1]
  (b)
• Glucose: Small, soluble molecule. [1]
• Can be easily transported in blood plasma to cells for respiration. [1]
• Starch: Large, insoluble molecule. [1]
• Does not affect the water potential of the cell (osmotic balance) and is compact for storage. [1]

20. [7]
(a)
(i) Phospholipid bilayer: (Label pointing to the double layer of lipids) [1]
(ii) Protein channel: (Label pointing to the transmembrane protein) [1]
(iii) Glycoprotein: (Label pointing to protein with carbohydrate chain) [1]
(b)

• The phospholipid bilayer is hydrophobic in the middle, preventing water-soluble substances/ions from passing through freely. [1]
• Protein channels/carriers allow specific ions/large molecules to pass through (facilitated diffusion/active transport). [1]
• The membrane is selectively/partially permeable, controlling what enters/exits. [1]
• Fluid nature allows flexibility and self-sealing. [1]

End of Marking Scheme