O Level Biology Plant Biology Quiz

O-Level Biology Quiz - Plant Biology: Answer Key

Total Marks: 40
Topic: Plant Biology (O-Level 6093, Theme III)

Section A: Short Answer

1. [1 mark]
Answer: Xylem
Teaching note: Xylem is the plant vascular tissue made of dead hollow cells that carries water and dissolved mineral ions upward from roots to leaves. Common mistake: confusing xylem with phloem (which carries sugars).

2. [1 mark]
Answer: Long and thin extension / large surface area / thin wall
Teaching note: A root hair cell is an epidermal cell with a hair-like projection. This increases surface area for absorption of water by osmosis and ions by active transport.

3. [1 mark]
Answer: Carbon dioxide + water → glucose + oxygen (light, chlorophyll)
Teaching note: Word equation for photosynthesis:
$\text{CO}_2 + \text{H}_2\text{O} \xrightarrow{\text{light, chlorophyll}} \text{glucose} + \text{O}_2$
Marks for correct reactants and products.

4. [1 mark]
Answer: Cools the plant / transports minerals / maintains water column / pulls water up
Teaching note: Transpiration is loss of water vapour from leaves; it creates a suction (transpiration pull) that helps move water and minerals up the plant and cools leaves.

5. [1 mark]
Answer: Carbon dioxide
Teaching note: CO₂ enters via stomata for photosynthesis; O₂ is a product that exits.

Section B: Structured Response

6. [3 marks]
Answer:

• Soil water has higher water potential than root hair cell sap. [1]
• Water moves by osmosis across partially permeable membrane into cell. [1]
• Then moves into xylem via symplast/apoplast pathways. [1]
  Teaching note: Osmosis = net movement of water from region of higher water potential to lower through partially permeable membrane. Mark for each clear step.

7. [4 marks]
Answer:

• Nitrate needed to make amino acids/proteins for growth. [1]
• Magnesium needed for chlorophyll synthesis. [1]
• Ions absorbed from soil by root hair cells. [1]
• Taken up by active transport (against gradient, uses ATP). [1]
  Teaching note: Mineral deficiency: lack of nitrate → stunted growth; lack of Mg → chlorosis (yellow leaves). Active transport requires energy from respiration.

8. [3 marks]
Answer:

• Without O₂, root cells cannot respire aerobically. [1]
• Less ATP produced. [1]
• Active transport of mineral ions reduced, so uptake falls. [1]
  Teaching note: Root hair active transport depends on mitochondrial ATP. Anaerobic conditions sharply cut ion uptake.

9. [3 marks]
Answer:

• Stomata open to let CO₂ in for photosynthesis. [1]
• Allow O₂ out. [1]
• Water vapour exits via stomata (transpiration). [1]
  Teaching note: Guard cells control stomatal aperture. Common trap: saying stomata “breathe” – plants do not breathe like animals.

10. [4 marks]
Answer:

• Broad flat shape → large surface area to capture light. [1]
• Thin → short diffusion distance for gases. [1]
• Palisade layer has many chloroplasts → absorbs light. [1]
• Spongy mesophyll air spaces → gas exchange. [1]
  Teaching note: Structure-function link is key in O-Level plant questions.

Section C: Data and Diagram Interpretation

11. [1 mark]
Answer: Palisade mesophyll / palisade layer
Teaching note: Palisade cells are packed with chloroplasts near upper surface to absorb most light.

12. [1 mark]
Answer: Air spaces between cells / loose arrangement
Teaching note: Air spaces increase surface area for CO₂ diffusion to cells.

13. [2 marks]
Answer: Transpiration rate increases with light intensity [1]; then levels off (plateau) after about 6 klx [1].
Teaching note: Describe both rising and plateau parts from graph.

14. [3 marks]
Answer:

• Higher light → stomata open wider → more evaporation. [1]
• More light → more photosynthesis → more water used. [1]
• Plateau because other factors limit (e.g. humidity, CO₂, max stomatal opening). [1]

15. [1 mark]
Answer: Fertiliser increased nitrate and magnesium content / plants absorbed more minerals.
Teaching note: Simple inference from table values.

16. [2 marks]
Working:
% increase = $\frac{\text{after} - \text{before}}{\text{before}} \times 100$
= $\frac{3.5 - 1.2}{1.2} \times 100 = \frac{2.3}{1.2} \times 100 = 191.7\%$ [2]
Answer: 192% (or 191.7%)
Teaching note: Show substitution. Common error: using wrong denominator.

17. [1 mark]
Answer: Xylem is dead with thick wall; phloem is living with sieve plates.
Teaching note: Any one valid structural difference from diagram.

18. [3 marks]
Answer:

• Transpiration loses water from leaves. [1]
• Creates tension pulling water up xylem. [1]
• Cohesion of water molecules transmits pull from top to roots. [1]
  Teaching note: Cohesion-tension theory; water column is continuous.

19. [2 marks]
Working:
Distance = 35 mm − 0 mm = 35 mm
Rate = $\frac{35 \text{ mm}}{10 \text{ min}} = 3.5 \text{ mm/min}$ [2]
Answer: 3.5 mm/min
Teaching note: Potometer measures water uptake as proxy for transpiration. Units required.

20. [2 marks]
Answer: Any two:

• Closes stomata [1]
• Leaves wilt to reduce surface area [1]
• Thicker cuticle / hairs [1]
  Teaching note: Two valid adaptations, 1 mark each.