Secondary 3 Biology Plant Biology Quiz

Secondary 3 Biology Quiz - Plant Biology

Answer Key

Section A: Multiple Choice

1. C — Cell wall [2]

• The cell wall is a rigid structure made of cellulose found only in plant cells. It provides mechanical support and maintains cell shape.
• Common mistake: Students may select A (cell membrane) because it is also involved in support, but the cell membrane is present in both plant and animal cells.

2. B — Plasmolysis [2]

• Plasmolysis occurs when a plant cell loses water by osmosis in a hypertonic solution, causing the cytoplasm and cell membrane to pull away from the cell wall.
• Common mistake: Students confuse osmosis (the process of water movement) with plasmolysis (the observable result).

3. C — Carbon dioxide [2]

• Carbon dioxide is a raw material (reactant) required for photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• Common mistake: Students may select B (oxygen), which is a product, not a raw material.

4. C — Xylem [2]

• Xylem tissue transports water and dissolved mineral ions from the roots to the rest of the plant. Phloem transports food (sucrose/amino acids).
• Common mistake: Students confuse xylem and phloem functions.

5. B — Phototropism [2]

• Phototropism is the growth response of a plant shoot towards light. The shoot bends towards the light source to maximise light absorption for photosynthesis.
• Common mistake: Students may select A (geotropism), which is a response to gravity, not light.

Section B: Short Answer and Structured Questions

6. [4 marks — 2 marks per valid difference]

Any TWO of the following:

(a) Palisade mesophyll cells have a cell wall; red blood cells do not. [2]

(b) Palisade mesophyll cells contain chloroplasts; red blood cells do not. [2]

(c) Palisade mesophyll cells have a large central vacuole; red blood cells do not. [2]

(d) Palisade mesophyll cells are rectangular/columnar in shape; red blood cells are biconcave disc-shaped. [2]

Marking note: Award 1 mark per correct difference stated. Do not accept vague answers such as "plant cells are different from animal cells" without specifying the structural feature.

7. [4 marks]

(a) [2 marks — ½ mark per correct label]

• A: Upper epidermis — outermost layer on top of leaf
• B: Palisade mesophyll — elongated cells beneath upper epidermis, densely packed with chloroplasts
• C: Spongy mesophyll — irregularly shaped cells with air spaces between them
• D: Stoma (plural: stomata) — small pore in lower epidermis, flanked by two guard cells

(b) [2 marks]

• Palisade mesophyll cells contain the most chloroplasts [1] and are positioned near the upper surface to receive the most light [1] for photosynthesis.
• Full answer: The palisade mesophyll is located near the upper surface because this is where light intensity is highest. These cells contain numerous chloroplasts to maximise the absorption of light energy for photosynthesis.

8. [2 marks]

• Transpiration is the loss of water vapour from the aerial parts of a plant (mainly through the stomata in the leaves) [1] by evaporation [1].
• Alternative acceptable: Transpiration is the process by which water evaporates from the surfaces of mesophyll cells and diffuses out of the leaf through stomata.
• Common mistake: Students describe transpiration as "breathing" or "sweating" — these terms are not scientifically accurate for plants.

9. [5 marks]

(a) [2 marks]

• As wind speed increases, the rate of transpiration increases [1]. The rate of increase slows down at higher wind speeds (the relationship is not directly proportional; it begins to level off above 1.5 m/s) [1].

(b) [3 marks]

• Wind removes the humid layer of air (boundary layer) surrounding the leaf surface [1].
• This increases the water vapour concentration gradient between the inside of the leaf (moist air spaces) and the surrounding air [1].
• As a result, water vapour diffuses out of the stomata at a faster rate, increasing the rate of transpiration [1].

10. [2 marks]

• Carbon dioxide + Water →(sunlight, chlorophyll)→ Glucose + Oxygen [2]
• Marking note: Award 1 mark for correct reactants and 1 mark for correct products. The condition (sunlight/chlorophyll) is not required for full marks but is good practice. Accept the word equation in either direction (reactants → products).

11. [5 marks]

(a) [2 marks]

• Covered part: Remains brown/yellow (no colour change with iodine) / no starch present [1]
• Uncovered part: Turns blue-black / starch present [1]

(b) [3 marks]

• The covered part received no light [1], so photosynthesis could not occur and no starch was produced [1].
• The uncovered part received light, enabling photosynthesis to take place, producing glucose which was converted to starch [1].
• Common mistake: Students may say "the covered part had no chlorophyll" — this is incorrect; the chlorophyll was still present but light was absent.

12. [3 marks]

• Xylem vessels are dead cells that have lost their cytoplasm and end walls [1], forming continuous hollow tubes [1]. This provides an unimpeded pathway for water and dissolved mineral ions to flow through with little resistance [1].
• Additional acceptable point: The walls of xylem vessels are thickened with lignin, which provides structural support and prevents the vessels from collapsing under the tension created by transpiration pull.

13. [4 marks — 1 mark per correct row entry]

Marking note: Award 1 mark for each correctly completed row. Accept equivalent phrasing.

14. [3 marks]

• Waterlogged soil has reduced oxygen availability in the air spaces of the soil [1].
• Root hair cells cannot carry out aerobic respiration efficiently, reducing the supply of ATP [1].
• Without sufficient ATP, active transport of mineral ions (such as nitrate ions needed for chlorophyll production) cannot occur efficiently, leading to yellowing of leaves (chlorosis) and stunted growth [1].
• Common mistake: Students may say "the roots cannot absorb water" — while waterlogged conditions can affect water uptake, the primary issue is the lack of oxygen for active transport of minerals.

15. [3 marks]

• Root hair cells have long, thin extensions (root hairs) that greatly increase the surface area available for absorption [1].
• They have a thin cell wall and cell membrane, providing a short diffusion distance for water and mineral ions to enter the cell [1].
• They maintain a higher concentration of solutes inside the cell compared to the soil water, creating a water potential gradient that draws water in by osmosis [1].
• Additional acceptable point: Root hair cells contain many mitochondria to provide ATP for active transport of mineral ions.

Section C: Data-Based and Extended Response Questions

16. [7 marks]

(a) [2 marks]

• As light intensity increases, the rate of photosynthesis increases [1]. At higher light intensities, the rate levels off and becomes constant (reaches a plateau) [1].

(b) [2 marks]

• At high light intensity, light is no longer the limiting factor [1]. Another factor (such as CO₂ concentration or temperature) becomes the limiting factor, so increasing light intensity further does not increase the rate of photosynthesis [1].

(c) [3 marks]

• At 0.12% CO₂, the maximum rate of photosynthesis is higher than at 0.04% CO₂ [1].
• This is because CO₂ is a raw material for photosynthesis [1]. A higher CO₂ concentration means more CO₂ molecules are available to combine with RuBP in the Calvin cycle, producing more glucose and thus a higher rate of photosynthesis [1].
• Acceptable alternative: At 0.04% CO₂, CO₂ is the limiting factor at lower light intensities, so the rate plateaus earlier. At 0.12% CO₂, more CO₂ is available, so the rate continues to increase until another factor becomes limiting.

17. [6 marks]

(a) [3 marks]

• Correctly labelled axes (Temperature on x-axis, Number of oxygen bubbles per minute on y-axis) [1]
• Appropriate scale used and all points correctly plotted [1]
• Smooth curve or line of best fit drawn through the points [1]
• Marking note: Deduct 1 mark if axes are not labelled or units are missing. Deduct 1 mark if the scale is inappropriate (e.g., does not use at least half the grid).

(b) [1 mark]

• Optimum temperature: approximately 30°C [1]
• Accept answers between 28–32°C based on the data.

(c) [2 marks]

• At 50°C, the enzymes involved in photosynthesis (e.g., RuBisCO and other Calvin cycle enzymes) become denatured [1].
• The active sites of the enzymes change shape, so substrates can no longer bind, and the rate of photosynthesis decreases significantly [1].
• Common mistake: Students may say "the plant died" — this is not necessarily true at 50°C; enzyme denaturation is the precise explanation.

18. [4 marks]

• Root pressure: Root cells actively transport mineral ions into the xylem, lowering the water potential in the xylem. Water follows by osmosis, creating a positive pressure that pushes water up the xylem (especially important in small plants and at night) [1].
• Capillary action: The narrow diameter of xylem vessels allows water to rise due to the adhesive forces between water molecules and the xylem walls, and cohesive forces between water molecules [1].
• Transpiration pull: Water evaporates from the mesophyll cells in the leaf and diffuses out through stomata. This creates a negative pressure (tension) in the xylem [1]. Water molecules are pulled up as a continuous column due to cohesion (hydrogen bonding between water molecules) and adhesion (attraction between water and xylem walls) [1].
• Note: Transpiration pull is the main driving force for water transport in tall plants. Root pressure and capillary action play supporting roles.

19. [4 marks]

(a) [2 marks]

• Shoot: Negative geotropism (or "growing away from gravity") [1]
• Roots: Positive geotropism (or "growing towards gravity") [1]

(b) [2 marks]

• The roots growing downwards (positive geotropism) anchor the plant firmly in the soil [1] and enable the roots to reach water and mineral ions deeper in the soil for absorption [1].
• Alternative acceptable: Growing towards gravity helps the roots grow into the soil for anchorage and access to water and minerals.

20. [6 marks]

(a) [3 marks]

1. Place the leaf in boiling water for about 1 minute to kill the cells and stop all chemical reactions [1].
2. Place the leaf in hot ethanol (using a water bath) to dissolve the chlorophyll, turning the leaf pale/white [1].
3. Rinse the leaf in warm water to soften it, then add iodine solution (iodine in potassium iodide) and observe the colour change [1].

• Safety note: Ethanol is flammable and must be heated using a water bath, not a direct flame.

(b) [3 marks]

• Green region: Turns blue-black [½]. The green region contained chlorophyll, which enabled photosynthesis to occur, producing glucose that was stored as starch. Starch turns blue-black with iodine solution [1].
• White region: Remains brown/yellow (no colour change) [½]. The white region lacked chlorophyll, so photosynthesis could not take place and no starch was produced. Without starch, the iodine solution does not change colour [1].
• Marking note: Award 1 mark for each correct prediction with explanation (green region: 1½ marks; white region: 1½ marks).

End of Answer Key