Secondary 3 Biology Plant Biology Quiz

Secondary 3 Biology Quiz - Plant Biology

Answer Key

1. C — Cell wall [1]
Common mistake: Students may select A (cell membrane) because it is present in both plant and animal cells. The cell wall is unique to plant cells and provides mechanical support.

2. B — Palisade mesophyll [1]
The palisade mesophyll contains the highest density of chloroplasts because it is positioned near the upper surface to maximise light absorption.

3. B — carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll) [1]
Common mistake: Students may confuse this with the equation for respiration (option A).

4. B — The uncovered part only [1]
Light cannot pass through the black paper, so photosynthesis cannot occur in the covered region. Without photosynthesis, no starch is produced, and the covered region does not turn blue-black with iodine.

5. C — Temperature [1]
On a cold winter day, enzymes involved in photosynthesis work slowly due to low temperature, making temperature the limiting factor despite high light intensity.

6. [2 marks — 1 mark per correct difference]

(i) Palisade mesophyll cells have a cell wall; red blood cells do not. [1]
(ii) Palisade mesophyll cells contain chloroplasts; red blood cells do not. [1]

Acceptable alternatives: Palisade cells have a large central vacuole (red blood cells do not); palisade cells are rectangular/regular in shape (red blood cells are biconcave discs); palisade cells have chloroplasts (red blood cells have haemoglobin instead). Award 1 mark per valid difference, max 2.

7. (a) [2 marks]

• Palisade mesophyll: upper tissue layer (tightly packed, elongated cells) [1]
• Spongy mesophyll: lower tissue layer (loosely packed cells with air spaces) [1]
• Stoma: small pore in the lower epidermis [1]
  Award 1 mark for each correctly labelled structure, max 2.

(b) [2 marks]
The palisade mesophyll is located near the upper surface to receive the most light [1]. This maximises the rate of photosynthesis because the palisade cells contain the highest concentration of chloroplasts [1].

8. (a) [1 mark]
To remove any starch already present in the leaf [1], so that any starch detected at the end of the experiment must have been produced during the experiment.

(b) [1 mark]
To act as a control / to provide a comparison between the covered (no light) and uncovered (light) regions on the same leaf [1].

(c) [2 marks]
Covered region: Remains brown/yellow (no colour change with iodine) / does not turn blue-black [1].
Uncovered region: Turns blue-black [1].

9. [3 marks — 1 mark per correct explanation]

(i) Xylem vessels are hollow and have no cell contents, which reduces resistance to the flow of water / allows water to flow freely through the lumen [1].

(ii) Xylem vessels have thick walls reinforced with lignin, which provides structural support / prevents the vessels from collapsing under the tension created by transpiration pull [1].

(iii) Xylem vessels are joined end to end to form continuous tubes, which allow water to be transported uninterrupted from the roots to the leaves [1].

10. (a) [2 marks]
The rate of water uptake increases from 06:00 to a peak at 12:00 [1], then decreases from 12:00 to 21:00 [1].

(b) [2 marks]
At 12:00, the light intensity is highest [1], which causes the stomata to open fully, increasing the rate of transpiration and therefore the rate of water uptake [1].

11. [2 marks]
Transpiration is the loss of water vapour from the aerial parts of a plant (mainly through stomata) [1].
Translocation is the transport of sucrose and amino acids (food substances) through the phloem from source to sink [1].

12. (a) [1 mark]
Transpiration [1].

(b) [2 marks]
The plant releases water vapour through transpiration via the stomata [1]. The water vapour condenses on the cooler inner surface of the bell jar, forming water droplets [1].

13. [4 marks — 2 marks per factor: 1 for naming, 1 for explanation]

Factor 1: Increased temperature
Explanation: Higher temperature increases the kinetic energy of water molecules, causing faster evaporation from the leaf surface, which increases the water vapour concentration gradient and thus the rate of transpiration.

Factor 2: Increased wind speed / air movement
Explanation: Wind removes the humid air (boundary layer) around the leaf, maintaining a steep water vapour concentration gradient between the inside and outside of the leaf, increasing the rate of transpiration.

Acceptable alternatives: Increased light intensity (causes stomata to open wider, increasing water loss); decreased humidity (increases the concentration gradient); low water availability in soil (triggers stomatal closure — but this decreases transpiration, so only accept if the student correctly identifies it as decreasing the rate). Award 1 mark for factor + 1 mark for correct explanation, max 4.

14. (a) [1 mark]
Vascular bundle / stele [1].

(b) [3 marks]

• Root hair cells have long, thin extensions (root hairs) that greatly increase the surface area for absorption of water and mineral ions [1].
• Root hair cells have a thin cell wall / cell membrane, which shortens the diffusion distance for water and mineral ions [1].
• Root hair cells have a large number of mitochondria / high metabolic activity, which provides energy (ATP) for active transport of mineral ions [1].

Acceptable alternative: Root hair cells have a lower water potential (due to dissolved solutes) compared to the soil solution, creating an osmotic gradient for water uptake.

15. (a) [2 marks]
Green region: Turns blue-black [1].
White region: Remains brown/yellow (no blue-black colour) [1].

(b) [2 marks]
The green region contains chlorophyll [1], which is necessary for photosynthesis to occur and produce starch. The white region lacks chlorophyll, so no photosynthesis occurs and no starch is produced [1].

16. [4 marks]

• The high salt concentration in the soil lowers the water potential of the soil solution [1].
• The water potential of the soil solution becomes lower (more negative) than the water potential of the cell sap in the root hair cells [1].
• Water moves by osmosis from a region of higher water potential (root cell) to a region of lower water potential (soil solution) [1].
• The plant cells lose water and become plasmolysed / the cell membrane pulls away from the cell wall, causing the plant to wilt [1].

Marking note: Students must reference water potential and/or osmosis to gain full marks. Award 1 mark per valid point, max 4.

17. (a) [2 marks]
As light intensity increases, the rate of photosynthesis increases [1]. The relationship is directly proportional / positive correlation [1].

(b) [3 marks]
At higher light intensity, more light energy is absorbed by chlorophyll in the chloroplasts [1]. This increases the rate of the light-dependent reactions, producing more ATP and NADPH [1], which drives the light-independent reactions to produce more glucose, and more oxygen is released as a by-product [1].

(c) [2 marks]
The rate of photosynthesis would increase further [1] because the light intensity would be even higher (closer to the light source), providing more energy for the light-dependent reactions [1].

Accept: Students may note that at very high intensities, the rate may plateau if another factor becomes limiting. Award 1 mark for a reasonable prediction with justification.

18. (a) [2 marks]
Xylem: Transports water and dissolved mineral ions from the roots to the leaves [1].
Phloem: Translocates sucrose and amino acids from source (e.g., leaves) to sink (e.g., roots, fruits, growing regions) [1].

(b) [3 marks]

• Sieve tube elements are living cells joined end to end to form continuous tubes, allowing the flow of sucrose and amino acids [1].
• Sieve plates (perforated end walls) allow the passage of substances between sieve tube elements [1].
• Companion cells are closely associated with sieve tube elements and contain many mitochondria, providing energy (ATP) for active loading of sucrose into the phloem [1].

(c) [3 marks]
Removing the bark (including phloem) disrupts / blocks the translocation of sucrose and amino acids downwards [1]. Sucrose and amino acids accumulate above the ring [1], causing the tissue to swell due to the build-up of organic nutrients / increased osmotic pressure drawing water into the tissue [1].

19. (a) [2 marks]
Independent variable: Presence or absence of carbon dioxide [1].
Dependent variable: Whether starch is produced (result of iodine test) [1].

(b) [2 marks — 1 mark each]
(i) Same light intensity / same distance from light source [1].
(ii) Same temperature / same type and size of plant / same duration of exposure [1].
Acceptable alternatives: Same species of plant, same size of bell jar, same volume of solution, same initial destarching procedure.

(c) [3 marks]
Bell Jar A: The leaf does not turn blue-black / remains brown-yellow [1] because sodium hydroxide absorbed the carbon dioxide, so photosynthesis could not occur and no starch was produced [1].
Bell Jar B: The leaf turns blue-black [1] because carbon dioxide was present, allowing photosynthesis to occur and starch to be produced.

20. [5 marks — award 1 mark per valid structural feature with explanation, max 5]

1. Thin leaf blade / broad, flat shape — Provides a large surface area for absorption of light and carbon dioxide, and shortens the diffusion distance for gases. [1]

2. Palisade mesophyll cells contain many chloroplasts — Maximises the absorption of light energy for photosynthesis. [1]

3. Spongy mesophyll has large air spaces — Allows carbon dioxide to diffuse rapidly to the photosynthetic cells and oxygen to diffuse out. [1]

4. Stomata on the lower epidermis — Allow gas exchange (CO₂ in, O₂ out) between the leaf and the atmosphere. [1]

5. Lower epidermis has more stomata than upper epidermis — Reduces water loss by transpiration (lower epidermis receives less direct sunlight). [1]

6. Waxy cuticle on upper epidermis — Reduces water loss by evaporation from the leaf surface. [1]

7. Xylem in the leaf veins — Supplies water and mineral ions to the photosynthetic cells. [1]

8. Phloem in the leaf veins — Translocates the products of photosynthesis (sucrose) away from the leaf to other parts of the plant. [1]

Award 1 mark per valid feature with correct explanation, max 5. Students must refer to at least 4 features to gain full marks.