O Level Biology Plant Biology Quiz

O-Level Biology Quiz - Plant Biology (Answer Key)

Total Marks: 40

Section A: Multiple Choice

1. C [1]
Explanation: Root hair cells are underground and do not perform photosynthesis, so they lack chloroplasts. Palisade cells are in the leaf and contain many chloroplasts. Both have cell walls, cytoplasm, and nuclei.

2. B [1]
Explanation: Photosynthesis produces oxygen as a by-product, which is released as gas bubbles.

3. A [1]
Explanation: Xylem transports water and mineral ions; Phloem transports organic nutrients like sucrose (translocation).

4. B [1]
Explanation: Iodine solution turns blue-black in the presence of starch. Ethanol removes chlorophyll to make the colour change visible.

5. C [1]
Explanation: At low light intensity, light is the limiting factor. Temperature is already "warm," so it is not limiting the rate at that specific point compared to light.

Section B: Structured Questions

6. Leaf Structure
(a)
B: Palisade mesophyll [1]
D: Stoma (or Stomata) [1]

(b)

• Contains many chloroplasts to maximise light absorption for photosynthesis. [1]
• Located near the upper surface of the leaf to receive maximum light. [1]

(c)

• Allows for gas exchange (entry of CO₂ and exit of O₂/water vapour). [1]

7. Osmosis in Potato
(a)

• The mass of the potato strip decreases. [1]

(b)

• The concentrated sugar solution has a lower water potential 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]

8. Transpiration Process

• Transpiration is the loss of water vapour from the aerial parts of the plant (mainly leaves). [1]
• Water evaporates from the surface of mesophyll cells into the air spaces. [1]
• Water vapour diffuses out of the leaf through the stomata down a concentration gradient. [1]

9. Temperature and Photosynthesis
(a)

• 35°C [1]

(b)

• High temperatures cause the enzymes involved in photosynthesis to denature. [1]
• The active site of the enzyme changes shape, so the substrate can no longer bind, stopping the reaction. [1]

10. Mineral Ion Absorption
(a)

• Active transport [1]

(b)

• Active transport moves substances against the concentration gradient (from low to high concentration). [1]
• This requires energy (ATP) produced by respiration in the mitochondria. [1]

11. Pollination Comparison

• Insect-pollinated Petals: Large / bright / coloured [1]
• Insect-pollinated Pollen: Sticky / spiky / large [1]
• Insect-pollinated Scent: Present / strong / sweet [1]

12. Pollen Tube Route

• The pollen grain lands on the stigma. [1]
• It germinates and grows a pollen tube down through the style. [1]
• The pollen tube enters the ovule through the micropyle and releases male gametes for fertilisation. [1]

13. Mineral Deficiencies
(a)

• To make amino acids / proteins / chlorophyll (for growth). [1]
  (Note: Nitrates are primarily for amino acids/proteins, but often linked to general growth. Magnesium is for chlorophyll.)

(b)

• The leaves will turn yellow (chlorosis). [1]
• Because magnesium is needed to make chlorophyll, and without it, photosynthesis is reduced. [1]

14. Translocation

• Translocation is the transport of organic nutrients (such as sucrose and amino acids). [1]
• It occurs in the phloem from sources (e.g., leaves) to sinks (e.g., roots, fruits). [1]

15. Potometer Experiment
(a)

• The rate of water uptake (which approximates the rate of transpiration). [1]

(b)

• The rate of bubble movement would increase. [1]
• The fan increases air movement/wind, which removes water vapour from around the leaf. [1]
• This maintains a steep concentration gradient for diffusion, increasing transpiration. [1]
  (Note: Only 2 marks available, so any two of the above points.)

Section C: Free Response Questions

16. Xylem vs Phloem Structure

• Xylem: Made of dead, hollow cells with no cytoplasm or nucleus. [1]
• Xylem: Walls are thickened with lignin for strength and waterproofing. [1]
• Phloem: Made of living cells (sieve tube elements and companion cells). [1]
• Phloem: Sieve plates have pores to allow flow of sap; companion cells provide metabolic support. [1]

17. CO₂ Entry and Use

• CO₂ enters the leaf through the stomata by diffusion. [1]
• It moves into the air spaces and dissolves in the moisture on the surface of mesophyll cells. [1]
• It diffuses into the chloroplasts. [1]
• In the chloroplasts, CO₂ is used in the light-independent reaction (Calvin cycle) to produce glucose/starch using energy from ATP and NADPH. [1]

18. Destarching

• To remove any existing starch stored in the leaves from previous photosynthesis. [1]
• This ensures that any starch detected after the experiment was produced during the experiment. [1]
• If not destarched, the result would be invalid as we wouldn't know if the starch was new or old. [1]

19. Self vs Cross Pollination

• Advantage of Self: Does not depend on external agents (wind/insects); ensures reproduction even if isolated. [1]
• Disadvantage of Self: No genetic variation; offspring are genetically identical, making them vulnerable to disease/environmental change. [1]
• Advantage of Cross: Produces genetic variation; allows for evolution and adaptation to changing environments. [1]
• Disadvantage of Cross: Depends on external agents; energy required to produce nectar/scents; chance of pollination is not guaranteed. [1]

20. Root Hair Cell Adaptation

• Large Surface Area: The long hair-like projection greatly increases the surface area for absorption of water and minerals. [1]
• Thin Cell Wall: The cell wall is thin, reducing the distance for water to diffuse/osmose into the cell. [1]
• High Solute Concentration: The cell sap has a lower water potential (higher solute concentration) than the soil water, facilitating water entry by osmosis. [1]
• Many Mitochondria: Provide energy (ATP) for the active transport of mineral ions. [1]