Q1. [1] medium thorough-understanding
Guard cells control whether stomatal pores are open or closed. On a hot, dry day a plant begins to wilt due to water loss. Which of the following best explains the stomatal response and its trade-off for the plant?
- A Guard cells shrink, closing the stomata; this prevents further water loss but also stops the entry of CO₂ needed for photosynthesis.
- B Guard cells swell, opening the stomata; this increases CO₂ uptake and simultaneously reduces water loss.
- C Guard cells shrink, closing the stomata; this stops water loss and increases the rate of photosynthesis by trapping CO₂ inside the leaf.
- D Guard cells swell, opening the stomata; this allows excess water vapour to leave and prevents the leaf from overheating without affecting photosynthesis.
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
Answer: A
Guard cells shrink, closing the stomata; this prevents further water loss but also stops the entry of CO₂ needed for photosynthesis.
Explanation
The passage (Chapter 5, Section 5.2.1) states that guard cells shrink to close stomatal pores when the plant does not need CO₂ or needs to conserve water. While this prevents wilting by reducing water loss, it simultaneously blocks CO₂ entry, which is essential for photosynthesis — this is the trade-off. Options B and D incorrectly state that guard cells swell (which opens stomata). Option C is wrong because closing stomata traps no extra CO₂ and does not increase photosynthesis.