A student seals a healthy potted plant inside an airtight glass jar and places it in bright sunlight for several hours. Predict and explain what will happen to the concentrations of CO₂ and O₂ inside the jar, keeping in mind that the plant carries out both photosynthesis and respiration.
Generated by claude-sonnet-4-6 · 2026-06-15 06:45 · grounding rag
Model Answer
In bright sunlight, the rate of photosynthesis exceeds the rate of respiration in a healthy plant.
- CO₂: Its concentration will decrease. Photosynthesis consumes CO₂, and since the jar is sealed, no fresh CO₂ enters. Although respiration releases CO₂, the amount used in photosynthesis is greater, so net CO₂ falls.
- O₂: Its concentration will increase. Photosynthesis releases O₂ as a by-product (splitting of water). Although respiration consumes O₂, photosynthesis produces far more, so net O₂ rises.
Eventually, if CO₂ is completely exhausted, photosynthesis will slow down and stop, leading to a stable or slightly declining O₂ level.
Source: Chapter 5, Section 5.2.1 (Autotrophic Nutrition) and Section 5.3 (Respiration)
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Explanation
- Examiners look for two clear predictions (CO₂ decreases, O₂ increases) each with a reason — that's the core of the 3 marks.
- The key concept is net gas exchange: in bright light, photosynthesis > respiration, so the plant is a net consumer of CO₂ and a net producer of O₂. The textbook (Section 5.3) explicitly states: "During the day, CO₂ generated during respiration is used up for photosynthesis; oxygen release is the major event at this time."
- Mentioning the sealed/airtight condition shows you understand why concentrations change — no external supply or escape.
- The final point about photosynthesis stopping when CO₂ is exhausted is a bonus insight but keep it brief.