Science (086) — AI-generated practice question

When stomata open during a sunny afternoon, CO₂ enters the leaf for photosynthesis. Simultaneously, O₂ produced by photosynthesis becomes directly available inside the leaf for aerobic respiration, without needing a separate gas-exchange mechanism. As the textbook states, "CO₂ generated during respiration is used up for photosynthesis… oxygen release is the major event at this time." Thus, the two processes mutually supply each other's raw materials through the same open stomata.

Unavoidable cost: Large amounts of water vapour are lost through the open stomata (transpiration). Since "large amounts of water can also be lost through these stomata," the plant cannot take in CO₂ without simultaneously losing water — this is the unavoidable trade-off.

Source: Chapter 5, Sections 5.2.1 and 5.3

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• The key insight examiners want is the mutual benefit: O₂ from photosynthesis feeds aerobic respiration, and CO₂ from respiration feeds photosynthesis — both via the same open stomata.
• The "unavoidable cost" must be water loss/transpiration — directly supported by the passage. Don't write vague answers like "energy loss."
• Quote or paraphrase the textbook lines to show you are grounding the answer in source material.
• At 3 marks: ~2 marks for the gas-exchange benefit explanation, ~1 mark for the cost.