A-Level Biology October/November 2024 Q2(a): Explain why it is important for plants to open and close their stomata in a daily rhythm.

1 answer

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  Stomata generally open during the day when light is available. This is essential to allow carbon dioxide to diffuse into the leaf from the atmosphere. This carbon dioxide is a required raw material for the light-independent stage of photosynthesis. Conversely, stomata close at night in the dark. This is a vital water-conserving mechanism, as it significantly reduces water loss by transpiration when photosynthesis is not occurring.

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  How the marks are awarded

  • B1 — Stating that stomata open during the day or in the light, as seen in the first sentence: "Stomata generally open during the day when light is available."
  • B1 — Linking stomatal opening to the intake of carbon dioxide for photosynthesis, as stated: "This is essential to allow carbon dioxide to diffuse into the leaf... for the light-independent stage of photosynthesis."
  • B1 — Explaining that closing stomata reduces water loss, as mentioned in the final sentence: "This is a vital water-conserving mechanism, as it significantly reduces water loss by transpiration..."

  Common mistakes

  • Being too vague, e.g., stating 'for gas exchange' without specifying that carbon dioxide is taken in for photosynthesis.
  • Only describing one part of the rhythm, for example explaining why stomata open but failing to explain the importance of them closing at night.
  • Confusing the primary reason for opening (CO₂ intake) with the release of oxygen, which is a by-product and not the driving reason.
  • Incorrectly stating that stomata are for 'breathing'.

  Examiner tip: For 'explain' questions, always try to link a biological structure or event to its specific physiological function and the environmental conditions that influence it.

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