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A-Level Biology May/June 2025 Q1(b): Some species in the Ranunculus genus are xerophytes. State and explain two adaptations…
A-Level Biology · Paper 9700/23 · May/June 2025 · Question 1(b) · [4 marks]
Some species in the Ranunculus genus are xerophytes. State and explain two adaptations of the leaves of xerophytic plants that reduce water loss.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
Adaptation 1
State: Xerophytic leaves may have stomata that are sunken in pits, often accompanied by hairs (trichomes).
Explanation: The pits and hairs trap a layer of still, moist air directly outside the stomata. This reduces the water potential gradient between the humid air spaces inside the leaf and the external atmosphere. As a result, the rate of diffusion of water vapour out of the leaf (transpiration) is significantly decreased.
Adaptation 2
State: The leaves are covered by a thick, waxy cuticle.
Explanation: The waxy cuticle is waterproof and largely impermeable to water. A thicker cuticle increases the diffusion distance for any water vapour that might escape through the epidermal surface. This minimises water loss through cuticular transpiration, which is particularly important when stomata are closed.
How the marks are awarded
- B2 — The first mark is for stating the adaptation of sunken stomata/hairs (A). The second mark is for correctly explaining that this traps humid air, which reduces the water potential gradient and thus lowers the rate of transpiration (E).
- B2 — The third mark is for stating the adaptation of a thick waxy cuticle (A). The fourth mark is for explaining that this increases the diffusion distance for water vapour or makes the leaf surface more impermeable to water, reducing cuticular transpiration (E).
Common mistakes
- Providing a vague explanation, such as 'sunken stomata prevent water loss', without explaining the mechanism involving humid air and the water potential gradient.
- Stating an adaptation but failing to provide any explanation, which would score a maximum of 2 out of 4 marks.
- Confusing the functions of different adaptations, for example, stating that a thick cuticle traps humid air.
- Describing an adaptation of a xerophyte that is not related to the leaves, such as extensive root systems, which would not be credited for this question.
Examiner tip: For 'state and explain' questions, always structure your answer by first naming the feature and then detailing the scientific principle or mechanism that makes it an effective adaptation.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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