Explain how the adaptations of a CAM plant allow it to fix carbon dioxide efficiently in an arid environment.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
CAM (Crassulacean Acid Metabolism) plants such as cacti and agaves are highly successful in hot, dry habitats where water availability is severely limited.
Model answer (4 marks)
1. Stomata open at night when temperatures are lower and humidity higher, allowing CO₂ uptake while minimising water loss by transpiration.
2. CO₂ is fixed by PEP carboxylase, which combines it with phosphoenolpyruvate to form oxaloacetate, subsequently converted to malate.
3. Malate is stored in the vacuole and released during the day; it is decarboxylated to release CO₂ for the Calvin cycle.
4. This temporal separation of gas exchange (night) and light‑dependent reactions (day) lets the plant photosynthesise efficiently while greatly reducing water loss, an essential adaptation to arid conditions.
2. CO₂ is fixed by PEP carboxylase, which combines it with phosphoenolpyruvate to form oxaloacetate, subsequently converted to malate.
3. Malate is stored in the vacuole and released during the day; it is decarboxylated to release CO₂ for the Calvin cycle.
4. This temporal separation of gas exchange (night) and light‑dependent reactions (day) lets the plant photosynthesise efficiently while greatly reducing water loss, an essential adaptation to arid conditions.
Examiner tips
- Use the word ‘stomata’ and ‘PEP carboxylase’ to show specific knowledge; link each step to water‑saving benefit.
- Show the sequence: night stomata open → CO₂ fixation → malate storage → day decarboxylation → Calvin cycle.
- Mention the key advantage: reduced transpiration in hot, dry habitats.
Common mistakes
- Confusing the order of CO₂ fixation and malate storage; or stating that stomata stay open all day.
- Using ‘photosynthesis’ without explaining the temporal separation or the role of PEP carboxylase.
Mark scheme (4 marks)
- Stomata open at night (when temperatures are lower and humidity is higher) to take in carbon dioxide, minimising water loss by transpiration.
- Carbon dioxide is fixed at night by PEP carboxylase combining CO₂ with phosphoenolpyruvate (PEP) to form oxaloacetate, which is then converted to malate (an organic acid) stored in the vacuole.
- During the day, stomata remain closed (conserving water), and malate is released from the vacuole and decarboxylated to release CO₂ for the Calvin cycle.
- This temporal separation of gas exchange (night) and the light-dependent reactions / Calvin cycle (day) allows the plant to photosynthesise effectively while greatly limiting water loss, an essential adaptation in arid conditions.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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