# Explain how the adaptations of a CAM plant allow it to fix carbon dioxide efficiently in an arid environment.

> IB DP Biology Higher Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks

> CAM (Crassulacean Acid Metabolism) plants such as cacti and agaves are highly successful in hot, dry habitats where water availability is severely limited.

## Mark scheme (4 marks)

1. Stomata open at night (when temperatures are lower and humidity is higher) to take in carbon dioxide, minimising water loss by transpiration.
2. 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.
3. During the day, stomata remain closed (conserving water), and malate is released from the vacuole and decarboxylated to release CO₂ for the Calvin cycle.
4. 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.

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