Which photosynthetic adaptation splits the carbon fixation process between day and night, reducing water loss in arid environments?

• A. C3
• B. Calvin cycle
• C. CAM
• D. C4

Answer: (C)

Crassulacean acid metabolism is the adaptation that splits carbon fixation between night and day to save water in dry environments. At night, stomata open when evaporation is lower, and CO2 is fixed into a four-carbon compound (malate) and stored in vacuoles using the enzyme PEP carboxylase. By day, the stomata close to reduce water loss, and the stored malate is decarboxylated to release CO2 for the Calvin cycle. This temporal separation keeps photosynthesis going while minimizing water loss, which is exactly the challenge arid-adapted plants face. Other options involve different strategies (Calvin cycle in the daytime for C3, or spatial separation for C4) and don’t describe the day–night split that CAM uses.