Yes, CAM (Crassulacean Acid Metabolism) plants do use Rubisco.
CAM plants utilize Rubisco in the Calvin cycle to fix carbon dioxide and produce sugars, just like other photosynthetic plants (C3 and C4). However, the timing of when Rubisco is used differs significantly. In CAM plants:
- Night: CAM plants open their stomata at night, allowing CO₂ to enter. This CO₂ is initially fixed by PEP carboxylase, forming a 4-carbon organic acid (malate). This malate is stored in vacuoles.
- Day: During the day, the stomata are closed to conserve water. The stored malate is transported out of the vacuole and decarboxylated, releasing CO₂. This CO₂ is then concentrated around Rubisco within the bundle sheath cells or mesophyll cells, facilitating the Calvin cycle and sugar production.
Key Takeaway: The controlled release of CO₂ from malate ensures a high concentration of CO₂ around Rubisco during the day when the stomata are closed, minimizing photorespiration. While the initial carbon fixation is done by PEP carboxylase at night, Rubisco is absolutely essential for the second carbon fixation stage within the Calvin Cycle during the day in CAM plants. Rubisco's activity is temporally separated from the initial CO2 uptake, but it remains a core enzyme in the photosynthetic pathway.
