Plants fix atmospheric CO2 into organic compounds via three main pathways:

1. C3 Pathway (Calvin Cycle): Commonest, RuBisCO fixes CO2 into 3-PGA.

2. C4 Pathway: Adapted for hot, dry conditions, spatially separates initial CO2 capture from the Calvin cycle.

3. CAM Pathway (Crassulacean Acid Metabolism): Adaptation for extreme aridity, temporally separating CO2 capture.

CAM Pathway and Stomatal Activity:
CAM is an adaptation for succulents and desert plants (e.g., cacti) to conserve water.

1. Nighttime CO2 Fixation:

   • Stomata open at night (cooler, less transpiration). CO2 enters, fixed by PEP carboxylase into 4-carbon malate.

   • Malate is stored in cell vacuoles.

2. Daytime CO2 Release and Calvin Cycle:

   • Stomata close during the day (hot, dry) to prevent water loss.

   • Stored malate is released from vacuoles, decarboxylated to release CO2 internally.

   • This internal CO2 then enters the Calvin cycle (C3 pathway) via RuBisCO in chloroplasts, producing sugars.

Role in Stomatal Activity: CAM's unique mechanism dictates stomatal opening only at night. This significantly reduces water loss by transpiration, enabling plants to photosynthesize and survive in harsh, arid environments where C3/C4 plants would dehydrate. This water-conservation strategy often leads to slower growth rates due to limited daytime CO2 availability.