تسجيل الدخول

تسجيل

Photorespiration and the C4 and CAM Pathways:- In CAM Plants, CO2 Capture and Rubisco Action Are Temporally Separated

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  770

2026-06-25

40

Photorespiration and the C₄ and CAM Pathways:- In CAM Plants, CO₂ Capture and Rubisco Action Are Temporally Separated

Succulent plants such as cactus and pineapple, which are native to very hot, very dry environments, have another variation on photosynthetic CO₂ fixation, which reduces loss of water vapor through the pores (stomata) by which CO₂ and O₂ must enter leaf tissue. Instead of separating the initial trapping of CO₂ and its fixation by rubisco across space (as do the C₄ plants), they separate these two events over time. At night, when the air is cooler and moister, the stomata open to allow entry of CO₂, which is then fixed into oxaloacetate by PEP carboxylase. The oxaloacetate is reduced to malate and stored in the vacuoles, to protect cytosolic and plastid enzymes from the low pH produced by malic acid dissociation. During the day the stomata close, pre venting the water loss that would result from high day time temperatures, and the CO₂ trapped overnight in malate is released as CO₂ by the NADP-linked malic en zyme. This CO₂ is now assimilated by the action of rubisco and the Calvin cycle enzymes. Because this method of CO₂ fixation was first discovered in stonecrops, perennial flowering plants of the family Crassulaceae, it is called crassulacean acid metabolism, and the plants are called CAM plants.