Cam plants avoid photorespiration by two mechanisms: 1) PEP carboxylase and 2) a separation in time of carbon fixation and the Calvin cycle (light independent reactions). Background: Photorespiration refers to the dual affinity of RuBP Carboxylase (aka Rubisco) for carbon dioxide and oxygen. For effective photosynthesis RuBP Carboxylase would fix carbon dioxide into organic molecules. However if oxygen is present in the cell oxygen competes with carbon dioxide for the active site of Rubisco. If Rubisco binds oxygen rather than carbon dioxide the quantity of carbon dioxide fixed is reduced thus lowering the effectiveness of photosynthesis. Answer Part I: C4 and CAM plants have over evolutionary time evolved a different enzyme to serve in the (initial) fixation of carbon dioxide. PEP Carboxylase has a higher affinity for carbon dioxide than Rubisco and serves to fix carbon dioxide into an organic intermediate molecule. The presence of PEP Carboxylase in C4 and CAM plants reduces photorespiration. Answer Part II: Both C4 and CAM plants ultimately release carbon dioxide from organic intermediates to the Calvin Cycle and Rubisco but avoid photorespiration by sequestering the Calvin Cycle out of the way of oxygen. Let’s focus on CAM plants here. CAM plants well adapted to dry environments open their stomata at night to fix carbon dioxide into organic intermediates. (Cooler nighttime temperatures reduce water lost through the stomata). Carbon dioxide is stored in organic intermediates through the night then carbon dioxide is released to the Calvin cycle in the morning as the light comes up (the Calvin Cycle requires ATP and NADPH from the light-dependent reactions). But … by the morning the stomata are closed. The concentration of carbon dioxide within the cells will be much higher than oxygen. Photorespiration will be reduced as carbon dioxide not oxygen binds Rubisco’s active site.
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