A CAM cycle uses approximately 5.5 to 6.5 ATP for every CO2 molecule fixed photosynthetically. This energy is required due to the extra enzymatic reactions and compartmentation complexities inherent in the CAM pathway, as contrasted with C3 photosynthesis.
| Plant Type | ATP Usage (per CO2 fixed) | NADPH Usage (per CO2 fixed) |
|---|---|---|
| C3 | 3 | 2 |
| C4 | 4-5 | 2 |
| CAM | 5.5-6.5 | 2 |
The higher ATP requirement in CAM plants is due to the following factors:
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Nocturnal CO2 Fixation: CAM plants open their stomata at night, fixing CO2 into a 4-carbon acid (usually malate), which is stored in the vacuole. This initial CO2 fixation requires energy.
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Daytime Decarboxylation: During the day, the stored 4-carbon acid is decarboxylated, releasing CO2 for use in the Calvin cycle. The regeneration of the CO2 acceptor molecule also requires ATP.
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Compartmentation: The CAM pathway involves spatial separation of initial CO2 fixation and the Calvin cycle within the mesophyll cells, requiring transport of metabolites across membranes, which consumes ATP.
In summary, the CAM pathway provides an adaptation to arid environments by minimizing water loss, but this comes at the expense of increased ATP usage compared to other photosynthetic pathways like C3.
