How does CAM photosynthesis differ from C4 in terms of stomatal behavior and timing?

How does CAM photosynthesis differ from C4 in terms of stomatal behavior and timing?

• A. CAM opens stomata during the day and C4 fixes CO2 at night.
• B. CAM opens stomata at night to fix CO2 into organic acids; during the day stomata close; CO2 released from acids is fixed by the Calvin cycle. C4 uses spatial separation; stomata operate during the day.
• C. CAM uses spatial separation of steps between mesophyll and bundle-sheath cells; stomata stay open all day; C4 fixes CO2 at night.
• D. CAM relies on RuBisCO in the stroma; C4 uses the chloroplast thylakoids.

Answer: (B)

The question tests how CAM and C4 plants control CO2 uptake and stomatal opening in time. In CAM plants, stomata open at night to fix CO2 into organic acids, which are stored overnight. During the day, the stomata stay closed to conserve water, and CO2 is released from those stored acids to feed the Calvin cycle. In contrast, C4 plants open their stomata during the day and fix CO2 first in mesophyll cells with PEP carboxylase to form four-carbon acids, which are transported to bundle-sheath cells where CO2 is released for the Calvin cycle. This combination gives CAM high water-use efficiency via nocturnal CO2 uptake, while C4 uses spatial separation to concentrate CO2 during the day. The other statements mix up the timing or the type of separation: CAM does not fix CO2 during the day, nor does C4 fix CO2 at night, and CAM’s distinction is temporal (night vs day) rather than spatial, unlike C4.

The question tests how CAM and C4 plants control CO2 uptake and stomatal opening in time. In CAM plants, stomata open at night to fix CO2 into organic acids, which are stored overnight. During the day, the stomata stay closed to conserve water, and CO2 is released from those stored acids to feed the Calvin cycle. In contrast, C4 plants open their stomata during the day and fix CO2 first in mesophyll cells with PEP carboxylase to form four-carbon acids, which are transported to bundle-sheath cells where CO2 is released for the Calvin cycle. This combination gives CAM high water-use efficiency via nocturnal CO2 uptake, while C4 uses spatial separation to concentrate CO2 during the day. The other statements mix up the timing or the type of separation: CAM does not fix CO2 during the day, nor does C4 fix CO2 at night, and CAM’s distinction is temporal (night vs day) rather than spatial, unlike C4.