Another “add-on” feature to C3 photosynthesis is crassulacean acid metabolism (CAM).The physiology of this pathway is almost identical to C4 photosynthesis, with the changes that follow:
1. PEP carboxylase still fixes CO2 to OAA, as in C4. Instead of malate, however, OAA is converted to malic acid. (This is a minor difference, since malate is merely the ionized form of malic acid). 2. Malic acid is shuttled to the vacuole of the cell (not moved out of the cell to bundle sheath cells as in regular C4). 3. Stomata are open at night. During the night, PEP carboxylase is active and malic acid accumulates in the cell’s vacuole. 4. Stomata are closed during the day (the reverse of other plants). At this time, malic acid is shuttled out of the vacuole and converted back to OAA (requiring 1 ATP to ADP), releasing CO2. The CO2 is now fixed by rubisco, and the Calvin-Benson cycle proceeds. The advantage of CAM is that photosynthesis can proceed during the day while the stomata are closed, greatly reducing H2O loss. As a result, CAM provides an adaptation for plants that grow in hot, dry environments with cool nights (such as deserts). The name crassulacean acid metabolism comes from the early discovery of CAM in the succulent plants of the family Crassulaceae and the discovery of the accumulation of malic acid in vacuoles during the night. In addition to the Crassulaceae, CAM is found among plants in over a dozen different families, including cacti.
1. PEP carboxylase still fixes CO2 to OAA, as in C4. Instead of malate, however, OAA is converted to malic acid. (This is a minor difference, since malate is merely the ionized form of malic acid). 2. Malic acid is shuttled to the vacuole of the cell (not moved out of the cell to bundle sheath cells as in regular C4). 3. Stomata are open at night. During the night, PEP carboxylase is active and malic acid accumulates in the cell’s vacuole. 4. Stomata are closed during the day (the reverse of other plants). At this time, malic acid is shuttled out of the vacuole and converted back to OAA (requiring 1 ATP to ADP), releasing CO2. The CO2 is now fixed by rubisco, and the Calvin-Benson cycle proceeds. The advantage of CAM is that photosynthesis can proceed during the day while the stomata are closed, greatly reducing H2O loss. As a result, CAM provides an adaptation for plants that grow in hot, dry environments with cool nights (such as deserts). The name crassulacean acid metabolism comes from the early discovery of CAM in the succulent plants of the family Crassulaceae and the discovery of the accumulation of malic acid in vacuoles during the night. In addition to the Crassulaceae, CAM is found among plants in over a dozen different families, including cacti.
