1. C4 Photosynthesis Improving upon photosynthetic efficiency

2. Evolved a special add on feature When CO2 enters the leaf, it is absorbed by the usual photosynthesizing cells, the mesophyll cells. Instead of being fixed by rubisco into PGA, the CO2 combines with PEP (phosphoenolpyruvate) to form OAA (oxaloacetate) The fixing enzyme is PEP carboxylase

3. C4 The first product of this pathway, has 4 carbon atoms, thus the name C4. OAA is converted to malate, and the malate is shuttled to specialized cells within the leaf, the bundle sheath cells. Here malate is converted to pyruvate and CO2 The pyruvate is then shuttled back to the mesophyll cells were one ATP is required to convert the pyruvate back to PEP.

4. C4 Then the process repeats. The overall effect is to move CO2 from mesophyll cells to the bundle sheath cells. The purpose for moving the CO2 to bundle sheath cells is to increase the efficiency of photosynthesis

5. C4 The bundle sheath cells surround the leaf veins and are themselves surrounded by densely packed mesophyll cells. Since bundle cells rarely make contact with an intercellular space, very little oxygen reaches them. When malate delivers CO2 to a bundle sheath cell, rubisco begins the Calvin cycle (C3 photosynthesis). Because little oxygen is present, rubisco can fix CO2 without competition for O2. thus, little photorespiration takes place, and photosynthesis is more efficient.

6. C4 Higher rate of photosynthesis among C4 plants allow them to reduce the time that the stomata are open, thereby, reducing H2O loss. C4 plants are found in hot, dry climates, where they possess an adaptive advantage over C3 plants. C4 plants—sugarcane and crab grasses are two examples

7. CAM photosynthesis Another “add on” feature to photosynthesis is crassulacean acid metabolism (CAM) The physiology of this pathway is almost identical to C4 photosynthesis. PEP carboxylase still fixes CO2 to OAA, as in C4 Instead of malate, however, OAA is converted to malic acid. Malic acid is shuttled to the vacuole of the cell(not moved out of the cell to bundle sheath cell as in C4)

8. CAM Stomata are open at night. During the night, PEP carboxylase is active and malic acid accumulates in the cell’s vacuole. Stomata are closed during the day (reverse of other plants). At this time, malic acid is shuttled out of the vacuole and converted back to OAA, releasing CO2. The CO2 is now fixed by rubisco, and the Calvin cycle proceeds.

9. CAM 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. Found in many families, including cacti