CO2 removed from malate enters C3 Calvin cycle, catalyzed by rubisco

2. C4 plants contain 2 types of photosynthetic cells: bundle-sheath cells surrounding a vein, and mesophyll cells that are located around the bundle-sheath cells • PEP carboxylase catalyzes the addition of a CO2 molecule to a 3-C PEP to form a 4-C oxaloacetate (OAA) in the cytoplasm of the mesophyll cell • OAA is converted to malate, which then diffuses from the mesophyll cells into the bundle-sheath cells through cell-to-cell connections called plasmodesmata • CO2 is removed from malate to form pyruvate which diffuses back into the mesophyll cell where it is converted to PEP, requiring the energy from an ATP molecule • C4 carbon fixation reduces photorespiration by pumping CO2 from mesophyll cells into bundle-sheath cells where rubisco brings them into the Calvin cycle, but it costs 2 ATP molecules to transport a CO2 molecule into a bundle-sheath cell, for a total of 18 additional ATP molecules to produce a glucose molecule (30 vs. 18 for C3) • Still advantageous in hot climates where photorespiration would convert more than ½ of the glucose back to CO2 • CO2 removed from malate enters C3 Calvin cycle, catalyzed by rubisco

3. CAM Plants (called Crassulacean Acid Metabolism as it was first discovered in the Crassulacea family of plants) water-storing plants (succulents) including those in the Crassulacea family, open their stomata at night and cl;ose them during the day (the opposite of other plants) • in the dark, succulents take in CO2 and incorporate it into C4 organic acids using PEP carboxylase organic acids are stored in vacuoles until morning until they release CO2 that enter the C3 Calvin cycle C4 plants separate the two steps of carbon fixation in different parts of the leaf while CAM plants separate the steps by time