The first step is carried out by an enzyme present only in mesophyll cells called PEP carboxylase. This enzyme adds CO2 to phosphoenolpyruvate (PEP), forming the four-carbon product oxaloacetate. PEP carboxylase has a much higher affinity for CO2 than does rubisco and no affinity for O2. Therefore, PEP carboxylase can fix carbon efficiently when rubisco cannot: that is, when it is hot and dry and stomata are partially closed, causing CO2 concentration in the leaf to fall and O2 concentration to rise. After the C4 plant fixes carbon from CO2, the mesophyll cells export their four-carbon products (malate in the example shown in Figure 10.20) to bundle-sheath cells through plasmodesmata (see Figure 6.31). Within the bundle-sheath cells, the four-carbon compounds release CO2, which is reassimilated into organic material by rubisco and the Calvin cycle. The same reaction regenerates pyruvate, which is transported to mesophyll cells. There, ATP is used to convert pyruvate to PEP, allowing the reaction cycle to continue; this ATP can be thought of as the "price" of concentrating CO2 in the bundle-sheath cells. To generate this extra ATP, bundle-sheath cells carry out cyclic electron flow, the process described earlier in this chapter (see Figure 10.16). In fact, these cells contain PS I but no PS II, so cyclic electron flow is their only photosynthetic mode of generating ATP.
