FIGURE 12-50 Leaf anatomy of C₄ plants and the C₄ pathway. (a) In C₄ plants, bundle sheath cells line the vascular bundles containing the xylem and phloem. Mesophyll cells, which are adjacent to the substomal air spaces, can assimilate CO₂ into four-carbon molecules at low ambient CO₂ concentrations and deliver those molecules to the interior bundle sheath cells. The bundle sheath cells contain abundant chloroplasts and are the sites of photosynthesis and sucrose synthesis. Sucrose is carried to the rest of the plant via the phloem. In C₃ plants, which lack bundle sheath cells, the Calvin cycle operates in the mesophyll cells to fix CO₂. (b) The key enzyme in the C₄ pathway is phosphoenolpyruvate carboxylase, which assimilates CO₂ to form oxaloacetate in mesophyll cells. Decarboxylation of malate or other C₄ intermediates in bundle sheath cells releases CO₂, which enters the standard Calvin cycle (see Figure 12-48, top).