Bundle sheath cells are specialized cells found in C4 plants that lie below a layer of mesophyll cells (Figure 17.25). The bundle sheath cells are the sites within C4 plants where the Calvin cycle enzymes are located. By being beneath the mesophyll cells, the bundle sheath cells have a lower oxygen concentration and a steady supply of CO2 from the mesophyll cells (Figure 17.26). The combination of these factors helps reduce the amount of wasteful oxgenase activity of rubisco, known as photorespiration.