Photorespiration is an inefficient process occurring in plants under conditions where CO2 levels are low. Certain plants called C4 plants have evolved an additional photosynthetic pathway that helps conserve CO2 released by photorespiration. This pathway is called the C4 cycle because it incorporates CO2 into an intermediate with four carbons (oxaloacetate). By contrast, the Calvin cycle is sometimes called the C3 cycle because the reaction of CO2 with ribulose-1,5-bisphosphate yields two molecules of 3-phosphoglycerate, a three - carbon compound.

The C4 cycle is found in important crop species, such as maize and sugarcane, and is important in tropical plants, which are exposed to intense sunlight and high temperatures. Photorespiration is most active under these conditions.

C4 plants concentrate their Calvin cycle photosynthesis in specialized bundle sheath cells, which lie below a layer of mesophyll cells (Figure 17.25). The mesophyll cells, which are most directly exposed to external CO2, contain the enzymes of the C4 cycle. See Figure 17.26 for the reactions of the C4 cycle. The key reaction, which is the capture of CO2 into oxaloacetate, is catalyzed by the enzyme phosphoenolpyruvate carboxylase and occurs in the mesophyll cells.

Unlike ribulose bisphosphate carboxylase (rubisco) from the Calvin cycle, phosphoenolpyruvate carboxylase has no oxygenase activity. As a result, capturing CO2 into oxaloacetate serves as a mechanism for delivering CO2 to the Calvin cycle process in the bundle sheath cells, not for initiating photorespiration. In this way, high levels of CO2 can be maintained in the bundle sheath cells, favoring CO2 fixation rather than photorespiration. Even if photorespiration does occur, CO2 released in that process may be readily returned to the Calvin cycle by the C4 pathway, instead of being lost.

Note that the C4 cycle costs 2 additional ATPs for every CO2 fixed. This price may seem steep, but it appears to be worth paying under circumstances when photorespiration would dominate.