The C 4 Cycle

Photorespiration is an inefficient process occurring in plants under conditions where CO 2 levels are low. Certain plants called C 4 plants have evolved an additional photosynthetic pathway that helps conserve CO 2 released by photorespiration. This pathway is called the C 4 cycle because it incorporates CO 2 into an intermediate with four carbons (oxaloacetate). By contrast, the Calvin cycle is sometimes called the C 3 cycle because the reaction of CO 2 with ribulose-1,5-bisphosphate yields two molecules of 3-phosphoglycerate, a three - carbon compound.

The C 4 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.

C 4 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 CO 2, contain the enzymes of the C 4 cycle. See Figure 17.26 for the reactions of the C 4 cycle. The key reaction, which is the capture of CO 2 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 CO 2 into oxaloacetate serves as a mechanism for delivering CO 2 to the Calvin cycle process in the bundle sheath cells, not for initiating photorespiration. In this way, high levels of CO 2 can be maintained in the bundle sheath cells, favoring CO 2 fixation rather than photorespiration. Even if photorespiration does occur, CO 2 released in that process may be readily returned to the Calvin cycle by the C 4 pathway, instead of being lost.

Note that the C 4 cycle costs 2 additional ATPs for every CO 2 fixed. This price may seem steep, but it appears to be worth paying under circumstances when photorespiration would dominate.

See also: Photorespiration

INTERNET LINK: Carbon Fixation in C 4 Plants