9.7: C4 Pathway and CAM
Some plants, like sugar cane and corn, that grow in hot conditions, use an alternative process called the C4 pathway to fix carbon. The cycle begins with CO2 from the atmosphere entering mesophyll cells where it is used to generate oxaloacetate—a four-carbon molecule—from phosphoenolpyruvate (PEP). Oxaloacetate is then converted to malate and transported to bundle sheath cells, where the oxygen concentration is low. There, CO2 is released from malate and enters the Calvin Cycle where it is converted into sugars. The CAM pathway is carried out in plants like cacti that also need to conserve water during the day. CAM plants let CO2 into the leaves at night and produce malate that is stored in vacuoles until the following day. The malate is then released from vacuoles and processed in the Calvin Cycle. The C4 pathway separates the different processes locally, while the CAM pathway separates them chronologically.
The C4 Pathway
Some plants, like corn and sugarcane, have evolved alternative ways to fix carbon that help avoid water loss in hot, dry environments. One such method is the C4 pathway. In the first step, CO2 enters mesophyll cells, and the enzyme phosphoenolpyruvate (PEP) carboxylase adds it to the 3-carbon compound PEP to form the 4-carbon compound oxaloacetate. Oxaloacetate is then converted into an organic acid called malate.
Subsequently, malate is transported into bundle sheath cells deep in the leaf where the oxygen concentration is low. Malate is broken down, releasing a molecule of CO2 that then enters the Calvin Cycle where the enzyme rubisco converts it into sugar. The C4 pathway offers an advantage in hot, arid conditions as the plants will close their stomata to conserve water. As a result, they can keep the oxygen concentration low and therefore favor the binding of CO2 to rubisco rather than O2. When the oxygen concentration is higher, rubisco binds O2 instead of CO2—a process termed photorespiration—which would halt photosynthesis and consume energy.
The CAM Pathway
Other plants, like cacti and pineapple, use the crassulacean acid metabolism (CAM) pathway to fix carbon. CAM plants primarily open their stomata at night to prevent water loss during the hot day. At night CO2 enters the mesophyll cells, where it combines with PEP to form oxaloacetate and eventually malate. Malate is then stored in vacuoles until the next day when it is released from vacuoles and enters the Calvin Cycle. The first stages of photosynthesis proceed during the day as they are light-dependent, while the light-independent reactions of the Calvin cycle take place during the night. In this manner, CAM plants separate CO2 fixation and sugar synthesis by using different times of the day.