מסלול C4 וֹCAM

Overview

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.