Most plants use the C3 pathway for carbon fixation. However, some plants, such as sugar cane, corn, and cacti that grow in hot conditions, use alternative pathways to fix carbon and conserve energy loss due to photorespiration. Photorespiration is the process that occurs when the oxygen concentration is high. Under such conditions, the rubisco enzyme in the Calvin cycle binds O2 instead of CO2, which halts photosynthesis and consumes energy.
C4 Pathway
The C4 pathway is used by plants such as corn and sugarcane. In the first step, CO2 enters the mesophyll cells, and the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) adds it to the 3-carbon compound PEP to form the 4-carbon compound oxaloacetate. Oxaloacetate is then converted into organic acids, predominantly malate. Malate is then transported into the bundle sheath cells deep in the leaf, where the oxygen concentration is low. It is here that malate is broken down to release a molecule of CO2. CO2 then enters the Calvin cycle and is converted into sugar with the help of the enzyme RuBisCo.
In hot, arid conditions, the physical separation of carbon fixing and the Calvin cycle is advantageous, as the plants can 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.
CAM Pathway
Other plants, such as cacti and pineapple, use the crassulacean acid metabolism (CAM) pathway to fix carbon. CAM plants open their stomata at night to prevent water loss during hot days. Once the CO2 enters the mesophyll cells, it combines with PEP to form oxaloacetate and eventually malate.
Malate is then stored in vacuoles until the next day when it is released to enter the Calvin cycle. So, the carbon fixing occurs at night, and the Calvin cycle, fuelled by the light-dependent reactions, proceeds during the day. In this manner, CAM plants separate carbon fixation and sugar synthesis across different times of the day.
Some plants that grow in very hot environments have evolved strategies to overcome water loss and fix carbon efficiently.
Two examples are corn plants that use the C4 pathway and cacti that use the Crassulacean acid metabolism or CAM pathway.
In C4 plants, carbon fixation occurs in the mesophyll cells. The enzyme PEP carboxylase combines carbon dioxide with phosphoenolpyruvate to form oxaloacetate.
Oxaloacetate is then enzymatically converted to malate or another organic acid. Next, these organic acids are transported into the bundle sheath cells to be broken down into CO2 and pyruvate. The CO2 enters the Calvin cycle and pyruvate replenishes the PEP.
Unlike C4 plants where CO2 fixation and the Calvin cycle are separated in space, CAM plants separate these two steps in time. CAM plants only open their stomata for CO2 intake at night to prevent water loss.
The carbon is fixed into a molecule of malate and stored in the vacuoles of mesophyll cells until daylight. The CO2 is released during the day time and the Calvin cycle proceeds along with the light reactions.
