ACS Biochemistry Practice Exam

C4 plants fix CO2 primarily into oxaloacetate as part of their adaptation to high light intensities and temperatures, which helps to minimize photorespiration. In the C4 pathway, when CO2 enters the leaf, it first combines with phosphoenolpyruvate (PEP) to form oxaloacetate, a four-carbon compound. This reaction is catalyzed by the enzyme PEP carboxylase, which has a high affinity for CO2 and effectively excludes O2.

The formation of oxaloacetate allows C4 plants to concentrate CO2 in a specialized cell type (bundle sheath cells) where the Calvin cycle takes place. There, the oxaloacetate is converted to malate, which can then be transported into the bundle sheath cells, where it is decarboxylated to release CO2 for use in the Calvin cycle.

By fixing CO2 into oxaloacetate first, C4 plants are able to reduce the oxygenation reaction that leads to photorespiration, which is more prominent in C3 plants. Thus, the role of oxaloacetate in the initial fixation of CO2 is crucial for the efficiency of photosynthesis in C4 plants, particularly in conditions where