One of the steps in the photosynthesis process is dark reaction where the carbondioxide is reduced to glucose via Calvin cycle and this process is independent of light. Calvin cycle requires high concentration of carbondioxide which is taken in from air through stomata. Plants in the dry tropical region show leaf anatomy called Kranz anatomy having two types of tissue - bundle sheath and mesophyll and follow the ${\mathrm{C}}_4$ pathway to compensate for carbondioxide.

In the mesophyll cell, phosphoenol pyruvate accepts the carbodixode to form a four-carbon compound-oxaloacetic acid. This is transported to the bundle sheath cell where it undergoes decarboxylation releasing carbon dioxide to be used in ${\mathrm{C}}_3$ cycle and pyruvate. The glucose is synthesized in the bundle sheath cells.