The diagram shows hatch and slack pathway.

The C4 pathway occurs through cooperation between bundle sheath cells and mesophyll cells.

1. The first step takes place in the mesophyll cell, where the phosphoenolpyruvate carboxylase enzyme catalyses the formation of oxaloacetate from phosphoenolpyruvate and carbon dioxide.

2. Oxaloacetate reduces to malic acid (a C4 compound) in the presence of the malate dehydrogenase enzyme.

3. The malic acid was then transferred to the bundle sheath cell through plasmodesmata.

4. In the bundle sheath cell, malic acid is decarboxylated in the presence of the malic enzyme and releases carbon dioxide.

5. By the decarboxylation of malic acid, a 3-carbon compound, i.e., pyruvate, is formed. Carbon dioxide enters the Calvin cycle, and pyruvate returns back to the mesophyll cells.

6. In the mesophyll cell, pyruvate, in the presence of the pyruvate phosphate dikinase enzyme, is converted into phosphoenolpyruvate, and the cycle repeats.