Question

Describe the mechanism of Hatch and Slack pathway in $C_4$ plants.

Answer 1

Hatch and Slack pathway is a cyclic process. The primary $C{O}_2$ acceptor is a three-carbon molecule phosphoenolpyruvate (PEP) and it is present in mesophyll cells. The enzyme that catalyses this $C{O}_2$ fixation is PEP carboxylase or PEPcase. The mesophyll cells pf $C_4$ plants lack the enzyme RuBisCO. The 4-carbon oxaloacetic acid (OAA) is formed in the mesophyll cells. It is then converted to other four-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, these are then transported to the bundle sheath cells. In the bundle sheath cells, these $C_4$ acids are broken down to release $C{O}_2$ and three-carbon molecule. The $C{O}_2$ released in the bundle sheath cells enters the $C_3$ or the Calvin pathway. The bundle sheath cells are rich in an enzyme RuBisCO but lack PEPcase. The three-carbon molecule is transported back to the mesophyll cells where it is converted to PEP again, thus completing the cycle.