Question

The enzyme that catalyses carbon dioxide fixation in $C_4$ plants is

• A. RuBP carboxylase
• B. PEP carboxylase
• C. Carbonic anhydrase
• D. Carboxydismutase

Answer 1

Answer: (B)

PEP carboxylase

Most of the plants that are adapted to dry tropical regions have the $C_4$ pathway. These plants are called $C_4$ plants. Sugarcane, Maize, Sorghum, Corn, etc., are the examples of these plants. Carbon fixation in these plants occurs through Hatch and Slack pathway. It is also known as $C_4$ pathway. It is a cyclic process. It occurs in chloroplasts. It was discovered by two Australian scientists Hatch and Slack. Carbon dioxide fixation is done in this pathway. The primary $C{O}_2$ acceptor is a three-carbon molecule phosphoenolpyruvate (PEP) and it is present in mesophyll cells. The enzyme that catalyzes this $C{O}_2$ fixation is PEP carboxylase or PEPcase. The mesophyll cells of $C_4$ plants lack the enzyme RuBisCO. The 4-carbon oxaloacetic acid (OAA) is formed in the mesophyll cells which is further converted into the malic acid or aspartic acid and then transported into bundle sheath cells. In the bundle sheath cells, these $C_4$ acids are broken down to release $C{O}_2$ and a three-carbon molecule.