AP BIOLOGY

Detailed explanation-1: -The enzyme that catalyzes this CO2 fixation is PEP carboxylase or PEPcase. The mesophyll cells of C4 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.

Detailed explanation-2: -The carbon-fixation pathway begins in the mesophyll cells, where carbon dioxide is converted into bicarbonate, which is then added to the three-carbon acid phosphoenolpyruvate (PEP) by an enzyme called phosphoenolpyruvate carboxylase.

Detailed explanation-3: -(1) An active, photosynthetically‐driven, CO2 capture system In C4 plants this role is taken by PEP carboxylase operating in the cytosol of the specialized mesophyll cells. A specialized C4 isoform of PEP carboxylase is expressed in very high amounts (c.

Detailed explanation-4: -The enzyme ribulose bisphosphate carboxylase oxygenase or RuBisCO catalyse the carboxylation of RuBP. It is the first step of the Calvin cycle and the carbon fixation reaction into stable organic intermediate 3-phosphoglyceric acid or PGA.

Detailed explanation-5: -In photosynthetic tissues, PEPC participates in the exchange of CO2 and water between a leaf and the atmosphere through its role in guard cell metabolism. Additionally, in C4 species PEPC catalyzes the first carboxylation reaction of C4 photosynthesis in the mesophyll cells (Kanai and Edwards, 1999).