Question

Choose the correct sequence of steps occuring in the $C_4$ pathway.

(A) Decarboxylation of malic acid

(B) Carboxylation of PEP

(C) Reduction of OAA

(D)Phosphorylation of pyruvate

• A. (B), (C), (A), (D)
• B. (A), (B), (C), (D)
• C. (C), (A), (B), (D)
• D. (D), (B), (C), (A)

Answer 1

The correct option is A (B), (C), (A), (D)

In the $C_4$ pathway, phosphoenol pyruvate (PEP) accepts carbon dioxide and forms oxaloacetic acid (OAA) in mesophyll cells. OAA is then reduced to form to malic acid, which is transported to bundle sheath cells. In the bundle sheath cells, malic acid is decarboxylated to form pyruvic acid and carbon dioxide. This carbon dioxide enters the calvin cycle. The pyruvic acid is sent back to the mesophyll cells, where it is phosphorylated to regenerate PEP.