Discussion
Which one of the following molecules captures CO2 in the C4 cycle?
Answer: C
The CO2 acceptor of the C4 cycle is phosphoenolpyruvate.
In the mesophyll cells of C4 plants, PEP carboxylase adds bicarbonate (from CO2) to phosphoenolpyruvate to form the four-carbon oxaloacetate. The C4 acid is then carried to the bundle sheath, decarboxylated, and the released CO2 enters the Calvin cycle there.
Oxaloacetate is the product of that fixation, not the acceptor. Ribulose-1,5-bisphosphate is the acceptor of the C3 (Calvin) cycle, carboxylated by Rubisco. 1,3-Bisphosphoglycerate is an intermediate of glycolysis and of the Calvin cycle's reduction phase and captures no CO2.
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