[Campbell Biology P.250] During the Calvin cycle, three molecules of CO2 enter the cycle. Ba... | Practice Question
During the Calvin cycle, three molecules of CO2 enter the cycle. Based on the information provided, what is the fate of the carbon atoms from these three CO2 molecules in terms of net carbohydrate gain and regeneration?
A: All three CO2 molecules are incorporated into a single molecule of glucose, which then exits the cycle.
B: The carbons from three CO2 molecules combine with three RuBP molecules to form six molecules of G3P, with one G3P molecule being a net gain for carbohydrate synthesis and the other five regenerating RuBP.
C: Three molecules of CO2 are reduced to form three molecules of G3P, all of which exit the cycle as net carbohydrate.
D: Each CO2 molecule attaches to RuBP, forming a stable six-carbon intermediate that directly becomes G3P.
E: Rubisco catalyzes the formation of 1,3-bisphosphoglycerate from CO2 and RuBP, which then receives electrons from ATP to form G3P.
Explanation
The text explains that for every three molecules of CO2 that enter the cycle, six molecules of G3P are formed. However, only one molecule of G3P can be counted as a net gain for carbohydrate synthesis; the other five molecules are recycled to regenerate the three molecules of RuBP, ensuring the cycle continues.