Question

If four molecules of carbon dioxide enter the Calvin cycle (four "turns" of the cycle), how many G3P molecules are produced and how many are exported?

a) 8 G3P molecules produced, 2 exported
b) 4 G3P molecules produced, 2 exported
c) 6 G3P molecules produced, 4 exported
d) 2 G3P molecules produced, 4 exported

Answer 1

Eight G3P molecules are produced when four molecules of carbon dioxide enter the Calvin cycle, and one G3P molecule is exported out of these.

Step-by-step explanation:

If four molecules of carbon dioxide enter the Calvin cycle, resulting in four "turns" of the cycle, each turn of the Calvin cycle produces two G3P molecules. Thus, four turns will produce a total of eight G3P molecules. However, not all of these G3P molecules are exported. According to the information provided, for every three turns of the Calvin cycle, only one G3P molecule is exported, since it takes three turns to fix enough net carbon to export one G3P. Therefore, in four turns, you would expect more than one G3P to be exported, but less than two, as it would take six turns to export two G3P molecules. Given the options presented, the closest correct answer would be that eight G3P molecules are produced and one G3P molecule is exported (as the next turn will contribute to the second G3P being exported).