Calvin cycle must run 6 rounds to make 1 glucose. How many rounds are required?

• A. 4
• B. 5
• C. 6
• D. 8

Answer: (C)

In the Calvin cycle, each turn fixes one molecule of CO2 and, overall, every three CO2 fixed yield one net molecule of glyceraldehyde-3-phosphate (G3P) that can exit the cycle. To build glucose, you need two G3P molecules ( glucose is a 6-carbon sugar built from two 3-carbon units). Since only one G3P exits after every three CO2 fixed, you need six CO2 fixed to obtain two G3P. A large portion of the G3P produced is then used to regenerate the cycle’s RuBP, not to make glucose, which is why you can’t get glucose from just a few turns. Therefore, six rounds of the Calvin cycle are required to synthesize one glucose molecule.