How many CO2 molecules are fixed to synthesize one hexose (glucose) in the Calvin cycle, and how many turns are required?

• A. Two CO2 molecules fixed; two turns.
• B. Two CO2 molecules fixed; six turns.
• C. Six CO2 molecules fixed; eight turns.
• D. Six CO2 molecules fixed; six turns are required to produce one glucose.

Answer: (D)

Carbon fixation stoichiometry in the Calvin cycle shows how input CO2 translates into a six-carbon sugar. Three CO2 molecules fixed through Rubisco lead to one molecule of glyceraldehyde-3-phosphate (a three-carbon product) after the cycle’s steps. To build glucose, you need two of these three-carbon units, totaling six carbons. Therefore six CO2 molecules must be fixed, and the cycle must run six times to produce two G3P molecules that can be combined into one glucose. The process also uses ATP to regenerate the starting molecule RuBP, allowing continued CO2 fixation.