Final answer:
The Calvin cycle must turn three times to yield a single molecule of glyceraldehyde 3-phosphate (G3P), with each turn fixing one molecule of CO2.
Step-by-step explanation:
The Calvin cycle must turn three times for the fixation of three molecules of CO2 to yield a single molecule of glyceraldehyde 3-phosphate (G3P). Each turn of the cycle incorporates one molecule of CO2. Therefore, three molecules of CO2 are required to produce a three-carbon molecule of G3P.
This explains why it takes three turns of the Calvin cycle to produce the initial photosynthetic product of G3P. The process of carbon fixation in plants during the Calvin cycle is an integral part of photosynthesis, where the energy stored in ATP and NADPH is utilized to convert CO2 into organic compounds.
The correct option is, confirming the number of turns required to produce one molecule of G3P, is: b. 3