Final answer:
To regenerate RuBP in the Calvin cycle, three turns of the cycle are needed, producing enough G3P molecules, five of which are used to regenerate three RuBP molecules.
Step-by-step explanation:
The Calvin cycle, a part of photosynthesis that occurs in the stroma of chloroplasts, is responsible for converting carbon dioxide into organic molecules. For the cycle to function continually, the molecule ribulose-1,5-bisphosphate (RuBP) must be regenerated. Each turn of the Calvin cycle fixes one molecule of carbon dioxide, and after three turns, enough glyceraldehyde-3-phosphate (G3P) molecules are produced to regenerate RuBP. However, five out of six G3P molecules produced during these three turns are used to regenerate three molecules of RuBP. Thus, it takes three turns of the Calvin cycle to produce enough G3P to regenerate the RuBP needed for the cycle to continue.