Answer :
Final answer:
It takes three turns of the Calvin cycle to produce one net G3P because each turn produces two G3Ps but only one can be exported for carbohydrate synthesis, with the rest used to regenerate RuBP requiring additional ATP.
Explanation:
The reason it takes three turns of the Calvin cycle to produce one net glyceraldehyde-3-phosphate (G3P) is because during each cycle, three molecules of carbon dioxide (CO₂) combine with RuBP, and through a series of reactions, produce two G3Ps. However, only one G3P can be counted as a net gain, since the majority of the G3P molecules are utilized to regenerate RuBP, which is essential for the cycle to continue. Specifically, each turn of the cycle produces two molecules of G3P, thereby three cycles produce a total of six G3Ps.
Out of these six, five are used to regenerate three molecules of RuBP and one G3P is exported from the cycle for the formation of carbohydrates, such as glucose. Regenerating RuBP requires additional ATP, thus consuming more of the cell's energy resources.