Final answer:
The Calvin cycle takes three turns to produce one net G3P molecule because each turn fixes only one molecule of CO2, and one exported G3P is a net gain with five remaining G3Ps needed to regenerate RuBP.
Step-by-step explanation:
The Calvin cycle is the process plants use to fix carbon dioxide from the atmosphere into organic molecules. One essential aspect of this cycle is the production of glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that is the precursor for glucose and other carbohydrates in plants. It takes three turns of the Calvin cycle to produce one G3P because each turn of the cycle incorporates one molecule of CO2, producing two molecules of 3-phosphoglycerate (3-PGA), which are then converted into a pair of G3P molecules. However, only one G3P can be counted as a net gain because the remaining G3Ps are recycled to regenerate the five-carbon molecule ribulose 1,5-bisphosphate (RuBP) which is necessary for the cycle to continue. This regeneration requires further input of energy in the form of ATP.
Therefore, to summarize, each turn of the Calvin cycle fixes one molecule of CO2, producing two molecules of G3P, three turns are needed to produce three CO2 and six G3P in total, with one G3P being exported and the rest used to regenerate RuBP. Additionally, ATP is consumed in the process of regeneration.
In the context of a broader photosynthetic process, this constant regeneration and production is vital for the continuous synthesis of glucose and other carbohydrates that are crucial for plant growth and energy storage.