Final answer:
The 5-carbon molecule Ribulose-1,5-bisphosphate (RuBP) is regenerated from Glyceraldehyde-3-phosphate (G3P) during the third stage of the Calvin Cycle, enabling the cycle to continue and prepare for more CO₂ fixation.
Step-by-step explanation:
The regeneration of the 5-carbon molecule needed in step 1 of the Calvin Cycle is accomplished by the synthesis of Ribulose-1,5-bisphosphate (RuBP) from Glyceraldehyde-3-phosphate (G3P). This process occurs during the third step of the Calvin Cycle, where five G3P molecules (15 carbons in total) are used to regenerate three molecules of RuBP (also 15 carbons). This regeneration is crucial for the cycle to continue, as it allows the system to prepare for more CO₂ to be fixed. It requires ATP energy, which is provided by the light reactions of photosynthesis.
During each turn of the Calvin Cycle, carbon dioxide is fixed into a 3-Carbon molecule, 3-PGA, which is further reduced to G3P using electrons from NADPH. Out of the six G3P molecules produced after three turns of the cycle, only one is exported to form carbohydrates like glucose, while the remaining five G3P molecules are used to regenerate RuBP. In this process, three more molecules of ATP are used to power the regeneration reactions.