Here's how it works:
1. Carbon Fixation: The Calvin cycle begins with the enzyme rubisco attaching carbon dioxide to RuBP, forming a six-carbon unstable intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA).
2. Reduction: 3-PGA is then reduced to G3P using ATP and NADPH.
3. Regeneration: For every six molecules of CO2 fixed, the Calvin cycle produces 12 molecules of G3P. Only one molecule of G3P exits the cycle to be used for carbohydrate synthesis. The remaining 11 molecules are used to regenerate RuBP, the starting molecule of the cycle.
4. RuBP Regeneration: A complex series of reactions involving several enzymes uses those 11 G3P molecules to regenerate 6 molecules of RuBP. This regeneration is crucial for the continued operation of the Calvin cycle.
Therefore, G3P plays a central role in the Calvin cycle, not only as a product of carbon fixation and reduction but also as the precursor for the regeneration of RuBP, ensuring the cycle can continue to fix carbon dioxide.
