Answer :
Final answer:
Carbon dioxide (CO₂) must continually enter the Calvin cycle for the light-independent reactions to occur. It is initially fixed to RuBP by the enzyme RuBisCO to produce 3-PGA, which then participates in further reactions of the cycle.
Explanation:
The molecule that must enter the Calvin cycle continually for the light-independent reactions to take place is CO₂ (carbon dioxide). The Calvin cycle has three stages, starting with carbon fixation, where the enzyme RuBisCO catalyzes the addition of CO₂ to ribulose bisphosphate (RuBP). This results in the production of two molecules of 3-phosphoglycerate (3-PGA). The subsequent stages involve the reduction of these molecules to create glyceraldehyde-3-phosphate (G3P) using electrons from NADPH, and finally, the regeneration of RuBP so that the cycle can continue.
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