Final answer:
The Calvin cycle relies on ATP and NADPH from light reactions, which are produced through non-cyclic and cyclic electron flow to balance their supply for glucose synthesis.
Step-by-step explanation:
The reactions of the Calvin cycle require a switch between non-cyclic and cyclic electron flow during the light reactions because the reaction center chlorophyll a molecules need their electrons replaced after excitation. Non-cyclic photophosphorylation, using two photosystems, is the primary method in plants and algae to replace these electrons and generate the chemical energy carriers ATP and NADPH. Those carriers are then used in the Calvin cycle to fix carbon and assemble sugar molecules, with glucose as the final product.
However, at times when the light reactions produce more ATP than NADPH, plants may switch to cyclic electron flow to balance their ATP and NADPH production. This cyclic flow only involves photosystem I and results in the production of ATP without the generation of NADPH or the release of oxygen. This flexibility ensures that the Calvin cycle has a balanced supply of ATP and NADPH for synthesizing glucose from CO₂.