The NADP+ and ADP formed in this process return to the thylakoids to regenerate NADPH and ATP in the light reactions.
The final stage in the Calvin cycle is the regeneration of the CO2 acceptor RuBP. This involves a series of reactions that convert triose phosphate first to the 5-carbon intermediate Ru5P (ribulose 5- phosphate), then phosphorylate Ru5P to regenerate RuBP (ribulose-bisphosphate). This final step requires ATP formed in the light reactions.
Overall, for every three turns of the cycle one molecule of product (triose phosphate) is formed (3CO2:1G3P). The remaining 15 carbon atoms (5 G3P) re-enter the cycle to produce three molecules of RuBP.
The triose phosphate formed in the Calvin cycle can remain in the chloroplast where it is converted to starch. This is why chloroplasts form starch grains. Alternatively, triose phosphate can be exported from the chloroplast where it is converted to carbohydrates in the cytoplasm. Both reactions involve the release of phosphate. In the case of carbohydrates, the phosphate must be returned to the chloroplast to support continued photophosphorylation (ATP formation).
The net energy balance of six rounds of the Calvin cycle to produce one mole of hexose is thus:
6CO2 + 18ATP + 12NADPH + 12H2O → C6H12O6 + 18ADP + 18Pi + 12NADP+ + 6H+ (3:4)
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