Calvin cycle is a series of reactions that lead to the formation of glucose. The steps involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per
${{CO}{2}}$ molecule fixed. The fixation of six molecules of
${{CO}{2}}$ and six turns of the cycle are required for the removal of one molecule of glucose from the pathway.
Hence, for every ${{CO}_{2}}$ molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. It is. probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place. To make one molecule of glucose six turns of the cycle are required.