Calvin cycle or dark reaction of photosynthesis in which fixation of carbondioxide occurs to form glucose. It occurs in three steps carboxylation, reduction and regeneration. In carboxylation product of light reaction ATP and ${\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}\mathrm{H}}_2$ are not consumed, it is enzyme dependent process in which $\mathrm{C}{\mathrm{O}}_2$ is trapped into organic form.