The Calvin cycle is a metabolic pathway found in the stroma of the chloroplasts of plants. In this process, carbon enters in the form of carbon dioxide and leaves in the form of sugar. There are three main stages in the Calvin cycle. In the first, rubisco attaches carbon to RuBP. In the second, NADPH and ATP convert 3-PGA to G3P. In stage three, 5 G3P and 3 ATP regenerate 3 RuBP. The Calvin cycle occurs in the plant above.