Step in photosynthesis in which carbon dioxide is attached to 5-carbon sugar and then reduced to other sugars. The first part of the cycle involves the reduction of ribulose 5-phosphate to 2 molecules of glyceraldehyde-3-phosphate. The second part of the cycle involves carbohydrate rearrangements that allow regeneration of pentose phosphate and a net synthesis of carbohydrate. The net reaction is 6CO2 + 12NADPH + 18ATP 8 2 triose phosphate + 12NADP+ + 18ADP + 18Pi