As you can see in step ➊, carbon fixation, the enzyme rubisco attaches CO2 to RuBP . (Recall that carbon fixation refers to the initial incorporation of CO2 into organic com-pounds.) This unstable six-carbon molecule splits into two three-carbon molecules. In step ➋, reduction, ATP and NADPH are used to reduce the three-carbon molecule to G3P .
For this to be a cycle, RuBP must be regenerated. In step ➌, release of one molecule of G3P , you can see that for every three CO2 molecules fixed, one G3P molecule leaves the cycle as product. In step ➍, regeneration of RuBP , the remaining five G3P molecules are rearranged, using energy from ATP , to regenerate three molecules of RuBP .
For the synthesis of one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH molecules, which were pro-vided by the light reactions. Neither the light reactions nor the Calvin cycle alone can make sugar from CO2. Photosynthesis is an emergent property of the structural organization of a chloro-plast, which integrates the two stages of photosynthesis.