The Calvin cycle-the so-called dark reactions of photosynthesis-does not occur solely in the dark. In fact, the dark reactions are stimulated by light, but do not directly use the energy of light to function. Instead, they use the NADPH and ATP generated by the light reactions to fix atmospheric carbon dioxide into carbohydrates.

The dark reactions occur in the stroma of the chloroplast and are shown schematically in Figure 17.20. The cycle can be viewed as occurring in two stages. In stage I, carbon dioxide is trapped as a carboxylate and reduced to the aldehyde-ketone level found in sugars. In stage II, the molecule that accepts CO2 is regenerated. Note with each turn of the cycle in Figure 17.20 that one glyceraldehyde-3-phosphate exits the cycle to be made into hexose phosphates (such as glucose-6-phosphate) and other sugar moieties. Thus, the end products of the Calvin cycle are hexoses and a regenerated acceptor molecule.

Stage 1: The initial reaction, in which CO2 is first incorporated into ribulose-1,5-bisphosphate (RuBP), is shown here. This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase, more commonly known as rubisco. The end product of this reaction is two molecules of 3-phosphoglycerate, a gluconeogenesis and glycolysis intermediate.

Next, 3-phosphoglycerate is converted to 1,3 bisphosphoglycerate (BPG) and then to glyceraldehyde-3-phosphate (G3P) by the enzymes phosphoglycerate kinase and glyceradehyde-3-phosphate dehydrogenase, respectively. These two reactions are similar to the analogous ones that occur in gluconeogenesis.

To this point, one molecule of CO2 has been incorporated and two G3Ps have been made at the expense of 2 ATPs and 2 NADPHs from the light reactions. Because it takes 6 CO2 to make one complete glucose molecule, it takes 12 ATP and 12 NADPH to incorporate a complete glucose from CO2. Six CO2 molecules will also generate 12 G3Ps at this point (Figure 17.21). Two of these are used to make one glucose-phosphate compound (via gluconeogenesis) and the other 10 are used to regenerate the 6 molecules of RuBP that are necessary to bind to 6 CO2 molecules. Of the 10 G3Ps involved in regeneration of RuBP, 4 go through part of the gluconeogenesis cycle and form 2 molecules of fructose-6-phosphate (F6P).

Stage II: Many of the reactions of the Calvin cycle also occur in the pentose phosphate pathway. The two F6P molecules from the last step of stage I react with the remaining 6 G3Ps as shown in Figure 17.22. Two of the G3Ps are isomerized to dihydroxyacetone phosphate (DHAP). As shown in Figure 17.22, the enzymes transketolase, aldolase, phosphatase, and transketolase (again) generate intermediates after combining the G3Ps and DHAPs together. These intermediates include 4-xylulose-5-phosphates and 2 ribulose-5-phosphates. Conversion of the 4 xylulose-5-phosphates to 4 ribulose-5-phosphates occurs and the 6 ribulose-5-phosphates are phosphorylated by kinases to regenerate the 6 RuBPs. This last reaction requires 1 ATP per molecule converted, or a total of 6 ATPs. The overall equation for the Calvin cycle is shown in here. Note that complete synthesis of one molecule of glucose requires 18 ATPs and 12 NADPHs.

1. CO2 Fixation in Bacteria

2. CO2 Fixation in Plants