Dark reactions:

The dark reactions (also called the carbon-fixation reactions) use the NADPH and ATP produced through the light reactions to convert carbon dioxide into carbohydrate. The last products are starch and sucrose.

The key carbon fixation reaction is catalyzed through a huge enzyme known as ribulose bisphosphate carboxylase (frequently abbreviated to rubisco) which is situated in the stroma. The reaction condenses a carbon die oxide molecule with ribulose 1, 5- bisphosphate (a five-carbon molecule) to produce a transient six-carbon intermediate which rapidly hydrolyzes to two molecules of 3-phosphoglycerate shown in the figure:

Figure:  The rubisco reaction.

Rubisco is a very slow enzyme, fixing only three molecules of its substrate every second and thus a huge amount of this enzyme is required through each plant. Classically, rubisco accounts for 50 percent or so of the total protein in a chloroplast. Certainly, it is possibly the most abundant protein on earth!

The rubisco reaction forms element of a cycle of reactions that is known as the Calvin cycle, which leads to the reproduction  of ribulose 1,5-bisphosphate  (ready to fix other CO₂) and the net production  of glyceraldehyde  3-phosphate  for the synthesis of sucrose and starch. There are three molecules of CO₂ that must be fixed to generate one molecule of glyceraldehyde 3-phosphate (a three-carbon molecule).  The whole reaction for this is:

3CO₂+6NADPH+9ATP→glyceraldehyde 3-phosphate+6NADP⁺+9ADP+8Pi

Figure:   The Calvin cycle.

The several reactions are shown in Figure. Following the creation of six molecules of 3-phosphoglycerate, six NADPHs and six ATPs are used to produce six molecules of glyceraldehyde 3-phosphate.   Simply one of these molecules of glyceraldehyde 3-phosphate is then used to create fructose 6-phosphate and, ultimately, starch. The  another  five  molecules   of  glyceraldehyde 3-phosphate are transformed  in a series  of steps (including  transketolase, aldolase and five other enzymes) to three molecules of ribulose 5-phosphate  that are then phosphorylated  (using  three  molecules  of ATP)  to create  three  molecules  of ribulose  1,5- bisphosphate,  prepared for another  turn of the cycle. Therefore one complete turn of the cycle, trapping the three molecules of CO₂ as one molecule of glyceraldehyde  3- phosphate, requires 6+3= 9 ATPs and 6 NADPHs.