The Calvin/Benson cycle and biosynthesis:
The dark reaction is where the energy captured in the two photosystems from light in the light reactions is used to form carbohydrates and other organic compounds from carbon dioxide. This involves the reduction or "fixing" of carbon. This is the second half of our half reaction : 4nH+ + 4ne- +nCO2 -> (CHO)n + nH20.
The carbohydrates produced in this reaction make up the plant and serve as a way to store the energy captured in the light reaction in the form of chemical bonds. These bonds can then be broken in cellular respiration to produce energy in the form of ATP. Additionally these chain may be used in the production of micelles for walls, or converted into all the macromolecules needed by the plant.
Essential CO2 diffuses into the chloroplast stroma and is incorporated into a 5C sugar with two phosphate groups called RuBP. The enzyme that catalyzes this reaction is called rubisco and is probably the most abundant protein on earth. It makes up 16-25% of the soluble proteins in leaves. The diagrams of RUBISCO below come from the excellent German site:http://www.rrz.uni-hamburg.de/biologie/b_online/e42/11.htm.
Each molecule is made up of 8 large and 8 small subunits. This molecule has remained genetically conservative throughout evolution suprisingly considering its inefficiency under low CO2 conditions ( we'll address this consideration later)....
The reaction of CO2 with RuBP creates an unstable 6C compound that breaks into two 3C molecules 3-PGA.
A phosphate group is then cleaved from each molecule of 3-PGA, and ATP and NADPH from the light reactions are used to reduce the resulting molecule to glyceraldehyde-3-phosphate (G3P).
The unstable phosphate bonds of 1, 3-diphosphoglyceric acid prime this compound for the addition of high-energy electrons donated from NADPH.
This redox reaction reduces 1, 3-diphosphoglyceric acid to glyceraldehyde phosphate (the reaction is driven by hydrolysis of the phosphate bond transferred to the substrate from ATP in the previous reaction).
Electrons from NADPH reduce the carboxyl group of 3-phosphoglyceric acid to the carbonyl group of glyceraldehyde phosphate which stores more potential energy.
The diagram below from the German site:http://www.rrz.uni-hamburg.de/biologie/b_online/e42/11.htm. gives you an idea of further conversions
This series of reactions requires energy from the light reactions.