Dark reaction or Calvin cycle or C_3.

                                In the second phase of photosynthesis which is independent of light. The ATP and NADPH  are produced by the light reaction utilized in the dark reaction to reduce carbon dioxide to carbohydrate by a process called carbon fixation. It occurs in stoma of chloroplast. Dark reaction was discovered by calvin in 1954. Therefore, it is called calvin cycle or C₃ cycle. Dark reaction can be studied under three headlines.

1.Carboxylation:

              In this step, atmospheric CO₂ accepted by RUBP (Ribulose 1,5-diphosphate) in the presence of enzyme Rubisco. The three molecules of RUBP react with 3 molecules of CO₂ to form 3 molecules of 6 carbon unstable compound which is converted into PGA. PGA ( 3 carbon compound) is the first stable product. So, it is called C₃ cycle.

RUBP + CO₂ ————–> 6 carbon stable compound.

6 carbon unstable compund + H₂O  ———>  PGA (Phosphoglyceric acid)

2. Glycolytic reversal :

                         This process involves conversion of three PGA to glucose molecules in many steps. This Process is the reverse of the oxidation step in glycolysis. So, called glycolytic reversal. It involves following steps:

a) 3 PGA + ATP 1,3- diphosphoglyceric acid + ADP

b) 1,3- diphosphoglyceric acid

c) 3- Phosphoglyceraldehyde dihydroxy acetone-3-phosphate (DHAP).

d) DHAP + PGAL Fructose-1,6-diphosphate

e) Fructose-1,6-diphosphate + H₂O Fructose-6-Phosphate + H₃PO₄

f) Fructose-6-phosphate Glucose-6-phosphate

g) Glucose-6-phosphate Glucose + starch

3. Regeneration of RUBP

              Fructose-6-phosphate and 3-glyceraldehyde phosphate react to form erythose-6-phosphate and xyulose-5-phosphate. The erythose phosphate and dihydroxy acetone phosphate combine to form sedoheptolase-7-phosphate which regenerate RUBP in many reactions.

Fig: C₃ cycle or calvin cycle or dark reaction