Explain Calvin cycle of photosynthesis. Give the schematic representation of Calvin cycle.

Question

Explain Calvin cycle of photosynthesis.

OR

Describe the essential steps of dark reaction of photosynthesis.

OR

Give the schematic representation of Calvin cycle.

Write the schematic representation of C₃-pathway. 

Answer 1

Calvins cycle or dark reaction or C₃ cycle or thermochemical reaction forms the second step of photosynthesis which takes place independent of light. The steps involved here studied by Calvin using C_14. Dark reaction mainly includes the fixation of carbon to produce the carbohydrate. This occurs in the stroma and utilises the assimilatory powers produced during the light reaction.

The steps involved may be summarised as follows.

(1) First Phosphorylation:

The starting compound RuMP(Ribulose Mono Phosphate) is converted to RuBP(Ribulose biphosphate) utilising 6 molecules of ATP from the light reaction. 6 molecules of RuBP are obtained from 6 molecules of RuMP.

(2) Carbon fixation by RUBP:

The CO₂ is accepted by RuBP the primary acceptor to give 12 molecules of PGA(phosphoglycerate) a 3-carbon compound; The 6-carbon compound is highly unstable and dissociates to form the 3-carbon compound.

(3) Second Phosphorylation:

The 12 molecules of PGA utilise 12 ATP molecules to yield 12 Di PGA. The ATP used is produced during the light reaction.

(4) Reduction:

12 Di PGA combine with NADPH₂ of the light reaction and are reduced to 12 PGAL(phosphoglyceraldehyde) a-3-carbon compound. The other products are 12iP, 12NADP and H₂O. Out of the 12 PGAL molecules only 2 are transported to the cytoplasm and used for the formation of a sugar molecule (hexose) and the remaining two molecules are used to regenerate RuBP. 

Synthesis of sugar involves condensation of 2PGAL to give fructose 1-6- diphosphate which by dephosphorylation forms Fructose 6-Phosphate and by isomerization forms, glucose and finally is stored as sucrose.

Regeneration of RuBP involves the formation of intermediates like Erythrose monophosphate, xylulose monophosphate, Sedoheptulose monophosphate, Ribose monophosphate and finally Ribulose monophosphate-RuMP which gives RuBP. The overall equation of dark reaction is

6CO₂ + 6RuBP + 18 ATP + 12 NADPH₂ → C₆H₁₂O₆ +6RuBP + 18ip + 18ADP + 12 NADP