Calvin cycle or C3 cycle - Biosynthesis phase

• The Calvin cycle is associated with the dark reaction or biosynthesis phase of photosynthesis.

• During the calvin cycle, molecules like ATP and NADPH are used  to form glucose and other carbohydrate molecules. In Autotrophs, carbon dioxide is entered into the stroma of chloroplast. 

👌👌Remember - The light and dark  reaction of photosynthesis takes place in Grana and stroma of chloroplast respectively.

• Therefore stroma is the site of the dark  reactions where glucose is  synthesized by using carbon dioxide.

• Malvin Calvin found that carbon dioxide is fixed to form the glucose in plants.

• Calvin worked about the fixation of carbon dioxide  and described a cyclic pathway  by which carbon dioxide is fixed and stated that this  pathway is operated in a cyclic manner. 

• The Calvin cycle takes place in all photosynthetic plants. During Calvin cycle , first product is formed containing  three carbon atoms hence it is called C3 Pathway.

• The Calvin cycle can be understood by the  three steps - carboxylation, reduction and regeneration. 

Carboxylation or Fixation - 

• It  is the fixation of carbon dioxide. Carboxylation is the most important step of the Calvin cycle where CO2 is used  for the carboxylation of Ribulose bi phosohate.

• This reaction is catalysed by the enzyme Ribulose bisphosphate  carboxylase. At the end of this reaction, two molecules of 3 phosphoglyceric acid are formed.

• This enzyme  has an oxygenation activity  therefore it is called Ribulose bisphosphate carboxylase-oxygenase or RuBisCO. Ribulose bisphosphate has five atoms of carbon. RuBisCO  catalyzes a reaction between Carbon dioxide and RuBP.

• For each carbon dioxide molecule that reacts with one RuBP, two molecules of 3-phosphoglyceric acid  are formed. It has three carbons and one phosphate.

• The Calvin cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3 Phosphoglyceric acid.

Reduction

• The six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate. This step involves the use of six molecules of  ATP and NADPH.

• During this reaction, The phosphoglyceric acid gains the  electron and we know that reduction is the gain of an electron by an atom or molecule. Hence it is called reduction.

• Six molecules of both ATP and NADPH are used. By losing one molecule of phosphate, ATP is changed into to ADP whereas  when  NADPH is converted into NADP+

• Both energy and a hydrogen atom are lost. Both of these molecules return to the  light-dependent reactions to be reused.

Regeneration

• The glyceraldehyde 3 phosphate that is formed during the reduction stage of the Calvin cycle. Out of these molecules, one  molecule of glyceraldehyde  leaves the Calvin cycle and is sent to the cytoplasm for the  formation of other compounds required  by the plant.

• One is exported  from stroma to cytoplasm while the remaining molecules of glyceraldehyde 3 phosphate  remain in the cycle and are used to regenerate Ribulose bi phosphate for the continuation of calvin cycle.

• Three more molecules of ATP are used in these regeneration reactions.

Essential of Calvin Cycle.

• When one   molecule of carbon dioxide enters into the Calvin cycle, three  molecules of ATP and two molecule  of NADPH are required.  For the formation of  one molecule of glucose 6 turns of the  calvin cycle are required. 

• Therefore for the formation of one molecule of glucose ,  Six molecule of carbon dioxide,  Eighteen molecule of ATP and twelve molecule of  NADPH  are used during the Calvin cycle.