Dark reaction or Blackman’s reaction or Path of carbon in photosynthesis: Calvin Cycle or C3 Cycle

 

Dark reaction or Blackman’s reaction or Path of carbon in photosynthesis

This is the second step in the mechanism of photosynthesis. The chemical processes of photosynthesis occurring independent of light is called dark reaction. It takes place in the stroma of chloroplast. The dark reaction is purely enzymatic and it is slower than the light reaction. The dark reactions occur also in the presence of light. In dark reaction, the sugars are synthesized from CO₂. The energy poor CO₂ is fixed to energy rich carbohydrates using the energy rich compound, ATP and the assimilatory power, NADPH₂ of light reaction. The process is called carbon fixation or carbon assimilation. Since Blackman demonstrated the existence of dark reaction, the reaction is also called as Blackman’s reaction. In dark reaction two types of cyclic reactions occur

1.      Calvin cycle or C3 cycle

2.      Hatch and Slack pathway or C4 cycle

 

 

Calvin cycle or C3 cycle

It is a cyclic reaction occurring in the dark phase of photosynthesis. In this reaction, CO₂ is converted into sugars and hence it is a process of carbon fixation. The Calvin cycle was first observed by Melvin Calvin in chlorella, unicellular green algae. Calvin was awarded Nobel Prize for this work in 1961. Since the first stable compound in Calvin cycle is a 3 carbon compound (3 phosphoglyceric acid), the cycle is also called as C3 cycle. The reactions of Calvin’s cycle occur in three phases.

1.      Carboxylative phase

2.      Reductive phase

3.      Regenerative phase

 

1.  Carboxylative phase

Three molecules of CO₂ are accepted by 3 molecules of 5C compound viz., ribulose diphosphate to form three molecules of an unstable intermediate 6C compound. This reaction is catalyzed by the enzyme, carboxy dismutase

 

3 CO₂    +        3 Ribulose diphosphate  ----------------  3 unstable intermediate 6 carbon compound

                                                  *With the help of Carboxy dismutase

 

The three molecules of the unstable 6 carbon compound are converted by the addition of 3 molecules of water into six molecules of 3 phosphoglyceric acid. This reaction is also catalyzed by the enzyme carboxy mutase.

 

3 unstable intermediate 6 C       compound +  3 H₂O   ----------  3 phosphoglyceric acid

     

 

3 phosphoglyceric acid (PGA) is the first stable product of dark reaction of photosynthesis and since it is a 3 carbon compound, this cycle is known as C3 cycle.

2.  Reductive phase

Six molecules of 3PGA are phosphorylated by 6 molecules of ATP (produced in the light reaction) to yield 6 molecules of 1-3 diphospho glyceric acid and 6 molecules of ADP. This reaction is catalyzed by the enzyme, Kinase

 

                                                                    Kinase                            

     3    Phospho glyceric acid +    ATP     -----------------     1,3 diphospho glyceric acid  +      ADP  

Six molecules of 1, 3 diphosphoglyceric acid are reduced with the use of 6 molecules of NADPH₂ (produced in light reaction) to form 6 molecules of 3 phospho glyceraldehyde. This    reaction is catalysed by the enzyme, triose phosphate dehydrogenase.

           

3. Regenerative phase

In the regenerative phase, the ribose diphosphate is regenerated. The regenerative phase is called as pentose phosphate pathway or hexose monophophate shunt. It involves the following steps.

 

1.      Some of the molecules of 3 phospho glyceraldehyde into dihydroxy acetone phosphate. Both 3 phospho glyceraldehyde and dihydroxy acetone phosphate then unite in the presence of the enzyme, aldolase to form fructose, 1-6 diphosphate.

 

 

2.      Fructose 6 phosphate is converted into fructose 6 phosphate in the presence of phosphorylase

 

3.      Some of the molecules of 3 phospho glyceraldehyde instead of forming hexose sugars are diverted to regenerate ribulose 1-5 diphosphate

4.      3 phospho glyceraldehyde reacts with fructose 6 phosphate in the presence of enzyme transketolase to form erythrose 4 phosphate ( 4C sugar) and xylulose 5 phosphate(5C sugar)

 

 

5.      Erythrose 4 phosphate combines with dihydroxy acetone phosphate in the presence of the enzyme aldolase to form sedoheptulose 1,7 diphosphate(7C sugar)

 

6.      Sedoheptulose 1, 7 diphosphate loses one phosphate group in the presence of the enzyme phosphatase to form sedoheptulose 7 phosphate.

 

7.      Sedoheptulose  phosphate reacts with 3 phospho glyceraldehyde in the presence of transketolase to form xylulose 5 phosphate and ribose 5 phosphate ( both % c sugars)

8.      Ribose 5 phosphate is converted into ribulose 1, 5 diphosphate in the presence of enzyme, phosphopentose kinase and ATP. Two molecules of xylulose phosphate are also converted into one molecule of ribulose monophosphate. The ribulose monophosphate is phosphorylated by ATP to form ribulose diphosphate and ADP, thus completing Calvin cycle.

 

 

In the dark reaction, CO₂ is fixed to carbohydrates and the CO₂ acceptor ribulose diphosphate is regenerated. In Calvin cycle, 12 NADPH₂ and 18 ATPs are required to fix 6 CO₂ molecules into one hexose sugar molecule (fructose 6 phosphate).

 

Schematic diagram of light reaction and Calvin cycle

Thank You 

Vikas Kashyap :)