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Calvin cycle.

In the world of biology, Calvin cycle is known by many names like Light-independent reaction, Dark reaction and Calvin-Benson cycle. It is absolutely true that, this reaction is not dependent on light, but it can happen in both, presence and absence of light and that's the reason, few experts do not prefer the term dark reaction.
An another great plant-biochemistry expert named Andy Benson discovered carbon-fixation mechanism and helped Calvin in his discovery. Hence, Calvin-Benson cycle is also a popular name.
Let's understand what actually happens during the said process:

Whole Calvin cycle starts with a simple yet, one of the most important character called CO2(carbon dioxide). This CO2 after being permitted by stomata present in the lower epidermis, diffuses through intercellular spaces and reaches mesophyll layer inside the leaf. After reaching mesophyll, CO2 enters Stroma, present in the Chloroplasts. This stroma is the site where Calvin cycle takes place.

We are now in the Stroma, where ingredients(ATP+NADP) of Calvin cycle, from thylakoids meet CO2. 
Before coming on to the actual reaction, Let's go through important catalyst, reactants, and energy sources, which are as follows:
  • RubisCo: Abbreviation of Rubilose-biohosphate carboxylase an organic substance and a plant enzyme, which acts as a catalyst in the reaction between RuBP and CO2, This reaction between RuBP andCO2, fixes the atmospheric CO2 from inorganic to organic state.
  • RuBP: Abbreviation of Rubilose biphosphate an organic substance, which is present in the stroma ready to react with CO2 to form 3-PGA
  • 3-PGA: Abbreviation of 3-Phosphoglyceric acid.
  • GA3P: Abbreviation of Gylaceraldehyde 3-Phosphate.
  • ATP: Abbreviation of Adenosine triphosphate
  • ADP: Abbreviation of Adenosine diphosphate.
  • NADPH: Abbreviation of Nicotinamide adenine dineucleotide phosphate.

3 stages Of The Calvin cycle:

       Figure 1.1
Calvin cycle or Dark reaction or Light-independent reaction
   
The figure no. 1.1 shows Calvin cycle in action with all 3 stages: Fixation, Reduction, and Regeneration.

Let's study all 3 stages individually in detail:

Stage 1: Fixation:

       Figure 1.2
First stage of Calvin cycle.
  

As I said earlier, RuBisCo and RuBP are already present in the Stroma. When atmospheric CO2 reaches Stroma it reacts with RuBP in the presence of catalyst RuBisCo to form 3-GPA. Here in this stage the inorganic crabon from CO2 is fixed in to organic carbon in the form of organic compound 3-GPA. Hence, this stage is known as "Fixation". Also this reaction maintains the number of carbon atoms. Let's see how:
                                RuBisCo(catalyst)
     3CO2 + 6RuBP---------------------->6 3-GPA 
In this reaction the number of carbon atoms on LHS(left hand side) is equal to the RHS(right hand side) that is 18 carbon atoms.(Refer figure no. 1.2)

Stage 2: Reduction:

                    Figure 1.3
2nd stage of Calvin cycle
          

Once, 6 molecules of 3-GPA is formed gets reduced with the help of electrons from donors, 6ATP and 6NADPH, to form a 6 molecules of organic compound called, GA3P(Glyceraldehyde 3-Phoshate). ATP and NADPH is the most important energy source for the whole photosynthesis to occur. In this stage ATP releases one of its phosphate ion and gets converted to ADP; Similarly NADPH releases proton and energy to get converted into NADP+. Both of these conversion releases energy required for the reduction of 3-GPA to GA3P. Hence, this stage is known as "Reduction".
ADP and NADP+, again go back to light-dependent reactions to get reused and 
re-energized.(Refer figure no. 1.3)

Stage 3: Regeneration:

     Figure 1.4
Third stage of Calvin cycle.
 

RubisCo used during first stage is regenerated in this stage. Out of all molecules of GA3P, during this stage only one molecule of GA3P having six carbon atoms, is used to form organic compounds like glucose and fructose. And the rest of GA3P is repumped back to Calvin cycle to regenerate RubisCo which in turn prompts the system to get ready to fix more CO2 and thus, more ATP and NADPH is required, to continue the cycle.
Hence, this stage is known as "Regeneration". (Refer figure no. 1.4)

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