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The Calvin Cycle

Overview: The Calvin Cycle consists of reactions that are light-independent; in other words, they may occur in the absence of light. Also called the C3 cycle, these reactions operate by using the NADPH and ATP produced in the light reactions. The three major stages of the cycle are as follows: The carbon fixation reactions, the reductionr reactions, and the regeneration of RuBP.

 

Inputs: 6 CO2, 18 ATP, 12 NADPH, 12 H2O

 

Outputs: C6H12O6, 18 ADP, 18 Pi, 12 NADP+, 6 H2O, 12 H+, 6O2

 

 

 

 

 

 

 

 

 

 

 

 

Carbon Fixation Reactions

Reduction Reactions

Regeneration of RuBP

Regulation of Rubisco

Step One: Two 3-carbon compounds of 3-phosphoglycerate are formed by the enzyme rubisco. This happens when carbon dioxide is combined with a 5-carbon RuBP.

 

 

Main idea: 3CO2 + 3 ribulose biphosphate yields six 3-phosphoglycerates through the enzyme rubisco. 

 

**In the C3 cycle, this is a regulatory step. It controls the formation of sugars.

 

 

 

Step Two: 3-phosphoglycerate is phosphorylated into 1,3 biphosphoglycerate. This step is the first to use energy and is competed with the assisstance of phosphoglycerate kinase.

 

Step Three: Glyceraldehyde 3-phosphate is formed once 1,3 biphosphoglycerate is dephosphorylated and reduced. NADPH is used in this step.

 

Main idea: Six 1,3 biphosphoglycerates yield six glyceraldehyde 3-phosphates through the use of glyceraldehyde 3-phosphate dehydrogenase. In this process, 6NADPH are converted to 6NADP+ and 6Pi.

 

Step Four: One of the six glyceraldehyde 3-phosphates produces glucose; this happens through a number of steps. 6-carbon sugars are formed from 3-carbon sugars in this process.

 

Main idea: Glyceraldehyde 3-phosphate yields glucose and other sugars.

Step Five: RuBP is regenerated in order to keep the Calvin Cycle going. 3 ribulose biphosphates are formed from five glyceraldehyde 3-phosphates. More CO2 molecules are fixed, which generates six 3-carbon sugars. Energy is used in this step in the form of ATP.

 

Main idea: Five glyceraldehyde 3-phosphates yield 3 ribulose biphosphates. Three ATPs are converted to three ADPs.

GENERAL INFORMATION CONCERNING RUBISCO:

  • Probably the most abundant protein on Earth

  • 50% of a leaf's soluble proteins are rubisco

  • Has 8 large subunits and 8 small subunits

    • ​The whole enzyme complex is about 560 kDa

  • Large subunit- catalytic subunit that is made in the chloroplast

  • Small subunit- regulatory subunit that is made in the cytoplasm and sent to the chloroplast

  • Rubisco can fix CO2 and O2

 

Factors that regulate rubisco:

  1. Concentration of CO2 and O2 in the cell: As the CO2 levels decrease after the Calvin Cycle, rubisco fixes oxygen. Then, during photorespiration, CO2 is released.

  2. Mg2+ concentration: The level of Mg2+ increases in the stroma in order to assist in carbon fixation.

  3. pH: During the light reactions, the pH in the stroma increases from 7 to 8. This is the optimal pH.

  4. NADPH levels: The presence of NADPH prompts the activity of rubisco.

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