PACKET #33CHAPTER #10

The Calvin Cycle“The Dark Reactions”

Introduction

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The Calvin Cycle occurs in the stroma of the chloroplast and involves the usage of CO2, NADPH and ATP to make sugar.

Introduction II

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The Calvin Cycle has multiple steps that fall under three broad phases. Carbon Fixation Reduction Regeneration of CO2 Acceptor.

Each phase involves the use of one or more enzymes. However, only a select few will be mentioned in the

packet.

Introduction IIICalvin Cycle Phases

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Carbon Fixation Use of carbon dioxide

Reduction 1, 3 bisphosphoglycerate is reduced to the sugar G3P 6 ATP’s is used

Reduced using the electrons produced by the oxidation of NADPH

Regeneration of CO2 Acceptor This acceptor, RuBP, is then re-used in carbon

fixation to continue the next cycle of the Calvin Cycle

Carbon Fixation

Carbon Fixation I

Carbon fixation begins with three molecules of ribulose-1,5-bisphosphate (RuBP). Each RuBP contains 5

carbons.

Carbon Fixation II

The enzyme, ribulose bisphosphate carboxylase (rubisco), converts EACH molecule of RuBP into TWO molecules of 3-phosphoglycerates. Rubisco is the ONLY

enzyme used during carbon fixation and the process requires 5 steps.

Carbon Fixation III

Rubisco adds CO2 to EACH RuBP to produce a 6-carbon intermediate known as β-Keto acid.

Carbon Fixation III

Each β-Keto acid is split to produce two molecules of 3-phosphoglycerate.

Carbon Fixation IV

The formation of 3-phosphoglycerate includes the making of a 6-carbon intermediate known as β-Keto acid.

Carbon Fixation V

At the end of carbon fixation three molecules of RuBP will have been converted into six molecules of 3-phosphoglycerate via the enzyme rubisco.

Reduction

Reduction I

The phase begins with six molecules of 3-phosphoglycerate.

Reduction II

The enzyme phosphoglycerate kinase adds a phosphate to each 3-phosphoglycerate to produce 1,3-bisphosphoglycerate. There should be a total

of six molecules of 1,3-bisphosphoglycerate at the end of this step.

Reduction III

The enzyme phosphoglycerate kinase adds a phosphate to each 3-phosphoglycerate to produce 1,3-bisphosphoglycerate. There should be a total

of six molecules of 1,3-bisphosphoglycerate at the end of this step.

Reduction IV

The enzyme glyceraldehyde-3-phosphate dehydrogenase reduces each 1,3-bisphosphoglycerate to glyceraldehye-3-phosphate. There should be a total of

six molecules of glyceraldehyde-3-phosphate at the end of this step.

The electrons necessary for this step are extracted from NADPH.

Reduction V

One of the G3P’s are extracted from the cycle and await the cycle to repeat to produce another “extracted” G3P. The two extracted

G3P’s combine to make one glucose molecule.

Reduction VI

After the extraction of one of the G3P’s, the remaining five enter the final phase of the Calvin Cycle.

Regeneration of the CO2Acceptor (RuBP)

Regeneration of RuBP I

Through a series of complex steps, three molecules of RuBP are produced from five molecules of G3P.

Regeneration of RuBP II

The complicated steps in the re-production of RuBP.

Regeneration of RuBP III

The final step in the regeneration of RuBP requires the use of the enzyme phosphoribulokinase.

Phosphoribulokinase converts Ribulose-5-phosphate (Ru5P) into Ribulose-1, 5-bisphosphate. (RuBP) Three molecules of

RuBP will be produced from the five molecules of G3P.

Review

Review—The Big Picture

Calvin Cycle—Overall Inputs & Outputs

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Inputs CO2

H2O ATP NADPH

Outputs Sugars (Glucose) ADP P NADP+