Calvin Cycle• Chemical energy captured in ATP

and NADPH is used to “fix” carbon dioxide in the dark reaction

• 5C sugar + CO2 _ 2 x 3C sugars • CO2 is fixed into a 3 carbon sugar • Referred to as C3 photosynthesis • The enzyme that catalyzes this

reaction is called ribulose bisphosphate carboxylase, or rubisco

Importance of Calvin Cycle

• Removes 200 billion tons of CO2 from the air every year

• Rubisco is the most abundant protein on earth, 20 kgs for every human

• When forests are cut down and cleared, some of this capacity to remove CO2 from the atmosphere is lost, contributing to global warming and the greenhouse effect

The Calvin CycleThe Calvin Cycle•Light independent reaction

•Occurs in Stroma of chloroplast

•ATP and NADPH from light reaction used to reduce CO2 into carbohydrate molecules

•Divided into 3 stages

1. Carbon Fixation

2. Reduction reactions

3. Ribulose 1,5-bisphosphate (RuBP) regeneration

Phase 1: Carbon Fixation

• CO2 joins to RuBP (5C) to form a 6C intermediate

• This reaction is catalyzed by enzyme rubisco• Intermediate 6C molecule immediately splits into two 3C

molecules called PGA (3-phosphoglycerate)

• This reaction occurs 3 times, therefore 3 CO2 are used and 6 PGA are produced

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Phase 2: Reduction Reactions

• 6 PGA molecules are phosphorylated by ATP to produce 6 molecules of 1,3-bisphophoglycerate (1,3 BPG)

• 6 1,3 BPG are then reduced by NADPH to produce 6 glyceraldehyde 3-phosphate (G3P) – a sugar

• One G3P exits the cycle as a final product and the remaining 5 G3P are used in RuBP regeneration

• G3P that exited is used to synthesize larger sugars

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Phase 3: RuBP Regeneration

• 5 G3P are rearranged to form 3 molecules of RuBP• 3 ATP are used in this process

• The RuBP is now available to join with the next CO2 in the next cycle

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Calvin Cycle

3H2O

2H2O

2 Pi

3RuBP+3CO2+9ATP+6NADPH+5H2O9ADP+8Pi+6NADP++G3P+3RuBP

To Produce One G3P…

3 RuBP + 3 CO2 + 9 ATP + 6 NADPH + 5 H2O 9 ADP + 8 Pi + 6 NADP+ + G3P + 3 RuBP

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

Tutorial 8.3 Tracing the Pathway of CO2