The Calvin Cycle Making sugar…..sweet…..

What is the purpose? Calvin cycle uses ATP and NADPH to convert CO2 to sugar Occurs in the stroma Similar to Krebs in that it is a cycle and therefore your starting material is regenerated. Process is essentially the reverse of Krebs Actual product is GAP (G3P) which is used in the construction of many other things.

Phase 1 – Carbon Fixation CO2 is added to a 5-carbon molecule, ribulose 1,5-biphosphate (RuBP) and forms an unstable 6-carbon intermediate The intermediate splits into two 3-carbon molecules (PGA) The reaction up to this point must occur 3 times with three CO2 (3 carbons) molecules forming three RuBP molecules (3 x 5 carbons = 15 carbons) and it will produce 6 molecules of PGA (6 x 3 = 18 carbons)

Since the first thing that is produced in the Calvin Cycle is PGA, which is 3 carbons, we call this the C3 photosynthesis. Most plants are called C3 plants because they use this type of carbon fixation. These reactions are catalyzed by an enzyme called rubisco.

Phase 2 - Reduction Reactions Each of the six molecules of PGA is then phophorylated with ATP Then, a pair of electrons from each of six NADPH molecules reduces those phosphorylated molecules and make 6 molecules of G3P. One molecule of G3P exits the reaction, the other 5 continue.

Phase 3 - RuBP Regenartion The five G3P molecules (5 x 3carbons) then rearrange to become 3 RuBP molecules. 3 ATPs are used the RuBP then continues to fix more carbon. The G3P that was removed is then used to synthesize glucose and other carbohydrates. So, six turns of the cycle fixes enough carbon to make one glucose molecule.

Overall Equation 3 RuBP + 3CO2 + 9ATP + 6NADPH + 5H2O  9ADP + 8 NADP+ + G3P + 3 RuBP