– cyclic process in the stroma that converts carbon dioxide into carbohydrate molecules
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– three phases: 1) carbon fixation, 2) reduction reaction, 3) RuBP regeneration
Phase 1: Carbon Fixation
– C02 added to RuBP -> unstable 6-carbon intermediate -> 2 3-carbon PGAs
– required to occur three times, 3 CO2 + 3 RuBP -> 6 PGA
– called C3 photosynthesis because the first compound produced contains 3 carbon atoms; most plants
– reaction is catalyzed by the enzyme rubisco
Phase 2: Reduction Reactions
– each PGH phosphorylated by an ATP; 6 PGA + 6 ATP -> 6 1, 3-BPG
– six NADPH reduce (transfer 2 electrons) the six 1,3-BPG -> 6 G3P
– one molecule of G3P exits the cycle as a final product
Phase 3: RuBP Regeneration
– five remaining G3Ps used to regenerate three molecules of RuBP
– 5 G3P + 3 ATP -> RuBP
– RuBP reenters the cycle to fix more carbon and the process repeats itself
– the G3P that exit the cycle are used to synthesize larger sugars like glucose
– 3 CO2 must be fixed before 1 G3P can be removed from the cycle in order to maintain the pool of intermediate molecules needed to sustain the cycle
– 6 turns of the cycle are needed to fix enough CO2 to make one molecule of glucose
Self-Sufficient For Energy Needs
– synthesis of one G3P requires 9 ATP and 6 NADPH
– every 4 e– (from 2H2O) adds 12 protons to H+ reservoir -> 3 ATP
– these 4 e– also produce 2 NADPH
– these 3 ATP and 2 NADPH are used in the Calvin cycle for every G3P produced
Note: The amount of H+ needed to generate one ATP is an approximation, if this is the case the lack of ATP can be compensated by cyclic flow.
Note: The amounts of NADPH and NADP+ in the stroma regulate cyclic and noncyclic flow. When there is a lot of NADPH cyclic flow is favoured since there will be little available NADP+, slowing down noncylic flow.
Where’s the Glucose!?
– glucose is synthesized from G3P by a series of enzyme-catalyzed reactions in the stroma or the cytoplasm
– if more glucose is produced than the cell requires excess glucose is polymerized into molecules of amylase and amylopectin and store them as starch granules
– 1/3 of G3P goes on to become starch, the other 2/3 of G3P are converted to sucrose and transported to other cells of the plant