The Calvin Cycle

– cyclic process in the stroma that converts carbon dioxide into carbohydrate molecules

– 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

Result

– 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

Light reactions:

– 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