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

William Anderson (Schoolworkhelper Editorial Team)

William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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