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Understanding Photosynthesis: Light-Independent Reactions

Dive deep into the Calvin Cycle, where carbon is fixed, regenerated, and reduced to produce essential molecules in plants. Learn how CO2 is converted to carbohydrates using energy from ATP and NADPH.

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Understanding Photosynthesis: Light-Independent Reactions

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  1. Light Independent Reactions • Occurs in the stroma and converts carbon dioxide to carbohydrates using energy from ATP and NADPH • CO2 must be converted to organic compounds (this is called CO2 assimilation) – this is done in the • light independent reaction Carbon Fixation Regeneration of RuBP Reduction Ribulose 1,5-bisphosphate

  2. 1. Carbon Fixation CO2 + RuBP unstable C6  2 PGA • CO2 bonds to a pre-existing molecule in the stroma (Ribulose 1,5-bisphosphate… aka RuBP) • The resulting 6 carbon intermediate quickly changes to 2 three-carbon molecules of 3-phosphoglycerate (PGA) • This is catalyzed by Ribulosebisphosphatecarboxylase (Rubisco) - possibly the most abundant protein on earth

  3. 2. Reduction • The new PGA is phosphorlated by ATP and then reduced by NADPH creating Glyceraldehyde-3-phosphate • Some G3P goes on to make glucose and other carbohydrates • Most G3P goes on to replenish RuBP

  4. 3. Regenerating RuBP Energy, supplied by ATP is required to break and reform bonds to make the 5 carbon RuBP The Calvin cycle must be completed 6 times in order to synthesize one molecule of glucose. Of 12 G3P molecules that are produced in 6 cycles 10 are used for regeneration of RuBP, and 2 are used to produce one glucose molecule. Net equation is : 6 CO2 + 18 ATP + 12 NADPH + water  3 G3P + 16 Pi + 18 ADP + 12 NADP+

  5. Glyceraldehyde-3-phosphate Is used to produce: Glucose Sucrose Cellulose Oils AAs (along with nitrogen)

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