Calvin Cycle (Light Independent Photosynthetic Reactions)


Calvin Cycle

 1. This stage uses 2 products from the light-dependant stage: ATP and NADPH

2. CO2 diffuses into the stroma and combines with a 5 carbon compound RuBP. This produces an unstable 6-carbon molecule that instantly splits into 2, 3-carbon molecules of GP. The reaction is catalysed by the enzyme Rubisco. 

3. GP is reduced to GALP by NADPH (which acts as a hydrogen carrier, transferring H+ ions from water to carbon dioxide). This reaction is driven by the ATP produced via photophosphorylation. NADP is oxidised and available for further use in the light-dependant reactions. 

4. The combination of two molecules of GALP produces a molecule of glucose, which consists of carbon, oxygen and hydrogen. The carbon and oxygen have been obtained from carbon dioxide, and the hydrogen has been obtained from water (via photolysis). 

5. Most of the carbon atoms of GALP are recylcled to regenerate RuBP for the cycle to continue (5 out of 6 carbons), the 6th carbon of GALP is the building block of carbohydrates, which can be modified into several organic compounds including lipids, proteins and nucleic acids.

6. It takes 6 cycles of the Calvin Cycle to produce one molecule of glucose as only 1 carbon of GALP is used to form it per cycle whilst the other 5 are used to regenerate RuBP

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Diagram of Calvin Cycle

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