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AP Biology Light Independent Reactions – Calvin Cycle Photosynthesis Light Reaction Photosynthesis Calvin Cycle Calvin Cycle step 1 Carbon Fixation • Part 1 of the Calvin Cycle: – 3 Carbon Dioxides enter the cycle and combine with RuBP to create 3 very unstable 6 carbon molecules. – These 6-carbon molecules instantly break down into 6, 3-carbon molecules that are much more stable. – We now have all of the carbons in the cycle that are needed to make ½ of a glucose molecule. Calvin Cycle step 2 Reduction • Part 2 of the Calvin Cycle: – ATP and NADPH are now used to modify the original 3-carbon molecules in preparation for one of them to leave the cycle to make glucose. – This stage is called reduction because electrons are added to the 3-carbon molecules to transform them into the building blocks for glucose. – ADP and NADP+ are sent back to the ETC. Calvin Cycle step 3 Regeneration of Carbon Dioxide Acceptor • Part 3 of the Calvin Cycle: – The remaining 3-carbon molecules are further modified using ATP to get back to the starting material in the cycle. – If you trace the number of carbons through the cycle, you should end up with the same amount that you started with. – The cycle starts with 15 carbons and at the end of the third phase you should have 15 carbons. – What happens if the plant has no carbon dioxide? Calvin Cycle Intermediates C4 Plants and Photorespiration Photosynthetic cells of C4 plant leaf Mesophyll cell PEP carboxylase Mesophyll cell CO2 Bundlesheath cell The C4 pathway Oxaloacetate (4 C) PEP (3 C) Vein (vascular tissue) ADP Malate (4 C) ATP C4 leaf anatomy Stoma Bundlesheath cell Pyruvate (3 C) CO2 CALVIN CYCLE Sugar Vascular tissue CAM Plants and Photorespiration Sugarcane Pineapple CAM C4 CO2 Mesophyll cell Organic acid Bundlesheath cell CO2 CO2 incorporated into four-carbon Organic acid organic acids (carbon fixation) CO2 CALVIN CYCLE Sugar Spatial separation of steps CO2 Organic acids release CO2 to Calvin cycle Night Day CALVIN CYCLE Sugar Temporal separation of steps Stomata of Leaves & Transpiration