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The Calvin Cycle Named after Melvin Calvin (1911-1997) • The Calvin Cycle Carbon atoms from CO2 are bonded or “fixed” into organic molecules Carbon Fixation – the incorporation of CO2 into organic molecules Step#1 of 3 in the Calvin cycle CO2 enters through the stoma, diffuses into cytosol then it enters the stroma of the chloroplast. Enzyme combines CO2 with a five carbon compound RuBP This produces a six carbon compound This Splits into a pair of three carbon compounds PGA Step #2 – PGA is converted into another threecarbon molecule called PGAL. PGA receives a phosphate from ATP then the resulting compound receives a proton from NADPH this causes the received phosphate to be released producing PGAL The ending products are NADP+ and ADP Step #3 PGAL is converted back into RuBP This step requires ATP which is changed to ADP This step allows the Calvin cycle to continue Some PGAL Molecules are not converted into RuBP but they leave the cycle and make organic compounds C3 Pathway All Plants that fix carbon using the Calvin Cycle are C3 Plants Low CO2 or high O2 will slow the Calvin Cycle When the Stoma is closed there will be less CO2 When Photosynthesis is running it makes O2 C4 Pathway Fixes a four carbon molecule Corn, Sugar cane and Crabgrass (monocots) CAM Pathway Stands for Crassulacean Acid Metabolism Plants that live in arid environments Cactuses, and pineapple