The Calvin Cycle The Calvin Cycle is much like the Citric Acid Cycle in that the circular process regenerates the starting molecule at the end of the cycle. It differs in that it is an anabolic process consuming energy to produce a sugar molecule whereas the Citric Acid Cycle is catabolic producing energy as it consumes sugar

Calvin Cycle Calvin Cycle - 3 cycles are required to produce 1 3 carbon sugar (G3P) Carbon fixation – 3 CO 2 molecules are incorporated 1 at a time by the enzyme rubisco to ribulose biphosphate (RuBP, 5 carbon sugar) – the product immediately splits into two 3 carbon sugars, 3-phosphoglycerate Reduction – ATP is used to attach an additional PO 4 group to 3-phosphoglycerate – product is now 1,3-biphosphoglycerate – NADPH donates 2 e- to 1,3-biphosphoglycerate to reduce it to form glyceride-3-phosphate (G3P) – G3P is the same molecule produced in the 1st stage of gylcolysis Regeneration of RuBP – 5 G3P molecules (from the 3 cycles) are converted back to 3 RuBP

Summary Consumed (used) – 9 ATP – 6 NADPH – 3 CO 2 Produced – 1 G3P (through 3 cycles)  G = 114kcal/mol 6CO2 + 18ATP + 12 NADPH + 12H 2 O  C 6 H 12 O ADP + 18Pi + 12NADP H + + 6O 2 + 6H 2 O Note that the product is not glucose, fructose, or sucrose. Another step is required to fuse 2 G3P to form a 6 carbon sugar. – This process takes 6 turns of the cycle! – Glucose is formed from the fusion of 2 molecules of G3P

Alternate Pathways C 3 Plants (Rice, soy beans, grass) – 1st product is 3-phosphoglycerate – as stomata close on hot dry days the Calvin cycle fixes O2 instead of CO2 to produce CO2 called photorespiration – produces no sugar – rxn has a net consumption of ATP – decreases photosynthetic output by robbing Calvin cycle of organic material C 4 Plants (Sugar cane, corn) – have specialized tissues arranged around the veins – mesophyll cells store excess CO2 in organic molecules (oxaloacetate) by PEP carboxylase can later release the bound CO2 in dry hot conditions when the stomata are closed process minimizes photorespiration – Bundle-sheath cells place where Calvin Cycle takes place arranged around the veins to increase storage of sugar product CAM Plants (Succulent) – Stomata are closed during the day to minimize water loss – Uptake of CO2 at night and incorporation into organic acids – Calvin cycle operates during the day when the light cycle can produce ATP & NADHP for the Calvin cycle