Lecture 7- photosynthesis 3

Carbon dioxide is fixed p 3p 6 rubp p 3

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Unformatted text preview: n 5ixation - In phase 3 (Regeneration), more ATP is used to convert some of the of the pool of glyceraldehyde 3- phosphate back to RuBP, the acceptor for CO2, thereby completing the cycle. - For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3- phosphate (G3P). - For each G3P synthesized, the cycle spends nine molecules of ATP and six molecules of NADPH2. - The light reactions sustain the Calvin cycle by regenerating the ATP and NADPH2 9 Where do all the carbon atoms go? There are three parts to the Calvin Cycle: Carboxylation Sugar Biosynthesis RuBP regeneration Summary: Add 3 reactions 08:55 6 CO2 + 6 RuBP ==> 12 3-carbon sugars 2 3-carbon sugars ==> 1 glucose 10 3-carbon sugars ==> 6 5-carbon RuBP 6 CO2 ==> 1 glucose Gardiner 28 Gardiner 29 • Starch grains appear in the chloroplast (stroma) in day and are converted to sucrose at night Gardiner 1202B – Major transport sugar in plants – Or to starch – major storage carbohydrate BIOLOGY • G3P is transported from the chloroplast to the cytosol of the cell • Very little glucose is generated in photosynthesizing cells, rather most fixed carbon converted to sucrose 08:55 1202B 08:55 BIOLOGY Remember: The Calvin Cycle requires more ATP than NADPH hence the need for the ATP generated by cyclic photophosphorylation 30 10 Rubisco: ribulose-1,5-bisphosphate Carboxylase/oxygenase 3 CO2 1. Carbon dioxide is fixed P 3P 6 RuBP P 3-phosphoglycerate CALVIN CYCLE 3 ADP 3 ATP 6 ATP 2. 3-phosphoglycerate is reduced to G3P 3. RuBP is regenerated from G3P 6 ADP 6 NADPH 6 NADP+ + 6 Pi 5 G3P 6 G3P P 1 G3P Fig. 9.13a (only 3 CO2) Glucose Remember this: - In three turns of the Calvin Cycle, 3CO2 (3 carbons) are incorporated into 3 molecules of RuBP (15 carbons, which produces 6 molecules of 3- phosphoglycerate (18 carbons) - Each molecule is phosphorylated by a phosphate donated by ATP - 6 ATP are consumed to phosphorylate the 6 molecules of 3- phosphoglycerate, generating 6 molecules of 1,3- bisphosphoglycerate - Six molecules of NADPH are consumed in converted the 6 molecules of 1,3- bisphosophoglycerate into 6 molecules of G3P (18 carbons) - 5 molecules of G3P (totalling 15 carbons) are used to regenerate 3 RuBP molecules (15 carbons), which requires 3 molecules of ATP - The cycle generates one surplus molecule of G3P (three carbons) after every three turns - The Calvin Cycle requires a total of 9 molecules of ATP and 6 molecules of NADPH - ATP and NADPH are regenerated from ADP and NADP+ by the light reactions carbon dioxide + water + light energy ---> glucose + oxygen + water As a chemical reaction: CO2 + H2O + light energy ---> C6H12O6 + O2 + H2O INPUTS: - ATP, CO2, NADPH Summary of Calvin Cycle (for input of 3 CO2): 9 ATP consumed (9 ADP produced) 6 NADPH oxidized (6 NADP+ produced) 1/2 glucose (C3) produced (3 CO2 consumed) OUTPUTS: - ADP, NADP+, G3P NOTE: TWO "turns" of the Calvin Cycle will produce ONE GLUCOSE (C6H12O6) Note that carbon fixation is a highly endergonic reaction (18 ATP and 12 NADPH are consumed per glucose produced) 08:55 Gardiner 33 11 Respiration vs Photosynthesis • Photosynthesis is an anabolic endergonic reaction • Respiration is a catabolic exergonic reaction 08:55 34 Gardiner - Photosynthesis and Cellular Respiration are basically the reverse of each other - The reactants of photosynthesis (CO2 and H2O) are the products of cellular respiration - The reactants of cellular respiration (glucose and O2) are the products of photosynthesis - Both processes have key phosphorylation reactions involving photophosphorylation in photosynthesis and oxidative phosphorylation in cellular respiration, following by the chemiosmotic synthesis of ATP - G3P is found in the pathways of both processes - In photosynthesis, it is a product of the Calvin Cycle and is used for the synthesis of sugars and other organic fuel molecules - In cellular respiration, it is an intermediate generat...
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