Lecture 7- photosynthesis 3

A pair of electrons from nadph reduces each 13

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Unformatted text preview: phases. • In the carbon fixation phase, each CO2 molecule is attached to a fivecarbon sugar, ribulose bisphosphate (RuBP). Fig. 7.14 (one per turn) into one molecule of the 5ive- carbon sugar ribulose- 1,5- biphosphate (RuBP) to produce two molecules of the three carbon compounds 3- phosphoglycerate - The reaction is catalyzed by enzyme RuBP (Rubisco) 21 7 - Carbon 5ixation: attaching the carbon from carbon dioxide onto something else - In phase 1 (Carbon Fixation), CO2 is incorporated into a 5ive- carbon sugar named ribulose bisphosphate (RuBP). - The enzyme which catalyzes this 5irst step is RuBP carboxylase or rubisco. It is the most abundant protein in chloroplasts and probably the most abundant protein on Earth. - The product of the reaction is a six- carbon 08:55 Gardiner 22 intermediate which immediately splits in half to form two molecules of 3- phosphoglycerate • During reduction, each 3phosphoglycerate receives another phosphate group from ATP to form 1,3 bisphosphoglycerate. • A pair of electrons from NADPH reduces each 1,3 bisphosphoglycerate to G3P. – The electrons reduce a carboxyl group to a carbonyl group. 08:55 Gardiner - Phase 2: Reduction - Each molecule of 3- phosphoglycerate gets an additional phosphate added from the breakdown of ATP - Produces two molecules of 1,3- bisphosphoglycerate - Each of these molecules is simultaneously reduced by electrons from NADPH - This produces a molecule of glyceraldehyde- 3- phosphate (G3P) 23 - In phase 2 ( Reduction), ATP and NADPH2 from the light reactions are used to convert 3- phosphoglycerate to glyceraldehyde 3- phosphate, the three- carbon carbohydrate precursor to glucose and other sugars 08:55 Gardiner 24 8 • If our goal was to produce one G3P net, we would start with 3 CO2 (3C) and three RuBP (15C). • After fixation and reduction we would have six molecules of G3P (18C). – One of these six G3P (3C) is a net gain of carbohydrate. • This molecule can exit the cycle to be used by the plant cell. – The other five (15C) must remain in the cycle to regenerate three RuBP. 08:55 Gardiner 25 • In the last phase, regeneration of the CO2 acceptor (RuBP), these five G3P molecules are rearranged to form 3 RuBP molecules. • To do this, the cycle must spend three more molecules of ATP (one per RuBP) to complete the cycle and prepare for the next. 08:55 Gardiner 27 1202B • The G3P from the Calvin cycle is the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates. BIOLOGY – It “costs” three ATP and two NADPH per CO2. Gardiner - 26 • For the net synthesis of one G3P molecule, the Calvin recycle consumes nine ATP and six NAPDH. 08:55 - Phase 3: Regeneration - For each turn, two molecules of G3P are produced- a total of six carbon atoms - Five of these carbons are rearranged to regenerate the single molecule of RuBP required for the next round of carbo...
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This note was uploaded on 02/11/2014 for the course BIO 1202B taught by Professor Almanke during the Winter '12 term at UWO.

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