PCB3134 - Lecture 19 CO2 Fixation and Carbohydrate Synthesis

PCB3134 - Lecture 19 CO2 Fixation and Carbohydrate...

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Light reactions: Energy of light is conserved as “high energy” phosphoanhydride bonds of ATP reducing power of NADPH . Proteins & pigments responsible for the light reactions are in thylakoid (grana disc) membranes . grana disks (thylakoids) stroma compartment 2 outer membranes Chloroplast Photosynthesis takes place in chloroplasts. It includes light reactions and reactions that are not directly energized by light.
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Light-independent reactions (or “dark reactions”) -Calvin cycle
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The free energy of cleavage of ~P bonds of ATP , and reducing power of NADPH , are used to fix and reduce CO 2 to form carbohydrate . Enzymes and intermediates of the Calvin Cycle are located in the chloroplast stroma , a compartment analogous to the matrix of the mitochondrion. grana disks (thylakoids) stroma compartment 2 outer membranes Chloroplast Calvin Cycle (photosynthetic “dark reaction” pathway):
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After world war II, Melvin Calvin, along with his colleagues Andrew Benson and James Bassham used: 1. Long-lived radioactive isotope of carbon ( 14 C); 2. Two-dimensional paper chromatography. to study the green alga Chlorella cell suspension. Unlabeled CO 2 14 CO 2 Autoradiography Chromatography Hot alcohol
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Five seconds after injection of 14 CO 2 , one radioactive spot on the chromatogram predominated, which was determined to be 3-phosphoglycerate (PGA). Because the first intermediate was a three-carbon molecule, plants using this pathway to fix CO 2 are referred to as C 3 plants. Question: What is the compound to which CO 2 is covalently linked ?
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1. RuBP condenses with CO 2 to produce a 6-carbon molecule; 2. The 6-carbon molecule rapidly fragments into 2 molecules of PGA. Both reactions are carried out in the stroma by a large multisubunit enzyme, ribulose bisphosphate carboxylase (Rubisco). Rubisco is a very low efficient enzyme (3 CO
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PCB3134 - Lecture 19 CO2 Fixation and Carbohydrate...

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