26_printout - Announcements / review Lecture 26 Reading for...

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Unformatted text preview: Announcements / review Lecture 26 Reading for today: Reading for tomorrow: Assigned problems: pages 773-783, 851-857 pages 805-814, 836-840 N&C: Ch. 22: #1 SG: p.260: #2,3; p. 284: #1,3 Yesterday: ATP synthase Light reactions (photophosphorylation and photoreduction) Clarification: Proton accounting slide was for oxidative phosphorylation, not photophosphorylation Today: Carbon assimilation Nitrogen fixation Finally: Anabolism! Up to now our discussion of metabolism has been focused on catabolism. (how to break molecules down, and harvest the energy released) Next two lectures: ANABOLISM (synthesis). Also, after spring break – synthesis of macromolecules (DNA, RNA, Proteins) Anabolic pathways: Usually: So what high-energy and redox molecules are needed? Carbohydrate biosynthesis: Animals use 3-carbon sugars (trioses) to make larger carbohydrates. Eat carbohydrates and/or proteins: Plants and autotrophic bacteria – similar pathways as in animals, but also can make the 3-carbon precursors: WHATʼS THE CARBON SOURCE? THE SYNTHESIS IN PHOTOSYNTHESIS “Dark reactions” or “Carbon assimilation” or “Carbon fixation”: 6CO2 + 6H2O C6H12O6 + 6O2 HOW DOES THE GAS CO2 BECOME “FIXED” INTO CARBOHYDRATE? Melvin Calvin: 1 minute 14CO 2 fed to algae during photosynthesis After 1 minute, grind up algae, extract with solvent Use thin-layer chromatography to figure out which compounds have the 14C THIN LAYER CHROMATOGRAPHY Oh no! ~40 different compounds – not interpretable! Try again, this time after 5 sec: 5 seconds The Calvin Cycle of carbon assimilation Occurs in stroma of chloroplasts Notice stoichiometry of: 1)  Carbons 2)  Molecules Hopefully 3PG and G3P look familiar! Yes, plants have: glycolysis, gluconeogenesis, CAC, β-ox, and oxidative phosphorlyation! Where did these molecules come from? Synthesize just one G3P molecule: Yes, know structures, and what happens in each step Rubisco catalyzes the key step of the Calvin Cycle (Stage 1: fixation) Enzyme: “RUBISCO” = RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE/OXYGENASE “Most abundant enzyme on Earth!” CO2 + Rubisco 2x Rate-limiting step of Calvin Cycle (kcat = 3 s-1) To make Calvin Cycle stoichiometry work: 3 Ribulose 1,5-bisphosphate + 3 CO2 --> 6 3-PG Rubisco active site Is that a “common” amino acid? Mg2+ Formation of this A covalent bond is made between lysine-amine and CO2 to make carbamoyl-lysine Stage 2: Reduction of 3-PG to G3P Same reactions as part of: Were these regulated/bypass steps? Similar enzymes to glycoylsis/gluconeogenesis, but present in the stroma. No. NADP+ NADPH What happens to G3P? 1 out of 6 molecules of G3P produced can leave the Calvin cycle to be used by the cell: Glycolysis, gluconeogenesis, etc. 5 out of 6 molecules of the G3P remain in the Calvin cycle: converted to 3 molecules of ribulose 1,5-bisphosphate Stage 3: Regeneration of Ribulose 1,5-bisphosphate Again, part of this pathway is Like: G3P enters this series of steps several places. There are two different (but connected) routes to ribulose 1,5-bisphosphate Transketolase enzymes: Epimerase enzymes: Donʼt memorize this pathway, But do know the details I pointed out Total energy cost To make 1 glucose through Calvin Cycle and gluconeogenesis: making G3P from CO2: 9 ATP and 6 NADPH making 2 G3P from CO2: making glucose from 2 G3P: 2nd, 3rd bypass of gluconeogenesis: So, making glucose from CO2: assume NADPH like NADH: = ____ consumed for reductive synthesis of glucose from CO2 compare to 32 ATP made by oxidizing glucose to CO2 Pretty good, considering the number of steps! Air is 80% Nitrogen, but… Animals get nitrogen for anabolism by eating protein (from plants and other animals). Soil bacteria: many can “fix” N2 into ammonia. Many then convert ammonia into nitrate and nitrite. Plants get nitrogen by absorbing nitrate and nitrite and converting it to ammonia, or obtain ammonia directly from symbiotic bacteria in root “nodules” (legumes only). USA nitrogen fertilizer use (farming) Nitrogen and phosphate are usually limiting for growth (CO2, light, and water are abundant, cheap). Producing more food requires more fertilizer. Tons USA nitrogen fertilizer price index Making fertilizer is energy-intensive! Cost tied to cost of fossil-fuels. N2 + 3 H2 ⇌ 2 NH3 The challenge N2 + 3H2 2NH3 ΔGʼº = -34 kJ/mol VERY HIGH ΔG This reaction is favorable… HOWEVER: NN N + N ΔGʼº = +930 kJ/mol Gʼº N2 + 3H2 “Haber process”: 2NH3 The solution Nitrogenase complex: N2 + 10H+ + 8e- + 16ATP 2NH4+ + 16ADP + 16Pi + H2 Dinitrogenase / dinitrogenase reductase Reaction requires Dinitrogenase: Reduces N2 and H+ Dinitrogenase reductase: Reduces to dinitrogenase, Why would it be bad for O2 to interact with this enzyme? In root nodules (legumes): Leghemoglobin, produced by plant: ...
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This note was uploaded on 04/08/2011 for the course BIO BM 3350 taught by Professor Blakenship during the Spring '11 term at Cornell University (Engineering School).

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26_printout - Announcements / review Lecture 26 Reading for...

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