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Cellular Respiration and Fermentation chapter 9 Catabolic Pathways and Production of ATP Breakdown – exergonic process Cellular Respiration produces energy Two fundamental processes Substrate-level phosphorylation Oxidative phosphorylation -- ~90% 2 Synthesis of ATP – 2 Methods Substrate-level phosphorylation – Direct transfer of a phosphate to ADP to make ATP Enzyme Enzyme ADP P Substrate + Product ATP 3 Oxidative phosphorylation – production using energy derived from transfer of electrons & occuring via chemiosmosis 44 Redox Reactions Oxidation-reduction reactions -- redox reactions Oxidation – loss of electrons Reduction – gain of electrons becomes oxidized (loses electron) Xe- + Y X + Ye- becomes reduced (gains electron) Not reversible! 5 Reducing Agent -- electron donor – gets oxidized! Oxidizing Agent -- electron receptor – gets reduced! Leo Ger Loss of Electrons – Oxidation Gain of Electrons -- Reduction Oxidation Is Loss; Reduction Is Gain 6 Catabolic Pathways Two fundamental “kinds” Using organic molecules Using inorganic molecules (prokaryotes only) Breakdown of organic molecules Aerobic – requires oxygen Anaerobic (fermentation) – absence of oxygen 7 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Cellular Respiration Both aerobic and anaerobic pathways Any organic fuel carbohydrates, fats, and proteins Generally start with glucose C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat) 8 Organic Fuel is Oxidized ReDox process During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced: becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced 9 Stepwise Energy Harvest Glucose -- broken down in a series of steps Electron transport chain -- series of steps instead of one explosive reaction 10 LE 9-5 H2 + 1/2 O2 + 2H 1 /2 O2 1/2 O2 (from food via NADH) Explosive release of heat and light energy Free energy, G Free energy, G 2 H+ + 2 e– Controlled release of energy for synthesis of ATP ATP ATP ATP 2 e– 2 H+ H2O Uncontrolled reaction H2O Cellular respiration RESPIRATION C6H12O6 + O2 CO2 + H2O + ENERGY ATP 686 kcal/mole (180 grams) 12 Glycolysis: glucose to pyruvate Glycolysis -- glucose into two molecules of pyruvate Glycolysis -- occurs in the cytoplasm Energy investment phase Energy payoff phase Independent reaction 13 LE 9-8 Energy investment phase Glucose 2 ATP used 2 ADP + 2 P Glycolysis Citric acid cycle Oxidative phosphorylation Energy payoff phase ATP ATP ATP 4 ADP + 4 P 2 NAD+ + 4 e– + 4 H+ 4 ATP formed 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H2O 2 ATP 2 NADH + 2 H+ Glycolysis 10 steps – specific enzyme Energy Investment Glucose to 2 PGAL uses ATP Energy Payoff PGAL to Pyruvic Acid Produces ATP PGAL/ G3P – phosphoglyceraldehyde or glyceraldehyde-3-phosphate 15 GLYCOLYSIS Energy investment phase and splitting of glucose Two ATPs invested per glucose Glucose 2 ATP 3 steps 2 ADP Fructose-1,6-bisphosphate P P Glyceraldehyde Glyceraldehyde phosphate phosphate (G3P) (G3P) P PGAL P 16 Fig. 8-3, p. 175 Energy capture phase Four ATPs and two NADH produced per glucose P P (G3P) (G3P) NAD+ NAD+ NADH 2 ADP 5 steps NADH 2 ATP 2 ADP 2 ATP Pyruvate Pyruvate Net yield per glucose: Two ATPs and two NADH 17 Fig. 8-3, p. 175 Glycolysis Is Glycolysis really a part of Aerobic Cellular Respiration? Occurs in both Aerobic and Anaerobic Respiration! 18 Steps of Respiration 3? 4? 5? Glycolysis Formation of Acetyl Co-Enzyme A Krebs Cycle Electron Transport Chain Chemiosmosis 19 Glycolysis …. What next? Glucose Glycolysis CYTOSOL Pyruvate No O2 present Fermentation or Anaerobic respiration O2 present -- Aerobic cellular respiration MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle 20 Formation of Acetyl Co-A MITOCHONDRION CYTOSOL NAD+ NADH + H+ Acetyl Co A Pyruvate CO2 Coenzyme A Transport protein 21 Formation of Acetyl CoA 1 pyruvate molecule Acetyl group + coenzyme A loses 1 molecule of carbon dioxide produce acetyl CoA Carbon dioxide Pyruvate NAD+ CO2 Coenzyme A NADH 1 NADH produced per pyruvate Acetyl coenzyme A 22 Citric Acid Cycle Different terms The citric acid cycle The Krebs cycle The Tricarboxylic Acid Cycle Oxidizes acetyl co-A (fuel derived from pyruvate) Produced PER Acetyl Co-A 1 ATP 3 NADH 1 FADH2 2 CO2 23 LE 9-11 Pyruvate (from glycolysis, 2 molecules per glucose) CO2 NAD+ Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle FADH2 2 CO2 3 NAD+ 3 NADH + 3 H+ FAD ADP + P i ATP ATP Citric Acid Cycle 1 acetyl CoA enters cycle 2 C enter as acetyl CoA -- 2 leave as CO2 1 acetyl CoA combines with 4-C oxaloacetate forms 6-C citrate Citrate decomposed back to oxaloacetate transfers H atoms to 3 NAD+ , 1 FAD 1 ATP produced NADH and FADH2 -- electrons to electron transport chain 25 Acetyl coenzyme A Coenzyme A Citrate Oxaloacetate NADH NAD+ NAD+ CITRIC ACID CYCLE H2O NADH CO2 FADH2 5-carbon compound FAD GTP NADH GDP 4-carbon compound ADP CO2 ATP 26 Fig. 8-6, p. 179 Electron Transport Chain NADH and FADH2 account for most of the energy extracted from food Donate electrons to the electron transport chain Probably the most important step 27 Electron Transport Chain The electron transport chain generates no ATP Function -- break the large free-energy drop from food to O2 releasing energy in manageable amounts Moves electrons around Alternating oxidized and reduced states Maintains pH gradient Electrons drop in free energy until passed to O2 forming water 28 Electron Transport Chain Chemiosmosis 4 2 4 29 LE 9-13 NADH 50 Free energy (G) relative to O2 (kcal/mol) FADH2 40 FMN I Multiprotein complexes FAD Fe•S II Fe•S Q III Cyt b 30 Fe•S Cyt c1 Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylation: electron transport and chemiosmosis IV Cyt c Cyt a Cyt a3 20 10 0 2 H+ + 1/2 O2 H2O ATP 31 Chemiosmosis ✥ Paul Mitchell theorized in 1961 Nobel Prize in Chemistry in 1978 ✥ Energy stored in pH gradient ✥ 32 Chemiosmosis: The EnergyCoupling Mechanism Electron transfer -- proteins pump H+ from mitochondrial matrix to intermembrane space -- use of energy in a H+ gradient to drive cellular work Chemiosmosis H+ through channels in ATP synthase ATP synthase uses the exergonic flow to drive phosphorylation of ATP 33 34 35 CO2 CO2 H2O 36 Electron transport chain inhibitors Lack of organic fuel Cyanide & Carbon monoxide Rotenone & Amytal Blocks I Oligomycin Blocks IV Blocks ATP synthase Antimycin Blocks III 37 CYANIDE RESISTANT RESPIRATION Aerobic respiration is inhibited when the terminal electron carrier combines with cyanide, azide or certain other negatively charged ions This poisons the enzyme and stops electron transport 38 CYANIDE RESISTANT RESPIRATION Some plants, fungi and bacteria This pathway produces heat rather than ATPs but is aerobic (i.e., oxygen is the terminal electron acceptor) The heat produced by some plants has important ecological functions. 39 This seems to be important when electron transport is saturated 40 Energy is captured from light by Philodendron leaves and used for life processes and growth When it flowers -- heats to as high as 46 C (115 F). The heat protects the flowers from freezing at night and disperses compound that attract pollinators Light energy —> Heat 41 Skunk Cabbage Air temp – 5-50 ˚F Flower temp – 60-72 ˚F -15-10 ˚C 15-22 ˚C Some flowers generate 0.4 J heat/second/gram whereas hummingbirds generate 0.24 J heat/second/gram 42 An Accounting of ATP Production by Cellular Respiration During cellular respiration -- energy flows in this sequence: glucose NADH electron transport chain pH gradient ATP About 40% of the energy is transferred to ATP -making about 38 ATP 43 LE 9-16 Electron shuttles span membrane CYTOSOL 2 NADH Glycolysis Glucose 2 Pyruvate MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 Acetyl CoA 6 NADH Citric acid cycle + 2 ATP + 2 ATP by substrate-level phosphorylation by substrate-level phosphorylation Maximum per glucose: About 36 or 38 ATP 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis + about 32 or 34 ATP by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol Fermentation Facultative anaerobes -- they can survive using either fermentation or cellular respiration Pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes 45 Glucose Glycolysis CYTOSOL Pyruvate No O2 present Fermentation or Anaerobic respiration O2 present -- Aerobic cellular respiration MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle 46 Fermentation vs Respiration Glycolysis -- oxidizes glucose to pyruvate Different final electron acceptors Fermentation -- an organic molecule (such as pyruvate) Respiration -- O2 Aerobic respiration produces much more ATP 47 Types of Fermentation Fermentation -- glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Two types alcohol fermentation lactic acid fermentation 48 Alcohol Fermentation pyruvate is converted to ethanol in two steps, with the first releasing CO2 yeast -- brewing, winemaking, and baking 49 LE 9-17a 2 ADP + 2 P i Glucose 2 ATP Glycolysis 2 Pyruvate 2 NAD+ 2 Ethanol Alcohol fermentation 2 NADH + 2 H+ 2 CO2 2 Acetaldehyde Lactic Acid Fermentation pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 fungi and bacteria -- cheese and yogurt muscle cells -- lactic acid fermentation 51 LE 9-17b 2 ADP + 2 P i Glucose 2 ATP Glycolysis 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate 2 Lactate Lactic acid fermentation Glycolysis and metabolic pathways Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways 53 LE 9-19 Proteins Carbohydrates Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P NH3 Fats Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation The Versatility of Catabolism funnel electrons into cellular respiration wide range of carbohydrates amino groups can feed glycolysis or the citric acid cycle fats -- glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) a gram of fat > 2X ATP as an oxidized gram of carbohydrate 55 FIGURE 6.1 56 Biosynthesis (Anabolic Pathways) The body uses small molecules to build other substances These small molecules may come directly from food, from glycolysis, or from the citric acid cycle 57 58 Regulation of Cellular Respiration: Feedback Mechanisms most common mechanism for control If ATP concentration drop -- respiration speeds up Enzyme control at strategic points in the catabolic pathway 59 LE 9-20 Glucose AMP Glycolysis Fructose-6-phosphate – Stimulates + Phosphofructokinase – Fructose-1,6-bisphosphate Inhibits Inhibits Pyruvate ATP Citrate Acetyl CoA Citric acid cycle Oxidative phosphorylation PHOTORESPIRATION Some plants increase their use of oxygen when CO2 gets too low, a process known as photorespiration. Photorespiration interferes with photosynthesis and causes decreased crop yield 61 62 Summary Reaction Complete oxidation of glucose C6H12O6 + 6 O2 + 6 H2O → 6 CO2 + 12 H2O + energy (36 to 38 ATP) [40% of potential energy -> ATP 60% -> wasted (heat)] 63 Summary Reaction Glycolysis C6H12O6 + 2 ATP + 2 ADP + 2 Pi + 2 NAD+ → 2 pyruvate + 4 ATP + 2 NADH + H2O 64 Summary Reaction Conversion of pyruvate to acetyl CoA 2 pyruvate + 2 coenzyme A + 2 NAD+ → 2 acetyl CoA + 2 CO2 + 2 NADH 65 Summary Reaction Citric acid cycle 2 acetyl CoA + 6 NAD+ + 2 FAD + 2 ADP + 2 Pi + 2 H2O → 4 CO2 + 6 NADH + 2 FADH2 + 2 ATP + 2 CoA 66 Summary Reactions Hydrogen atoms in Electron Transport Chain NADH + 3 ADP + 3 Pi + 12 O2 → NAD+ + 3 ATP + H2O FADH2 + 2 ADP + 2 Pi + 12 O2 → FAD + 2 ATP + H2O 67 Summary Reaction Lactate fermentation C6H12O6 → 2 lactate + 2 ATP + NAD+ Pyruvate → NAD+ + lactate/lactic acid 68 Summary Reaction Alcohol fermentation C6H12O6 → 2 CO2 + 2 C2H5OH + 2 ATP + NAD+ Pyruvate -> NAD+ + ethanol + CO2 69 70