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Cellular Respiration: Harvesting Chemical Energy Chapter 9 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 3. Explain the role of the electron transport chain in cellular respiration 2. Name and describe the three stages of cellular respiration; for each, state the region of the eukaryotic cell where it occurs and the products that result 1. Explain how redox reactions are involved in energy exchanges You should be able to: Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 6. Distinguish between obligate and facultative anaerobes 5. Distinguish and explain fermentation and anaerobic respiration 4. Explain where and how the respiratory electron transport chain creates a proton gradient Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Some animals, such as the giant panda, obtain energy by eating plants, and some animals feed on other organisms that eat plants • Living cells require energy from outside sources Overview: Life Is Work Fig. 9-1 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work • Photosynthesis generates O2 and organic molecules, which are used in cellular respiration • Energy flows into an ecosystem as sunlight and leaves as heat Fig. 9-2 ATP Cellular respiration in mitochondria Photosynthesis in chloroplasts Organic +O molecules 2 Heat energy ATP powers most cellular work CO2 + H2O ECOSYSTEM Light energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Several processes are central to cellular respiration and related pathways Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2 • Aerobic respiration consumes organic molecules and O2 and yields ATP • Fermentation is a partial degradation of sugars that occurs without O2 • The breakdown of organic molecules is exergonic Catabolic Pathways and Production of ATP 6 CO2 + 6 H2O + Energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings C6H12O6 + 6 O2 (ATP + heat) • Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: • Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • This released energy is ultimately used to synthesize ATP • The transfer of electrons during chemical reactions releases energy stored in organic molecules Redox Reactions: Oxidation and Reduction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) • In oxidation, a substance loses electrons, or is oxidized • Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions The Principle of Redox Fig. 9-UN1 becomes reduced (gains electron) becomes oxidized (loses electron) Fig. 9-UN2 becomes reduced becomes oxidized Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • An example is the reaction between methane and O2 • Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds • The electron receptor is called the oxidizing agent • The electron donor is called the reducing agent Oxygen (oxidizing agent) becomes oxidized Reactants Methane (reducing agent) Fig. 9-3 Carbon dioxide becomes reduced Products Water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced: Oxidation of Organic Fuel Molecules During Cellular Respiration Fig. 9-UN3 becomes reduced becomes oxidized Fig. 9-UN4 Dehydrogenase Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP • As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration • Electrons from organic compounds are usually first transferred to NAD+, a coenzyme • In cellular respiration, glucose and other organic molecules are broken down in a series of steps Stepwise Energy Harvest via NAD+ and the Electron Transport Chain Fig. 9-4 NAD+ 2[H] Nicotinamide (oxidized form) + Oxidation of NADH Reduction of NAD+ Dehydrogenase 2 e– + 2 H+ 2 e– + H+ Nicotinamide (reduced form) NADH + H+ H+ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The energy yielded is used to regenerate ATP • O2 pulls electrons down the chain in an energyyielding tumble • Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction • NADH passes the electrons to the electron transport chain Fig. 9-5 (a) Uncontrolled reaction Explosive release of heat and light energy H2 + 1/2 O2 (b) Cellular respiration 2H (from food via NADH) Controlled release of + – 2H + 2e energy for synthesis of ATP 1/ O 2 2 1/ O 2 2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings – Oxidative phosphorylation (accounts for most of the ATP synthesis) – The citric acid cycle (completes the breakdown of glucose) – Glycolysis (breaks down glucose into two molecules of pyruvate) • Cellular respiration has three stages: The Stages of Cellular Respiration: A Preview ATP Substrate-level phosphorylation Cytosol Pyruvate Glycolysis Electrons carried via NADH Glucose Fig. 9-6-1 ATP Substrate-level phosphorylation Substrate-level phosphorylation Mitochondrion Citric acid cycle Electrons carried via NADH and FADH2 ATP Cytosol Pyruvate Glycolysis Electrons carried via NADH Glucose Fig. 9-6-2 ATP Oxidative phosphorylation Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation: electron transport and chemiosmosis ATP Mitochondrion Citric acid cycle Electrons carried via NADH and FADH2 ATP Cytosol Pyruvate Glycolysis Electrons carried via NADH Glucose Fig. 9-6-3 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation • Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration ADP Enzyme P Substrate Fig. 9-7 Product + ATP Enzyme Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings – Energy payoff phase – Energy investment phase • Glycolysis occurs in the cytoplasm and has two major phases: • Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Fig. 9-8 Glucose 2 NAD+ + 4 e– + 4 H+ 4 ATP formed – 2 ATP used Net 2 NAD+ + 4 e– + 4 H+ 4 ADP + 4 P Energy payoff phase 2 ADP + 2 P Glucose Energy investment phase formed used 2 NADH + 2 H+ 2 ATP 2 Pyruvate + 2 H2O 2 Pyruvate + 2 H2O 2 NADH + 2 H+ 4 ATP 2 ATP Fig. 9-9-1 1 Hexokinase Glucose-6-phosphate ADP ATP Glucose 1 Hexokinase Glucose-6-phosphate ADP ATP Glucose Fig. 9-9-2 1 Hexokinase Fructose-6-phosphate 2 Phosphoglucoisomerase Glucose-6-phosphate ADP ATP Glucose Fructose-6-phosphate 2 Phosphoglucoisomerase Glucose-6-phosphate Fig. 9-9-3 1 Hexokinase 3 Phosphofructokinase Fructose1, 6-bisphosphate ADP ATP Fructose-6-phosphate 2 Phosphoglucoisomerase Glucose-6-phosphate ADP ATP Glucose Phosphofructokinase 3 Fructose1, 6-bisphosphate ADP ATP Fructose-6-phosphate 1 Hexokinase Glucose 3 Phosphofructokinase 5 Isomerase Glyceraldehyde3-phosphate 4 Aldolase Fructose1, 6-bisphosphate ADP ATP Fructose-6-phosphate 2 Phosphoglucoisomerase Glucose-6-phosphate ADP ATP Dihydroxyacetone phosphate Fig. 9-9-4 Dihydroxyacetone phosphate Isomerase 5 Glyceraldehyde3-phosphate Aldolase 4 Fructose1, 6-bisphosphate Fig. 9-9-5 6 Triose phosphate dehydrogenase 2 Pi 2 1, 3-Bisphosphoglycerate 2 NADH + 2 H+ 2 NAD+ 6 Triose phosphate dehydrogenase 2 Pi 2 1, 3-Bisphosphoglycerate + 2 H+ 2 NADH 2 NAD+ Glyceraldehyde3-phosphate Fig. 9-9-6 6 Triose phosphate dehydrogenase 2 Pi 2 7 Phosphoglycerokinase 3-Phosphoglycerate 2 ATP 2 ADP 2 1, 3-Bisphosphoglycerate 2 NADH + 2 H+ 2 NAD+ 2 7 Phosphoglycerokinase 3-Phosphoglycerate 2 ATP 2 ADP 2 1, 3-Bisphosphoglycerate Fig. 9-9-7 6 Triose phosphate dehydrogenase 2 Pi 2 2 7 Phosphoglycerokinase 2-Phosphoglycerate Phosphoglyceromutase 8 3-Phosphoglycerate 2 ATP 2 ADP 2 1, 3-Bisphosphoglycerate 2 NADH + 2 H+ 2 NAD+ 2 2 2-Phosphoglycerate 8 Phosphoglyceromutase 3-Phosphoglycerate Fig. 9-9-8 6 Triose phosphate dehydrogenase 2 Pi 2 2 2 7 Phosphoglycerokinase 9 Enolase Phosphoenolpyruvate 2 H2O 2-Phosphoglycerate Phosphoglyceromutase 8 3-Phosphoglycerate 2 ATP 2 ADP 2 1, 3-Bisphosphoglycerate 2 NADH + 2 H+ 2 NAD+ 2 2 Phosphoenolpyruvate 2 H2O Enolase 9 2-Phosphoglycerate Fig. 9-9-9 6 2 Pi Triose phosphate dehydrogenase 2 H2O Enolase 9 2-Phosphoglycerate Phosphoglyceromutase 8 3-Phosphoglycerate 7 Phosphoglycerokinase 2 2 ATP Pyruvate 10 Pyruvate kinase 2 Phosphoenolpyruvate 2 ADP 2 2 2 ATP 2 ADP 2 1, 3-Bisphosphoglycerate 2 NADH + 2 H+ 2 NAD+ 2 ADP 2 Pyruvate 10 Pyruvate kinase Phosphoenolpyruvate 2 ATP 2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis • In the presence of O2, pyruvate enters the mitochondrion Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules CYTOSOL Transport protein Pyruvate Fig. 9-10 1 CO2 NAD+ 3 Coenzyme A 2 NADH + H+ Acetyl CoA MITOCHONDRION Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn • The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix Fig. 9-11 NAD+ FAD + H+ NADH FADH2 Pyruvate ATP CoA ADP + P i Citric acid cycle Acetyl CoA CoA CoA CO2 + 3 H+ 3 NADH 3 NAD+ 2 CO2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain • The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle • The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate • The citric acid cycle has eight steps, each catalyzed by a specific enzyme Fig. 9-12-1 Citric acid cycle Oxaloacetate 1 Citrate CoA—SH Acetyl CoA Fig. 9-12-2 Citric acid cycle Oxaloacetate 1 Citrate CoA—SH Acetyl CoA 2 Isocitrate H2O Fig. 9-12-3 Citric acid cycle Oxaloacetate 1 Citrate CoA—SH Acetyl CoA 2 NAD+ 3 -Ketoglutarate CO2 NADH + H+ Isocitrate H2O Fig. 9-12-4 Citric acid cycle Oxaloacetate 1 2 Succinyl CoA NAD+ NADH + H+ 3 CO2 -Ketoglutarate CO2 NADH + H+ Isocitrate H2O NAD+ 4 CoA—SH Citrate CoA—SH Acetyl CoA Fig. 9-12-5 ATP ADP GTP GDP Succinate 5 Pi CoA—SH Citric acid cycle Oxaloacetate 1 2 Succinyl CoA NAD+ NADH + H+ 3 CO2 -Ketoglutarate CO2 NADH + H+ Isocitrate H2O NAD+ 4 CoA—SH Citrate CoA—SH Acetyl CoA Fig. 9-12-6 FADH2 FAD 5 ATP ADP Pi CoA—SH GTP GDP Succinate 6 Fumarate Citric acid cycle Oxaloacetate 1 2 Succinyl CoA NAD+ NADH + H+ 3 CO2 -Ketoglutarate CO2 NADH + H+ Isocitrate H2O NAD+ 4 CoA—SH Citrate CoA—SH Acetyl CoA Fig. 9-12-7 H2O FADH2 7 FAD 5 ATP ADP Pi CoA—SH Citric acid cycle GTP GDP Succinate 6 Fumarate Malate Oxaloacetate 1 2 Succinyl CoA 4 NAD+ NADH + H+ 3 CO2 -Ketoglutarate CO2 NADH + H+ Isocitrate H2O NAD+ CoA—SH Citrate CoA—SH Acetyl CoA Fig. 9-12-8 H2O FADH2 7 NAD+ 8 FAD Fumarate 5 ATP ADP Pi CoA—SH Citric acid cycle GTP GDP Succinate 6 1 Oxaloacetate Malate NADH +H+ 2 Succinyl CoA 4 NAD+ NADH + H+ 3 CO2 -Ketoglutarate CO2 NADH + H+ Isocitrate H2O NAD+ CoA—SH Citrate CoA—SH Acetyl CoA Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation • Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O • The carriers alternate reduced and oxidized states as they accept and donate electrons • Most of the chain’s components are proteins, which exist in multiprotein complexes • The electron transport chain is in the cristae of the mitochondrion The Pathway of Electron Transport Fig. 9-13 0 10 20 30 40 50 FMN 2 e– Fe•S NAD+ NADH Q Cyt b Fe•S FAD 2 e– Fe•S FAD FADH2 Cyt c1 Cyt a Cyt a3 I V H 2O 2 H+ + 1/2 O2 2 e– (from NADH or FADH2) Cyt c Multiprotein complexes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The chain’s function is to break the large freeenergy drop from food to O2 into smaller steps that release energy in manageable amounts • The electron transport chain generates no ATP • Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2 • Electrons are transferred from NADH or FADH2 to the electron transport chain Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP • H+ then moves back across the membrane, passing through channels in ATP synthase • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space Chemiosmosis: The Energy-Coupling Mechanism Fig. 9-14 ATP Stator MITOCHONDRIAL MATRIX i ADP + P Catalytic knob Internal rod Rotor H+ INTERMEMBRANE SPACE Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • The H+ gradient is referred to as a protonmotive force, emphasizing its capacity to do work • The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis H+ (carrying electrons from food) NADH Protein complex of electron carriers Fig. 9-16 NAD+ FAD V 2 H+ + 1/2O2 Cyt c H+ Oxidative phosphorylation 1 Electron transport chain FADH2 Q H+ H2O H+ ATP synthase ATP 2 Chemiosmosis ADP + P i H+ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP glucose NADH electron transport chain proton-motive force ATP • During cellular respiration, most energy flows in this sequence: An Accounting of ATP Production by Cellular Respiration 2 NADH Electron shuttles span membrane + 2 ATP Glucose 2 Pyruvate Glycolysis CYTOSOL Fig. 9-17 Maximum per glucose: 2 Acetyl CoA 2 NADH or 2 FADH2 2 NADH About 36 or 38 ATP + 2 ATP Citric acid cycle 6 NADH + about 32 or 34 ATP Oxidative phosphorylation: electron transport and chemiosmosis 2 FADH2 MITOCHONDRION Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP • Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) • Most cellular respiration requires O2 to produce ATP Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Fermentation uses phosphorylation instead of an electron transport chain to generate ATP • Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Two common types are alcohol fermentation and lactic acid fermentation • Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Types of Fermentation Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Alcohol fermentation by yeast is used in brewing, winemaking, and baking • In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 Fig. 9-18 2 NAD+ 2 NAD+ (b) Lactic acid fermentation 2 Lactate 2 ATP 2 CO2 2 Pyruvate 2 NADH + 2 H+ 2 Pyruvate 2 Acetaldehyde 2 NADH + 2 H+ Glycolysis 2 ADP + 2 Pi Glucose 2 ATP Glycolysis (a) Alcohol fermentation 2 Ethanol Glucose 2 ADP + 2 Pi 2 Ethanol Glucose 2 NAD+ 2 ATP 2 CO2 2 Pyruvate 2 Acetaldehyde 2 NADH + 2 H+ Glycolysis 2 ADP + 2 P i (a) Alcohol fermentation Fig. 9-18a Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce • Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt • In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 Fig. 9-18b 2 NAD+ 2 ATP 2 NADH + 2 H+ Glycolysis (b) Lactic acid fermentation 2 Lactate Glucose 2 ADP + 2 P i 2 Pyruvate Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule • The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration • Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate Fermentation and Aerobic Respiration Compared Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes • Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration • Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 Fig. 9-19 Ethanol or lactate No O2 present: Fermentation CYTOSOL Acetyl CoA Citric acid cycle MITOCHONDRION O2 present: Aerobic cellular respiration Pyruvate Glycolysis Glucose Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere • Glycolysis occurs in nearly all organisms The Evolutionary Significance of Glycolysis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle • Glycolysis accepts a wide range of carbohydrates • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration The Versatility of Catabolism Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate • Fatty acids are broken down by beta oxidation and yield acetyl CoA • Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) Fig. 9-20 NH3 Sugars Amino acids Oxidative phosphorylation Citric acid cycle Acetyl CoA Pyruvate Glyceraldehyde-3- P Glucose Glycolysis Carbohydrates Proteins Glycerol Fatty acids Fats Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • These small molecules may come directly from food, from glycolysis, or from the citric acid cycle • The body uses small molecules to build other substances Biosynthesis (Anabolic Pathways) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway • If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down • Feedback inhibition is the most common mechanism for control Regulation of Cellular Respiration via Feedback Mechanisms Fig. 9-21 ATP Inhibits – Oxidative phosphorylation cycle Citric acid Acetyl CoA Pyruvate Fructose-1,6-bisphosphate Phosphofructokinase Glycolysis Fructose-6-phosphate Glucose Citrate Inhibits – Stimulates + AMP