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9/25/2011 Chapter 9 – Cellular Respiration and Fermentation Overview: Life Is Work Living cells require energy from outside sources Some animals, obtain energy by eating plants, and some animals feed on other organisms that eat plants © 2011 Pearson Education, Inc. Figure 9.1 Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O2 and organic molecules, which are used in cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work © 2011 Pearson Education, Inc. Figure 9.2 Catabolic pathways yield energy by oxidizing organic fuels Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2 H2O Cellular respiration in mitochondria ATP Organic O2 molecules Several processes are central to cellular respiration and related pathways ATP powers most cellular work Heat energy © 2011 Pearson Education, Inc. 1 9/25/2011 Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O2 Aerobic respiration consumes organic molecules and O2 and yields ATP Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2 © 2011 Pearson Education, Inc. Cellular Respiration Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat) © 2011 Pearson Education, Inc. Redox Reactions: Oxidation and Reduction The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP The Principle of Redox Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions • In oxidation, a substance loses electrons, or is oxidized In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Figure 9.UN01 The electron donor is called the reducing agent The electron receptor is called the oxidizing agent becomes oxidized (loses electron) becomes reduced (gains electron) © 2011 Pearson Education, Inc. 2 9/25/2011 Oxidation of Organic Fuel Molecules During Cellular Respiration Figure 9.UN03 During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced becomes oxidized becomes reduced © 2011 Pearson Education, Inc. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain In cellular respiration, glucose and other organic molecules are broken down in a series of steps Figure 9.4 NAD NADH Dehydrogenase Reduction of NAD (from food) Electrons from organic compounds are usually first transferred to NAD+, a coenzyme Nicotinamide (oxidized form) Oxidation of NADH Nicotinamide (reduced form) As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP © 2011 Pearson Education, Inc. Figure 9.UN04 The Role of NADH in Cellular Respiration NADH passes the electrons to the electron transport chain Dehydrogenase Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction O2 pulls electrons down the chain in an energyyielding tumble The energy yielded is used to regenerate ATP © 2011 Pearson Education, Inc. 3 9/25/2011 Figure 9.5 The Stages of Cellular Respiration: A Preview Free energy, G Free energy, G H2 1/2 O2 Explosive release of heat and light energy 1/ O 2H 2 2 (from food via NADH) Controlled release of 2 H+ 2 e energy for synthesis of ATP ATP Glycolysis (breaks down glucose into two molecules of pyruvate) ATP The pyruvate oxidation and citric acid cycle (completes the breakdown of glucose) ATP 2 e 1/ 2 2 H+ H2 O O2 H2 O (a) Uncontrolled reaction Harvesting of energy from glucose has three stages Oxidative phosphorylation (accounts for most of the ATP synthesis) (b) Cellular respiration © 2011 Pearson Education, Inc. Figure 9.UN05 Figure 9.6-1 Electrons carried via NADH 1. Glycolysis (color-coded teal throughout the chapter) 2. Pyruvate oxidation and the citric acid cycle (color-coded salmon) 3. Oxidative phosphorylation: electron transport and chemiosmosis (color-coded violet) Glycolysis Glucose Pyruvate MITOCHONDRION CYTOSOL ATP Substrate-level phosphorylation Figure 9.6-2 Figure 9.6-3 Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolysis Glucose Pyruvate CYTOSOL Pyruvate oxidation Acetyl CoA Citric acid cycle MITOCHONDRION Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolysis Glucose Pyruvate CYTOSOL Pyruvate oxidation Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis MITOCHONDRION ATP ATP ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation 4 9/25/2011 Oxidative Phosphorylation The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions BioFlix: Cellular Respiration © 2011 Pearson Education, Inc. Substrate-level Phosphorylation © 2011 Pearson Education, Inc. Figure 9.7 Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration Enzyme A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP Enzyme ADP P Substrate ATP Product © 2011 Pearson Education, Inc. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Figure 9.8 Energy Investment Phase Glucose 2 ADP 2 P Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases Energy Payoff Phase 4 ADP 4 P Energy investment phase Energy payoff phase Glycolysis occurs whether or not O2 is present 2 ATP used 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+ © 2011 Pearson Education, Inc. 5 9/25/2011 Figure 9.9a Figure 9.9b Glycolysis: Energy Investment Phase Fructose 6-phosphate Glycolysis: Energy Investment Phase Glucose ATP Glucose 6-phosphate ADP ATP Fructose 1,6-bisphosphate ADP Phosphofructokinase Fructose 6-phosphate Aldolase 3 Hexokinase Phosphoglucoisomerase 1 Dihydroxyacetone phosphate 2 Glyceraldehyde 3-phosphate Isomerase Figure 9.9c 4 To step 6 5 Figure 9.9d Glycolysis: Energy Payoff Phase Glycolysis: Energy Payoff Phase 2 ATP 2 NADH 2 NAD +2 2 H2O 2 ADP H 2 2 2 2 ATP 2 ADP 2 2 2 Triose phosphate dehydrogenase 6 Phosphoglyceromutase Phosphoglycerokinase 2Pi 1,3-Bisphosphoglycerate 7 3-Phosphoglycerate After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed 3-Phosphoglycerate 8 Enolase 2-Phosphoglycerate 9 Pyruvate kinase Phosphoenolpyruvate (PEP) 10 Pyruvate Oxidation of Pyruvate to Acetyl CoA Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle This step is carried out by a multienzyme complex that catalyses three reactions © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 6 9/25/2011 Coenzyme A Figure 9.10 MITOCHONDRION CYTOSOL CO2 Coenzyme A 1 3 2 NADH + H NAD Pyruvate Acetyl CoA Transport protein The Citric Acid Cycle Figure 9.11 Pyruvate CO2 NAD CoA The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2 NADH + H Acetyl CoA CoA CoA The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn, there will be two turns for every glucose that went into glycolysis Citric acid cycle 2 CO2 3 NAD FADH2 3 NADH FAD + 3 H ADP + P i ATP © 2011 Pearson Education, Inc. The Citric Acid Cycle Figure 9.12-1 Acetyl CoA • The citric acid cycle has eight steps, each catalyzed by a specific enzyme CoA-SH 1 Oxaloacetate • The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate • The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle Citrate Citric acid cycle • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain © 2011 Pearson Education, Inc. 7 9/25/2011 Figure 9.12-2 Figure 9.12-3 Acetyl CoA Acetyl CoA CoA-SH CoA-SH H2O 1 H2O 1 Oxaloacetate Oxaloacetate 2 2 Citrate Citrate Isocitrate Isocitrate NAD Citric acid cycle Citric acid cycle NADH 3 + H CO2 -Ketoglutarate Figure 9.12-4 Figure 9.12-5 Acetyl CoA Acetyl CoA CoA-SH CoA-SH H2O 1 H2O 1 Oxaloacetate Oxaloacetate 2 2 Citrate Citrate Isocitrate Isocitrate NAD Citric acid cycle NAD Citric acid cycle NADH 3 + H CO2 NADH 3 + H CO2 CoA-SH CoA-SH -Ketoglutarate -Ketoglutarate 4 4 CoA-SH 5 CO2 NAD Succinate + H Succinyl CoA CO2 NAD NADH NADH Pi GTP GDP + H Succinyl CoA ADP ATP Figure 9.12-6 Figure 9.12-7 Acetyl CoA Acetyl CoA CoA-SH CoA-SH H2O 1 H2O 1 Oxaloacetate Oxaloacetate 2 2 Malate Citrate Citrate Isocitrate Isocitrate NAD Citric acid cycle Fumarate NAD Citric acid cycle NADH 3 + H CO2 7 H2O Fumarate CoA-SH CoA-SH -Ketoglutarate 4 5 NAD FAD Succinate GTP GDP ADP ATP Pi Succinyl CoA 4 6 CoA-SH FADH2 NADH + H CO2 + H CO2 -Ketoglutarate 6 NADH 3 CoA-SH 5 FADH2 NAD FAD Succinate GTP GDP Pi Succinyl CoA CO2 NADH + H ADP ATP 8 9/25/2011 Figure 9.12-8 Figure 9.12a Acetyl CoA CoA-SH NADH H2O 1 + H Acetyl CoA NAD Oxaloacetate 8 2 Malate CoA-SH Citrate Isocitrate NAD Citric acid cycle 7 H2O NADH 3 + H CO2 Fumarate H2O 1 CoA-SH -Ketoglutarate Oxaloacetate 4 6 CoA-SH 2 5 FADH2 NAD FAD Succinate NADH Pi + H Succinyl CoA GTP GDP CO2 Citrate ADP Isocitrate ATP Figure 9.12b Figure 9.12c Fumarate Isocitrate NAD 6 NADH 3 CoA-SH + H CO2 5 FADH2 FAD CoA-SH -Ketoglutarate Succinate 4 Pi GTP GDP CO2 NAD Succinyl CoA ADP NADH ATP + H Succinyl CoA Figure 9.12d During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis NADH + H NAD 8 Oxaloacetate • NADH and FADH2 account for most of the energy extracted from food Malate • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation 7 H2O Fumarate © 2011 Pearson Education, Inc. 9 9/25/2011 Figure 9.13 The Pathway of Electron Transport NADH 50 2 e The electron transport chain is in the inner membrane (cristae) of the mitochondrion NAD FADH2 Free energy (G) relative to O2 (kcal/mol) 2 e Most of the chain’s components are proteins, which exist in multiprotein complexes The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O 40 Multiprotein complexes FAD I FMN Fe S Fe S II Q III Cyt b Fe S 30 Cyt c1 IV Cyt c Cyt a Cyt a3 20 2 e 10 (originally from NADH or FADH2) 2 H + 1/2 O2 0 H2 O © 2011 Pearson Education, Inc. Chemiosmosis: The Energy-Coupling Mechanism The Pathway of Electron Transport Electrons are transferred from NADH or FADH2 to the electron transport chain Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2 Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space H+ then moves back across the membrane, passing through the proton, ATP synthase The electron transport chain generates no ATP directly ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Figure 9.15 Figure 9.14 INTERMEMBRANE SPACE H Stator H H Rotor H Protein complex of electron carriers Q I IV III II FADH2 FAD NADH H Cyt c 2 H + 1/2O2 ATP synthase H2O NAD ADP P i (carrying electrons from food) ATP Internal rod Catalytic knob H 1 Electron transport chain Oxidative phosphorylation 2 Chemiosmosis ADP + Pi ATP MITOCHONDRIAL MATRIX 10 9/25/2011 The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work © 2011 Pearson Education, Inc. An Accounting of ATP Production by Cellular Respiration Figure 9.16 Electron shuttles span membrane During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP 2 NADH Glycolysis Glucose 2 Pyruvate MITOCHONDRION 2 NADH or 2 FADH2 2 NADH Pyruvate oxidation 2 Acetyl CoA 2 ATP Maximum per glucose: There are several reasons why the number of ATP is not known exactly 6 NADH 2 FADH2 Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis 2 ATP about 26 or 28 ATP About 30 or 32 ATP CYTOSOL © 2011 Pearson Education, Inc. Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen • Most cellular respiration requires O2 to produce ATP • Without O2, the electron transport chain will cease to operate In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP © 2011 Pearson Education, Inc. 11 9/25/2011 Anaerobic Respiration and Fermentation Types of Fermentation Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, for example sulfate Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP Two common types are alcohol fermentation and lactic acid fermentation Which is used by yeast? © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Alcohol Fermentation In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 Alcohol fermentation by yeast is used in brewing, winemaking, and baking Animation: Fermentation Overview Right-click slide / select “Play” © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Figure 9.17a Lactic Acid Fermentation 2 ADP 2 P i Glucose 2 ATP In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 Glycolysis 2 Pyruvate 2 NAD 2 NADH 2 H 2 CO2 Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation © 2011 Pearson Education, Inc. 12 9/25/2011 Figure 9.17b 2 ADP 2 P i Glucose Comparing Fermentation with Anaerobic and Aerobic Respiration 2 ATP Glycolysis 2 NAD 2 NADH 2 H 2 Pyruvate 2 Lactate All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule (b) Lactic acid fermentation © 2011 Pearson Education, Inc. Figure 9.18 Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 Glucose CYTOSOL Pyruvate No O2 present: Fermentation Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes Glycolysis O2 present: Aerobic cellular respiration MITOCHONDRION Ethanol, lactate, or other products Acetyl CoA Citric acid cycle © 2011 Pearson Education, Inc. The Evolutionary Significance of Glycolysis Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere Very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP Glycolysis and the citric acid cycle connect to many other metabolic pathways Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways Glycolysis is a very ancient process © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 13 9/25/2011 The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) Glycolysis accepts a wide range of carbohydrates Fatty acids are broken down by oxidation and yield acetyl CoA Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate © 2011 Pearson Education, Inc. Figure 9.19 © 2011 Pearson Education, Inc. Proteins Carbohydrates Amino acids Sugars Fats Biosynthesis (Anabolic Pathways) Glycerol Fatty acids Glycolysis Glucose The body uses small molecules to build other substances Glyceraldehyde 3- P NH3 Pyruvate Acetyl CoA These small molecules may come directly from food, from glycolysis, or from the citric acid cycle Citric acid cycle Oxidative phosphorylation © 2011 Pearson Education, Inc. Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition is the most common mechanism for control Figure 9.20 Glucose AMP Glycolysis Fructose 6-phosphate Phosphofructokinase Fructose 1,6-bisphosphate Inhibits Inhibits If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Pyruvate ATP Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway Stimulates Citrate Acetyl CoA Citric acid cycle Oxidative phosphorylation © 2011 Pearson Education, Inc. 14 9/25/2011 Figure 9.UN07 Oxidation of Pyruvate to Acetyl CoA and Citric Acid Cycle Glycolysis Glycolysis Outputs Inputs Inputs Outputs 2 Pyruvate 2 Acetyl CoA Glycolysis Glucose 2 Pyruvate 2 ATP 2 NADH 2 Oxaloacetate 2 ATP 8 NADH 6 CO2 2 FADH2 Citric acid cycle Summary of Cellular Respiration VCAC: Cellular Processes: Electron Transport Chain One molecule of glucose is broken down and 32 - 38 ATP are generated. Oxygen is used by the electron transport chain Carbon dioxide is produced by the Transition Reaction Krebs cycle Summary of Cellular Respiration Glycolysis: Starts the process by taking in glucose. Produces 2 ATP The Pyruvate Oxidation produces CO2 and NADH The Citric acid cycle: Gives 2 ATP but also produces lots of NADH and FADH2. Produces CO2. Summary of Cellular Respiration Electron transport chain Takes electrons from NADH and FADH2 and uses them to produce ATP using the ATP synthase molecule. Requires oxygen. Oxygen is the final electron acceptor on the electron transport chain 15 9/25/2011 Which stage produces CO2 Which stage uses O2 Tr El ec t ro n Tr ro n ec t El Which stage produces the most ATP 1. Glycolysis 2. Krebs Cycle 3. Electron Transport Chain C Kr e an sp bs ly co Tr ro n ec t El El ec t ro n Tr G an sp or t. .. yc le 33% 33% 33% or t. .. C bs Kr e G ly co ly si s yc le 33% 33% 33% ly si s Which stage produces the most NADHs 1. Glycolysis 2. Krebs Cycle 3. Electron Transport Chain or t. .. Kr e an sp bs ly co C ly si s yc le 33% 33% 33% an sp Kr e G 1. Glycolysis 2. Krebs Cycle 3. Electron Transport Chain or t. .. C bs ly co ly si s yc le 33% 33% 33% G 1. Glycolysis 2. Krebs Cycle 3. Electron Transport Chain Which process requires oxygen? Glucose is the only molecule that can be broken down for energy in cellular respiration at io en t sp Fe rm re bi c ce llu la r ce llu la r er o A na er ob ic A ls e Fa ue Tr n 33% 33% 33% 1. Anaerobic cellular respiration 2. Aerobic cellular respiration 3. Fermentation re s. .. 50% i.. . 50% 1. True 2. False 16 9/25/2011 Important Concepts Know what Aerobic Cellular respiration, Anaerobic cellular respiration and Anaerobic Fermentation are and the differences between them. Know the overall reaction of aerobic cellular respiration and what molecule is oxidized and what is reduced Know the four steps of aerobic cellular respiration, what happens in each step, what is the starting molecules are, what comes out of each step, where in the cell does each step occur. How many ATP, NADH and FADH2 are produced in each step Important Concepts What is the role of NADH and FADH2 How is ATP made in the ETC, what is the role of ATPsynthase? Know the overall picture of cellular respiration What is the role of oxygen in cellular respiration, what steps produce carbon dioxide Know Anaerobic Fermentation, what happens, what are the end products, when is it used, what organisms use it and the different reactions depending on the organisms (ie yeast vs animals). 17