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Chapter 9 Cellular Respiration: Harvesting Chemical Energy PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Life Is Work • Living cells require energy from outside sources • Some animals, such as the giant panda, obtain energy by eating plants; others feed on organisms that eat plants Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration • Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-2 Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic + O molecules 2 CO2 + H2O Cellular respiration in mitochondria ATP powers most cellular work Heat energy 3 Key Pathways of Cellular Respiration 1. Glycolysis 2. Citric Acid Cycle 3. Oxidative Phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels A. Catabolic Pathways and Production of ATP 1. The breakdown of organic molecules is exergonic 2. Fermentation is a partial degradation of sugars that occurs without oxygen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Catabolic Pathways and Production of ATP 3. Cellular respiration consumes oxygen and organic molecules and yields ATP 4. Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + heat) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings B. Redox Reactions: Oxidation and Reduction 1. The transfer of electrons during chemical reactions releases energy stored in organic molecules 2. This released energy is ultimately used to synthesize ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C. The Principle of Redox 1. Chemical reactions that transfer electrons between reactants are called oxidationreduction reactions, or redox reactions 2. Oxidation a. substance loses electrons becomes oxidized (loses electron) b. is oxidized Xe- + Y X + Ye- becomes reduced (gains electron) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C. The Principle of Redox 3. Reduction a. substance gains electrons b. is reduced (the amount of positive charge is reduced) becomes oxidized (loses electron) Xe- + Y X + Ye- becomes reduced (gains electron) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4. The electron donor is called the reducing agent 5. The electron receptor is called the oxidizing agent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 6. Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds • An example is the reaction between methane and oxygen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-3 Products Reactants becomes oxidized CH4 2 O2 + CO2 C Energy 2 H2O + becomes reduced H H + H O O O C O H O H Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water H D. Oxidation of Organic Fuel Molecules During Cellular Respiration 1. During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced: becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings D. Oxidation of Organic Fuel Molecules During Cellular Respiration 2. In general, organic molecules with an abundance of hydrogen are excellent fuels because their bonds release energy when electrons fall down their energy gradient (usually toward oxygen) 3. Activation energy barrier holds back the flood of electrons to a lower energy state a. enzymes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings E. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain 1. In cellular respiration, glucose and other organic molecules are broken down in a series of steps 2. Electrons from organic compounds are usually first transferred to NAD+ a. coenzyme b. electron acceptor - functions as an oxidizing agent during cellular respiration c. derived from vitamin niacin Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings E. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain 3. Energy is passed to NADH a. the reduced form of NAD+ b. NADH passes the electrons to the electron transport chain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-4 2 e– + 2 H+ NAD+ 2 e– + H+ H+ NADH Dehydrogenase + 2[H] (from food) Nicotinamide (oxidized form) + Nicotinamide (reduced form) H+ 4. the electron transport chain passes electrons in a series of steps instead of one explosive reaction a. Oxygen pulls electrons down the chain in an energy-yielding tumble Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 F. Cellular Respiration has three stages 1. Glycolysis (breaks down glucose into two molecules of pyruvate) a. A small amount of ATP is formed in glycolysis by substrate-level phosphorylation 1) substrate-level phosphorylation occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP [Animation listed on slide following figure] Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings F. Cellular Respiration has three stages 2. The citric acid cycle (completes the breakdown of glucose) a. a small amount of ATP is formed in citric acid cycle by substrate-level phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings F. Cellular Respiration has three stages 3. Oxidative phosphorylation (accounts for most of the ATP synthesis) a. The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions b. oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration c. inorganic phosphate is transferred to ADP [Animation listed on slide following figure] Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-6_1 Glycolysis Pyruvate Glucose Cytosol Mitochondrion ATP Substrate-level phosphorylation LE 9-6_2 Glycolysis Pyruvate Glucose Cytosol Citric acid cycle Mitochondrion ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation LE 9-6_3 Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolysis Pyruvate Glucose Cytosol Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation Animation: Cell Respiration Overview Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-7 Enzyme Enzyme ADP P Substrate + Product ATP Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate A. Glycolysis 1. “splitting of sugar” 2. breaks down glucose into two molecules of pyruvate 3. 2-3 carbon sugars Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate B. Glycolysis occurs in the cytoplasm C. two major phases: 1. Energy investment phase 2. Energy payoff phase Animation: Glycolysis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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+ LE 9-9a_1 Glucose ATP Hexokinase ADP Glucose-6-phosphate Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP LE 9-9a_2 Glucose ATP Hexokinase ADP Glucose-6-phosphate Phosphoglucoisomerase Fructose-6-phosphate ATP Phosphofructokinase ADP Fructose1, 6-bisphosphate Aldolase Isomerase Dihydroxyacetone phosphate Glyceraldehyde3-phosphate Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP LE 9-9b_1 2 NAD+ Triose phosphate dehydrogenase 2 NADH + 2 H+ 1, 3-Bisphosphoglycerate 2 ADP Phosphoglycerokinase 2 ATP 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate LE 9-9b_2 2 NAD+ Triose phosphate dehydrogenase 2 NADH + 2 H+ 1, 3-Bisphosphoglycerate 2 ADP Phosphoglycerokinase 2 ATP 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate 2 H2O Enolase Phosphoenolpyruvate 2 ADP Pyruvate kinase 2 ATP Pyruvate Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules A. Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-10 MITOCHONDRION CYTOSOL NAD+ NADH + H+ Acetyl Co A Pyruvate Transport protein CO2 Coenzyme A B. The citric acid cycle, also called the Krebs cycle C. takes place within the mitochondrial matrix D. The cycle oxidizes organic fuel derived from pyruvate, generating one ATP, 3 NADH, and 1 FADH2 per turn Animation: Electron Transport Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 1. The citric acid cycle has eight steps, each catalyzed by a specific enzyme 2. The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate 3. The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle 4. The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain E. Each glucose causes two turns of the cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-12_1 Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate Citric acid cycle LE 9-12_2 Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ a-Ketoglutarate NAD+ Succinyl CoA NADH + H+ CO2 LE 9-12_3 Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ Fumarate a-Ketoglutarate FADH2 NAD+ FAD Succinate GTP GDP ADP ATP Pi Succinyl CoA NADH + H+ CO2 LE 9-12_4 Glycolysis Citric acid cycle ATP ATP Oxidation phosphorylation ATP Acetyl CoA NADH + H+ H2O NAD+ Oxaloacetate Malate Citrate Isocitrate CO2 Citric acid cycle H2O NAD+ NADH + H+ Fumarate a-Ketoglutarate FADH2 NAD+ FAD Succinate GTP GDP ADP ATP Pi Succinyl CoA NADH + H+ CO2 Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis A. Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food B. These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C. The Pathway of Electron Transport 1. The electron transport chain is in the cristae of the mitochondrion 2. Most of the chain’s components are proteins, which exist in multiprotein complexes 3. The carriers alternate reduced and oxidized states as they accept and donate electrons 4. Electrons drop in free energy as they go down the chain and are finally passed to O2, forming water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 5. The electron transport chain generates no ATP 6. The chain’s function is to break the large freeenergy drop from food to O2 into smaller steps that release energy in manageable amounts Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings D. Chemiosmosis: The Energy-Coupling Mechanism 1. Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space 2. H+ then moves back across the membrane, passing through channels in ATP synthase 3. ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-14 INTERMEMBRANE SPACE H+ H+ H+ H+ H+ H+ A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient. H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ ADP + P ATP i MITOCHONDRAL MATRIX Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. 4. The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis 5. The H+ gradient is referred to as a protonmotive force a. emphasizing its capacity to do work b. the force drives H+ back across the membrane through H+ channels Animation: Fermentation Overview Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-15 Inner mitochondrial membrane Glycolysis Citric acid cycle ATP ATP Oxidative phosphorylation: electron transport and chemiosmosis ATP H+ H+ H+ H+ Intermembrane space Cyt c Protein complex of electron carriers Q IV III I ATP synthase II Inner mitochondrial membrane FADH2 NADH + H+ 2H+ + 1/2 O2 H2O FAD NAD+ Mitochondrial matrix ATP ADP + P i (carrying electrons from food) H+ Electron transport chain Electron transport and pumping of protons (H+), Which create an H+ gradient across the membrane Oxidative phosphorylation Chemiosmosis ATP synthesis powered by the flow of H+ back across the membrane E. An Accounting of ATP Production by Cellular Respiration 1. During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP 2. About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen A. Cellular respiration requires O2 to produce ATP B. Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) C. In the absence of O2, glycolysis couples with fermentation to produce ATP D. Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings E. Two common types of fermentation 1. alcoholic fermentation a. pyruvate is converted to ethanol in two steps, with the first releasing CO2 b. Alcohol fermentation by yeast is used in brewing, winemaking, and baking Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 2. Lactic acid fermentation a. pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 b. 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-17b 2 ADP + 2 P i Glucose 2 ATP Glycolysis 2 NAD+ 2 NADH + 2 H+ 2 CO2 2 Pyruvate 2 Lactate Lactic acid fermentation F. Fermentation and Cellular Respiration Compared 1. Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate 2. The processes have different final electron acceptors: an organic molecule (such as pyruvate) in fermentation and O2 in cellular respiration 3. Cellular respiration produces much more ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4. 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 9-18 Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration MITOCHONDRION Ethanol or lactate Acetyl CoA Citric acid cycle G. The Evolutionary Significance of Glycolysis 1. Glycolysis occurs in nearly all organisms 2. Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways A. Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings B. The Versatility of Catabolism 1. Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration 2. Glycolysis accepts a wide range of carbohydrates 3. Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings B. The Versatility of Catabolism 4. Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) a. An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 C. Biosynthesis (Anabolic Pathways) 1. The body uses small molecules to build other substances 2. These small molecules may come directly from food, from glycolysis, or from the citric acid cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings D. Regulation of Cellular Respiration via Feedback Mechanisms 1. Feedback inhibition is the most common mechanism for control 2. If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down 3. Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway ** the energy that keeps us alive is released, not produced by cellular respiration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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