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Chapter 9 Concepts Chapter 9 Cellular Respiration and Fermentation Dr. Wendy Sera Houston Community College Biology 1406 1. Catabolic pathways yield energy by oxidizing organic fuels. 2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate. 3. After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules. 4. During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Life Is Work 5. Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen. 6. Glycolysis and the citric acid cycle connect to many other metabolic pathways. Living cells require energy from outside sources Some animals, such as the giraffe, obtain energy by eating plants (herbivores/primary consumers), and some animals feed on other organisms that eat plants (carnivories/secondary consumers) Figure 9.1—How do these leaves power the work of life for this giraffe? © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Energy Flow and Nutrient Cycling in Ecosystems Figure 9.2— Energy flow and chemical recycling in ecosystems Light energy Energy flows into an ecosystem as sunlight and leaves as heat ECOSYSTEM Photosynthesis generates O2 and organic molecules (e.g., glucose), which are used in cellular respiration CO2 + H2O Photosynthesis in chloroplasts Cellular respiration in mitochondria Cells use chemical energy stored in organic molecules to regenerate ATP, which powers cellular work ATP Organic O molecules + 2 ATP powers most cellular work Heat energy © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 1 BioFlix: The Carbon Cycle Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels Catabolic pathways release stored energy by breaking down complex molecules Electron transfers (i.e., redox reactions) play a major role in these pathways These processes are central to cellular respiration © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O2 and yields very little ATP Aerobic respiration consumes organic molecules and O2 and yields ATP Anaerobic respiration is similar to aerobic respiration, but consumes compounds other than O2 and yields same amount of ATP as aerobic 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) © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Redox Reactions: Oxidation and Reduction The Principle of Redox The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP 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) © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 2 becomes oxidized (loses electron) becomes oxidized becomes reduced (gains electron) © 2014 Pearson Education, Inc. becomes reduced © 2014 Pearson Education, Inc. Figure 9.3—Methane combustion as an energy-yielding redox reaction The electron donor is called the reducing agent Reactants Products becomes oxidized The electron receptor is called the oxidizing agent becomes reduced Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds An example is the reaction between methane and O2 (or the burning of any fuel!) Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Oxidation of Organic Fuel Molecules During Cellular Respiration Stepwise Energy Harvest via NAD+ and the Electron Transport Chain During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced becomes oxidized becomes reduced In cellular respiration, glucose and other organic molecules are broken down in a series of steps Electrons from organic compounds are usually first transferred to NAD+, a coenzyme 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 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 3 Figure 9.4—NAD+ as an electron shuttle 2 e− + 2 H+ How does NAD+ trap electrons from glucose and other organic molecules? 2 e− + H+ NAD+ Dehydrogenase NADH • Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons & 2 protons) from the substrate (e.g., glucose), thereby oxidizing it. • The enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD+ • The other proton is released as a hydrogen ion (H+) H+ Reduction of NAD+ 2[H] (from food) Oxidation of NADH Nicotinamide (oxidized form) H+ Nicotinamide (reduced form) dehydrogenase © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 9.5—An introduction to electron transport chains H2 + ½ O2 2H NADH passes the electrons to the electron transport chain The energy yielded is used to regenerate ATP Explosive release Mitochondria: Chemical Energy Conversion ATP ATP 2 e− ½ O2 2 H+ H2O H2O (a) Uncontrolled reaction © 2014 Pearson Education, Inc. Controlled release of energy ATP Free energy, G O2 pulls electrons down the chain in an energyyielding tumble Free energy, G Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction 2 H+ + 2 e− ½ O2 + (b) Cellular respiration © 2014 Pearson Education, Inc. Figure 6.17—The mitochondrion: site of cellular respiration Intermembrane space Outer membrane Mitochondria are in nearly all eukaryotic cells They have a smooth outer membrane and an inner membrane folded into cristae The inner membrane creates two compartments: intermembrane space and mitochondrial matrix Free ribosomes in the mitochondrial matrix Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix Inner membrane Cristae Matrix Cristae present a large surface area for enzymes that synthesize ATP © 2014 Pearson Education, Inc. 0.1 µm © 2014 Pearson Education, Inc. 4 The Stages of Cellular Respiration: A Preview Harvesting of energy from glucose has three stages: Glycolysis (breaks down glucose into two molecules of pyruvate) Figure 9.6—An overview of cellular respiration (step 1) Electrons via NADH GLYCOLYSIS Glucose Pyruvate The citric acid cycle (completes the breakdown of glucose) CYTOSOL Oxidative phosphorylation (accounts for most of the ATP synthesis) MITOCHONDRION ATP Substrate-level © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 9.6—An overview of cellular respiration (step 2) Electrons via NADH GLYCOLYSIS Glucose Electrons via NADH and FADH2 PYRUVATE OXIDATION CITRIC ACID CYCLE Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION Figure 9.6—An overview of cellular respiration (step 3) Electrons via NADH GLYCOLYSIS Glucose Electrons via NADH and FADH2 PYRUVATE OXIDATION CITRIC ACID CYCLE Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) ATP ATP ATP ATP ATP Substrate-level Substrate-level Substrate-level Substrate-level Oxidative © 2014 Pearson Education, Inc. BioFlix: Cellular Respiration © 2014 Pearson Education, Inc. Oxidative vs. Substrate-level Phosphorylation The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation In total, for each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 5 Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Figure 9.7—Substrate-level phosphorylation Glycolysis (“sugar splitting”) breaks down glucose into two molecules of pyruvate Enzyme Glycolysis occurs in the cytoplasm and has two major phases Enzyme ADP Energy investment phase P ATP Substrate Energy payoff phase Product Glycolysis occurs whether or not O2 is present © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 9.8—The energy input and output of glycolysis Energy Investment Phase Glucose GLYCOLYSIS: Energy Investment Phase 2 ADP + 2 P ATP used 2 Figure 9.9—A closer look at glycolysis Energy Payoff Phase 4 ADP + 4 P 4 ATP Glucose 6-phosphate ATP formed Glucose Fructose 6-phosphate ADP 2 NAD+ + 4 e− + 4 H+ 2 NADH + 2 H+ Phosphoglucoisomerase Hexokinase 2 Pyruvate + 2 H2O 1 2 Net Glucose 4 ATP formed − 2 ATP used 2 NAD+ + 4 e− + 4 H+ 2 Pyruvate + 2 H2O 2 ATP 2 NADH + 2 H+ © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 9.9 (cont.)—A closer look at glycolysis Figure 9.9 (cont.)—A closer look at glycolysis GLYCOLYSIS: Energy Investment Phase GLYCOLYSIS: Energy Payoff Phase Glyceraldehyde 3-phosphate (G3P) Fructose ATP 6-phosphate ADP Glyceraldehyde 3-phosphate (G3P) Fructose 1,6-bisphosphate Phosphofructokinase 3 © 2014 Pearson Education, Inc. Aldolase 4 Dihydroxyacetone phosphate (DHAP) Aldolase 4 2 NADH 2 H+ 2 Isomerase Triose phosphate 2 dehydrogenase 5 6 Isomerase 5 2 NAD+ Dihydroxyacetone phosphate (DHAP) 2 ATP 2 ADP 2 Phosphoglycerokinase 1,3-Bisphosphoglycerate 7 3-Phosphoglycerate © 2014 Pearson Education, Inc. 6 Concept 9.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules Figure 9.9 (cont.)—A closer look at glycolysis GLYCOLYSIS: Energy Payoff Phase 2 H2O 2 2 2 Phosphoglyceromutase 3-Phosphoglycerate 8 Enolase Pyruvate kinase 9 2-Phosphoglycerate In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed 2 ATP 2 ADP 2 Phosphoenolpyruvate (PEP) 10 Pyruvate © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Oxidation of Pyruvate to Acetyl CoA Before the citric acid cycle can begin, however, pyruvate must first be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle This step is carried out by a multi-enzyme complex that catalyzes three reactions © 2014 Pearson Education, Inc. PYRUVATE OXIDATION CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION © 2014 Pearson Education, Inc. Figure 9.10—Oxidation of pyruvate to acetyl CoA, the step before the citric acid cycle MITOCHONDRION CYTOSOL CO2 Coenzyme A 3 1 NAD+ NADH + H+ The Citric Acid Cycle The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2 The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn (per pyruvate molecule) 2 Pyruvate GLYCOLYSIS Acetyl CoA (or double the numbers above if you are calculating for 1 glucose molecule!) Transport protein © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 7 PYRUVATE OXIDATION Figure 9.11—An overview of pyruvate oxidation and the citric acid cycle Pyruvate (from glycolysis, 2 molecules per glucose) CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA GLYCOLYSIS PYRUVATE OXIDATION CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION CoA CITRIC ACID CYCLE 2 CO2 3 NAD+ FADH2 ATP CoA FAD 3 NADH + 3 H+ ADP + P i ATP © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 9.12—A closer look at the citric acid cycle The Citric Acid Cycle, cont. Acetyl CoA CoA-SH The citric acid cycle has eight steps, each catalyzed by a specific enzyme NADH + H+ NAD+ Oxaloacetate 8 The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate (i.e., citric acid) H2O Citrate 7 These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation NAD+ NADH + H+ 3 CO2 CoA-SH 4 6 -Ketoglutarate CoA-SH FADH2 Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food Isocitrate CITRIC ACID CYCLE Fumarate The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis 2 Malate The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle © 2014 Pearson Education, Inc. H2O 1 5 NAD+ CO2 FAD Succinate Pi GTP GDP ADP Succinyl CoA NADH + H+ ATP © 2014 Pearson Education, Inc. The Pathway of Electron Transport The electron transport chain is in the inner membrane (cristae) of the mitochondrion Most of the chain’s components are proteins, which exist in multi-protein 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 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 8 NADH GLYCOLYSIS PYRUVATE OXIDATION Free energy (G) relative to O2 (kcal/mol) 50 OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE ATP 2 e− NAD+ FADH2 40 2 e− FAD Fe•S II I FMN Fe•S Multiprotein complexes Q III Cyt b 30 Figure 9.13— Free-energy change during electron transport Fe•S Cyt c1 IV Cyt c Cyt a 20 10 0 Cyt a3 2 e− (originally from NADH or FADH2) 2 H+ + ½ O2 H2O © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. The Pathway of Electron Transport, cont. 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 The electron transport chain generates no ATP directly It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts © 2014 Pearson Education, Inc. Figure 9.14— ATP synthase, a molecular mill Chemiosmosis: The Energy-Coupling Mechanism 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 protein complex, ATP synthase 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 © 2014 Pearson Education, Inc. INTERMEMBRANE H+ SPACE Rotor Stator Video: ATP Synthase 3-D Structure, Top View Internal rod Catalytic knob ADP + Pi MITOCHONDRIAL MATRIX © 2014 Pearson Education, Inc. ATP © 2014 Pearson Education, Inc. 9 Video: ATP Synthase 3-D Structure, Side View Figure 9.15—Chemiosmosis couples the electron transport chain to ATP synthesis Protein complex of electron carriers ATP synthase H+ H+ H+ H+ Cyt c IV Q III I II FADH2 FAD NADH (carrying electrons from food) 2 H+ + ½ O2 NAD+ H 2O ADP + P i ATP H+ 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Chemiosmosis = A Proto-Motive Force 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 An Accounting of ATP Production by Cellular Respiration 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 There are several reasons why the number of ATP is not known exactly © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 9.16—ATP yield per molecule of glucose at each stage of cellular respiration Electron shuttles span membrane CYTOSOL 2 NADH 2 Pyruvate PYRUVATE OXIDATION 2 Acetyl CoA + 2 ATP Maximum per glucose: © 2014 Pearson Education, Inc. 6 NADH 2 NADH GLYCOLYSIS Glucose MITOCHONDRION 2 NADH or 2 FADH2 2 FADH2 CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) + 2 ATP + about 26 or 28 ATP Concept 9.5: 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 anaerobic respiration or fermentation to produce ATP About 30 or 32 ATP © 2014 Pearson Education, Inc. 10 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 Lactic acid fermentation © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 9.17—Fermentation Alcohol Fermentation In alcohol fermentation, pyruvate is converted to ethanol in two steps: 2 ADP + 2 P i ATP 2 GLYCOLYSIS Glucose 1. The first step releases CO2 2 Pyruvate 2. The second step produces ethanol 2 NAD+ Alcohol fermentation by yeast is used in brewing, winemaking, and baking 2 NADH + 2 H+ 2 Ethanol 2 CO2 2 Acetaldehyde (a) Alcohol fermentation © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 9.17—Fermentation Lactic Acid Fermentation In lactic acid fermentation, pyruvate is reduced by NADH, forming lactate as an end product, with no release of CO2 2 ADP + 2 P i Glucose Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt 2 ATP GLYCOLYSIS 2 NAD+ 2 NADH + 2 H+ Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce 2 Pyruvate 2 Lactate (b) Lactic acid fermentation © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 11 Animation: Fermentation Overview Comparing Fermentation with Anaerobic and Aerobic Respiration 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 However, the processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation, O2 in aerobic respiration, or something else in anaerobic respiration Cellular respiration produces 30-32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Glucose CYTOSOL Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 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 Pyruvate No O2 present: Fermentation O2 present: Aerobic cellular respiration MITOCHONDRION Ethanol, lactate, or other products Figure 9.18— Pyruvate as a key juncture in catabolism Acetyl CoA CITRIC ACID CYCLE © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. The Evolutionary Significance of Glycolysis Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere Glycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways 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 is a very ancient process © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 12 The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Proteins Carbohydrates Amino acids Sugars Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle Fatty acids Glyceraldehyde 3- P NH3 Pyruvate Acetyl CoA Fatty acids are broken down by beta oxidation and yield acetyl CoA An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate Glycerol GLYCOLYSIS Glucose Glycolysis accepts a wide range of carbohydrates Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) Fats Figure 9.19—The catabolism of various molecules from food CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Biosynthesis (Anabolic Pathways) Regulation of Cellular Respiration via Feedback Mechanisms The body uses small molecules to build other substances Feedback inhibition is the most common mechanism for metabolic control These small molecules may come: If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down 1. directly from food, 2. from glycolysis, Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway 3. or from the citric acid cycle. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Glucose GLYCOLYSIS Fructose 6-phosphate AMP Stimulates Phosphofructokinase Fructose 1,6-bisphosphate Inhibits Inhibits Pyruvate Citrate ATP Acetyl CoA Figure 9.20—The control of cellular respiration CITRIC ACID CYCLE Oxidative phosphorylation © 2014 Pearson Education, Inc. 13