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Chapter 9 Cellular Respiration and Fermentation Dr. Wendy Sera Houston Community College Biology 1406 © 2014 Pearson Education, Inc. Chapter 9 Concepts 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. 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. © 2014 Pearson Education, Inc. 1 Life Is Work 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. Energy Flow and Nutrient Cycling in Ecosystems Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O2 and organic molecules (e.g., glucose), which are used in cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP, which powers cellular work © 2014 Pearson Education, Inc. Figure 9.2— Energy flow and chemical recycling in ecosystems Light energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Cellular respiration in mitochondria ATP Organic O molecules + 2 ATP powers most cellular work Heat energy © 2014 Pearson Education, Inc. 2 BioFlix: The Carbon Cycle © 2014 Pearson Education, Inc. 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. 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 © 2014 Pearson Education, Inc. 3 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. 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 © 2014 Pearson Education, Inc. 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) © 2014 Pearson Education, Inc. 4 becomes oxidized (loses electron) becomes reduced (gains electron) © 2014 Pearson Education, Inc. becomes oxidized becomes reduced © 2014 Pearson Education, Inc. The electron donor is called the reducing agent The electron receptor is called the oxidizing agent 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!) © 2014 Pearson Education, Inc. 5 Figure 9.3—Methane combustion as an energy-yielding redox reaction Reactants Products becomes oxidized becomes reduced Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water © 2014 Pearson Education, Inc. Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced becomes oxidized becomes reduced © 2014 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 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. 6 Figure 9.4—NAD+ as an electron shuttle 2 e− + 2 H+ 2 e− + H+ NAD+ Dehydrogenase NADH H+ Reduction of NAD+ 2[H] (from food) Oxidation of NADH Nicotinamide (oxidized form) H+ Nicotinamide (reduced form) © 2014 Pearson Education, Inc. How does NAD+ trap electrons from glucose and other organic molecules? • 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+) dehydrogenase © 2014 Pearson Education, Inc. NADH passes the electrons to the electron transport chain 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 © 2014 Pearson Education, Inc. 7 Figure 9.5—An introduction to electron transport chains H2 + ½ O2 2H Controlled release of energy ATP Free energy, G Free energy, G 2 H+ + 2 e− Explosive release ½ O2 + ATP ATP 2 e− ½ O2 2 H+ H2O H2O (a) Uncontrolled reaction (b) Cellular respiration © 2014 Pearson Education, Inc. Mitochondria: Chemical Energy Conversion 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 Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix Cristae present a large surface area for enzymes that synthesize ATP © 2014 Pearson Education, Inc. Figure 6.17—The mitochondrion: site of cellular respiration Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix 0.1 µm © 2014 Pearson Education, Inc. 8 The Stages of Cellular Respiration: A Preview Harvesting of energy from glucose has three stages: Glycolysis (breaks down glucose into two molecules of pyruvate) The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis) © 2014 Pearson Education, Inc. Figure 9.6—An overview of cellular respiration (step 1) Electrons via NADH GLYCOLYSIS Glucose Pyruvate CYTOSOL MITOCHONDRION ATP Substrate-level © 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 ATP ATP Substrate-level Substrate-level © 2014 Pearson Education, Inc. 9 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 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. 10 Figure 9.7—Substrate-level phosphorylation Enzyme Enzyme ADP P ATP Substrate Product © 2014 Pearson Education, Inc. Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis (“sugar splitting”) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases Energy investment phase Energy payoff phase Glycolysis occurs whether or not O2 is present © 2014 Pearson Education, Inc. Figure 9.8—The energy input and output of glycolysis Energy Investment Phase Glucose 2 ATP used 2 ADP + 2 P Energy Payoff Phase 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+ © 2014 Pearson Education, Inc. 11 Figure 9.9—A closer look at glycolysis GLYCOLYSIS: Energy Investment Phase Glucose 6-phosphate ATP Glucose Fructose 6-phosphate ADP Phosphoglucoisomerase Hexokinase 1 2 © 2014 Pearson Education, Inc. Figure 9.9 (cont.)—A closer look at glycolysis GLYCOLYSIS: Energy Investment Phase Glyceraldehyde 3-phosphate (G3P) Fructose ATP 6-phosphate ADP Fructose 1,6-bisphosphate Isomerase 5 Aldolase Phosphofructokinase 4 3 Dihydroxyacetone phosphate (DHAP) © 2014 Pearson Education, Inc. Figure 9.9 (cont.)—A closer look at glycolysis GLYCOLYSIS: Energy Payoff Phase Glyceraldehyde 3-phosphate (G3P) Aldolase 4 2 NAD+ 2 NADH 2 H+ 2 Isomerase Triose phosphate 2 dehydrogenase 5 6 Dihydroxyacetone phosphate (DHAP) 2 ATP 2 ADP 2 Phosphoglycerokinase 1,3-Bisphosphoglycerate 7 3-Phosphoglycerate © 2014 Pearson Education, Inc. 12 Figure 9.9 (cont.)—A closer look at glycolysis GLYCOLYSIS: Energy Payoff Phase 2 H2O 2 2 Phosphoglyceromutase 3-Phosphoglycerate 8 2-Phosphoglycerate 2 Enolase 9 2 ATP 2 ADP 2 Pyruvate kinase Phosphoenolpyruvate (PEP) 10 Pyruvate © 2014 Pearson Education, Inc. Concept 9.3: 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 © 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. 13 GLYCOLYSIS 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 2 Pyruvate NAD+ NADH + H+ Acetyl CoA Transport protein © 2014 Pearson Education, Inc. 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) (or double the numbers above if you are calculating for 1 glucose molecule!) © 2014 Pearson Education, Inc. 14 GLYCOLYSIS CITRIC ACID CYCLE PYRUVATE OXIDATION OXIDATIVE PHOSPHORYLATION ATP © 2014 Pearson Education, Inc. PYRUVATE OXIDATION Pyruvate (from glycolysis, 2 molecules per glucose) CO2 NAD+ Figure 9.11—An overview of pyruvate oxidation and the citric acid cycle CoA NADH + H+ Acetyl CoA CoA CoA CITRIC ACID CYCLE FADH2 2 CO2 3 NAD+ CoA FAD 3 NADH + 3 H+ ADP + P i ATP © 2014 Pearson Education, Inc. The Citric Acid Cycle, cont. The citric acid cycle has eight steps, each catalyzed by a specific enzyme The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate (i.e., citric acid) The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain © 2014 Pearson Education, Inc. 15 Figure 9.12—A closer look at the citric acid cycle Acetyl CoA CoA-SH NADH + H+ H2O 1 NAD+ Oxaloacetate 8 2 Malate H2O Citrate Isocitrate NAD+ CITRIC ACID CYCLE 7 Fumarate CO2 CoA-SH 4 6 NADH + H+ 3 -Ketoglutarate CoA-SH FADH2 5 NAD+ CO2 FAD Succinate Pi GTP GDP ADP Succinyl CoA NADH + H+ ATP © 2014 Pearson Education, Inc. Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation © 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. 16 OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE PYRUVATE OXIDATION GLYCOLYSIS ATP © 2014 Pearson Education, Inc. NADH Free energy (G) relative to O2 (kcal/mol) 50 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. 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. 17 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. Figure 9.14— ATP synthase, a molecular mill INTERMEMBRANE H+ SPACE Rotor Stator Internal rod Catalytic knob ADP + Pi MITOCHONDRIAL MATRIX ATP © 2014 Pearson Education, Inc. Video: ATP Synthase 3-D Structure, Top View © 2014 Pearson Education, Inc. 18 Video: ATP Synthase 3-D Structure, Side View © 2014 Pearson Education, Inc. 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. 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 © 2014 Pearson Education, Inc. 19 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. Figure 9.16—ATP yield per molecule of glucose at each stage of cellular respiration Electron shuttles span membrane CYTOSOL 2 NADH 6 NADH 2 NADH GLYCOLYSIS Glucose MITOCHONDRION 2 NADH or 2 FADH2 2 Pyruvate PYRUVATE OXIDATION 2 Acetyl CoA + 2 ATP Maximum per glucose: 2 FADH2 CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) + 2 ATP + about 26 or 28 ATP About 30 or 32 ATP © 2014 Pearson Education, Inc. 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 © 2014 Pearson Education, Inc. 20 Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, for example sulfate Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP © 2014 Pearson Education, Inc. Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Two common types are: Alcohol fermentation Lactic acid fermentation © 2014 Pearson Education, Inc. Alcohol Fermentation In alcohol fermentation, pyruvate is converted to ethanol in two steps: 1. The first step releases CO2 2. The second step produces ethanol Alcohol fermentation by yeast is used in brewing, winemaking, and baking © 2014 Pearson Education, Inc. 21 Figure 9.17—Fermentation 2 ADP + 2 P i ATP 2 GLYCOLYSIS Glucose 2 Pyruvate 2 NAD+ 2 NADH + 2 H+ 2 Ethanol 2 CO2 2 Acetaldehyde (a) Alcohol fermentation © 2014 Pearson Education, Inc. Lactic Acid Fermentation In lactic acid fermentation, pyruvate is reduced by NADH, forming lactate as an end product, with no release of 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 © 2014 Pearson Education, Inc. Figure 9.17—Fermentation 2 ADP + 2 P i Glucose 2 ATP GLYCOLYSIS 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate 2 Lactate (b) Lactic acid fermentation © 2014 Pearson Education, Inc. 22 Animation: Fermentation Overview © 2014 Pearson Education, Inc. 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. 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 © 2014 Pearson Education, Inc. 23 Glucose CYTOSOL 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. 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 is a very ancient process © 2014 Pearson Education, Inc. Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Glycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways © 2014 Pearson Education, Inc. 24 The Versatility of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating 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 © 2014 Pearson Education, Inc. Proteins Carbohydrates Amino acids Sugars Fats Glycerol Fatty acids GLYCOLYSIS Glucose Glyceraldehyde 3- P NH3 Pyruvate Acetyl CoA Figure 9.19—The catabolism of various molecules from food CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION © 2014 Pearson Education, Inc. Biosynthesis (Anabolic Pathways) The body uses small molecules to build other substances These small molecules may come: 1. directly from food, 2. from glycolysis, 3. or from the citric acid cycle. © 2014 Pearson Education, Inc. 25 Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition is the most common mechanism for metabolic control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway © 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. 26