CH 7: Cell Respiration and Fermentation Overview Living cells require energy from outside sources Some animals obtain energy by eating plants, and some animals feed on other organisms Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O and organic molecules, which are used as fuel for cellular respiration Cells use chemical energy stored in organic molecules to regenerate, which powers work Figure 7. ECOSYSTEM CO + H O Light energy Photosynthesis in chloroplasts Cellular respiration in mitochondria Heat energy Organic molecules + O powers most cellular work Concept 7.1: Catabolic pathways yield energy by oxidizing organic fuels Several processes are central to cellular respiration and related pathways The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O Aerobic respiration consumes organic molecules and O and yields Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O 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 C 6 H 1 O 6 + 6 O 6 CO + 6 H O + Energy ( + heat) 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 Chemical reactions that transfer electrons between reactants are called -reduction reactions, or redox reactions In, a substance loses electrons, or is oxidized In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) becomes oxidized (loses electron) becomes reduced (gains electron) The electron donor is called the reducing agent The electron acceptor 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 O 1
Figure 7.3 Reactants Products Oxidation of Organic Fuel Molecules During Cellular Respiration becomes oxidized becomes reduced Redox reactions that move electrons closer to electronegative atoms, like oxygen, release chemical energy that can be put to work During cellular respiration, the fuel (such as glucose) is oxidized, and O is reduced Organic molecules with an abundance of hydrogen, like carbohydrates and fats, are excellent fuels Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water As hydrogen (with its electron) is transferred to oxygen, energy is released that can be used in sythesis Figure 7.UN03 Overall Cellular Respiration Formula becomes oxidized becomes reduced Stepwise Energy Harvest via 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, a coenzyme (nicotinamide adenine dinucleotide) As an electron acceptor, functions as an oxidizing agent during cellular respiration Each (the reduced form of ) represents stored energy used to synthesize e + e Nicotinamide (oxidized form) Dehydrogenase Reduction of + [H] (from food) Oxidation of Nicotinamide (reduced form) passes the electrons to the electron transport chain The electron transport chain passes electrons in a series of steps O pulls electrons down the chain yielding energy The energy is used to regenerate
Figure 7.5 H + ½ O ½ O The Stages of Cellular Respiration: A Preview Free energy, G Explosive release Free energy, G + e Electron transport chain e Controlled release of energy ½ O Harvesting of energy from glucose has three stages (breaks down glucose into two molecules of pyruvate) and the citric (completes the breakdown of glucose) phosphorylation (accounts for most of the synthesis) H O H O (a) Uncontrolled reaction (b) Cellular respiration Animation: Cellular Respiration Figure 7.UN05 Electrons via Electrons via and FADH 1.. (color-coded teal throughout the chapter) and the citric (color-coded salmon) Acetyl CoA phosphorylation: electron transport and chemiosmosis 3. phosphorylation: electron transport and chemiosmosis (color-coded violet) CYTOSOL MITOCHONDRION Substrate-level Substrate-level phosphorylation accounts for almost 90% of the generated by cellular respiration A smaller amount of is formed in glycolysis and the citric by substrate-level phosphorylation For each molecule of glucose degraded to CO and water by respiration, the cell makes up to 3 molecules of Enzyme Enzyme Concept 7.: harvests chemical energy by oxidizing glucose to pyruvate ( sugar splitting ) breaks down glucose into two molecules of pyruvate occurs in the cytoplasm and has two major phases Energy investment phase Energy payoff phase occurs whether or not O is present P Substrate + substrate-level phosphorylation Product 3
Figure 7.UN06 Figure 7.8 Energy Investment Phase + P used phosphorylation Energy Payoff Phase 4 + 4 P 4 formed + 4 e + 4 + Net 4 formed used + 4 e + 4 + H O + H O + Figure 7.9a Glyceraldehyde 3-phosphate (G3P) Hexokinase 1 6-phosphate + : Energy Investment Phase Phosphoglucoisomerase Fructose 6-phosphate 3 Phosphofructokinase : Energy Payoff Phase Fructose 1,6-bisphosphate Aldolase 4 Glyceraldehyde 3-phosphate (G3P) Dihydroxyacetone phosphate (DHAP) H O Isomerase 5 Concept 7.3: After pyruvate is oxidized, the citric completes the energy-yielding of organic molecules In the presence of O, pyruvate enters the mitochondrion (in eukaryotic cells), where the of glucose is completed Before the citric can begin, pyruvate must be converted to acetyl coenzyme A (acetyl CoA), which links glycolysis to the citric Triose phosphate P i dehydrogenase 6 Phospho- Phospho- Enolase glycerokinase glyceromutase kinase 9 1,3-Bisphospho- 7 3-Phospho- 8 -Phospho- Phosphoenol- 10 glycerate glycerate glycerate pyruvate (PEP) Figure 7.UN07 Figure 7.10 (from glycolysis, molecules per glucose) CYTOSOL CO CoA phosphorylation Acetyl CoA MITOCHONDRION CoA CoA FADH FAD + P i CO 3 3 + 3 4
The citric has eight steps, each catalyzed by a specific enzyme The citric, also called the Krebs, completes the breakdown of pyruvate to CO The oxidizes organic fuel derived from pyruvate, generating 1, 3, and 1 FADH per turn (flavin adenine dinucleotide) 1) The acetyl group of acetyl CoA joins the by combining with oxaloacetate, forming citrate The next seven steps decompose the citrate back to oxaloacetate, making the process a The and FADH produced by the relay electrons extracted from food to the electron transport chain Figure 7.UN08 Figure 7.11-6 Acetyl CoA 1 H O phosphorylation H O 7 8 Malate Oxaloacetate Fumarate Citrate 3 Isocitrate CO α-ketoglutarate 6 4 FADH FAD Succinate GTP 5 GDP P i Succinyl CoA CO formation Figure 7.11a Figure 7.11b Acetyl CoA Start: Acetyl CoA adds its two-carbon group to oxaloacetate, producing citrate; this is a highly exergonic reaction. Isocitrate 3 CO Redox reaction: Isocitrate is oxidized; is reduced. CO release Oxaloacetate 1 Citrate H O Isocitrate Succinyl CoA 4 CO α-ketoglutarate CO release Redox reaction: After CO release, the resulting four-carbon molecule is oxidized (reducing ), then made reactive by addition of CoA. 5
Figure 7.11c Figure 7.11d Fumarate Redox reaction: Malate is oxidized; is reduced. 6 8 Oxaloacetate FADH Redox reaction: Succinate is oxidized; FAD is reduced. FAD Succinate GTP GDP 5 P i Succinyl CoA formation H O 7 Malate Fumarate Concept 7.4: During oxidative phosphorylation, chemiosmosis couples electron transport to synthesis Following glycolysis and the citric, and FADH account for most of the energy extracted from food These two electron carriers donate electrons to the electron transport chain, which powers synthesis via oxidative phosphorylation 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 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 O, forming H O Figure 7.UN09 Electrons are transferred from or FADH to the electron transport chain phosphorylation: electron transport and chemiosmosis Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O The electron transport chain generates no directly It breaks the large free-energy drop from food to O into smaller steps that release energy in manageable amounts 6
Figure 7.1 Free energy (G) relative to O (kcal/mol) 50 40 30 0 10 0 e FMN Fe S I Q FADH e Fe S Cyt b FAD II Fe S III Cyt c 1 Multiprotein complexes Cyt c Cyt a IV Cyt a 3 e (originally from or FADH ) + ½ O Chemiosmosis: The Energy-Coupling Mechanism Electron transfer in the electron transport chain causes proteins to pump from the mitochondrial matrix to the intermembrane space then moves back across the membrane, passing through the protein complex, synthase synthase uses the exergonic flow of to drive phosphorylation of This is an example of chemiosmosis, the use of energy in a gradient to drive cellular work H O Figure 7.13 INTERMEMBRANE SPACE Rotor Stator Internal rod Catalytic knob MITOCHONDRIAL MATRIX + P i Figure 7.14 The energy stored in a gradient across a membrane couples the redox reactions of the electron transport chain to synthesis Protein complex of electron carriers Cyt c The gradient is referred to as a proton-motive force, emphasizing its capacity to do work I Q III II FADH FAD IV + ½ O H O synthase (carrying electrons from food) + P i 1 Electron transport chain Chemiosmosis phosphorylation 7
Figure 7.UN09 phosphorylation: electron transport and chemiosmosis An Accounting of Production by Cellular Respiration During cellular respiration, most energy flows in the following sequence: glucose electron transport chain proton-motive force About 34% of the energy in a glucose molecule is transferred to during cellular respiration, making about 3 The number of molecules is not known exactly One reason -energy lost when molecules must be transported Figure 7.15 CYTOSOL Electron shuttles span membrane or FADH Acetyl CoA + + 6 FADH MITOCHONDRION phosphorylation: electron transport and chemiosmosis + about 6 or 8 Concept 7.5: Fermentation and anaerobic respiration enable cells to produce without the use of oxygen Most cellular respiration requires O to produce Without O, the electron transport chain will cease to operate In that case, glycolysis couples with fermentation or anaerobic respiration to produce Maximum per glucose: About 30 or 3 Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate, which can be reused by glycolysis Two common types are alcohol fermentation and lactic fermentation Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O, for example, sulfate Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate In alcohol fermentation, pyruvate is converted to ethanol in two steps The first step releases CO from pyruvate, and the second step reduces acetaldehyde to ethanol Alcohol fermentation by yeast is used in brewing, winemaking, and baking 8
Figure 7.16 Ethanol + P i + Acetaldehyde CO + P i + Lactate In lactic fermentation, pyruvate is reduced by, forming lactate as an end product, with no release of CO Lactic fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic fermentation to generate when O is scarce (a) Alcohol fermentation (b) Lactic fermentation Comparing Fermentation with Anaerobic and Aerobic Respiration All use glycolysis (net = ) to oxidize glucose and harvest chemical energy of food In all three, 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 O in cellular respiration Cellular respiration produces 3 per glucose molecule; fermentation produces per glucose molecule Obligate anaerobes carry out only fermentation or anaerobic respiration and cannot survive in the presence of O 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 Figure 7.17 CYTOSOL No O present: Fermentation Ethanol, lactate, or other products O present: Aerobic cellular respiration Acetyl CoA MITOCHONDRION The Evolutionary Significance of Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere Very little O was available in the atmosphere until about.7 billion years ago, so early prokaryotes likely used only glycolysis to generate is an ancient process 9
Concept 7.6: and the citric connect to many other metabolic pathways Gycolysis and the citric are intersections to many catabolic and anabolic pathways Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration accepts a wide range of carbohydrates Proteins must be digested to amino s and amino groups must be removed before amino s can feed glycolysis or the citric NH 3 Proteins Amino s Carbohydrates Sugars Glyceraldehyde 3- P Acetyl CoA Fats Glycerol Fatty s An oxidized gram of fat produces more than twice as much as an oxidized gram of carbohydrate Fats are digested to glycerol (used in glycolysis) and fatty s Fatty s are broken down by beta and yield acetyl CoA phosphorylation Biosynthesis (Anabolic Pathways) The body uses small molecules to build other substances Some of these small molecules come directly from food; others can be produced during glycolysis or the citric 10