Sinh học - Chapter 9: Cellular respiration and fermentation

Cellular Respiration and FermentationChapter 9Overview: Life Is WorkLiving cells require energy from outside sourcesSome animals, such as the chimpanzee, obtain energy by eating plants, and some animals feed on other organisms that eat plants© 2011 Pearson Education, Inc.Energy flows into an ecosystem as sunlight and leaves as heatPhotosynthesis generates O2 and organic molecules, which are used in cellular respirationCells use chemical energy stored in organic molecules to regenerate ATP, which powers work© 2011 Pearson Education, Inc.Figure 9.2LightenergyECOSYSTEMPhotosynthesisin chloroplastsCellular respirationin mitochondriaCO2  H2O O2OrganicmoleculesATP powersmost cellular workATPHeatenergyConcept 9.1: Catabolic pathways yield energy by oxidizing organic fuelsSeveral processes are central to cellular respiration and related pathways© 2011 Pearson Education, Inc.Catabolic Pathways and Production of ATPThe breakdown of organic molecules is exergonicFermentation is a partial degradation of sugars that occurs without O2Aerobic respiration consumes organic molecules and O2 and yields ATPAnaerobic respiration is similar to aerobic respiration but consumes compounds other than O2© 2011 Pearson Education, Inc.Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respirationAlthough 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 ReductionThe transfer of electrons during chemical reactions releases energy stored in organic moleculesThis released energy is ultimately used to synthesize ATP© 2011 Pearson Education, Inc.The Principle of RedoxChemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactionsIn oxidation, a substance loses electrons, or is oxidizedIn reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced)© 2011 Pearson Education, Inc.Figure 9.UN01becomes oxidized(loses electron)becomes reduced(gains electron)Figure 9.UN02becomes oxidizedbecomes reducedThe electron donor is called the reducing agentThe electron receptor is called the oxidizing agentSome redox reactions do not transfer electrons but change the electron sharing in covalent bondsAn example is the reaction between methane and O2© 2011 Pearson Education, Inc.Figure 9.3ReactantsProductsEnergyWaterCarbon dioxideMethane(reducingagent)Oxygen(oxidizingagent)becomes oxidizedbecomes reducedOxidation of Organic Fuel Molecules During Cellular RespirationDuring cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced© 2011 Pearson Education, Inc.Figure 9.UN03becomes oxidizedbecomes reducedStepwise Energy Harvest via NAD+ and the Electron Transport ChainIn cellular respiration, glucose and other organic molecules are broken down in a series of stepsElectrons from organic compounds are usually first transferred to NAD+, a coenzymeAs an electron acceptor, NAD+ functions as an oxidizing agent during cellular respirationEach NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP© 2011 Pearson Education, Inc.Figure 9.4Nicotinamide(oxidized form)NAD(from food)DehydrogenaseReduction of NADOxidation of NADHNicotinamide(reduced form)NADHFigure 9.UN04DehydrogenaseNADH passes the electrons to the electron transport chainUnlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reactionO2 pulls electrons down the chain in an energy-yielding tumbleThe energy yielded is used to regenerate ATP© 2011 Pearson Education, Inc.Figure 9.5(a) Uncontrolled reaction(b) Cellular respirationExplosiverelease ofheat and lightenergyControlledrelease ofenergy forsynthesis ofATPFree energy, GFree energy, GH2  1/2 O22 H1/2 O21/2 O2H2OH2O2 H+  2 e2 e2 H+ATPATPATPElectron transportchain(from food via NADH)The Stages of Cellular Respiration: A PreviewHarvesting of energy from glucose has three stagesGlycolysis (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)© 2011 Pearson Education, Inc.Figure 9.UN05Glycolysis (color-coded teal throughout the chapter)1.Pyruvate oxidation and the citric acid cycle(color-coded salmon)2.Oxidative phosphorylation: electron transport andchemiosmosis (color-coded violet)3.Figure 9.6-1Electronscarriedvia NADHGlycolysisGlucosePyruvateCYTOSOLMITOCHONDRIONATPSubstrate-levelphosphorylationFigure 9.6-2Electronscarriedvia NADHElectrons carriedvia NADH andFADH2CitricacidcyclePyruvateoxidationAcetyl CoAGlycolysisGlucosePyruvateCYTOSOLMITOCHONDRIONATPATPSubstrate-levelphosphorylationSubstrate-levelphosphorylationFigure 9.6-3Electronscarriedvia NADHElectrons carriedvia NADH andFADH2CitricacidcyclePyruvateoxidationAcetyl CoAGlycolysisGlucosePyruvateOxidativephosphorylation:electron transportandchemiosmosisCYTOSOLMITOCHONDRIONATPATPATPSubstrate-levelphosphorylationSubstrate-levelphosphorylationOxidative phosphorylationThe process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions© 2011 Pearson Education, Inc.Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respirationA smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylationFor each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP © 2011 Pearson Education, Inc.Figure 9.7SubstrateProductADPPATPEnzymeEnzymeConcept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvateGlycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvateGlycolysis occurs in the cytoplasm and has two major phasesEnergy investment phaseEnergy payoff phaseGlycolysis occurs whether or not O2 is present© 2011 Pearson Education, Inc.Figure 9.8Energy Investment PhaseGlucose2 ADP  2 P4 ADP  4 PEnergy Payoff Phase2 NAD+  4 e  4 H+ 2 Pyruvate  2 H2O 2 ATP used4 ATP formed2 NADH  2 H+ NetGlucose2 Pyruvate  2 H2O 2 ATP2 NADH  2 H+ 2 NAD+  4 e  4 H+ 4 ATP formed  2 ATP usedFigure 9.9-4Glycolysis: Energy Investment PhaseATPATPGlucoseGlucose 6-phosphateFructose 6-phosphateFructose 1,6-bisphosphateDihydroxyacetonephosphateGlyceraldehyde3-phosphateTostep 6ADPADPHexokinasePhosphogluco-isomerasePhospho-fructokinaseAldolaseIsomerase12345Figure 9.9-9Glycolysis: Energy Payoff Phase2 ATP2 ATP2 NADH2 NAD+ 2 H 2 P i2 ADP1,3-Bisphospho-glycerate3-Phospho-glycerate2-Phospho-glyceratePhosphoenol-pyruvate (PEP)Pyruvate2 ADP2222 H2OPhospho-glycerokinasePhospho-glyceromutaseEnolasePyruvatekinase678910TriosephosphatedehydrogenaseConcept 9.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic moleculesIn the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed© 2011 Pearson Education, Inc.Oxidation of Pyruvate to Acetyl CoABefore the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycleThis step is carried out by a multienzyme complex that catalyses three reactions© 2011 Pearson Education, Inc.Figure 9.10PyruvateTransport proteinCYTOSOLMITOCHONDRIONCO2Coenzyme ANAD+ HNADHAcetyl CoA123The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn© 2011 Pearson Education, Inc.The Citric Acid CycleFigure 9.11PyruvateNADNADH+ HAcetyl CoACO2CoACoACoA2 CO2ADP + P iFADH2FADATP3 NADH3 NADCitricacidcycle+ 3 HThe citric acid cycle has eight steps, each catalyzed by a specific enzymeThe acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrateThe next seven steps decompose the citrate back to oxaloacetate, making the process a cycleThe NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain© 2011 Pearson Education, Inc.Figure 9.12-41Acetyl CoACitrateIsocitrate-KetoglutarateSuccinylCoACitricacidcycleNADHNADH+ H+ HNADNADH2O324CoA-SHCO2CoA-SHCO2OxaloacetateFigure 9.12-8NADH1Acetyl CoACitrateIsocitrate-KetoglutarateSuccinylCoASuccinateFumarateMalateCitricacidcycleNADNADHNADHFADH2ATP+ H+ H+ HNADNADH2OH2OADPGTPGDPP iFAD3245678CoA-SHCO2CoA-SHCoA-SHCO2OxaloacetateConcept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesisFollowing glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from foodThese two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation© 2011 Pearson Education, Inc.The Pathway of Electron TransportThe electron transport chain is in the inner membrane (cristae) of the mitochondrionMost of the chain’s components are proteins, which exist in multiprotein complexesThe carriers alternate reduced and oxidized states as they accept and donate electronsElectrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O© 2011 Pearson Education, Inc.Figure 9.13NADHFADH22 H + 1/2 O22 e2 e2 eH2ONADMultiproteincomplexes(originally from  NADH or FADH2) IIIIIIIV50403020100Free energy (G) relative to O2 (kcal/mol)FMNFeSFeSFADQCyt bCyt c1Cyt cCyt aCyt a3FeSElectrons are transferred from NADH or FADH2 to the electron transport chainElectrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2The electron transport chain generates no ATP directlyIt breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts© 2011 Pearson Education, Inc.Chemiosmosis: The Energy-Coupling MechanismElectron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane spaceH+ then moves back across the membrane, passing through the proton, ATP synthase ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATPThis is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work© 2011 Pearson Education, Inc.Figure 9.14INTERMEMBRANE SPACERotorStatorHInternalrodCatalyticknobADP+P iATPMITOCHONDRIAL MATRIXFigure 9.15Proteincomplexof electroncarriers(carrying electronsfrom food)Electron transport chainOxidative phosphorylationChemiosmosisATPsynth-aseIIIIIIIVQCyt cFADFADH2NADHADP  P iNADH2 H + 1/2O2HHH21HH2OATPThe energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesisThe 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 RespirationDuring cellular respiration, most energy flows in this sequence: glucose  NADH  electron transport chain  proton-motive force  ATPAbout 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATPThere are several reasons why the number of ATP is not known exactly© 2011 Pearson Education, Inc.Figure 9.16Electron shuttlesspan membraneMITOCHONDRION2 NADH2 NADH2 NADH6 NADH2 FADH22 FADH2or 2 ATP 2 ATP about 26 or 28 ATPGlycolysisGlucose2 PyruvatePyruvate oxidation2 Acetyl CoACitricacidcycleOxidativephosphorylation:electron transportandchemiosmosisCYTOSOLMaximum per glucose:About30 or 32 ATPConcept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygenMost cellular respiration requires O2 to produce ATPWithout O2, the electron transport chain will cease to operateIn that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP© 2011 Pearson Education, Inc.Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, for example sulfateFermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP© 2011 Pearson Education, Inc.Types of FermentationFermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysisTwo common types are alcohol fermentation and lactic acid fermentation© 2011 Pearson Education, Inc.In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2Alcohol fermentation by yeast is used in brewing, winemaking, and baking© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Animation: Fermentation Overview Right-click slide / select “Play”2 ADP  2 P i2 ATPGlucoseGlycolysis2 Pyruvate2 CO22 NAD2 NADH2 Ethanol2 Acetaldehyde(a) Alcohol fermentation2 HFigure 9.17aIn lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurtHuman muscle cells use lactic acid fermentation to generate ATP when O2 is scarce© 2011 Pearson Education, Inc.(b) Lactic acid fermentation2 Lactate2 Pyruvate2 NADHGlucoseGlycolysis2 ADP  2 P i2 ATP2 NAD2 HFigure 9.17bComparing Fermentation with Anaerobic and Aerobic RespirationAll use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of foodIn all three, NAD+ is the oxidizing agent that accepts electrons during glycolysisThe processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respirationCellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule © 2011 Pearson Education, Inc.Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respirationIn a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes© 2011 Pearson Education, Inc.Figure 9.18GlucoseCYTOSOLGlycolysisPyruvateNo O2 present:FermentationO2 present: Aerobic cellular respirationEthanol,lactate, orother productsAcetyl CoAMITOCHONDRIONCitricacidcycleThe Evolutionary Significance of GlycolysisAncient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphereVery little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATPGlycolysis is a very ancient process© 2011 Pearson Education, Inc.Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathwaysGycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways© 2011 Pearson Education, Inc.The Versatility of CatabolismCatabolic pathways funnel electrons from many kinds of organic molecules into cellular respirationGlycolysis accepts a wide range of carbohydratesProteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle© 2011 Pearson Education, Inc.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 CoAAn oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate© 2011 Pearson Education, Inc.Figure 9.19CarbohydratesProteinsFattyacidsAminoacidsSugarsFatsGlycerolGlycolysisGlucoseGlyceraldehyde 3- PNH3PyruvateAcetyl CoACitricacidcycleOxidativephosphorylationBiosynthesis (Anabolic Pathways)The body uses small molecules to build other substancesThese small molecules may come directly from food, from glycolysis, or from the citric acid cycle© 2011 Pearson Education, Inc.Regulation of Cellular Respiration via Feedback MechanismsFeedback inhibition is the most common mechanism for controlIf ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows downControl of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway© 2011 Pearson Education, Inc.Figure 9.20PhosphofructokinaseGlucoseGlycolysisAMPStimulatesFructose 6-phosphateFructose 1,6-bisphosphatePyruvateInhibitsInhibitsATPCitrateCitricacidcycleOxidativephosphorylationAcetyl CoAFigure 9.UN06InputsOutputsGlucoseGlycolysis2 Pyruvate  2ATP 2 NADHFigure 9.UN07InputsOutputs2 Pyruvate2 Acetyl CoA2 OxaloacetateCitricacidcycle2268ATPNADHFADH2CO2