Chapter 9 lecture notes BIOM first year PDF

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Chapter 9 lecture notes BIOM first year...

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Chapter9PathwaysThatHarvestChemicalEnergy

Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

“obesityepidemic.”Dramaticincreasesindiseasesassociatedwithobesity—amongthem diabetes,heartdisease,andcancer—haveunderstandablyalarmedthemedicalestablishment Whilenutritionalsciencehasshownthatnotallfoodsaffectallpeopleinthesameways,in manycasesthesignificantexcessbodyweightofobesepeoplecanbepreventedorreduced iftheyeatless.Itisamatterofenergy:ifweeatmoreenergy-yieldingmoleculesthanwe needtobuildourbodiesandtofuelactivitiessuchasbrainfunctionsandphysicalactivity, wewillstoretheunneededenergyasfat.Thereisanevolutionaryadvantageinstoringfat: theenergyintheC—CandC—Hbondsinfatcanbeusedlaterwhenfoodisscarce.But excessfathasadverseconsequences. Notallfat(adipose)tissuesarethesame.Whiteadiposetissue(sometimesreferredtosimply as“whitefat”)isusedprimarilytostoreenergy.Brownadiposetissuehashigh concentrationsofmitochondria,whichhaveiron-containingpigments.Whenenergy-rich moleculesinbrownfatarecatabolized,thestoredenergyisreleasednotaschemicalenergy butasheat.ThecellsinbrownfatmakeaproteincalledUCP1(uncouplingprotein1)that insertsintotheinnermembranesofmitochondria,makingthempermeabletoprotons(H+). ThegeneralimpermeabilityofthesemembranestoH+iskeytocouplingthecatabolismof moleculessuchasfatstothereleaseoftheirstoredchemical-bondenergytomakeATP.Ifthe membranesbecomepermeabletoH+,thiscouplingislostandthestoredenergyisreleased asheat.

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Humaninfantsarebornwithagreatdealofbrownfatintheirbackandshoulders.Because infantshaveahighsurfacearea-to-volumeratio,theytendtolosealotofheat.Onewaythey keepwarmistoproduceheatintheirbrownfattissues.Asachildgrowsup,thebrownfat contentofthebodyisreduced.Adultshavemostlywhitefat,whichhaslessUCP1and generateslessheatwhenthefatiscatabolized.Biologistsrecentlyfoundbrownfatinadults, particularlyintheupperchestandneck,butalsoscatteredelsewhereinthebody,mixedin withmuchmoreprevalentwhitefat.Theroleofthistissueisunderintensestudy. QandA 1. Howaremitochondriarelatedtoobesity?(Findtheansweronpage192.)

Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

KEYCONCEPT9.1 CellsHarvestChemicalEnergyfromGlucose Oxidation LearningObjectives 9.1.1Describetheprinciplesthatgovernmetabolicpathwaysincells. 9.1.2Identifythechangeinenergythatoccursduringanoxidation–reductionreaction. 9.1.3Describetherolesofelectronacceptorsanddonorsinbiologicalredoxreactions. 9.1.4Analyzebiologicalredoxreactionstoidentifytheoxidizingandreducingagents involved.

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Energyisstoredinthechemicalbondsofmolecules,anditcanbereleasedandtransformed bythemetabolicpathwaysoflivingcells.Ametabolicpathwayconsistsofacoordinated seriesofbiochemicalreactionsthatconvertmoleculesintoothermolecules.Asyoureadthis chapter,keepinmindfiveprinciplesthatgovernmetabolicpathways: 1. Acomplexchemicaltransformationoccursinaseriesofseparatereactionsthatforma metabolicpathway. 2. Eachreactioniscatalyzedbyaspecificenzyme. 3. Manymetabolicpathwaysaresimilarinallorganisms,frombacteriatohumans. 4. Ineukaryotes,manymetabolicpathwaysarecompartmentalized,withcertainreactions occurringinsidespecificorganelles,orevenspecificregionsofanorganelle. 5. Somekeyenzymesineachmetabolicpathwaycanbeinhibitedoractivatedtoalterthe rateofthepathway. ConnecttheConcepts Theprinciplesofenergytransformationsinlivingandnonlivingsystemsarediscussedin KeyConcept8.1.

Cellsobtainenergyfromglucose Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Incells,chemicalenergyreleasedfrombondsofmoleculesisusedtomakeATP(seeKey Concept8.2),whichinturndrivesendergonicreactions.Photosyntheticcellsandorganisms useenergyfromsunlighttosynthesizetheirownenergy-storingmolecules,aswewill describeinChapter10.Whileothercarbohydrates,aswellaslipidsandproteins,canbeused asenergysources,themostcommonchemicalfuelincellsisthemonosaccharideglucose. Asyouwillseethroughoutthischapter,toreleasetheirenergytomakeATP,theother moleculesareconvertedintoglucoseorintermediatecompoundsthatcanenterintothe variouspathwaysofglucosemetabolism. AsyousawinKeyConcept2.3,thefamiliarprocessofcombustion(burning)issimilarto thechemicalprocessesthatreleaseenergyincells.Ifglucoseisburnedinaflameorisina typicalcell,itreactswithoxygengas(O2),formingcarbondioxideandwaterandreleasing energyintheformofheat.Thebalancedequationforthecompletereactionis C6H12O6+6O2→6CO2+6H2O+freeenergy(ΔG=–686kcal/mol) Thisisanoxidation–reductionreaction(moreonthisshortly),inwhichglucoseloses electrons(becomesoxidized)andoxygengainsthem(becomesreduced).Thelargefree energychange(ΔG)isnegative,indicatingthattheoverallconversionishighlyexergonic andcandrivetheendergonicformationofalotofATPfromADPandphosphate: ADP+Pi+freeenergy→ATP(ΔG=+7.3kcal/mol)

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Threecatabolicprocessesharvesttheenergyinthechemicalbondsofglucose:glycolysis, cellularrespiration,andfermentation(Figure9.1).Allthreeprocessesinvolvepathways madeupofmanydistinctchemicalreactions.

Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Figure9.1EnergyforLife Manyprokaryotesandalleukaryotesobtaintheirenergyfromthemolecules ultimatelyproducedbyphotosynthesis,whichconvertssolarenergyintochemicalbondenergy.Often,organismsconvertmoleculesintoglucose,whichthey metabolizetotrapenergyinATP. Theimagesincludesalabeledgraphicofthesunwithitsraysshiningdirectlyontoagreen boxlabeled“photosynthesis.”agrayarrowleadsdowntoahexagonmarked“Glucose"ona bluefieldbearingthelabel“CO2+H2O”.Underglucoseisagrayarrowmarked“glycolysis thatleadstoawhiteboxmarked“Pyruvate(3-carbonmolecule)"twoarrowspointaway fromthebox.Aredarrowpointsleftandbelowitsays“Aeorobic(O2present/Cellular respiration"Beneaththearrowpointingtotherightitsays“Anaerobic(O2 absent)/Fermentation.”Theattributesofcellularrespirationarelistedas-“Complete oxidation,Wasteproducts:H2O,CO2,Netenergytrappedperglucose32ATP.”The attributesofFermentationarelistedtotherightas“Incompleteoxidation,WasteProducts: lacticacidorethanolandCO2,Netenergytrappedperglucose:2” Q:

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1. Doalleukaryotesperformalltheprocessesinthefigure? 1. Glycolysisbeginsglucosecatabolism.Throughaseriesofchemicalrearrangements, glucoseisconvertedtotwomoleculesofthethree-carbonproductpyruvate,andasmall amountofenergyiscapturedinusableforms.Glycolysisisananaerobicprocess becauseitdoesnotrequireO2. 2. CellularrespirationusesO2fromtheenvironmentandthusisaerobic.Eachpyruvate moleculeiscompletelyconvertedintothreemoleculesofCO2throughasetofcatabolic pathwaysincludingpyruvateoxidation,thecitricacidcycle,andanelectrontransport chain.Intheprocess,agreatdealoftheenergystoredinthecovalentbondsofpyruvate iscapturedtoformATP. 3. FermentationdoesnotinvolveO2(itisanaerobic).Withtheexceptionofmany microorganisms,fermentationconvertspyruvateintolacticacidorethylalcohol (ethanol),bothofwhicharestillrelativelyenergy-richmolecules.Becausethe breakdownofglucoseisincomplete,muchlessenergyisreleasedwhenglycolysisis coupledtofermentationthanwhenitiscoupledtocellularrespiration.

Redoxreactionstransferelectronsandenergy

Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Thecoenzymenicotinamideadeninedinucleotide(NAD+)actsasanelectroncarrierinredox reactions(Figure9.4A).NAD+existsintwochemicallydistinctforms,oneoxidized (NAD+)andtheotherreduced(NADH)(Figure9.4B).Bothformsparticipateinredox reactions.Thereductionreaction NAD++H++2e−→NADH

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isactuallythetransferofaproton(thehydrogenion,H+)andtwoelectrons,whichare releasedbytheaccompanyingoxidizationreaction.

Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Figure9.4NAD+/NADHIsanElectronCarrierinRedoxReactions (A)NAD+isanimportantelectronacceptorinredoxreactions,andthusits reducedform,NADH,isanimportantenergyintermediaryincells.Thebluelines showtheflowofelectrons.(B)MostoftheNAD+molecule(left)remains unchangedbytheredoxreaction. (A)CompoundAisinitiallyshownwithahydrogen.NAD+isreducedtoNADHwhenit acceptsthishydrogenfromcompoundA.ThisinturnoxidizescompoundA.NADHis oxidizedtoNAD+whenittransfersahydrogentocompoundB.Thisinturnreduces compoundB.BlackarrowsindicatehowtheredoxreactionsforNAD+/NADHarecyclical, whilebluearrowsindicatethedirectionofthereaction.(B)Oneprotonandtwoelectronsare transferredtotheringstructureofNAD+duringreduction.Asaresult,apositively-charged nitrogengainsanelectronandbecomesneutral,whileoneoftheringcarbonsgainsan electronandanotherhydrogen.Thelocationofthedouble-bondsintheringstructureisalso altered.Double-bondswiththenitrogenandcarbonthateachacceptanelectronarelost. Instead,adouble-bondisformedbetweentwodifferentring-carbons.Therestofthe moleculeremainsunchanged. Theelectronsdonotremainwiththecoenzyme.Oxygenishighlyelectronegativeandreadily acceptselectronsfromNADH.TheoxidationofNADHbyO2(whichoccursinseveral steps) NADH+H++½O2→NAD++H2O

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isexergonic,withastandardfreeenergychangeatpH7(ΔG0)of–52.4kcal/mol(–219 kJ/mol).Notethattheoxidizingagentappearshereas“½O2”insteadof“O.”Thisnotation emphasizesthatitismolecularoxygen,O2,thatactsastheoxidizingagent. JustasATPcanbethoughtofasapackageof7.3kcal/mol(30.5kJ/mol)offreeenergy, NADHcanbethoughtofasalargerpackageoffreeenergy(52.4kcal/mol;seeabove). NAD+isacommonelectroncarrierincells,butnottheonlyone.Anothercarrier,flavin adeninedinucleotide(FAD),alsotransferselectronsduringglucosemetabolism. ConnecttheConcepts

KeyConcept8.4describestheroleofcoenzymes,smallmoleculesthatassistinenzymecatalyzedreactions.ADPactsasacoenzymewhenitpicksupenergyreleasedinan exergonicreactionandpackagesittoformATP.

Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

KEYCONCEPT9.2 InthePresenceofOxygen,GlucoseIsFully Oxidized LearningObjectives 9.2.1Describetheenergyinputsandoutputsofglycolysis. 9.2.2Explainhowpyruvateoxidationconnectsthetransformationsofglycolysisandthe citricacidcycle. 9.2.3Compareandcontrastglycolysisandthecitricacidcycle.

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InthepresenceofO2(aerobic),thepathwaysofglucosecatabolismoxidizeglucose completelytoCO2andH2O.Initially,theglycolysisreactionsconvertthesix-carbonglucose moleculetotwothree-carbonpyruvatemolecules(Figure9.6).Pyruvateisthenconvertedto CO2inasecondseriesofreactionsbeginningwithpyruvateoxidationandfollowedbythe citricacidcycle.InadditiontogeneratingCO2,theoxidationeventsarecoupledwiththe reductionofelectroncarriers,mostlyNAD+.Wewillbeginourconsiderationofthe catabolismofglucosewithacloserlookatglycolysis.

Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Figure9.6GlycolysisConvertsGlucoseintoPyruvate Glucoseisconvertedtopyruvateintenenzyme-catalyzedsteps.Alongtheway, twoATPareused(Steps1and3),twoNAD+arereducedtotwoNADH(Step6), andfourATPareproduced(Steps7and10). Thefirstthreestepsconvertglucosetofructose1,6-bisphosphate.Thenexttwosteps,cleave fructose1,6-bisphosphate,whichhassixcarbons,intotwomoleculesofglyceraldehyde3phosphate,whicheachhavethreecarbons.Thenextfivesteps,convertthetwo glyceraldehyde3-phosphatemoleculesintotwopyruvatemolecules.Theearlysteps(1and 3)areendergonicandrequireenergyfromATPhydrolysis,whilethelaterstepsareexergonic andreleaseenergy,formingATP(steps7and10)andNADH(step6).Thechemical structuresforglucose,fructose1,6-bisphosphate,glyceraldehyde3-phosphate,andpyruvate areshown.

Intheglycolysispathway,glucoseispartially oxidized

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Glycolysistakesplaceinthecytoplasmandinvolvestenenzyme-catalyzedreactions.During glycolysis,someofthecovalentbondsbetweencarbonandhydrogenatomsintheglucose moleculeareoxidized,releasingsomeofthestoredenergy.Theproductsaretwomolecules ofpyruvate(pyruvicacid),twomoleculesofATP,andtwomoleculesofNADH.Glycolysis canbedividedintotwostages:theinitialenergy-investingreactionsthatconsumeATP,and theenergy-harvestingreactionsthatproduceATPandNADH(seeFigure9.6). Tohelpyouunderstandtheprocesswithoutgettingintoextensivedetail,wefocusontwo consecutivereactionsinthispathway(Figure9.7).Steps6and7areexamplesoftwotypes ofreactionsthatoccurrepeatedlyinglycolysisandinmanyothermetabolicpathways: oxidation–reduction,whereinenergyreleasedbyglucoseoxidationistrappedviathe reductionofNAD+toNADH;andsubstrate-levelphosphorylation,whereinenergyreleased issufficienttotransferaphosphatedirectlyfromthesubstratetoADP,formingATP.

Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookcentral.proquest.com Created from unimelb on 2021-02-28 20:22:00

Copyright © 2020. Macmillan Learning. All rights reserved. Hillis, D., Heller, H. C., Hacker, S. D., Hall, D., Sadava, D., & Laskowski, M. (2020). Life : The science of biology. ProQuest Ebook Central http://ebookce...