细胞生物学双语教案3(线粒体结构与功能)

1细胞生物学双语教案3（线粒体结构与功能）MitochondrialSubstructureMitochondriacontaintwomembranes,separatedbyaspace.Botharethetypicalunitmembrane(railroadtrack)instructure.Insidethespaceenclosedbytheinnermembraneisthematrix.ThisappearsmoderatelydenseandonemayfindstrandsofDNA,ribosomes,orsmallgranulesinthematrix.Themitochondriaareabletocodeforpartoftheirproteinswiththesemoleculartools.Theabovecartoonshowsthediagramofthemitochondrialmembranesandtheenclosedcompartments.Howaremitochondriaorganizedtobepowerhouses?Thefoodweeatisoxidizedtoproducehigh-energyelectronsthatareconvertedtostoredenergy.Thisenergyisstoredinhighenergyphosphatebondsinamoleculecalledadenosinetriphosphate,orATP.ATPisconvertedfromadenosinediphosphatebyaddingthephosphategroupwiththehigh-energybond.Variousreactionsinthecellcaneitheruseenergy(wherebytheATPisconvertedbacktoADP,releasingthehighenergybond)orproduceit(wherebytheATPisproducedfromADP).Stepsfromglycolysistotheelectrontransportchain.Whyaremitochondriaimportant?LetsbreakdowneachofthestepssoyoucanseehowfoodturnsintoATP2energypacketsandwater.Thefoodweeatmustfirstbeconvertedtobasicchemicalsthatthecellcanuse.Someofthebestenergysupplyingfoodscontainsugarsorcarbohydrates...bread,forexample.Usingthisasanexample,thesugarsarebrokendownbyenzymesthatsplitthemintothesimplestformofsugarwhichiscalledglucose.Then,glucoseentersthecellbyspecialmoleculesinthemembranecalled“glucosetransporters”.Onceinsidethecell,glucoseisbrokendowntomakeATPintwopathways.Thefirstpathwayrequiresnooxygenandiscalledanaerobicmetabolism.Thispathwayiscalledglycolysisanditoccursinthecytoplasmoutsidethemitochondria.Duringglycolysis,glucoseisbrokendownintopyruvate.Otherfoodslikefatscanalsobebrokendownforuseasfuel(seefollowingcartoon).Eachreactionisdesignedtoproducesomehydrogenions(electrons)thatcanbeusedtomakeenergypackets(ATP).However,only4ATPmoleculescanbemadebyonemoleculeofglucoserunthroughthispathway.Thatiswhymitochondriaandoxygenaresoimportant.WeneedtocontinuethebreakdownprocesswiththeKreb’scycleinsidethemitochondriainordertogetenoughATPtorunallthecellfunctions.Theeventsthatoccurinsideandoutsidemitochondriaarediagrammedintheabovecartoon.PyruvateiscarriedintothemitochondriaandthereitisconvertedintoAcetylCo-AwhichenterstheKreb'scycle.Thisfirstreactionproducescarbondioxidebecauseitinvolvestheremovalofonecarbonfromthepyruvate.3HowdoestheKreb'scyclework?ThewholeideabehindrespirationinthemitochondriaistousetheKreb’scycle(alsocalledthecitricacidcycle)togetasmanyelectronsoutofthefoodweeataspossible.Theseelectrons(intheformofhydrogenions)arethenusedtodrivepumpsthatproduceATP.TheenergycarriedbyATPisthenusedforallkindsofcellularfunctionslikemovement,transport,entryandexitofproducts,division,etc.Thefollowingexplanationisverysimpleandfocusesononlythepathwayfrompyruvatethroughthecycle.However,itillustratestheprocessanditsfunctions.ToruntheKreb'scycle,youneedseveralimportantmoleculesinadditiontoalltheenzymes.Consultyourtextfordetailsabouttheenzymesthemselves.Thispresentationwillfocusontheelectrondonors,carriersandacceptors.First,youneedpyruvate,whichismadebyglycolysisfromglucose.Next,youneedsomecarriermoleculesfortheelectrons.Therearetwotypesofthese:oneiscallednicotinamideadeninedinucleotide(NAD+)andtheotheriscalledflavinadeninedinucleotide(FAD+).Thethirdmolecule,ofcourse,isoxygen.Pyruvateisa3carbonmolecule.Afteritentersthemitochondria,itisbrokendowntoa2carbonmoleculebyaspecialenzyme(seetextformoredetailsaboutthebiochemistryofeachstep).Thisreleasescarbondioxide.The2carbonmoleculeiscalledAcetylCoAanditenterstheKreb’scyclebyjoiningtoa4carbonmoleculecalledoxaloacetate.Oncethetwomoleculesarejoined,theymakea6carbonmoleculecalledcitricacid(2carbons+4carbons=6carbons).ThatiswheretheCitricacidcyclegotitsname....fromthatfirstreactionthatmakescitricacid.Citricacidisthenbrokendownandmodifiedinastepwisefashion(seetextfordetails)and,asthishappens,hydrogenionsandcarbonmoleculesarereleased.ThecarbonmoleculesareusedtomakemorecarbondioxideandthehydrogenionsarepickedupbyNADandFAD(seebelow).Eventually,theprocessproducesthe4carbonoxaloacetateagain.Thereasontheprocessiscalledacycle,isbecauseitendsupalwayswhereitstarted....withoxaloacetateavailabletocombinewithmoreacetylcoA.Whatis“oxidativephosphorylation”?First,somebasicdefinitions.Whenyoutakehydrogenionsorelectronsawayfromamolecule,you“oxidize”thatmolecule.Whenyougivehydrogenions4orelectronstoamolecule,you“reduce”thatmolecule.Whenyougivephosphatemoleculestoamolecule,you“phosphorylate”thatmolecule.So,oxidativephosphorylation(verysimply)meanstheprocessthatcouplestheremovalofhydrogenionsfromonemoleculeandgivingphosphatemoleculestoanothermolecule.Howdoesthisapplytomitochondria?AstheKreb’scycleruns,hydrogenions(orelectrons)aredonatedtothetwocarriermoleculesin4ofthesteps.TheyarepickedupbyeitherNADorFADandthesecarriermoleculesbecomeNADHandFADH(becausetheynowarecarryingahydrogenion).Thefollowingcartoonshowswhathappensnext.Theseelectronsarecarriedchemicallytotherespiratoryorelectrontransportchainfoundinthemitochondrialcristae(seecartoonsabov