酵母,实验报告,英文版

ChengyangWang

EffectofTypeofSugaronGlycolysisandFermentationinYeast

Introduction:

Manycellsgothroughglycolysisandfermentationtobreakglucosedowntocarbondioxideandethanol(Blacketal.158and172).Inglycolysis,glucoseisconvertedtopyruvate,andinfermentation,pyruvateisconvertedtocarbondioxideandethanol(Blacketal.158and172).Glycolysisandfermentationprovidethesecells,suchasyeast,inthisexperiment,withenergy(Peck).However,yeastsdonotutilizeallkindsofsugarinthesamerate.Somesugarscanevennotbeutilizedbyyeasts(Peck).

Thisexperimentaimedtostudywhethercertaintypesofsugarscanbeutilizedbyyeast,ifso,whatisthedifferentratethattheyeastutilizethem.Theratewasstudiedbymeasuringtheratethatthesystemsproducedcarbondioxide,oneoffinalproductsoffermentation.Thesugarsbeinginvestigatedwerefructose,galactose,sucrose,maltose,andsactose.Ratethatyeastutilizeddifferentsugarswerecomparedwiththeratethattheyeastutilizedglucose.Thegroupofyeaststreatedwithglucosewaspositivecontrolandthatofyeastnottreatedbyanysugarwasnegativecontrol.

Inglycolysis,glucoseisturnedtofructose-6-phosphate,whichisthephosphorylatedformofglucose(Blacketal.159).Therefore,weexpectedthatyeastwouldutilizefructoseinsimilarrateasitutilizesglucose.Galactoseisageometricisomerglucose,whichisnotinvolvedintheprocessofglycolysis.Hencewehypothesizedthatgalactosecannotbeutilizedbyyeast.Sinceeachdisaccharidemoleculecontainstwosubunitsofmonosaccharidemonomers,yeastshoulddecomposedisaccharidesintomonosaccharidesbeforegoingthroughglycolysis.Therefore,wehypothesizedthatyeastwouldutilizedisaccharidesinslowerratethanitwouldutilizemonosaccharidesduetoonemoreprocessthatitneedsintheoverallprocess.Amongdisaccharides,theyeastwoulddecomposesucroseandmaltoseinidenticalratessincetheformeriscomposedofglucosemonomerandfructosemonomerandthelatteroneiscomposedoftwoglucosemonomer.Wealsohypothesizedthatyeastwoulddecomposelactoseinaslowerratesinceweexpectedthatgalactose,oneofitstwomonomers,cannotbeutilizedinglycolysis.

MaterialsandMethods:

Inthisexperiment,westudiedthetypesofsugarsthatyeastscanutilizeandthedifferencesinratethatyeastsutilizesugarsbyapplyingfructose,galactose,sucrose,maltose,andlactosetofivegroupsofyeastrespectively.Thepositivecontrolwassetasthegroupofyeastthatwastreatedwithglucoseandthenegativecontrolwassetasthegroupofyeastthatwasonlytreatedwithbufferandpurewater(Peck).Becausecarbondioxidewasafinalproductoftheoverallreaction,amountofcarbondioxideproducedwasproportionaltotheamountofsugarbeingconsumed.Therefore,theratesthatyeastsutilizedsugarswereindicatedbyamountofcarbondioxidebeingproducedinacertaintimeperiod.Wemeasuredtheamountofcarbondioxideproducedbyusingarespirometer.Theincreaseinvolumeofgasbubbleintherespirometerindicatedthevolumeofcarbondioxideproduced.Theratethatyeastutilizedsugarswerecalculatedbydividingvolumeofcarbondioxideproduced(inmilliliter)bytime(inhour).Thisvaluewaspositivelyproportionaltotheratethatyeastutilizedthesugar.WeavoidedradicalchangeinpHvalueduringtheexperimentbyaddingsomekindofbuffersolutionsintoalltesttubes(Peck).

Results:

Figure1:Therelationshipbetweentheratesthatyeastsutilizedsugarsandtypeofsugarsaddedtoyeasts.Therateisdeterminedbychangeinvolumeofthebubbleinrespirometerdividedbytimethatyeastcontactedwithsugar.Thegroupthattreatedwithyeastwasthepositivecontrolinthisexperiment.Theerrorbarsrepresentsstandarderrors.

Theratesthatyeastsutilizedfructoseandsucrosewerestatisticallyidenticaltotheratethatitutilizedglucose,becausethedifferencesinrateswerelessthanonestandarderror(figure1).Theratethatyeastutilizedmaltosewasahalfoftheratethatitutilizedglucose.Thechangesinvolumeofgasbubbleinrespirometersofthegroupoflactoseandgalactosearestatisticallysametothechangeinnegativecontrol.

Discussion:

Theyeastscouldgothroughfermentationprocesswithfructose,sucrose,andmaltose,butcouldnotgothroughfermentationwithgalactoseandlactose,becausethechangesinvolumeofrespirometeringroupstreatedwithgalactoseandlactosewerestatisticallysametochangeinvolumeofthenegativecontrolgroup,inwhichnofermentationwouldhappenbecausenosugarswereadded.However,theratethatyeastutilizedmaltosewashalfoftheratethatitutilizedglucose.

Thehypothesisaboutmonosaccharideswasprovedtobecorrectbecausetheyeastcouldnotutilizegalactoseandutilizedglucoseandfructoseinthesamerate.Buttheexperimentalresultsregardingyeastutilizationofdisaccharidesweredifferentfromthehypothesisbecausesucrose,whichwasexpectedtobeutilizedmoreslowlythanmonosaccharides,wasutilizedinthesamerateasglucosewas,andtheyeastwasnotabletoutilizelactose.Wecanconcludethatyeastcanutilizefructose,glucose,andsucroseinthesamerateandutilizemaltoseatahalfratethanitutilizedtheformersugars.Itcannotundergofermentationwithgalactoseandlactose.Thedifferencesbetweenresultandhypothesissuggestthatthereareotherfactorsdeterminingifyeastcanutilizesugarsandtheratethatyeastutilizethem.

Sinceglycolysisisaprocessinwhichglucoseisturnedintopyruvate,theglucoseisnecessaryforglycolysistohappen(Blacketal.158).Therefore,iftheyeastdoesn’thaveenzymesrequiredtoconvertotherkindsofsugarstoglucose,glycolysiscannothappen.Sinceyeastcanutilizemaltoseandsucrose,itmusthaveenzymethatcanconvertmaltoseandsucroseintoglucose.Similarly,asyeastcannotutilizelactoseorgalactose,wecandeducethatitlackstheenzymenecessarytoconvertlactoseandgalactosetoglucose.However,fructoseisanexceptioninthisexperiment.Fructose-6-phosphate,thephoshphyratedformoffructose,ispartofglycolysisprocess(Blacketal.159).Hencetheyeastcanundergoglycolysiswithoutconvertingittoglucose.Therelativelylowrateofutilizingmaltosemaybecausedbylowlevelofenzymesthatconvertmaltosetoglucoseinyeast.Iftheyeastconvertsmaltosetoglucosemoreslowlythanitconvertssucrose,thentheamountofglucosethatcanundergoglycolysiswillbeless,andtheratethatyeastutilizesmaltosewillhencebeconsequentlyslow.

Asglycolysiscanonlytakeplaceincytoplasm,thesugarmoleculesmustfirstentertheyeastcells(Blacketal.159).Sincesugarmoleculesarepolar,theyarenotlikelytoenterthecytoplasmthroughdiffusion.Therefore,yeastshouldhaveproteinsthathelptransfercertainkindofsugarmoleculesintoitscytoplasm.Itispossiblethatyeastdoesn’thavespecifictypeofproteinthatcanhelptransportgalactoseandlactoseintocytoplasm,andhencecannotundergotheprocessofglycolysis.Thelowratethattheyeastutilizedmaltosemightbecausedbythelowratethatmaltoseistransportedbythespecificproteinthattransportsthemaltose.

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