《细胞破碎方法》PPT课件.ppt

Chapter4CellDisruption第四章细胞破碎,4.1CellMembranes细胞膜,4.0Summary概述,4.2ChemicalMethods化学破碎方法,4.3mechanicaldisruption机械破碎,4.0Summary概述,Bioseparationsusuallybeginwiththeseparationofbiomassfrombroth.TheseparationcommonlyusesfiltrationorcentrifugationasdescribedinChapters2and3.Inmanycases,thedesiredproductisinthebroth.Antibiotics,arecommonlyinthebroth;soareextracellularenzymes,manypolysaccharides,andmostaminoacids.Inallthesecases,theseparatedbrothcanbetreatedtoisolateandpurifytheproduct,asabyproduct.生物分离的第一步是将生物机体从发酵液中分离，通常使用过滤和离心等方法，这在第二、三章中已有陈述。大多数情况下，抗生素，胞外酶，一些多糖，及氨基酸等目标产物存在于在发酵液中。在上述过程中，需被分离的发酵液可被看作一种副产物来处理，以此分离和纯化产物。,Insomecases,theproductsofinterestarenotinthebrothbutareinthebiomass.Inparticular,mostproteinsproducedinquantitybygeneticallymanipulatedbacteriaarenotexcretedintothebroth,butareprecipitatedwithinthecell.Lipidsandsomeantibioticsarealsotrappedinthebiomass.Inafewcases,likebakersyeast,thedesiredproductisthecellmassitself.Inafewothers,desiredproductslikesteroidscanbeextractedwithoutrupturingthecells.Inmanycases,theproductistrappedinthebiomass:Itisintracellular.有些目标产物不在发酵液中，而是存在于生物体中。尤其是由基因工程菌产生的大多数蛋白质不会被分泌到发酵液中，而是在细胞内沉积。脂类物质和一些抗生素也是包含在生物体中。还有一些目标产物就是细胞本身，如面包酵母。还有，产物如类固醇不必通过细胞破碎提取。大多数情况下，产物还是包裹在生物体内，属胞内产物。,Releasingthistrappedmaterialusuallyinvolvesrupturingthecellwall,andisthesubjectofthisshortchapter.Themethodsofcellrupturehavelargelybeendevelopedinbiochemistry,andhenceareofsmallscale.Theirapplicationtothelargerscaleoperationsimpliedbygeneticengineeringisspeculative.Theequipmentwhichisusedisnotdesignedforbiotechnology.使胞内产物释放出来一般需要破碎细胞壁，这也是这一章的主题。细胞破碎的方法在生物化学领域中得到了很广泛的运用，但多数在小规模生产中，在大规模生产尤其是基因工程中应用极少。,Inthischapter,webegininSection4.1byreviewingthestructureofthecellwall.Wethenconsiderthemethodsforcellrupturemostlikelytobeusefulatlargerscales.Thesemethods,listedinTable4.0-1,areconvenientlysplitintotwogroups,describedas“chemical”and“mechanical.”InthechemicalmethodsdetailedinSection4.2,thecellmembraneisrupturedbyosmoticpressure,dissolvedbydetergents,orenfeebledbyorganicsolvents.Thesechemicalcasesareoftengentlesothattheproductsarenotirreversiblydenaturedevenwhilethecellsareruptured.Scale-upiseasy:Ifwewanttotreat10timesthebiomass,weneedtouse10timesasmuchchemical.4.1节回顾一下细胞壁的结构。从中可知，在大规模生产中，细胞破碎方法是十分有用的。我们很容易将这些方法划分为两大类，化学法和机械法，表4.0-1中已经列出。化学法我们将在4.2节作详细介绍，有渗透冲击法、表面活性剂增溶法或有机溶剂溶解法。这些化学法较温和，细胞破坏后产物也不会不可逆的变性，规模也容易放大：如果需要处理10倍量的生物有机体，只需要加入10倍的化学药品剂量。,TABLE4.0-1.ChemicalCellDisintegrationTechniques,表4.0-1.细胞化学破碎法,TABLE4.0-1.MechanicalCellDisintegrationTechniques,表4.0-1.细胞物理破碎法,ThemechanicalmethodsdescribedinSection4.3aredominatedbyhomogenizationandbyballmillgrinding.Inbothcases,cellsarebrokenbyhighmechanicalshear.Heatgenerationisaproblemandscale-upisobscure:Ifwefindthatamethodworksatalaboratoryscale,weoftenhavelittleideahowitwillworkatalargescale.Asaresult,thispartofthechapterismoreaguidetoplanningexperimentsthanasynopsisofengineeringanalysis.4.3节机械法主要有匀浆法和研磨法。这两种方法中，细胞通过高的机械剪切力被破坏。热量的产生是该法的一个缺点，而且不容易规模放大:如果一种方法可以在实验室规模下操作，我们并不知道这种方法是否在大规模生产中可行。所以，这部分内容仅作为实验的指导，而不是工程分析。,4.1CellMembranes细胞膜,Atthispoint,wepausebrieflytoexplorethephysicalstructureofmicrobialmembranesandthecomplexityoftheproblemswhichweface.Atpresent,knowledgeofthisgeneralstructuredoesnotprovideadirectguidetomethodsofcellrupture.Inthefuture,itmay.Asaresult,wefeelthatasynopsishasmerit.Inthissynopsis,weemphasizeGram-negativeprocaryotes.本节主要探寻细胞膜的物质结构及我们所面对的几个复杂问题。现在所有的关于细胞膜结构的基本知识，并不能为细胞破碎方法提供基本的引导。也许将来可以。正因如此，对此进行提纲切领的介绍很有必要。主要强调的是革兰氏阴性原核生物。其细胞结构中没有细胞核：基因物质位于单链DNA上。典型的生物是大肠杆菌，是生物技术研究的主体。通过这种细胞生产了很多细胞重组的产物。,Suchcellshavenonuclei:TheirgeneticfactorsarecarriedinasinglestrandofDNA.ThebestexampleofthistypeofcellisEscherichiacoli,thehostorganismformanyeffortsinbiotechnology.Thisisthecellwhichproducesmostrecombinantproductsdevelopedtodate.此细胞结构没有细胞核：基因在单链DNA上。典型的是大肠杆菌，是生物技术研究的主体。通过这种细胞生产了很多细胞重组的产物。,ThebasiccellenvelopeforGram-negativecells,showninFigure4.1-1a,hasthreelayers.Theouterlayer,about8nmthick,consistsofapolymercontainingbothproteinandlipopolysaccharide.Thesecondthinnerlayerispeptidoglycan.Belowthissecondlayerisagap,calledtheperiplamicspace,whichisalso8nmthick.Enzymesareoftenlocatedinthisgap.Gram-positiveprocaroytes,showninFigure4.1-1aremissingthefirstouterlayer,buthaveboththesecondpeptidog1ycanlayerandtheperiplasmicspace.革兰氏阴性细胞结构如图4.1-1所示。有三层：最外层约8mm厚，酶大多数镶嵌在这层膜上。革兰氏阳性原核生物缺少最外层结构，但有第二层肽聚糖层和胞浆空间。,Gram-negativeprocaryotes革兰氏阴性原核生物（E.coli）,Thethirdmembranelayer,calledtheplasmamembraneortheinnermembrane,iscommontobothGram-positiveandGram-negativeorganisms.Itconsistslargelyofphospholipids,butalsocontainsdispersedproteinsandmetalions.第三层为浆膜层或内膜层，革兰氏阴性菌和革兰氏阳性都有这一结构，该层主要由磷脂组成，还有分散的蛋白质分子和金属离子。,Theselipidmoleculeshavetwoparts,ahydrophobicpartandthehydrophilicpartThehydrophobicpartor“tail”oftencontainstwoalkylgroups;andthehydrophilicpartor“head”oftenincludesachargedgroup,azwitterion,oranalcohol.Inthisinnermembrane,thephospho1ipidtailsaggregatetogether,andtheheadsareexposedtothewater,asshowninFigure4.1-2Theresultisalame11arbi1ayer.磷脂分子由疏水和亲水基团组成。疏水基团尾部有两烷基基团，亲水基团头部有两性基团，或者说是醇基，在内膜层中，磷脂的尾部聚集在一起，头部暴露在水中，如图4.1-2所示，形成磷脂双分子层。,structureofmicrobialmembranes细胞膜结构,Thesethreelayershavedifferentfunctions.Theoutermembranesandthepeptidoglycanlayerprovidemechanicalstrength;itistheirrupturewhichisacentralsubjectofthischapter.Theweakerplasmamembrane-theinnermostlayer-controlsthepermeabilityofthecell,includingtransportofnutrientsintothecellsinteriorandexportofmetabolitesintothesurroundingsolution.这三层有不同功能。外层及肽聚糖层使细胞有一定的机械强度；破坏该层是本章讨论的重点。浆膜层和细胞内膜控制细胞的渗透压，即将营养物质运送到细胞内，将代谢物排出胞外。,Thiscellinterior,calledthecytoplasm,isanaqueoussolutionofsalts,sugars,aminoacids,andbipo1ymers.Thebiopo1ymersincludeproteins,manyofthemenzymes;ribonucleicacid(RNA);anddeoxyribonucleicacid(DNA).细胞膜内部包含的物质称为细胞质，它是一种液态的溶液，溶有很多盐、糖、氨基酸和生物聚和物。生物聚和物包括蛋白质、酶、RNA、DNA。,Innaturallyoccurringprocaryotes,theproteinsareinsolution;butinmanygeneticallymodifiedprocaryotes,excessproteinissynthesizedandprecipitatedwithinthecytoplasm.Often,weseektorupturethecellwallsolelytorecoverthisprotein.Inothercases,wewanttoremoveonlysomeofthelayerstoreleasespecificenzymes.一些自然生长的细胞中蛋白质都溶解在细胞质中，在一些基因控制细胞中，合成的过量蛋白质在胞浆中沉降。有时细胞壁的目的就是为了获得蛋白质，有时则为了使细胞释放出特殊的酶。,Eucaryoticcells,thosewithtruenuclei,arestructurallymorecomplicatedthanprocaryotes.Thesecells,exemplifiedbytheyeastinFigure4.1-3,haveamembranesurroundingthecellwhichissimilarinstructuretotheprocaryotemembrane.真核细胞，具有真正的细胞核，其结构要比原核生物复杂的多，以图4.1-3中所示的酵母菌为例，和原核细胞一样真核细胞也具有一层细胞膜。,动物细胞,植物细胞模式图,Eucaryotemembranesapparentlydocontainsteroids,whichareabsentinprocaryotemembranes.Inadditiontothissurroundingmembrane,thecellscontainorganelles真核生物的细胞膜中含有固醇，而原核生物中没有。除了细胞膜，真核细胞具有一些有特殊作用的细胞器。,Eachorganellehasaspecializedfunction:Forexample,themitochondriaareresponsibleforrespirarionandthenucleuscontainstheDNA(aschromosomes).Eachstructureissurroundedbyamembranewhichissimilartotheinnerplasmamembraneofprocaryotes.Theincreasedstructuralcomplexityoftheeucaryotesisnotmirroredbyincreasedchemicalcomplexity.每个细胞器也都由一层膜包着与原核细胞的内膜浆膜层比较类似。真核生物的结构复杂并不反映其性质也很复杂。,4.2CHEMICALMETHODS化学方法,ThechemicalmethodsofcellrupturelistedinTable4.0-1aredominatedbyosmoticshock,detergentso1ubilization,andlipiddissolution.Thesethreemethodsaredescribedinthefollowingparagraphs.Beforethisdescription,wewanttogivesomebriefgeneralizationsabouttheothertwomethods:enzymedigestionandalkalitreatment.表4.0-1介绍了主要的几种化学方法，有渗透冲击法，表面活性剂增溶法、脂溶法。首先简单的介绍一下酶消化法和碱处理法。,Bothenzymedigestionandalkalitreatmentareeffectivemethodsofcellrupture,buteachhasamajordisadvantage.酶消化法和碱处理法都是细胞破碎的有效方法，但是也都有各自的缺点。,Thedisadvantageofenzymedigestionisthecostoftheenzymeswhich.Frequentlymakesthismethodprohibitiveatalargescale.Thisisunfortunate,becausethemethodisbothgentleandselective.Itisnothingmorethanaddingtoacellsuspensionenzymeswhichreactquicklywiththecellwallsanddestroythem.Becausetheenzymesselectivelycatalyzethesecellwallreactions,theyreactonlysparinglywithothersoluteswithinthecell.酶消化法的缺点在于酶的价格昂贵限制了在大规模生产中的使用，虽然条件温和、具有选择性。细胞悬浮液中加入酶能迅速和细胞壁反应并破坏它们。酶选择性的催化细胞壁反应，不破坏细胞内的其它物质。,Alkalitreatmentistheantithesisofenzymedigestion,foritisharshratherthangentle,nonselectiveratherthanspecific,andcheapratherthanexpensive.Alkaliaddedtoacellsuspensionreactswiththecellwallsinanumberofways,includingthesaponificationoflipidsinthecellwalls.碱处理法和酶消化法相反，反应激烈，不具选择性，而且较便宜。碱加入细胞悬浮液中后和细胞壁进行了多种反应，包括使磷脂皂化。,Inasense,thetreatmentisanextremeexampleofso1ubilization,foritconvertscellwallcomponentsintodetergents.Includingproteindenaturations.Asaresult,themethodtendsbothtorupturecellwallsandtodestroyproducts.Eventhoughitischeap,alkalitreatmentislessusefulthanthethreemethodswhicharedetailednext.该操作是细胞增溶的很好的例子，因为它使细胞壁的成分溶于表面活性剂，也使蛋白质变性。该法不仅破坏了细胞壁也破坏了产物，因此即便很便宜，碱处理也是一种很不常用的方法。,A、OsmoticShock(渗透冲击法),Thesimplestofthethreemajorchemicalmethodsisosmoticshock.Thisisnothingmorethandumpingagivenvolumeofcellsintopurewater-oftenabouttwicethevolumeofcells.Thecellsswellbecausetheycontainsoluteswhichcauseanosmoticflowofwaterintothecells.Insomecases,theyswellsomuchthattheyburst.Theircontents,releasedintothesurroundingsolution,cannowbeseparatedusingthemethodsinfollowingchapters.三种主要细胞破碎化学方法中最简单的是渗透冲击法。此法将一定体积的细胞液加到2倍体积的水中，细胞中溶质浓度高，水不断进入细胞，使细胞膨胀，最后导致破裂。细胞破裂后释放到周围环境中的胞内物可用后面章节介绍的方法分离。,Thesusceptibilityofcellstolysisdependsstronglyontheirtype.Redbloodcellsareeasilylysed.Animalcellsoftencanbelysed,butonlyafteranimaltissuehasbeenmechanicallymincedorhomogenized,asdescribedinSection4.3.Plantcellsaremuchmoredifficultto1yse,fortheircellwallsoftencontainstrongwoodymaterialwhichisrelativelyimpermeabletoosmoticflow.细胞破碎的难易决定于其类型，红血球细胞容易溶破，动物细胞只有当其组织被用4.3节所介绍的方法机械切碎或匀浆后才易溶破。植物细胞很难溶破，因为植物细胞中含有大量的木质成分，通过渗透流很难渗透。,Theosmoticflowinvolvedcomesfromtheosmoticpressure,whichissurprisinglylarge.Toestimatethesizeofthispressure,weturntothedefinitionofchemicalequilibrium,whichstatesthatthechemicalpotentialofwaterH20mustbeconstant：渗透流的动力来自渗透压，渗透压可能很大。可以通过化学平衡来估算，水的化学电势是常数，即：H20(outside)=H20(inside)(4.2-1),Thechemicalpotentialofthepurewateroutsidemustincludeareferencevalueandapressurecorrection;thecorrespondingpotentialinsideinvolvesareferencevalue,pressurecorrection,andacorrectionforsolutionconcentration.Foranidealincompressiblesolution,thesecorrectionsconvertEq.(4.2-1)intothefollowing.细胞外纯水的化学电势包括标准化学电势和压力修正项；与之平衡的胞内的化学电势包括三项，即标准化学电势、压力修正相和浓度修正项。对于理想的不可压缩溶液，这些修正相和式（4.2-1）可改写为：0H20+VH2oPout=0H20+VH2oPin+RTln(1-x1)(4.2-2),Where0H20isthereferencevalue,-VH2oisthepartialmolarvolumeofwater,andx1isthetotalmolefractionofsolutesinsidethecell.Ifthecellscontentsareadilutesolution,-VH2oisalmostequaltothemolarvolumeofwater-VH2o,andx1issmall0H20标准化学电势VH2o水的偏摩尔体积x1细胞内所有溶质的总摩尔分数若包内物为稀溶液，其偏摩尔体积等于纯水的摩尔体积。且x1很小时就有：,ThusPout-Pin=(RT/-VH2o)ln(1-x1)=(RT/-VH2o)(-x1-)=-RTc1+(4.2-3)ThisrelationiscalledvantHoffslaw.(该式称为范特苛夫定律。),Thusatequilibrium,thepressureoutsidethecellmustbelessthanthatinside.Ifthisisnotthecase,thenwaterwillflowintothecellandpotentiallylysesthecell.WecanuseEq.(4.2-3)toestimatethesizeofthepressure.Manycellshaveasoluteconcentrationroughlyequivalentto0.1MNaCl,or0.2Msolutes.由该方程知细胞内压力必须小于细胞外压力，否者，水会流入细胞内，溶破细胞。我们可以采用等式4.2-3估算出渗透压的大小。许多细胞内溶质浓度大约为0.1MNaCl或0.2M溶质。,Thisosmoticpressureis,forus,alwayssurprisinglylarge.Thislargepressureiswhatcanleadtocellrupture.由此可见渗透压必须很大，才导致了细胞的破碎。,B、Solubilization增溶法,Thesecondmajormethodofchemicallyrupturingcellsissolubilizationbydetergents.Typically,aconcentrateddetergentsolutionisaddedtoabouthalfthesolutionsvolumeofcells.Thedetergentdisruptsthecellmembrane.Theresultingsuspensioncanbecentrifugedtoremovecellfragments,andthenrunthroughanadsorptioncolumnoranextractortoisolatetheproduct.第二种是利用表面活性剂的增溶法。最典型的是将体积为细胞体积两倍的某浓度的表面活性剂加入到细胞中。表面活性剂能将细胞壁破碎，制成的悬浮液可用离心分离除去细胞碎片，再用吸附柱或萃取剂分离制得产品。,Thereasonthatsolubilizationiseffectiveliesindetergentchemistry.ThischemistrydependsondetergentstructureslikethoseinFigure4.2-1.Allthesestructureshaveahydrophilicportion,whichisoftenionic,andahydrophobicpart,whichisfrequentlyahydrocarbon.方法有效在于表面活性剂的化学性质，化学性质由图4.2-1所示的化学结构表示。结构中有一个亲水基团，通常是离子；一个疏水基团，通常是烃基。,Asaresult,thesedetergentsareallamphipathic,capableofinteractingwithbothwaterandlipid.表面活性剂通常是两性的，既能和水作用也能和脂作用。,Thisamphipathicnatureholdswhetherthedetergentsareanionic,cationic,ornonionic.Alltypesareillustratedinthefigure.Sodiumdode-cylsulfate(SDS),whichistheexamplegiven,isthemostwidelystudiedoftheanionicdetergents.Theanionicmaterialsalsoincludethesoaps,whicharethesaltsoffattyacids.无论表面活性剂是阴离子、阳离子还是非离子型，都是两性的。SDS（十二烷基磺酸钠）是典型的阴离子表面活性剂。阴离子表面活性剂还包括肥皂（脂肪酸盐）。,Becausesoapsdependonacarboxylicacidgroup,theyareeffectivedetergentsonlyathighpH,wherethisgroupremainsionized.Theyareineffectiveinhardwater,wherecalciumionscanreactwiththemtoforminsolubleprecipitates.由于肥皂的增溶作用依赖于羧基，因此只有在Ph值较高，羧基解离的情况下，肥皂才是有效的表面活性剂。在硬水中，由于Ca2+与羧酸基团形成不可溶的沉淀，而使肥皂失效。,Thedisadvantagesofconventionalsoapscanbeavoidedbyreplacingthecarboxylicacidgroupwithasulfate.Thesulfateinmostcurrentlaundrydetergents,againthosepurchasedatthesupermarket,isattachedtoalinearalkylchain.Thesulfatecanalsobeattachedtoabenzenering,makingasulfonatelikethatshowninFigure4.2-1.Suchsulfonatesaremoreeffectiveatcellrupturethanalkylsulfatesare;perhapsbecauseofthis,sulfonatesarenoteasilydegradedmicrobiologicallyandsoarenolongercommonlyusedinlaundrydetergents.可通过将肥皂的羧基用硫酸盐代替来改变传统肥皂的缺点。硫酸盐代替的肥皂是目前洗衣店常用及超市中常见的表面活性剂。硫酸盐可连在苯环上，结构如图4.2-1所示。这种硫酸盐表面活性剂比碱对细胞破碎的作用更好，磺酸盐不易被微生物降解，所以洗衣店中并不常用。,Cationicdetergentslargelyarebasedontetraalkylammoniumsalts.Cetyltrimethylammoniumbromide,theexampleinFigure4.2-1,istypical:Itcontainsonelongalkyltail(the16carbon”cetyl”)and3methylgroups,allattachedtoapositivelychargednitrogenatom.Thecounterionisoftenahalide.Inthesupermarket,thesedetergentsaremostfrequentlyfoundinshampoos,correctlysuggestingthattheywillofferamoregentlemeansofcellrupture.阳离子表面活性剂主要是烷基胺盐。图4.2-1中的十二烷基溴胺是典型的例子。它有一个长烷烃链（十六烷基）和三个甲基，都连接在一个带正电的氮原子上，负离子通常是卤素，市场上常做洗发剂出售。细胞破碎时条件较温和。,Inpassingweshouldnotethatcationicdetergentswithtwolongchainsareoftenoilsoluble,andareimplicatedintheformationofmembranepores.Theyhaverarelybeentestedasaroutetocellrupture.过去认为带有两个长尾的阳离子表面活性剂溶于油，和细胞膜孔的形成有关，但是很少用于细胞破碎。,Nonionicdetergents,ofwhichthecommercialmaterialTritonXisanexample,aremostcommonlybasedonwatersolublepolymers.Theyarelesswelldefinedchemically.Inthesupermarket,theyshowupindishwash-ingliquidswhoseadvertisingissuchacurseoneveningtelevision.Again,moleculesofthesedetergentshaveahydrophobicportionandahydrophilicone.Now,however,thehydrophilicpartisnotasulfateoratetraalkylam-monium,butanalcohol.非离子表面活性以氚核物质X为例，它是溶于水的聚合物。经济价值不明确，市场上用于做洗碗剂。同样，这种清洁剂既有亲水基团又有疏水基团。亲水基团不是硫酸盐，不是烷基胺盐而是乙醇。,Thekeyaspectofeachofthesedetergentsistheirabilitytosolubilizelipidsincellwalls,andthusrupturethesewalls.TheamountofthissolubilizationisshownschematicallyinFIGURE4.2-2.Inhighlydilutedetergentsolution,lipidssuddenlybegintogointosolution;abovethishigherconcentration,thesolubilityoflipidsvarieslinearlywiththedetergentconcentration.Otherpropertiesbesidestheamountsolubilizedalsochangeatthissameparticularconcentration:thesurfacetensionhitsaconstantvalue,andtheresponseofadetergentselectiveelectrodesuddenlychanges.这类表面活性剂起作用在于能溶解细胞壁中的脂，进行细胞破碎。图4.2-2表示了该表面活性增溶的原理。在高度稀释洗涤剂中，脂会溶解在溶液中；超过这个浓度溶解的物质的量与浓度成正比。还有，表面活性剂的表面张力是常数，表面活性的选择电极可以突然改变。,AmountSolubilized,SurfaceTension,MonomerActivity,DetergentConcentration,Fig4.2-2Typicaldetergentproperties,Therationalebothforsolubilizationandfortheseotherphenomenaisthatdetergentsformsmallaggregatescalledmicelles.Thesemicellesarecommonlyvisualizedasacollectionofhydrophobictailssurroundedbyashellorskinofthehydrophilicheadgroups.Solubilizedlipidisencapsulatedwithinthemicellesthemicellescore,Asaresult,micellesareviewedratherlikeanorange:Theskinofhydrophilicheadgroupsisattachedtoapulpofhydrophobictails,andcontainsajuiceofsolubilizedlipid.增溶和表面活性的其它现象的原理在于：清洁剂形成小的聚和物称为胶囊。微囊可看成是疏水尾部聚合在一起的，被一层壳或者是亲水的头包围着。增溶的脂被微囊包围，并存在于囊核心。微囊象橘子，橘皮是亲水的头部，内皮是疏水的尾部，橘汁就是内溶的脂。,Asanexampleoftheseexperiments,weturntotheruptureofcellwallswhitsodiumtaurocholate,showninFigure4.2-1.Thisspeciesisonecomponentofbile,whichisthebodysdetergent.Bilesphysiologicfunctionistosolubilizefatsintheintestine.Thensweptawaytobemetabolized.Interestingly,almostalllivinganimalsusethesebilesaltsastheirdetergent;onecuriousexceptionisthelobster,whichusesanalkylsulfate.以牛黄胆酸钠为例，如图4.2-1所示。它是胆汁的成分之一，是人体的“去污剂”。胆汁的生理机能是增溶小肠内的脂肪。这种清除可以称之为代谢。几乎所有的生物都用这种胆酸盐作为人体去污剂，除了牡蛎是用烷基硫酸盐的。,Bilesaltsareunusuallyeffectiveatrupturingcellwalls.Unfortunately,theyareexpensive,andsowilloftenneedtobereplacedbysomethingcheaper.Still,theyarealovelyexampleofwhydetergentswork.Toseewhy,weagainconsider