24 CELLS
ITSTARTSWITHasinglecell.Thefirstcellsplitstobeetwoawobeefourandsoon.Afterjustforty-sevendoublings,youhavetenthousandtrillion(10,000,000,000,000,000)cellsinyourbodyandarereadytforthasahumanbeing.
1Andeveryohosecellsklywhattodotopreserveandnurtureyoufromthemomentofceptiontoyourlastbreath.
Youhavesfromyourcells.Theyknowfarmoreaboutyouthanyoudo.Eaecarriesacopyofthepletegeiccode—theinstruanualforyourbody—soitknowsnotonlyhowtodoitsjobbuteveryotherjobinthebody.Neverinyourlifewillyouhavetoremiokeepasadenosiriphosphatelevelsortofindaplacefortheextrasquirtoffolicacidthat’sjustuedlyturnedup.Itwilldothatforyou,andmillionsmorethingsbesides.
Everycellinnatureisathingofwonder.Eventhesimplestarefarbeyondthelimitsofhumay.Tobuildthemostbasicyeastcell,forexample,youwouldhavetominiaturizeaboutthesamenumberofposasarefoundinaBoeing777jetlinerandfitthemintoaspherejustfivemisacross;thensomehowyouwouldhavetopersuadethatspheretoreproduce.
Butyeastcellsareasnothingparedwithhumancells,whicharenotjustmorevariedandplicated,butvastlymorefasatingbecauseoftheirplexiions.
Yourcellsareatryoftenthousandtrillioncitizens,eachdevotediensivelyspecificwaytoyouroverallwell-being.Thereisn’tathingtheydon’tdoforyou.Theyletyoufeelpleasureandformthoughts.Theyeostandandstretdcaper.Wheheyextractthenutrients,distributetheenergy,andcarryoffthewastes—allthosethingsyoulearnedaboutinjunihschoolbiology—buttheyalsoremembertomakeyouhungryinthefirstpladrewardyouwithafeelingofwell-beingafterwardsothatyouwon’tfettoeatagain.Theykeepyourhairgrowing,yourearswaxed,yourbrailypurring.Theymanageeveryerofyourbeing.Theywilljumptoyourdefeheinstantyouarethreateheywilluatinglydieforyou—billionsofthemdosodaily.Andnotonallyouryearshaveyouthankedevehem.Soletustakeamomentnardthemwiththewonderandappreciationtheydeserve.
Weuandalittleofhowcellsdothethingstheydo—howtheylaydownfatormanufactureinsulinageinmanyoftheotheraecessarytomaintainaplicatedentitylikeyourself—butonlyalittle.Youhaveatleast200,000differenttypesofprotein1Actually,quitealotofcellsarelostintheprocessofdevelopment,sothenumberyouemergewithisreallyjustaguess.Dependingonwhichsourceyousultthenumbervarybyseveralordersofmagnitude.Thefigureoftenthousandtrillion(orquadrillion)isfrulisandSagan,1986.
labawayinsideyou,andsofarweuandwhatnomorethanabout2pertofthemdo.(Othersputthefigureatmorelike50pert;itdepends,apparently,onwhatyoumeanby“uand.”)Surprisesatthecellularlevelturnupallthetime.Innature,nitricoxideisaformidabletoxinandapoofairpollution.Sostistswerenaturallyalittlesurprisedwhen,inthemid-1980s,theyfounditbeingproduacuriouslydevotedmannerinhumancells.Itspurposewasatfirstamystery,buttheistsbegantofinditallovertheplace—trollingtheflowofbloodandtheenergylevelsofcells,attagcersandotherpathogens,regulatingthesenseofsmell,evenassistinginpeions.Italsoexplainedwhynitroglye,thewell-knownexplosive,soothestheheartpainknownasangina.(Itisvertedintonitricoxideinthebloodstream,relaxingthemuscleliningsofvessels,allowingbloodtoflowmorefreely.)Ihespaceofadecadethisonegassysubstafromextraoxintoubiquitouselixir.
Youpossess“somefewhundred”differenttypesofcell,acctotheBelgianbiochemistChristiandeDuve,andtheyvaryenormouslyinsizeandshape,fromnervecellswhosefilamentsstretchtoseveralfeettotiny,disc-shapedredbloodcellstotherod-shapedphotocellsthathelptogiveusvision.Theyalsoeinasumptuouslywiderangeofsizes—nowheremorestrikinglythanatthemomentofception,whenasiingspermfrontsay-fivethousandtimesbiggerthanit(whichratherputsthenotionofmalequestintoperspective).Onaverage,however,ahumancellisabouttwentymiswide—thatisabouttwohuhsofamillimeter—whichistoosmalltobeseenbutroomyenoughtoholdthousandsofplicatedstructureslikemitodria,andmillionsuponmillionsofmolecules.Iliteralway,cellsalsovaryinliveliness.Yourskincellsarealldead.It’sasomewhatgallingnotiontoreflectthateveryinchofyoursurfaceisdeceased.Ifyouareanaverage-sizedadultygingaroundaboutfivepoundsofdeadskin,ofwhichseveralbilliontinyfragmentsaresloughedoffeachday.Runafingeralongadustyshelfandyouaredraatternverylargelyinoldskin.
Mostlivingcellsseldomlastmorethanamonthorso,buttherearesomenotableexceptions.Livercellssurviveforyears,thoughtheposwithinthemmayberenewedeveryfewdays.Braincellslastaslongasyoudo.Youareissuedahundredbillionorsoatbirth,andthatisallyouareevergoingtoget.Ithasbeeimatedthatyoulosefivehuhemanhour,soifyouhaveanyseriousthinkingtodotherereallyisn’tamomenttowaste.Thegoodnewsisthattheindividualposofyourbraincellsarestantlyrehat,aswiththelivercells,nopartofthemisactuallylikelytobemorethanaboutamonthold.Ihasbeensuggestedthatthereisn’tasiofanyofus—notsomuchasastraymolecule—thatartofusnineyearsago.Itmaynotfeellikeit,butatthecellularlevelweareallyoungsters.
ThefirstpersontodescribeacellwasRobertHooke,whomwelastenteredsquabblingwithIsaaewtonovercreditfortheiionoftheinversesquarelaw.Hookeachievedmanythingsinhissixty-eightyears—hewasbothanaplishedtheoretiandadabhandatmakingingeniousandusefulinstruments—butnothinghedidbroughthimgreateradmirationthanhispopularbookMicrophagia:orSomePhysiologicalDescriptionsofMiniatureBodiesMadebyMagnifyingGlasses,produ1665.Itrevealedtoanentedpubliiverseoftheverysmallthatwasfarmorediverse,crowded,andfiructuredthananyonehadevereclosetoimagining.
AmongthemicroscopicfeaturesfirstidentifiedbyHookewerelittlechambersinplantsthathecalled“cells”becausetheyremindedhimofmonks’cells.Hookecalculatedthataone-inchsquareofcorkwouldtain1,259,712,000ofthesetinychambers—thefirstappearanceofsuchaverylargenumberanywhereinsce.Microscopesbythistimehadbeenaroundfeionorso,butwhatsetHooke’sapartweretheirteicalsupremacy.Theyachievedmagnificationsofthirtytimes,makihelastwordieenth-turyopticalteology.
SoitcameassomethingofashockwhenjustadecadelaterHookeahermembersofLondon’sRoyalSocietybegantoreceivedrawingsasfromaeredlinendraperinHollandemployingmagnificationsofupto275times.Thedraper’snamewasAntonivanLeeuwehoughhehadlittleformaleducationandnobackgroundinsce,heerceptiveanddedicatedobserverandateicalgenius.
Tothisdayitisnotknownhowhegotsuchmagnifitmagnificationsfromsimplehandhelddevices,whichwerelittlemorethawoodendowelswithatinybubbleofglassembeddedinthem,farmorelikemagnifyingglassesthanwhatmostofusthinkofasmicroscopes,butreallynotmuchlikeeither.Leeuwenhoekmadeanewinstrumentforeveryexperimentheperformedandwasextremelysecretiveabouthisteiques,thoughhedidsometimesoffertipstotheBritishonhowtheymightimprovetheirresolutions.
2Overaperiodoffiftyyears—beginning,remarkablyenough,whenhewasalreadypastforty—hemadealmosttwohundredreportstotheRoyalSociety,allwritteninLowDutch,theonlytongueofwhichhewasmaster.Leeuwenhoekofferednointerpretations,butsimplythefactsofwhathehadfound,apaniedbyexquisitedrawings.Hesesonalmosteverythingthatcouldbeusefullyexamined—breadmold,abee’sstinger,bloodcells,teeth,hair,hisownsaliva,excrement,andsemen(theselastwithfretfulapologiesfortheirunsavorynature)—nearlyallofwhichhadneverbeenseenmicroscopicallybefore.
Afterhereportedfinding“animalcules”inasampleofpepperwaterin1676,themembersoftheRoyalSocietyspentayearwiththebestdeviglishteologycouldproduceseargforthe“littleanimals”beforefinallygettingthemagnificatiht.WhatLeeuwenhoekhadfoundwereprotozoa.Hecalculatedthattherewere8,280,000ofthesetinybeingsinasingledropofwater—morethanthenumberofpeopleinHolland.Theworldteemedwithlifeinwaysandhatnoonehadpreviouslysuspected.
InspiredbyLeeuwenhoek’sfantastidings,antopeerintomicroscopeswithsuchkeehattheysometimesfoundthingsthatweren’tinfactthere.OnerespectedDutchobserver,NicolausHartsoecker,wasvincedhesaw“tinypreformedmen”inspermcells.Hecalledthelittlebeings“homunculi”andforsometimemanypeoplebelievedthatallhumans—indeed,allcreatures—weresimplyvastlyinflatedversionsoftinybutpleteprecursorbeings.Leeuwenhoekhimselfoccasionallygotcarriedawaywithhisenthusiasms.
Inoneofhisleastsuccessfulexperimeriedtostudytheexplosivepropertiesofgunpowderbyasmallblastatcle;henearlyblindedhimselfintheprocess.
2LeeuwenhoekwasclosefriendswithanotherDelftnotable,theartistJanVermeer.Inthemid-1660s,Vermeer,whopreviouslyhadbeenapetentbutnotoutstandingartist,suddenlydevelopedthemasteryoflightandperspectiveforwhichhehasbeeedeversihoughithasneverbeenproved,ithaslongbeensuspectedthatheusedacameraobscura,adeviceforprojegimagesontoaflatsurfacethroughalens.NosuchdevicewaslistedamongVermeerspersonaleffectsafterhisdeath,butithappensthattheexecutorofVermeersestatewasherthanAntonivanLeeuwehemostsecretivelens-makerofhisday.
In1683Leeuwenhoekdiscoveredbacteria,butthatwasaboutasfarasprogresscouldgetfortheturyandahalfbecauseofthelimitationsofmicroscopeteology.Notuntil1831wouldanyonefirstseethenucleusofacell—itwasfoundbytheScottishbotanistRobertBrown,thatfrequentbutalwaysshadowyvisitortothehistoryofsce.Brown,wholivedfrom1773to1858,calleditnucleusfromtheLatinnucula,meaninglittlenutorkernel.
Notuntil1839,however,didanyonerealizethatalllivingmatteriscellular.ItwasTheodorSn,aGerman,whohadthisinsight,anditwasnotonlyparativelylate,asstifisightsgo,butnotwidelyembracedatfirst.Itwasn’tuntilthe1860s,andsomelandmarkworkbyLouisPasteurinFrahatitwasshownclusivelythatlifeotarisespontaneouslybutmustefrompreexistingcells.Thebeliefbecameknownasthe“celltheory,”anditisthebasisofallmodernbiology.
Thecellhasbeenparedtomanythings,from“aplexchemicalrefinery”(bythephysicistJamesTrefil)to“avast,teemiropolis”(thebiochemistGuyBrown).Acellisbothofthosethingsaher.Itislikearefineryinthatitisdevotedtochemicalactivityonagrandscale,andlikeametropolisinthatitiscrowdedandbusyandfilledwithiionsthatseemfusedandrandombutclearlyhavesomesystemtothem.Butitisamuchmhtmarishplacethananycityorfactorythatyouhaveeverseen.Tobeginwiththereisnoupordownihecell(gravitydoesn’tmeaningfullyapplyatthecellularscale),andnotanatom’swidthofspaceisuhereisactivityeverywhereandaceaselessthrumofelectriergy.Youmayerriblyelectrical,butyouare.Thefoodweeatandtheoxygenwebreathearebihecellsiricity.Thereasonwedon’tgiveeachothermassiveshocksorscorchthesofawheisthatitisallhappeningonatinyscale:amere0.1voltstravelingdistancesmeasuredinnaers.
However,scalethatupanditwouldtranslateasajoltoftwentymillionvoltspermeter,aboutthesameasthechargecarriedbythemainbodyofathuorm.
Whatevertheirsizeorshape,nearlyallyourcellsarebuilttofuallythesameplan:
theyhaveanoutergormembrane,anucleuswhereinresidesthenecessarygeiationtokeepyougoing,andabusyspacebetweewocalledthecytoplasm.Themembraneisnot,asmostofusimagi,adurable,rubberyg,somethingthatyouwouldneedasharppintoprick.Rather,itismadeupofatypeoffattymaterialknoid,whichhastheapproximatesistency“ofalightgradeofmaeoil,”toquoteSherwinB.Nuland.Ifthatseemssurprisinglyinsubstantial,bearinmindthatatthemicroscopiclevelthingsbehavedifferently.Toanythingonamolecularscalewaterbeesakindofheavy-dutygel,andalipidislikeiron.
Ifyoucouldvisitacell,youwouldn’tlikeit.Blownuptoascaleatwhiswereaboutthesizeofpeas,acellitselfwouldbeasphereroughlyhalfamileacross,andsupportedbyaplexframewirderscalledthecytoskeleton.Withinit,millionsuponmillionsofobjects—somethesizeofbasketballs,othersthesizeofcars—wouldwhizaboutlikebullets.Therewouldn’tbeaplaceyoucouldstandwithoutbeingpummeledandrippedthousandsoftimeseverysedfromeverydire.Evenforitsfull-timeoctstheinsideofacellisahazardousplace.EachstrandofDNAisonaverageattaagedonceevery8.4seds—tenthousandtimesinaday—bychemicalsandentsthatwhatoorcarelesslyslicethroughit,andeachofthesewoundsmustbeswiftlystitchedupifthecellisnottoperish.
Theproteinsareespeciallylively,spinning,pulsating,andflyingintoeachotheruptoabilliontimesased.Ehemselvesatypeofprotein,dasheverywhere,perfuptoathousandtasksased.Likegreatlyspeededupworkerants,theybusilybuildandrebuildmolecules,haulingapieceoffthisone,addingapiecetothatone.Somemonitorpassingproteinsandmarkwithachemicalthosethatareirreparablydamagedorflawed.Oncesoselected,thedoomedproteinsproceedtoastructurecalledaproteasome,wheretheyarestrippeddownandtheirposusedtobuildnewproteins.Sometypesofproteiforlessthanhalfanhour;otherssurviveforweeks.Butallleadexistehatareinceivablyfrenzied.AsdeDuvehemolecularworldmustnecessarilyremaiirelybeyondthepowersinationowingtotheincrediblespeedwithwhichthingshappeninit.”
Butslowthingsdown,toaspeedatwhichtheiionsbeobserved,andthingsdon’tseemquitesounnerving.Youseethatacellisjustmillionsofobjects—lysosomes,endosomes,ribosomes,ligands,peroxisomes,proteinsofeverysizeandshape—bumpingintomillionsofotherobjedperfmuasks:extragenergyfromnutrients,assemblingstructures,gettingridofwaste,wardingoffintruders,sendingandreceivingmessages,makingrepairs.Typicallyacellwilltainsome20,000differenttypesofprotein,andoftheseabout2,000typeswilleachberepresentedbyatleast50,000molecules.“Thismeans,”saysNuland,“thatevenifwetonlythosemoleculespresentinamountsofmorethan50,000each,thetotalisstillaveryminimumof100millionproteinmoleculesineachcell.Suchastaggeringfiguregivessomeideaoftheswarmingimmensityofbiochemicalactivitywithinus.”
Itisallanimmenselydemandingprocess.Yourheartmustpump75gallonsofbloodanhour,1,800gallonseveryday,657,000gallonsihat’senoughtofillfourOlympic-sizedswimmingpools—tokeepallthosecellsfreshlyoxygenated.(Andthat’satrest.Duringexercisetheratecreaseasmuchassixfold.)Theoxygenistakenupbythemitodria.Thesearethecells’powerstations,andthereareaboutathousandoftheminatypicalcell,thoughthenumbervariessiderablydependingonwhatacelldoesandhowmuergyitrequires.
Youmayrecallfromanearlierchapterthatthemitodriaarethoughttohaveinatedascaptivebacteriaandthattheynowliveessentiallyaslodgersinourcells,preservingtheirowistrus,dividingtotheirowable,speakingtheirownlanguage.Youmayalsorecallthatweareatthemercyoftheirgoodwill.Here’swhy.Virtuallyallthefoodandoxygenyoutakeintoyourbodyaredelivered,afterprocessing,tothemitodria,wheretheyarevertedintoamoleculecalledadenosiriphosphate,orATP.
YoumaynothaveheardofATP,butitiswhatkeepsyougoing.ATPmoleculesareessentiallylittlebatterypacksthatmovethroughthecellprovidingenergyforallthecell’sprocesses,andyougetthroughalotofit.Atanygive,atypicalcellinyourbodywillhaveaboutonebillionATPmoleculesinit,andintwominuteseveryohemwillhavebeendraineddryandanotherbillionwillhavetakentheirplace.EverydayyouproduduseupavolumeofATPequivalenttoabouthalfyourbodyweight.Feelthewarmthofyourskin.That’syourATPatwork.
Whencellsarenolongerheydiewithwhatlybecalledgreatdignity.Theytakedownallthestrutsandbuttressesthatholdthemtogetherandquietlydevourtheirpoparts.Theprocessisknotosisrammedcelldeath.Everydaybillionsofyourcellsdieforyourbeandbillionsofothersupthemess.Cellsalsodieviolently—forinstance,wheed—butmostlytheydiebecausetheyaretoldto.
Indeed,ifnottoldtolive—ifnotgivensomekindofactiveinstrufromanothercell—cellsautomaticallykillthemselves.Cellsneedalotofreassurance.
When,asoccasionallyhappens,acellfailstoexpireintheprescribedmanner,butratherbeginstodivideandproliferatewildly,wecalltheresultcer.cercellsarereallyjustfusedcells.Cellsmakethismistakefairlyregularly,butthebodyhaselaboratemeismsfordealingwithit.Itisonlyveryrarelythattheprocessspiralsoutoftrol.Onaverage,humanssufferoalmalignancyforeaillionbillioncelldivisions.
cerisbadlueverypossibleseheterm.
Thewonderofcellsisnotthatthingsoccasionallyg,butthattheymanageeverythingsosmoothlyfordecadesatastretch.Theydosobystantlysendingandmonitstreamsofmessages—acacophonyofmessages—fromallaroundthebody:
instrus,queries,corres,requestsforassistance,updates,noticestodivideorexpire.
Mostofthesesignalsarrivebymeansofcourierscalledhormones,chemititiessuchasinsulin,adrenalirogen,aosteroveyinformationfromremoteoutpostslikethethyroidandendlands.Stillothermessagesarrivebytelegraphfromthebrainorfrioersinaprocesscalledparaesignaling.Finally,cellsunicatedirectlywiththeirneighborstomakesuretheirasarecoordinated.
erhapsmostremarkableisthatitisalljustrandomfrantica,asequenceofendlessentersdirectedbynothingmorethaalrulesofattraandrepulsion.
Thereisclearlynothinkingpresencebehindanyoftheasofthecells.Italljusthappens,smoothlyaedlyandsoreliablythatseldomareweevensciousofit,yetsomehowallthisproduotjustorderwithinthecellbutaperfectharmhtacrosstheanism.
Inwaysthatwehavebarelybeguntouand,trillionsupontrillionsofreflexivechemicalreasadduptoamobile,thinking,deakingyou—or,etothat,aratherlessreflectivebutstillincrediblyanizeddule.Everylivingthing,neverfet,isawonderofatomigineering.
Indeed,sanismsthatwethinkofasprimitiveenjoyalevelofcellularanizationthatmakesourownlookcarelesslypedestrian.Disassemblethecellsofasponge(bypassihroughasieve,forinstahendumpthemintoasolution,andtheywillfindtheirwaybacktogetherandbuildthemselvesintoaspongeagain.Youdothistothemoverandover,andtheywilldoggedlyreassemblebecause,likeyouandmeaherlivingthing,theyhaveoneoverwhelmingimpulse:totiobe.
Andthat’sbecauseofacurious,determined,barelyuoodmoleculethatisitselfnotaliveandforthemostpartdoesn’tdoanythingatall.WecallitDNA,andtobegintouanditssupremeimportaosdtousweogoback160yearsorsotoVictorianEnglandandtothemomehenaturalistCharlesDarwinhadwhathasbeencalled“thesiideathatanyonehaseverhad”—andthen,forreasonsthattakealittleexplaining,lockeditawayinadrawerforthefifteenyears.松语文学www.16sy.coM免费小说阅读
1Andeveryohosecellsklywhattodotopreserveandnurtureyoufromthemomentofceptiontoyourlastbreath.
Youhavesfromyourcells.Theyknowfarmoreaboutyouthanyoudo.Eaecarriesacopyofthepletegeiccode—theinstruanualforyourbody—soitknowsnotonlyhowtodoitsjobbuteveryotherjobinthebody.Neverinyourlifewillyouhavetoremiokeepasadenosiriphosphatelevelsortofindaplacefortheextrasquirtoffolicacidthat’sjustuedlyturnedup.Itwilldothatforyou,andmillionsmorethingsbesides.
Everycellinnatureisathingofwonder.Eventhesimplestarefarbeyondthelimitsofhumay.Tobuildthemostbasicyeastcell,forexample,youwouldhavetominiaturizeaboutthesamenumberofposasarefoundinaBoeing777jetlinerandfitthemintoaspherejustfivemisacross;thensomehowyouwouldhavetopersuadethatspheretoreproduce.
Butyeastcellsareasnothingparedwithhumancells,whicharenotjustmorevariedandplicated,butvastlymorefasatingbecauseoftheirplexiions.
Yourcellsareatryoftenthousandtrillioncitizens,eachdevotediensivelyspecificwaytoyouroverallwell-being.Thereisn’tathingtheydon’tdoforyou.Theyletyoufeelpleasureandformthoughts.Theyeostandandstretdcaper.Wheheyextractthenutrients,distributetheenergy,andcarryoffthewastes—allthosethingsyoulearnedaboutinjunihschoolbiology—buttheyalsoremembertomakeyouhungryinthefirstpladrewardyouwithafeelingofwell-beingafterwardsothatyouwon’tfettoeatagain.Theykeepyourhairgrowing,yourearswaxed,yourbrailypurring.Theymanageeveryerofyourbeing.Theywilljumptoyourdefeheinstantyouarethreateheywilluatinglydieforyou—billionsofthemdosodaily.Andnotonallyouryearshaveyouthankedevehem.Soletustakeamomentnardthemwiththewonderandappreciationtheydeserve.
Weuandalittleofhowcellsdothethingstheydo—howtheylaydownfatormanufactureinsulinageinmanyoftheotheraecessarytomaintainaplicatedentitylikeyourself—butonlyalittle.Youhaveatleast200,000differenttypesofprotein1Actually,quitealotofcellsarelostintheprocessofdevelopment,sothenumberyouemergewithisreallyjustaguess.Dependingonwhichsourceyousultthenumbervarybyseveralordersofmagnitude.Thefigureoftenthousandtrillion(orquadrillion)isfrulisandSagan,1986.
labawayinsideyou,andsofarweuandwhatnomorethanabout2pertofthemdo.(Othersputthefigureatmorelike50pert;itdepends,apparently,onwhatyoumeanby“uand.”)Surprisesatthecellularlevelturnupallthetime.Innature,nitricoxideisaformidabletoxinandapoofairpollution.Sostistswerenaturallyalittlesurprisedwhen,inthemid-1980s,theyfounditbeingproduacuriouslydevotedmannerinhumancells.Itspurposewasatfirstamystery,buttheistsbegantofinditallovertheplace—trollingtheflowofbloodandtheenergylevelsofcells,attagcersandotherpathogens,regulatingthesenseofsmell,evenassistinginpeions.Italsoexplainedwhynitroglye,thewell-knownexplosive,soothestheheartpainknownasangina.(Itisvertedintonitricoxideinthebloodstream,relaxingthemuscleliningsofvessels,allowingbloodtoflowmorefreely.)Ihespaceofadecadethisonegassysubstafromextraoxintoubiquitouselixir.
Youpossess“somefewhundred”differenttypesofcell,acctotheBelgianbiochemistChristiandeDuve,andtheyvaryenormouslyinsizeandshape,fromnervecellswhosefilamentsstretchtoseveralfeettotiny,disc-shapedredbloodcellstotherod-shapedphotocellsthathelptogiveusvision.Theyalsoeinasumptuouslywiderangeofsizes—nowheremorestrikinglythanatthemomentofception,whenasiingspermfrontsay-fivethousandtimesbiggerthanit(whichratherputsthenotionofmalequestintoperspective).Onaverage,however,ahumancellisabouttwentymiswide—thatisabouttwohuhsofamillimeter—whichistoosmalltobeseenbutroomyenoughtoholdthousandsofplicatedstructureslikemitodria,andmillionsuponmillionsofmolecules.Iliteralway,cellsalsovaryinliveliness.Yourskincellsarealldead.It’sasomewhatgallingnotiontoreflectthateveryinchofyoursurfaceisdeceased.Ifyouareanaverage-sizedadultygingaroundaboutfivepoundsofdeadskin,ofwhichseveralbilliontinyfragmentsaresloughedoffeachday.Runafingeralongadustyshelfandyouaredraatternverylargelyinoldskin.
Mostlivingcellsseldomlastmorethanamonthorso,buttherearesomenotableexceptions.Livercellssurviveforyears,thoughtheposwithinthemmayberenewedeveryfewdays.Braincellslastaslongasyoudo.Youareissuedahundredbillionorsoatbirth,andthatisallyouareevergoingtoget.Ithasbeeimatedthatyoulosefivehuhemanhour,soifyouhaveanyseriousthinkingtodotherereallyisn’tamomenttowaste.Thegoodnewsisthattheindividualposofyourbraincellsarestantlyrehat,aswiththelivercells,nopartofthemisactuallylikelytobemorethanaboutamonthold.Ihasbeensuggestedthatthereisn’tasiofanyofus—notsomuchasastraymolecule—thatartofusnineyearsago.Itmaynotfeellikeit,butatthecellularlevelweareallyoungsters.
ThefirstpersontodescribeacellwasRobertHooke,whomwelastenteredsquabblingwithIsaaewtonovercreditfortheiionoftheinversesquarelaw.Hookeachievedmanythingsinhissixty-eightyears—hewasbothanaplishedtheoretiandadabhandatmakingingeniousandusefulinstruments—butnothinghedidbroughthimgreateradmirationthanhispopularbookMicrophagia:orSomePhysiologicalDescriptionsofMiniatureBodiesMadebyMagnifyingGlasses,produ1665.Itrevealedtoanentedpubliiverseoftheverysmallthatwasfarmorediverse,crowded,andfiructuredthananyonehadevereclosetoimagining.
AmongthemicroscopicfeaturesfirstidentifiedbyHookewerelittlechambersinplantsthathecalled“cells”becausetheyremindedhimofmonks’cells.Hookecalculatedthataone-inchsquareofcorkwouldtain1,259,712,000ofthesetinychambers—thefirstappearanceofsuchaverylargenumberanywhereinsce.Microscopesbythistimehadbeenaroundfeionorso,butwhatsetHooke’sapartweretheirteicalsupremacy.Theyachievedmagnificationsofthirtytimes,makihelastwordieenth-turyopticalteology.
SoitcameassomethingofashockwhenjustadecadelaterHookeahermembersofLondon’sRoyalSocietybegantoreceivedrawingsasfromaeredlinendraperinHollandemployingmagnificationsofupto275times.Thedraper’snamewasAntonivanLeeuwehoughhehadlittleformaleducationandnobackgroundinsce,heerceptiveanddedicatedobserverandateicalgenius.
Tothisdayitisnotknownhowhegotsuchmagnifitmagnificationsfromsimplehandhelddevices,whichwerelittlemorethawoodendowelswithatinybubbleofglassembeddedinthem,farmorelikemagnifyingglassesthanwhatmostofusthinkofasmicroscopes,butreallynotmuchlikeeither.Leeuwenhoekmadeanewinstrumentforeveryexperimentheperformedandwasextremelysecretiveabouthisteiques,thoughhedidsometimesoffertipstotheBritishonhowtheymightimprovetheirresolutions.
2Overaperiodoffiftyyears—beginning,remarkablyenough,whenhewasalreadypastforty—hemadealmosttwohundredreportstotheRoyalSociety,allwritteninLowDutch,theonlytongueofwhichhewasmaster.Leeuwenhoekofferednointerpretations,butsimplythefactsofwhathehadfound,apaniedbyexquisitedrawings.Hesesonalmosteverythingthatcouldbeusefullyexamined—breadmold,abee’sstinger,bloodcells,teeth,hair,hisownsaliva,excrement,andsemen(theselastwithfretfulapologiesfortheirunsavorynature)—nearlyallofwhichhadneverbeenseenmicroscopicallybefore.
Afterhereportedfinding“animalcules”inasampleofpepperwaterin1676,themembersoftheRoyalSocietyspentayearwiththebestdeviglishteologycouldproduceseargforthe“littleanimals”beforefinallygettingthemagnificatiht.WhatLeeuwenhoekhadfoundwereprotozoa.Hecalculatedthattherewere8,280,000ofthesetinybeingsinasingledropofwater—morethanthenumberofpeopleinHolland.Theworldteemedwithlifeinwaysandhatnoonehadpreviouslysuspected.
InspiredbyLeeuwenhoek’sfantastidings,antopeerintomicroscopeswithsuchkeehattheysometimesfoundthingsthatweren’tinfactthere.OnerespectedDutchobserver,NicolausHartsoecker,wasvincedhesaw“tinypreformedmen”inspermcells.Hecalledthelittlebeings“homunculi”andforsometimemanypeoplebelievedthatallhumans—indeed,allcreatures—weresimplyvastlyinflatedversionsoftinybutpleteprecursorbeings.Leeuwenhoekhimselfoccasionallygotcarriedawaywithhisenthusiasms.
Inoneofhisleastsuccessfulexperimeriedtostudytheexplosivepropertiesofgunpowderbyasmallblastatcle;henearlyblindedhimselfintheprocess.
2LeeuwenhoekwasclosefriendswithanotherDelftnotable,theartistJanVermeer.Inthemid-1660s,Vermeer,whopreviouslyhadbeenapetentbutnotoutstandingartist,suddenlydevelopedthemasteryoflightandperspectiveforwhichhehasbeeedeversihoughithasneverbeenproved,ithaslongbeensuspectedthatheusedacameraobscura,adeviceforprojegimagesontoaflatsurfacethroughalens.NosuchdevicewaslistedamongVermeerspersonaleffectsafterhisdeath,butithappensthattheexecutorofVermeersestatewasherthanAntonivanLeeuwehemostsecretivelens-makerofhisday.
In1683Leeuwenhoekdiscoveredbacteria,butthatwasaboutasfarasprogresscouldgetfortheturyandahalfbecauseofthelimitationsofmicroscopeteology.Notuntil1831wouldanyonefirstseethenucleusofacell—itwasfoundbytheScottishbotanistRobertBrown,thatfrequentbutalwaysshadowyvisitortothehistoryofsce.Brown,wholivedfrom1773to1858,calleditnucleusfromtheLatinnucula,meaninglittlenutorkernel.
Notuntil1839,however,didanyonerealizethatalllivingmatteriscellular.ItwasTheodorSn,aGerman,whohadthisinsight,anditwasnotonlyparativelylate,asstifisightsgo,butnotwidelyembracedatfirst.Itwasn’tuntilthe1860s,andsomelandmarkworkbyLouisPasteurinFrahatitwasshownclusivelythatlifeotarisespontaneouslybutmustefrompreexistingcells.Thebeliefbecameknownasthe“celltheory,”anditisthebasisofallmodernbiology.
Thecellhasbeenparedtomanythings,from“aplexchemicalrefinery”(bythephysicistJamesTrefil)to“avast,teemiropolis”(thebiochemistGuyBrown).Acellisbothofthosethingsaher.Itislikearefineryinthatitisdevotedtochemicalactivityonagrandscale,andlikeametropolisinthatitiscrowdedandbusyandfilledwithiionsthatseemfusedandrandombutclearlyhavesomesystemtothem.Butitisamuchmhtmarishplacethananycityorfactorythatyouhaveeverseen.Tobeginwiththereisnoupordownihecell(gravitydoesn’tmeaningfullyapplyatthecellularscale),andnotanatom’swidthofspaceisuhereisactivityeverywhereandaceaselessthrumofelectriergy.Youmayerriblyelectrical,butyouare.Thefoodweeatandtheoxygenwebreathearebihecellsiricity.Thereasonwedon’tgiveeachothermassiveshocksorscorchthesofawheisthatitisallhappeningonatinyscale:amere0.1voltstravelingdistancesmeasuredinnaers.
However,scalethatupanditwouldtranslateasajoltoftwentymillionvoltspermeter,aboutthesameasthechargecarriedbythemainbodyofathuorm.
Whatevertheirsizeorshape,nearlyallyourcellsarebuilttofuallythesameplan:
theyhaveanoutergormembrane,anucleuswhereinresidesthenecessarygeiationtokeepyougoing,andabusyspacebetweewocalledthecytoplasm.Themembraneisnot,asmostofusimagi,adurable,rubberyg,somethingthatyouwouldneedasharppintoprick.Rather,itismadeupofatypeoffattymaterialknoid,whichhastheapproximatesistency“ofalightgradeofmaeoil,”toquoteSherwinB.Nuland.Ifthatseemssurprisinglyinsubstantial,bearinmindthatatthemicroscopiclevelthingsbehavedifferently.Toanythingonamolecularscalewaterbeesakindofheavy-dutygel,andalipidislikeiron.
Ifyoucouldvisitacell,youwouldn’tlikeit.Blownuptoascaleatwhiswereaboutthesizeofpeas,acellitselfwouldbeasphereroughlyhalfamileacross,andsupportedbyaplexframewirderscalledthecytoskeleton.Withinit,millionsuponmillionsofobjects—somethesizeofbasketballs,othersthesizeofcars—wouldwhizaboutlikebullets.Therewouldn’tbeaplaceyoucouldstandwithoutbeingpummeledandrippedthousandsoftimeseverysedfromeverydire.Evenforitsfull-timeoctstheinsideofacellisahazardousplace.EachstrandofDNAisonaverageattaagedonceevery8.4seds—tenthousandtimesinaday—bychemicalsandentsthatwhatoorcarelesslyslicethroughit,andeachofthesewoundsmustbeswiftlystitchedupifthecellisnottoperish.
Theproteinsareespeciallylively,spinning,pulsating,andflyingintoeachotheruptoabilliontimesased.Ehemselvesatypeofprotein,dasheverywhere,perfuptoathousandtasksased.Likegreatlyspeededupworkerants,theybusilybuildandrebuildmolecules,haulingapieceoffthisone,addingapiecetothatone.Somemonitorpassingproteinsandmarkwithachemicalthosethatareirreparablydamagedorflawed.Oncesoselected,thedoomedproteinsproceedtoastructurecalledaproteasome,wheretheyarestrippeddownandtheirposusedtobuildnewproteins.Sometypesofproteiforlessthanhalfanhour;otherssurviveforweeks.Butallleadexistehatareinceivablyfrenzied.AsdeDuvehemolecularworldmustnecessarilyremaiirelybeyondthepowersinationowingtotheincrediblespeedwithwhichthingshappeninit.”
Butslowthingsdown,toaspeedatwhichtheiionsbeobserved,andthingsdon’tseemquitesounnerving.Youseethatacellisjustmillionsofobjects—lysosomes,endosomes,ribosomes,ligands,peroxisomes,proteinsofeverysizeandshape—bumpingintomillionsofotherobjedperfmuasks:extragenergyfromnutrients,assemblingstructures,gettingridofwaste,wardingoffintruders,sendingandreceivingmessages,makingrepairs.Typicallyacellwilltainsome20,000differenttypesofprotein,andoftheseabout2,000typeswilleachberepresentedbyatleast50,000molecules.“Thismeans,”saysNuland,“thatevenifwetonlythosemoleculespresentinamountsofmorethan50,000each,thetotalisstillaveryminimumof100millionproteinmoleculesineachcell.Suchastaggeringfiguregivessomeideaoftheswarmingimmensityofbiochemicalactivitywithinus.”
Itisallanimmenselydemandingprocess.Yourheartmustpump75gallonsofbloodanhour,1,800gallonseveryday,657,000gallonsihat’senoughtofillfourOlympic-sizedswimmingpools—tokeepallthosecellsfreshlyoxygenated.(Andthat’satrest.Duringexercisetheratecreaseasmuchassixfold.)Theoxygenistakenupbythemitodria.Thesearethecells’powerstations,andthereareaboutathousandoftheminatypicalcell,thoughthenumbervariessiderablydependingonwhatacelldoesandhowmuergyitrequires.
Youmayrecallfromanearlierchapterthatthemitodriaarethoughttohaveinatedascaptivebacteriaandthattheynowliveessentiallyaslodgersinourcells,preservingtheirowistrus,dividingtotheirowable,speakingtheirownlanguage.Youmayalsorecallthatweareatthemercyoftheirgoodwill.Here’swhy.Virtuallyallthefoodandoxygenyoutakeintoyourbodyaredelivered,afterprocessing,tothemitodria,wheretheyarevertedintoamoleculecalledadenosiriphosphate,orATP.
YoumaynothaveheardofATP,butitiswhatkeepsyougoing.ATPmoleculesareessentiallylittlebatterypacksthatmovethroughthecellprovidingenergyforallthecell’sprocesses,andyougetthroughalotofit.Atanygive,atypicalcellinyourbodywillhaveaboutonebillionATPmoleculesinit,andintwominuteseveryohemwillhavebeendraineddryandanotherbillionwillhavetakentheirplace.EverydayyouproduduseupavolumeofATPequivalenttoabouthalfyourbodyweight.Feelthewarmthofyourskin.That’syourATPatwork.
Whencellsarenolongerheydiewithwhatlybecalledgreatdignity.Theytakedownallthestrutsandbuttressesthatholdthemtogetherandquietlydevourtheirpoparts.Theprocessisknotosisrammedcelldeath.Everydaybillionsofyourcellsdieforyourbeandbillionsofothersupthemess.Cellsalsodieviolently—forinstance,wheed—butmostlytheydiebecausetheyaretoldto.
Indeed,ifnottoldtolive—ifnotgivensomekindofactiveinstrufromanothercell—cellsautomaticallykillthemselves.Cellsneedalotofreassurance.
When,asoccasionallyhappens,acellfailstoexpireintheprescribedmanner,butratherbeginstodivideandproliferatewildly,wecalltheresultcer.cercellsarereallyjustfusedcells.Cellsmakethismistakefairlyregularly,butthebodyhaselaboratemeismsfordealingwithit.Itisonlyveryrarelythattheprocessspiralsoutoftrol.Onaverage,humanssufferoalmalignancyforeaillionbillioncelldivisions.
cerisbadlueverypossibleseheterm.
Thewonderofcellsisnotthatthingsoccasionallyg,butthattheymanageeverythingsosmoothlyfordecadesatastretch.Theydosobystantlysendingandmonitstreamsofmessages—acacophonyofmessages—fromallaroundthebody:
instrus,queries,corres,requestsforassistance,updates,noticestodivideorexpire.
Mostofthesesignalsarrivebymeansofcourierscalledhormones,chemititiessuchasinsulin,adrenalirogen,aosteroveyinformationfromremoteoutpostslikethethyroidandendlands.Stillothermessagesarrivebytelegraphfromthebrainorfrioersinaprocesscalledparaesignaling.Finally,cellsunicatedirectlywiththeirneighborstomakesuretheirasarecoordinated.
erhapsmostremarkableisthatitisalljustrandomfrantica,asequenceofendlessentersdirectedbynothingmorethaalrulesofattraandrepulsion.
Thereisclearlynothinkingpresencebehindanyoftheasofthecells.Italljusthappens,smoothlyaedlyandsoreliablythatseldomareweevensciousofit,yetsomehowallthisproduotjustorderwithinthecellbutaperfectharmhtacrosstheanism.
Inwaysthatwehavebarelybeguntouand,trillionsupontrillionsofreflexivechemicalreasadduptoamobile,thinking,deakingyou—or,etothat,aratherlessreflectivebutstillincrediblyanizeddule.Everylivingthing,neverfet,isawonderofatomigineering.
Indeed,sanismsthatwethinkofasprimitiveenjoyalevelofcellularanizationthatmakesourownlookcarelesslypedestrian.Disassemblethecellsofasponge(bypassihroughasieve,forinstahendumpthemintoasolution,andtheywillfindtheirwaybacktogetherandbuildthemselvesintoaspongeagain.Youdothistothemoverandover,andtheywilldoggedlyreassemblebecause,likeyouandmeaherlivingthing,theyhaveoneoverwhelmingimpulse:totiobe.
Andthat’sbecauseofacurious,determined,barelyuoodmoleculethatisitselfnotaliveandforthemostpartdoesn’tdoanythingatall.WecallitDNA,andtobegintouanditssupremeimportaosdtousweogoback160yearsorsotoVictorianEnglandandtothemomehenaturalistCharlesDarwinhadwhathasbeencalled“thesiideathatanyonehaseverhad”—andthen,forreasonsthattakealittleexplaining,lockeditawayinadrawerforthefifteenyears.松语文学www.16sy.coM免费小说阅读
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