细胞的基本特性.docx

细胞的基本特性.docx

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细胞的基本特性

BASICPROPERTIESOFCELLS

Justasplantsandanimalsarealive,sotooarecells.Life,infact,isthemostbasicpropertyofcells,andcellsarethesmallestunitstoexhibitthisproperty.Unlikethepartsofacell,whichsimplydeteriorateifisolated,wholecellscanberemovedfromaplantoranimalandculturedinalaboratorywheretheywillgrowandreproduceforextendedperiodsoftime.Ifmistreated,theymaydie.

Deathcanalsobeconsideredoneofthemostbasicpropertiesoflife,becauseonlyalivingentityfacesthisprospect.Remarkably,cellswithinthebodygenerallydie“bytheirownhand”—thevictimsofaninternalprogramthatcausescellsthatarenolongerneededorcellsthatposeariskofbecomingcanceroustoeliminatethemselves.

ThefirstcultureofhumancellswasbegunbyGeorgeandMarthaGeyofJohnsHopkinsUniversityin1951.ThecellswereobtainedfromamalignanttumorandnamedHeLacellsafterthedonor,HenriettaLacks.HeLacells—descendedbycelldivisionfromthisfirstcellsample—arestillbeinggrowninlaboratoriesaroundtheworldtoday.Becausetheyaresomuchsimplertostudythancellssituatedwithinthebody,cellsgrowninvitro（i.e.,inculture,outsidethebody）havebecomeanessentialtoolofcellandmolecularbiologists.Infact,muchoftheinformationthatwillbediscussedinthisbookhasbeenobtainedusingcellsgrowninlaboratorycultures.

Wewillbeginourexplorationofcellsbyexaminingafewoftheirmostfundamentalproperties.

CellsAreHighlyComplexandOrganized

Complexityisapropertythatisevidentwhenencountered,butdifficulttodescribe.Forthepresent,wecanthinkofcomplexityintermsoforderandconsistency.

Themorecomplexastructure,thegreaterthenumberofpartsthatmustbeintheirproperplace,thelesstoleranceoferrorsinthenatureandinteractionsoftheparts,andthemoreregulationorcontrolthatmustbeexertedtomaintainthesystem.

Cellularactivitiescanberemarkablyprecise.DNAduplication,forexample,occurswithanerrorrateoflessthanonemistakeeverytenmillionnucleotidesincorporated—andmostofthesearequicklycorrectedbyanelaboraterepairmechanismthatrecognizesthedefect.

Duringthecourseofthisbook,wewillhaveoccasiontoconsiderthecomplexityoflifeatseveraldifferentlevels.Wewilldiscusstheorganizationofatomsintosmall-sizedmolecules;theorganizationofthesemoleculesintogiantpolymers;andtheorganizationofdifferenttypesofpolymericmoleculesintocomplexes,whichinturnareorganizedintosubcellularorganellesandfinallyintocells.Aswillbeapparent,thereisagreatdealofconsistencyateverylevel.

Eachtypeofcellhasaconsistentappearancewhenviewedunderahigh-poweredelectronmicroscope;thatis,itsorganelleshaveaparticularshapeandlocation,fromoneindividualofaspeciestoanother.Similarly,eachtypeoforganellehasaconsistentcompositionofmacromolecules,whicharearrangedinapredictablepattern.

Considerthecellsliningyourintestinethatareresponsibleforremovingnutrientsfromyourdigestivetract.

Theepithelialcellsthatlinetheintestinearetightlyconnectedtoeachotherlikebricksinawall.Theapicalendsofthesecells,whichfacetheintestinalchannel,havelongprocesses（microvilli）thatfacilitateabsorptionofnutrients.Themicrovilliareabletoprojectoutwardfromtheapicalcellsurfacebecausetheycontainaninternalskeletonmadeoffilaments,whichinturnarecomposedofprotein（actin）monomerspolymerizedinacharacteristicarray.Attheirbasalends,intestinalcellshavelargenumbersofmitochondriathatprovidetheenergyrequiredtofuelvariousmembranetransportprocesses.Eachmitochondrioniscomposedofadefinedpatternofinternalmembranes,whichinturnarecomposedofaconsistentarrayofproteins,includinganelectricallypoweredATP-synthesizingmachinethatprojectsfromtheinnermembranelikeaballonastick.

Fortunatelyforcellandmolecularbiologists,evolutionhasmovedratherslowlyatthelevelsofbiologicalorganizasigtionwithwhichtheyareconcerned.Whereasahumanandacat,forexample,haveverydifferentanatomicalfeatures,thecellsthatmakeuptheirtissues,andtheorganellesthatmakeuptheircells,areverysimilar.TheactinfilamentportrayedinFigure1.3,Inset3,andtheATP-synthesizingenzymeofInset6arevirtuallyidenticaltosimilarstructuresfoundinsuchdiverseorganismsashumans,snails,yeast,andredwoodtrees.Informationobtainedbystudyingcellsfromonetypeoforganismoftenhasdirectapplicationtootherformsoflife.Manyofthemostbasicprocesses,suchasthesynthesisofproteins,theconservationofchemicalenergy,ortheconstructionofamembrane,areremarkablysimilarinalllivingorganisms.

CellsPossessaGeneticProgramandtheMeanstoUseIt

Organismsarebuiltaccordingtoinformationencodedinacollectionofgenes.Thehumangeneticprogramcontainsenoughinformation,ifconvertedtowords,tofillmillionsofpagesoftext.Remarkably,thisvastamountofinformationispackagedintoasetofchromosomesthatoccupiesthespaceofacellnucleus—hundredsoftimessmallerthanthedotonthisi.

Genesaremorethanstoragelockersforinformation:

theyconstitutetheblueprintsforconstructingcellularstructures,thedirectionsforrunningcellularactivities,andtheprogramformakingmoreofthemselves.Themolecularstructureofgenesallowsforchangesingeneticinformation（mutations）thatleadtovariationamongindividuals,whichformsthebasisofbiologicalevolution.Discoveringthemechanismsbywhichcellsusetheirgeneticinformationhasbeenoneofthegreatestachievementsofscienceinrecentdecades.

CellsAreCapableofProducingMoreofThemselves

Justasindividualorganismsaregeneratedbyreproduction,sotooareindividualcells.Cellsreproducebydivision,aprocessinwhichthecontentsofa“mother”cellaredistributedintotwo“daughter”cells.Priortodivision,thegeneticmaterialisfaithfullyduplicated,andeachdaughtercellreceivesacompleteandequalshareofgeneticinformation.Inmostcases,thetwodaughtercellshaveapproximatelyequalvolume.Insomecases,however,asoccurswhenahumanoocyteundergoesdivision,oneofthecellscanretainnearlyallofthecytoplasm,eventhoughitreceivesonlyhalfofthegeneticmaterial

CellsAcquireandUtilizeEnergy

Everybiologicalprocessrequirestheinputofenergy.VirtuallyalloftheenergyutilizedbylifeontheEarth’ssurfacearrivesintheformofelectromagneticradiationfromthesun.Theenergyoflightistrappedbylight-absorbingpigmentspresentinthemembranesofphotosyntheticcells.

Lightenergyisconvertedbyphotosynthesisintochemicalenergythatisstoredinenergy-richcarbohydrates,suchassucroseorstarch.Formostanimalcells,energyarrivesprepackaged,of-tenintheformofthesugarglucose.Inhumans,glucoseisreleasedbytheliverintothebloodwhereitcirculatesthroughthebodydeliveringchemicalenergytoallthecells.Onceinacell,theglucoseisdisassembledinsuchawaythatitsenergycontentcanbestoredinareadilyavailableform（usuallyasATP）thatislaterputtouseinrunningallofthecell’smyriadenergy-requiringactivities.Cellsexpendanenormousamountofenergysimplybreakingdownandrebuildingthemacromoleculesandorganellesofwhichtheyaremade.Thiscontinual“turnover,”asitiscalled,maintainstheintegrityofcellcomponentsinthefaceofinevitablewearandtearandenablesthecelltorespondrapidlytochangingconditions.

CellsCarryOutaVarietyofChemicalReactions

Cellsfunctionlikeminiaturizedchemicalplants.Eventhesimplestbacterialcelliscapableofhundredsofdifferentchemicaltransformations,noneofwhichoccursatanysignificantrateintheinanimateworld.Virtuallyallchemicalchangesthattakeplaceincellsrequireenzymes—moleculesthatgreatlyincreasetherateatwhichachemicalreactionoccurs.Thesumtotalofthechemicalreactionsinacellrepresentsthatcell’smetabolism.

CellsEngageinMechanicalActivities

Cellsaresitesofbustlingactivity.Materialsaretransportedfromplacetoplace,structuresareassembledandthenrapidlydisassembled,and,inmanycases,theentirecellmovesitselffromonesitetoanother.Thesetypesofactivitiesarebasedondynamic,mechanicalchangeswithincells,manyofwhichareinitiatedbychangesintheshapeof“motor”proteins.Motorproteinsarejustoneofmanytypesofmolecular“machines”employedbycellstocarryoutmechanicalactivities.

CellsAreAbletoRespondtoStimuli

Somecellsrespondtostimuliinobviousways;asingle-celledprotist,forexample,movesawayfromanobjectinitspathormovestowardasourceofnutrients.Cellswithinamulticellularplantoranimalrespondtostimulilessobviously.Mostcellsarecoveredwithreceptorsthatinteractwithsubstancesintheenvironmentinhighlyspecificways.Cellspossessreceptorstohormones,growthfactors,andextracellularmaterials,aswellastosubstancesonthesurfacesofothercells.Acell’sreceptorsprovidepathwaysthroughwhichexternalagentscanevokespecificresponsesintargetcells.Cellsmayrespondtospecificstimulibyalteringtheirmetabolicactivities,movingfromoneplacetoanother,orevencommittingsuicide.

CellsAreCapableofSelf-Regulation

Inadditiontorequiringenergy,maintainingacomplex,orderedstaterequiresconstantregulation.Theimportanceofacell’sregulatorymechanismsbecomesmostevidentwhentheybreakdown.Forexample,failureofacelltocorrectamistakewhenitduplicatesitsDNAmayresultina