剑桥雅思8第三套试题阅读部分PASSAGE3Word文档下载推荐.docx

剑桥雅思8第三套试题阅读部分PASSAGE3Word文档下载推荐.docx

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ReadingPassage3hassevenparagraphs,A-G.

ChoosethecorrectheadingforparagraphsB-Gfromthelistofheadingsbelow.

Writethecorrectnumber,i-x,inboxes27-32onyouranswersheet.

ListofHeadings

iThebiologicalclock

iiWhydyingisbeneficial

iiiTheageingprocessofmenandwomen

ivProlongingyourlife

vLimitationsoflifespan

viModesofdevelopmentofdifferentspecies

viiAstablelifespandespiteimprovements

viiiEnergyconsumption

ixFundamentaldifferencesinageingofobjectsandorganisms

xRepairofgeneticmaterial

ExampleAnswer

ParagraphAv

27ParagraphB

28ParagraphC

29ParagraphD

30ParagraphE

31ParagraphF

32ParagraphG

HOWDOESTHEBIOLOGICALCLOCKTICK?

AOurlifespanisrestricted.Everyoneacceptsthisas'

biologically'

obvious.'

Nothinglivesforever!

'

However,inthisstatementwethinkofartificiallyproduced,technicalobjects,productswhicharesubjectedtonaturalwearandtearduringuse.Thisleadstotheresultthatatsometimeorothertheobjectstopsworkingandisunusable（'

death'

inthebiologicalsense.Butarethewearandtearandlossoffunctionoftechnicalobjectsandthedeathoflivingorganismsreallysimilarorcomparable?

BOur'

dead'

productsare'

static'

closedsystems.Itisalwaysthebasicmaterialwhichconstitutestheobjectandwhich,inthenaturalcourseofthings,isworndownandbecomes'

older'

.Ageinginthiscasemustoccuraccordingtothelawsofphysicalchemistryandofthermodynamics.Althoughthesamelawholdsforalivingorganism,theresultofthislawisnotinexorableinthesameway.Atleastaslongasabiologicalsystemhastheabilitytorenewitselfit

couldactuallybecomeolderwithoutageing;

anorganismisanopen,dynamicsystemthroughwhichnewmaterialcontinuouslyflows.Destructionofoldmaterialandformationofnewmaterialarethusinpermanentdynamicequilibrium.Thematerialofwhichtheorganismisformedchangescontinuously.Thusourbodiescontinuouslyexchangeoldsubstancefornew,justlikeaspringwhichmoreorlessmaintainsitsformandmovement,butinwhichthewatermoleculesarealwaysdifferent.

CThusageinganddeathshouldnotbeseenasinevitable,particularlyastheorganismpossessesmanymechanismsforrepair.Itisnot,inprinciple,necessaryforabiologicalsystemtoageanddie.Nevertheless,arestrictedlifespan,ageing,andthendeatharebasiccharacteristicsoflife.Thereasonforthisiseasytorecognise:

innature,theexistentorganismseitheradaptorareregularlyreplacedbynewtypes.Becauseofchangesinthegeneticmaterial（mutationsthesehavenewcharacteristicsandinthecourseoftheirindividuallivestheyaretestedforoptimalorbetteradaptationtotheenvironmentalconditions.Immortalitywoulddisturbthissystem-itneedsroomfornewandbetterlife.Thisisthebasicproblemofevolution.

DEveryorganismhasalifespanwhichishighlycharacteristic.Therearestrikingdifferencesinlifespanbetweendifferentspecies,butwithinonespeciestheparameterisrelativelyconstant.Forexample,theaveragedurationofhumanlifehashardlychangedinthousandsofyears.Althoughmoreandmorepeopleattainanadvancedageasaresultofdevelopmentsinmedicalcareandbetternutrition,thecharacteristicupperlimitformostremains80years.Afurtherargumentagainstthesimplewearandteartheoryistheobservationthatthetimewithinwhichorganismsageliesbetweenafewdays（evenafewhoursforunicellularorganismsandseveralthousandyears,aswithmammothtrees.

EIfalifespanisageneticallydeterminedbiologicalcharacteristic,itislogicallynecessarytoproposetheexistenceofaninternalclock,whichinsomewaymeasuresandcontrolstheageingprocessandwhichfinallydeterminesdeathasthelaststepinafixedprogramme.Likethelifespan,themetabolicratehasfordifferentorganismsafixedmathematicalrelationshiptothebodymass.Incomparisontothelifespanthisrelationshipis'

inverted'

:

thelargertheorganismtheloweritsmetabolicrate.Againthisrelationshipisvalidnotonlyforbirds,butalso,similarlyonaveragewithinthesystematicunit,forallotherorganisms（plants,animals,unicellularorganisms.

FAnimalswhichbehave'

frugally'

withenergybecomeparticularlyold,forexample,crocodilesandtortoises.Parrotsandbirdsofpreyareoftenheldchainedup.Thustheyarenotableto'

experiencelife'

andsotheyattainahighlifespanincaptivity.Animalswhichsaveenergybyhibernationorlethargy（e.g.batsorhedgehogslivemuchlongerthanthosewhicharealwaysactive.Themetabolicrateofmicecanbereduce