托福阅读原文及翻译

托福阅读原文及翻译

托福阅读原文：

ElementsofLife

Thecreationofliferequiresasetofchemicalelementsformakingthe componentsofcells.LifeonEarthusesabout25ofthe92naturallyocc urringchemicalelements,althoughjust4oftheseelements—oxygen,carbon,hydrogen,andnitrogen—

makeupabout96percentofthemassoflivingorganisms.Thus,afirstr equirementforlifemightbethepresenceofmostoralloftheelements usedbylife.

Interestingly,thisrequirementcanprobablybemetbyalmostanywor ld.Scientistshavedeterminedthatallchemicalelementsintheuniv ersebesideshydrogenandhelium(andatraceamountoflithium)werep roducedbystars.Theseareknownasheavyelementsbecausetheyarehe avierthanhydrogenandhelium.Althoughalloftheseheavyelementsa requiterarecomparedtohydrogenandhelium,theyarefoundjustabou teverywhere.

Heavyelementsarecontinuallybeingmanufacturedbystarsandrelea sedintospacebystellardeaths,sotheiramountcomparedtohydrogen andheliumgraduallyriseswithtime.Heavyelementsmakeupabout2pe rcentofthechemicalcontent(bymass)ofoursolarsystem,theother9 8percentishydrogenandhelium.Insomeveryoldstarsystems,whichf ormedbeforemanyheavyelementswereproduced,theheavy-elementsharemaybelessthan0.1percent.Nevertheless,everystars ystemstudiedhasatleastsomeamountofalltheelementsusedbylife. Moreover,whenplanetesimals—

small,solidobjectsformedintheearlysolarsystemthatmayaccumul atetobecomeplanets—

condensewithinaformingstarsystem,theyareinevitablymadefromh eavyelementsbecausethemorecommonhydrogenandheliumremaingase ous.Thus,planetesimalseverywhereshouldcontaintheelementsnee dedforlife,whichmeansthatobjectsbuiltfromplanetesimals—planets,moons,asteroids,andcomets-alsocontaintheseelements.Thenatureofsolar-systemformationexplainswhyEarthcontainsalltheelementsneeded forlife,anditiswhyweexpecttheseelementstobepresentonotherwo rldsthroughoutoursolarsystem,galaxy,anduniverse.

Notethatthisargumentdoesnotchange,evenifweallowforlifeveryd ifferentfromlifeonEarth.LifeonEarthiscarbonbased,andmostbio logistsbelievethatlifeelsewhereislikelytobecarbonbasedaswel l.However,wecannotabsolutelyruleoutthepossibilityoflifewith anotherchemicalbasis,suchassiliconornitrogen.Thesetofelemen ts(ortheirrelativeproportions)usedbylifebasedonsomeotherele mentmightbesomewhatdifferentfromthatusedbycarbon-basedlifeonEarth.Buttheelementsarestillproductsofstarsandwo uldstillbepresentinplanetesimalseverywhere.Nomatterwhatkind soflifewearelookingfor,wearelikelytofindthenecessaryelement sonalmosteveryplanet,moon,asteroid,andcometintheuniverse.

Asomewhatstricterrequirementisthepresenceoftheseelementsinm oleculesthatcanbeusedasready-

madebuildingblocksforlife,justasearlyEarthprobablyhadanorga nicsoupofaminoacidsandothercomplexmolecules.Earth'sorganicm oleculeslikelycamefromsomecombinationofthreesources:chemica lreactionsintheatmosphere,chemicalreactionsneardeep-seaventsintheoceans,andmoleculescarriedtoEarthbyasteroidsan dcomets.Thefirsttwosourcescanoccuronlyonworldswithatmospher esoroceans,respectively.Butthethirdsourceshouldhavebroughts imilarmoleculestonearlyallworldsinoursolarsystem.