09年英语六级快速阅读实战练习.docx

09年英语六级快速阅读实战练习.docx

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09年英语六级快速阅读实战练习

09年6月英语六级快速阅读实战练习

 Abalanceddietisonethatprovidesanadequateintakeofenergyandnutrientsformaintenanceofthebodyandthereforegoodhealth.Adietcaneasilybeadequatefornormalbodilyfunctioning,yetmaynotbeabalanceddiet.

　　Carbohydrates

　　Carbohydratesarearapidsourceofenergy,theyarethebody'sfuel.Thebulkofabalanceddietshouldbemadefromcarbohydrates.Ifeateninanexcessofthedietaryrequirementscarbohydratesareeasilystoredasfatsinthecells,althoughcarbohydrateisthefirstsourceofenergyinthebody.

　　Anaverageadultrequiresabout12,000kJofenergyaday,mostofthisissuppliedbytherespirationofcarbohydratesinthecells.

　　Carbohydratesareusedprincipallyasarespiratorysubstrates,.tobeoxidisedtoreleaseenergyforactivetransport,macromoleculesynthesis,celldivisionandmusclecontraction.Carbohydratesaredigestedintheduodenumandileumandabsorbedasglucoseintocells.Sourcesofcarbohydratessuchasstarcharerice,potatoes,wheatandothercereals.Sugarsarealsocarbohydrates,sourcesofsugarsarerefinedsugar-sucrose,whichisafoodsweetenerandpreservativeandfruitsugars-fructose.Ifthedietlackscarbohydratestoresoffataremobilisedandusedasanenergysource.

　　Proteins

　　Proteinisnotadirectsourceofenergyinthebody,itisusedprimarilyforgrowthandrepairofbodytissueswhileremaininganenergysourceasalastresort.Proteinsfulfillawidevarietyofrolesinthebody.Theyarebrokendowninthestomachandintestinestoaminoacidswhicharethenabsorbed.Thebodycanonlyform8aminoacidstobuildproteinsfrom,thedietmustprovideEssentialAminoAcids（EAAs）whicharesynthesisedintoproteinswhichcanbestructural,.collageninbone,keratininhair,myosinandactininmuscle;metabolicenzymes,hemoglobin,protectiveantibodiesandcommunicativehormones.

　　Sourcesofproteinincludemeat,fish,eggsandpulses.Thedietneedstoprovide8EAAsasthebodyisunabletosynthesisproteinswithoutthesemolecules.2otheraminoacidsaresynthesisedfromEAAssoifthedietlackstheoriginalEAAstheseothertwowillnotbepresenteither.Phenylalanineisconvertedtotyrosineandmethionineisconvertedtocysteine.Cellsdrawuponapoolofaminoacidsforproteinsynthesiswhicheithercomefromdietaryproteindigestedandabsorbedinthegutandthebreakdownofbodyproteinsuchasmuscle.However,unlikefatsandcarbohydratesthereisnostoreofaminoacidsforcellstodrawon,anyaminoacidinexcessofimmediatebodilyrequirementsisbrokendownintoureaandexcreted.Itisthereforeimportanttomaintainthedietaryintakeofproteineveryday.Ifthebodylacksprotein,musclewastingoccursasmuscleisbrokendown.

　　Ifproteinislackedinadietapersondevelopskwashiorkorwhichiscausedwhenhighlevelsofcarbohydratesareeatentoovercomethelackofproteininthediet.Onesymptomofkwashiorkoristheabnormalcollectionoffluidaroundtheabdomenduetothelackofproteinintheblood.Thebodycannotretainwaterbyosmosisandfluidaccumulatesintissuescausingthemtobecomewaterlogged.

　　Vitamins

　　Vitaminscannotbesynthesisedbythebodysomustbesuppliedbydiet.Vitaminshavenocommonstructureorfunctionbutareessentialinsmallamountsforthebodytobeabletoutiliseotherdietarycomponentsefficiently.

　　Vitaminsfallintotwocategories,fatsolublevitaminssuchasvitaminA,D,EandKwhichareingestedwithfattyfoodsandwatersolublevitaminssuchastheBgroupvitaminsandvitaminC.Vitaminsareknownasmicronutrientsbecauseonlysmallquantitiesarerequiredforahealthydiet,infactfatsolublevitaminscanbetoxicinhighconcentrations,forexamplethebodystoresvitaminA,orretinol,intheliverasitistoxicifkeptinhighconcentrationsinthebloodstream,adoseofmorethan3300mgofvitaminAcanbeconsideredtoxic.WatersolublevitaminssuchasvitaminCandBgroupsvitaminscanbeexcretedintheurineifinexcessinthediet.

Vitaminscarryoutawiderangeoffunctionsandpreventspecificdeficiencydiseases.Adietthatlacksacertainvitaminisnotabalanceddiet,vitaminshavevitalrolesinthemaintenanceofahealthybody.

　　Anexampleofadeficiencyiswhenthedietdoesnotcontainenough,oranyvitaminA.VitaminAisfoundinsomeanimalfoodssuchasmilk,eggs,liverandfishliveroils,relatedcompoundssuchascarotenoids.bcarotene,areinawidevarietyofvegetablessuchascabbages,carrotsandspinach.

　　VitaminAisessentialtotheproperfunctioningoftheretinaintheeyeandtheepithelialtissues.AlackofvitaminAresultsindry,roughskin,inflammationoftheeyes,adryingorscarringofthecornea-xerophthalmia,whichoccurswhenthesecretionoflubricatingtearsisstopped,theeyelidsbecomeswollenandstickywithpus.Mucoussurfacesoftheeyemaybecomeerodedallowinginfectiontosetin,leadingtoulcerationanddestructionofthecornea.Nightblindness-aninabilitytoseeindimlightcanalsooccur.Rodcellsintheretinaoftheeyedetectlightoflowintensity,theyconvertvitaminAintoapigment,rhodopsin,whichisbleachedwhenlightenterstheeye.Rodcellsresynthesisrhodopsin,butifthereisadeficiencyofthevitamin,rodcellscannolongerfunctionandtheresultisnightblindness.Epithelialcellsuseretinoltomakeretinoicacid,anintracellularmessengerusedincelldifferentiationandgrowth.Withoutretinoicacidepithelialcellsarenotmaintainedproperlyandthebodybecomessusceptibletoinfections,particularlymeaslesandinfectionsoftherespiratorysystemandgut.

　　Xenophthalmiaiscommonamongchildrenwho'sdietsconsistofmainlycerealswithlittlemeatorfreshvegetables,thisiscommoninIndonesia,Bangladesh,IndiaandthePhilippines.

　　VitaminD,orcalciferol,isanotherfatsolublesteroidvitaminwhichfunctionstostimulatecalciumuptakefromthegutanditsdepositioninbone.vitaminDactsasahormonewhenconvertedbyenzymesinthegutandliverintoanactiveformof"activevitaminD",whichstimulatesepithelialcellsintheintestinetoabsorbcalcium.vitaminDisthereforeessentialingrowingchildren'sdietstoenablethegrowthofstrongbones.WithoutadequateamountsofvitaminDchildrencandeveloprickets,whichisthedeformationofthelegscausedwhentheylackcalciumtostrengthenthebones.InadultsalackofvitaminDinthedietcanleadtoosteomalacia,aprogressivesofteningoftheboneswhichcanmakethemhighlysusceptibletofracture.

　　VitaminDismadebythebodywhenexposedtosunlightandisstoredinthemuscles,however,iftheskinisrarelyexposedtothesunlightorisdarklittlevitaminDisproduced.FoodssuchaseggsandoilyfishareallrichinvitaminD.

　　VitaminK,phylloquinone,isfoundindarkgreenleafyvegetablessuchasspinachandkale.Itisafatsolublevitaminwhichisinvolvedintheclottingprocessofblood.IntheintestinesbacteriasynthesizeanumberofimportantclottingfactorswhichneedvitaminK.WithoutvitaminKcutscanfailtohealandinternalbleedingcanoccur.

　　VitaminCisawatersolublevitamin,knownchemicallyasascorbicacid.Itisfoundincitrusfruitssuchasorangesandlemons,andalsoinpotatoesandtomatoes.ThemainfunctionofvitaminCistheformationofconnectivetissuessuchascollagen.Itisalsoknowntobeanantioxidantwhichhelpstoremovetoxinsandaidstheimmunesystem.AlackofvitaminCleadstoScurvy,aconditionexperiencedbysailorsonlongjourneyswhentheydidnothavefruitintheirdiets.Scurvycausespainful,bleedinggums.AsvitaminCiswatersoluble,itisnottoxicinhighdosesasitcanbeexcretedintheurine,veryhighdosescanhowevercausediarrhea.

　　Bgroupvitaminshaveawiderangeofrolesactingasco-enzymesinmetabolicpathways.Theyarefoundinmostplantandanimaltissuesinvolvedinmetabolism,thereforefoodssuchasliver,yeastanddairyproductsareallrichinBgroupvitamins.DeficiencyofBgroupvitaminsincludedermatitis,fatigueandmalformationofredbloodcells.

　　1.Whenwehavetoomuchofcarbohydrates,theyaremostlikelybestoredassugarunits.

　　2.Intheprocessofdigestion,carbohydratesarefinallyturnedintocells.

3.TheEssentialAminoAcidswhichbuildpartofproteinscanbeformedbyhumanbody.

　　4.Theultimatecauseofkwashiorkorislackofprotein,ratherthanlackofcarbohydrates.

　　5.Therearegenerallytwodifferentkindsofvitamins,theyare__________________________.

　　6.Nightblindnessisadiseasenormallycausedbylackof_____________.

　　7.OneofthemainfunctionofvitaminDistopreventadultsfrom________________.

　　8.AlthoughthehumanbodyproducesvitaminDnormally,itfailstodosoifthereisnotenough______________.

　　9.ThereasonwhyvitaminCisseenasanantioxidantisthatitdrives___________outofthebody.

　　10.IfyouareinlackofBgroupvitamins,youshouldturnto________________________.

　　1.（N）

　　2.（N）

　　3.（N）

　　4.（Y）

　　5.（fatsolublevitaminsandwatersolublevitamins）

　　6.（vitaminA）

　　7.（fracture/breakingtheirbones）

　　8.（sunlight/sunlightexposure/exposingtosunlight）

　　9.（toxins）

　　10.（plantandanimaltissuesinvolvedinmetabolism/liver,yeastanddairyproducts）

 Ourgalaxyisagiganticagglomerationofstarsandplanetswhosenumberswillprobablyneverbeenknown.Currentlyweestimatethisnumbertobeaboutthirtybillion.Scientistshaveestimatedthatthediameterofourgalaxy,ifitweretobetraveled,wouldtakeusaboutfiftythousandlightyearsandthethicknesstobeaboutfifteentotwentyligh