SATII生物部分资料.docx
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SATII生物部分资料.docx
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SATII生物部分资料
SAT是ScholasticAptitudeTest的缩写,是申请几乎所有美国大学必须参加的考试。
通常,希望继续接受高等教育的高中生需要参加SAT考试,并且SAT考试得分是获取奖学金的重要标准之一。
大部分美国大学要求SAT作为录取的条件并根据SAT得分授予奖学金。
Theconnectionsbetweentheatomsinacompoundarecalledchemicalbonds.Atomsformbondsbysharingtheirelectronswitheachother,relyingonthepowerofelectricchargetokeepthemselvesattached.Moleculesandcompoundscanalsobondwitheachother.Importantbondsbetweenatomsarecovalentandionicbonds.Bondsbetweenmoleculesorcompoundsarecalleddipole-dipolebonds.
Covalentbonds
Bondsformedthroughthemoreorlessequalsharingofelectronsbetweenatomsareknownascovalentbonds.
Iftheelectronsinacovalentbondaresharedequally,theresultingbondiscalledanonpolarcovalentbond.Whenoneatompullsthesharedelectronstowarditselfalittlemoretightlythantheother,theresultingcovalentbondissaidtobeapolarbond.Inapolarbond,theatomthatpullselectronstowarditselfgainsaslightnegativecharge(becauseelectronshaveanegativecharge).Sincetheotheratompartiallylosesanelectron,itgainsaslightpositivecharge.Forexample,theatomsinwaterformpolarbondsbecauseoxygen,whichhaseightprotonsinitsnucleus,hasagreaterpullonelectronsthanhydrogen,whichhasonlyoneproton.
IonicBonds
Polarcovalentbondsinvolvetheunequalsharingofelectrons.Thisinequalityisbroughttoanextremeinabondingarrangementcalledanionicbond.Inanionicbond,oneatompullsthesharedelectronsawayfromtheotheratomentirely.Ionicbondsarestrongerthanpolarbonds.
Oneexampleofionicbondingisthereactionbetweensodium(Na)andchlorine(Cl)toformtablesalt(NaCl).Thechlorineatomstealsanelectronfromthesodiumatom.Becauseitlosesanelectron,thesodiumatomdevelopsachargeof+1.Thechlorineatomhasachargeof–1,sinceitgainedanelectron.
Dipole-DipoleBonds
Asseeninpolarcovalentcompounds,duetotheunequalsharingofelectrons,somemoleculeshaveaslightlypositiveandaslightlynegativeendtothem,oradipole(di-pole=twomagneticpoles).Thesecompoundscanformweakbondswithoneanotherwithoutcombiningtogethercompletelytocreatenewcompounds.Thistypeofbonding,knownasdipole-dipoleinteraction,takesplaceswhenthepositivelychargedendofonepolarcovalentcompound(d+)comesincontactwiththenegativelychargedendofanotherpolarcovalentcompound(d–):
Dipole-dipoleinteractionsaremuchweakerthanthebondswithinmolecules,buttheyplayaveryimportantroleinthechemistryoflife.Perhapsthemostimportantdipole-dipolebondinbiochemistry(andontheSATIIBiology)isthedipole-dipoleinteractionbetweenpositivelychargedhydrogenmoleculesandnegativelychargedoxygenmolecules.Thisreactionissoimportant,itgetsitsownspecialname:
hydrogenbond.ThesebondsaccountformanyoftheexceptionalpropertiesofwaterandhaveimportanteffectsonthestructureofproteinsandDNA.
Thecellsofallorganisms,prokaryoticandeukaryoticalike,aresurroundedbyathinsheetcalledthecellmembrane.Thisbarrierkeepscellularmaterialsinandforeignobjectsout.Themembraneiskeytothelifeofthecell.Byregulatingwhatgetsintoandoutofthecell,themembranemaintainstheproperchemicalcomposition,whichiscrucialtothelifeprocessesthecellcarriesout.
StructureoftheCellMembrane
Thecellmembraneismadeupoftwosheetsofspecialfatmoleculescalledphospholipids,placedontopofeachother.
Thisarrangementisknownasaphospholipidbilayer.Phospholipidmoleculesnaturallyarrangeinbilayersbecausetheyhaveauniquestructure.Thelongchainsofcarbonandhydrogenthatformthetailofthismoleculedonotdissolveinwater;theyaresaidtobehydrophobicor“waterfearing.”Thehydrophilicphosphorousheadsareattractedtowater.Formingabilayersatisfiesthewaterpreferencesofboththe“heads”and“tails”ofphospholipids:
thehydrophilicheadsfacethewateryregionsinsideandoutsidethecell,andthehydrophobictailsfaceeachotherinawater-freejunction.Thebilayerformsspontaneouslybecausethissituationissofavorable.
TheFluidMosaicModel
Phospholipidsformthefundamentalstructureofthecellmembrane,buttheyarenottheonlysubstancefoundthere.Accordingtothefluid-mosaicmodelofthecellmembrane,specialproteinscalledmembraneproteinsfloatinthephospholipidbilayerlikeicebergsinasea.
Theseaofphospholipidmoleculesandgatekeepermembraneproteinsisinconstantmotion.Themembrane’sfluiditykeepsthecellfromfracturingwhenplacedunderstrain.
TransportThroughtheCellMembrane
Themostimportantpropertyofthecellmembraneisitsselectivepermeability:
somesubstancescanpassthroughitfreely,butotherscannot.Smallandnonpolar(hydrophobic)moleculescanfreelypassthroughthemembrane,butchargedionsandlargemoleculessuchasproteinsandsugarsarebarredpassage.Theselectivepermeabilityofthecellmembraneallowsacelltomaintainitsinternalcompositionatnecessarylevels.
Moleculesthatcanpassfreelythroughthemembranefollowconcentrationgradients,movingfromthehigherconcentrationareatotheregionoflowerconcentration.Theseprocessestakenoenergyandarecalledpassivetransport.Themoleculesthatcannotpassfreelyacrossthephospholipidbilayercanbecarriedacrossthemembraneinvariousprocessesthatrequireenergyandarethereforecalledactivetransport.
PassiveTransport
Therearethreemaintypesofpassivetransport:
diffusion,facilitateddiffusion,andosmosis.Infact,osmosisissimplythetermgiventothediffusionofwater.
Diffusion
Intheabsenceofotherforces,substancesdissolvedinwatermovenaturallyfromareaswheretheyareabundanttoareaswheretheyarescarce—aprocessknownasdiffusion.Ifthereisahigherconcentrationofcarbondioxidegasdissolvedinthewaterinsidethecellthaninthewateroutsidethecell,carbondioxidewillnaturallyflowoutfromthecelluntilitsdistributionisbalanced,withoutanyenergyrequiredfromthecell.
Nonpolarandsmallpolarmoleculescanpassthroughthecellmembrane,sotheydiffuseacrossitinresponsetoconcentrationgradients.Carbondioxideandoxygenaretwomoleculesthatundergothissimplediffusionthroughthemembrane.
Thesimplediffusionofwaterisknownasosmosis.Becausewaterisasmallpolarmolecule,itundergoessimplediffusion.SATIIBiologyproblemsonosmosiscanbetricky:
watermovesfromareaswhereitisinhighconcentrationtoareaswhereitisinlowconcentration.Remember,however,thatwaterisfoundinlowconcentrationsinplaceswheretherearemanydissolvedsubstances,calledsolutes.Therefore,watermovesfromplaceswheretherearefewdissolvedsubstances(knownashypotonicsolutions)toplaceswheretherearemanydissolvedsubstances(hypertonicsolutions).Anisotonicsolutionisoneinwhichtheconcentrationisthesameasthatfoundinsideacell,meaningosmoticpressureinbothsidesisequal.
Immersingcellsinunusuallyhypotonicorhypertonicsolutionscanbedisastrous:
watercanrushintocellsinhypotonicconditions,causingthemtofillupsofastthattheyburst.Tocombatthispossibility,manycellsthatneedtosurviveinfreshwaterenvironmentspossesscontractilevacuolestopumpoutexcesswater.
FacilitatedDiffusion
Certaincompoundsimportanttothefunctioningofthecell,suchasions,cannotenterthecellthroughsimplediffusionbecausetheycannotpassthroughthecellmembrane.Aswithwater,thesesubstances“want”toenterthecelliftheconcentrationgradientdemandsit.Forthatreason,cellshavedevelopedawayforsuchcompoundstobypassthecellmembraneandflowintothecellonthebasisofconcentration.Thecellhasproteinchannelsthroughthephospholipidmembrane.Thechannelscanopenandclosebasedonproteinmembranes.Whenclosed,nothingcangetthrough.Whenopen,theproteinchannelsallowcompoundstopassthroughalongtheconcentrationgradient,whichisdiffusion.
ActiveTransport
Quiteoften,cellshavetotransportasubstanceacrossthecellmembraneagainstthenormalconcentrationgradient.Inthesecases,cellsuseanotherclassofmembraneproteins.Insteadofrelyingondiffusion,theseproteinsactivelypumpcompoundsinthedirectionthecellwantsthemtogo,aprocessthatrequiresenergy.Cellscanturnactivetransportonandoffasneeded.
EndocytosisandExocytosis
Cellsuseyetanothertypeoftransporttomovelargeparticlesthroughthecellmembrane.Inexocytosis,wasteproductsthatneedtoberemovedfromthecellareplacedinvesiclesthatthenfusewiththecellmembrane,releasingtheircontentsintothespaceoutsidethecell.Endocytosisistheoppositeofexocytosis:
thecellmembraneengulfsasubstancethecellneedstoimportandthenpinchesoffintoavesiclethatisinsidethecell.
Therearetwokindsofendocytosis:
inphagocytosisthecelltakesinlargesolidfoodparticlesthatitthendigests.Inpinocytosis,thecelltakesindropsofcellularfluidcontainingdissolvednutrients.
Sometimesatomsgivetheirelectronsupaltogetherinsteadofsharingtheminachemicalbond.Thisprocessisknownasdisassociation.Water,forinstance,dissociatesbythefollowingformula:
Thehydrogenatomgivesupanegativelychargedelectron,gainingapositivecharge,andtheOHcompoundgainsanegativelychargedelectron,takingonanegativecharge.TheH+isknownasahydrogenionandOH–ionisknownasahydroxideion.
Thedisassociationofwaterproducesequalamountsofhydrogenandhydroxideions.However,thedisassociationofsomecom
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