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BIOC40H3 (14)
Lecture

Lecture 2

4 Pages
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Department
Biological Sciences
Course Code
BIOC40H3
Professor
Connie Soros

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Plant Physiology – Lecture 2  Waterisessentialinthelifecycleof plants  SincephotosynthesisutilizesCO2thatenterstheleaves viaopenstomates onthelowerleaf surface, the leavesarealsoexposedtowaterloss and thethreatof dehydrationthroughthe common diffusional pathway  Water must beabsorbedbytherootsandtransportedthroughtheplant bodytoprevent leafdesiccation  Balancingthe uptake, transport, and lossofwatertotheatmosphereisanimportantchallengeforplants  Most(approx. 97%) ofthewaterabsorbedbyplant rootsevaporatesfromleaf surfaces –thisprocessiscalledtranspiration  Oftheremainingwater, about 2%isusedforgrowth andthelast 1%isconsumedinthe biologicalreactionsofphotosynthesis andother metabolicprocesses TURGOR PRESSURE  Remember that plant cellshave cellwallswhichanimal cellsdo not  The presence ofcellwalls hasanimportantimpacton water balancewithinthe plant  Cellwallsallow plant cellsto build uplargeinternal hydrostatic pressures calledturgorpressure  Inaplantcell,theturgorpressurepushestheplasmamembraneagainst therigidcellwalland provides aforcefor cell expansion THE STRUCTUREAND PROPERTIES OFWATER  The polarityandtetrahedral shapeofwatermoleculesgivethem hydrogenbondingabilitywhichinturn giveswateritsuniquephysical properties  Forexample, itisanexcellent solvent, has ahighspecific heat, anunusuallyhighlatentheat of vaporization anda hightensilestrength HYDROGEN BONDING  The oneO2 andtwoH2atomsofwater are connectedbycovalent bonds (a chemical bondthatinvolves sharing ofpairsofelectronsbetween atoms)  Theoxygenatom hasahigher electronegativitythanthehydrogenatom andattractstheelectronsofthe covalentbondresultinginapartial negativechargeat the oxygenendofthemolecule and aslightly positivechargeat eachhydrogen  Asaresult, waterisa polarmolecule (negativeinthe centreandpositiveat theends, withanunequalsharing ofelectronscausingthe “bent” structure)  Becauseofthe chargeand structure, water alsocanform anunusually largenumber ofintermolecularhydrogenbonds(four)for amoleculeofits size. Oneat each hydrogen atomandtwoattheoxygenatom.  These strongattractiveforcesbetweenmolecules ofwater giverisetowater’spotencyas asolvent, high surfacetensionand capillaryforces THERMAL PROPERTIESOFWATER  The extensivehydrogenbondingbetweenwatermoleculesresultsin water having botha high specific heat capacity and ahighlatentheat of vaporization  Specificheatcapacity istheheatenergyrequiredtoraisethetemperature of asubstancebya set amount  When thetemperatureofwater israised, thewatermolecules vibrate fasterandstronger(increasedmolecular kineticE)and thehydrogen bonds act likerubberbands toabsorbsomeoftheheatenergy, leavinglessenergyavailabletoincreasemotion  By doingthis, waterrequiresamuch largerheatinput thanmany other liquidstoraiseitstemperature.Thisisimportantforplants because it helps to buffertemperaturefluctuations.  The latentheat of vaporizationisthe energyrequiredtoseparatemoleculesfromtheliquid phaseandmovethem tothe gasphase, without a changeoftemperature, a processthat occurs duringtranspiration  Transpirationistheprocess ofwater evaporationfrom plant foliage,mostlyfrom stomata. Thisprocess preventstheplanttissuefrom overheating and providesthemoisture necessaryto diffusecarbondioxide intoplant cells and enabletheemissionof oxygen WATER ASA SOLVENT  Waterisanexcellent solvent andis oftenreferredtoastheuniversalsolvent. Itcandissolve greater amountsofawider varietyofsubstancesthan other relatedsolvents.  Ionicsubstancesandpolar moleculesdissolve particularlywellinwater, e.g., salts, sugars, (eg. Thosethat contain polar –OH or –NH2groups) acids, alkalis, andsomegases – especiallyoxygenandcarbon dioxide, these areknownas hydrophilic (water-loving)substances, whilethosethat areimmisciblewithwater (e.g., fatsand oils), are knownas hydrophobic (water-repelling)substances.  Mostofthemajorcomponentsincells(proteinsDNA and polysaccharides)areeasilydissolvedinwater.  This propertyisalsoduetothehydrogenbondingabilityandpolarstructure ofwater, especiallyforsuch asmallmolecule ­ Example: Dissolvingasaltinwater- Table salt(NaCI) –isastrongioniccrystal. Theionic bondisformedbecauseoftheelectrostatic attractionbetweenoppositelychargedions. Inions, electronsofoneatom aretransferredtoanother atom. Theatom thatlosesthe electrons becomesa positivelychargedion, ora cation,whiletheone that gainselectronsbecomes anegatively chargedion, or an anion. When anioniccompoundis dissolvedbystirringitinwater,thepositive poles of thewatermolecules areattractedtotheanions. Whilethenegative polesof otherwater moleculesareattractedtothe cations, sothe polarwater molecules“pull”theions out ofthe crystal. Asaresult, theionic bonds eventuallybreakandionsarereleasedintothewaterWhenthesaltisdissolved, everyif hydration of thesalt,thecrystal breaks apart(dissolves), releasingtheionsintothe water Therefore, whentablesalt(NaCI)isdissolved, two hydratedionsappearinthewater:a positivelycationNa+ anda negativechargedanionCI-. Eachissurroundedwith ashell of closely attractedwatermoleculesthatprevent theions from formingintoacrystal again COHESIVECHARACTERISTICSOFWATER  Cohesion is the mutual attraction between water molecules as a result of extensive hydrogen bonding  A relatedpropertyis adhesion, whichisthe attraction ofwater to asolidphasesuchasacellwall or glasssurface, againduetotheformationof hydrogenbonds  Surfacetension isaforceexertedbywatermoleculesat theair-water interface, resultingfrom the cohesionandadhesion propertiesofwater molecules. Thisforceminimizesthesurfaceareaof theair-water interface ­ Surfacetensionisapropertythatallowsthesurfaceofliquidtoresist externalforces. ­ The cohesionforcesinliquidmolecules causesurfacetensionbecause eachwatermoleculeispulledin every directionby neighbouringwater moleculeswith anet forceof0.
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