水的特性与生物学意义

Chapter1PlantWaterMetabolismContentsinbrief:AbsorptionofwaterTransportofwaterLossofwater(transpiration)Irrigation§1水的结构与特性结构：水是极性分子，能形成分子间氢键2.水的特性1)Highboilingpoint(高沸点)

Compare:CH416CH3CH330CH3CH2CH344CH3CH2CH2CH358CH3(CH2)3CH372boilingpoint36℃

but:water18boilingpoint100℃reason:nH2O≒(H2O)n+heatgas2)Waterhashighspecificheat(高比热)Thepolarityofwaterandtheresultinghydrogenbondingamongwatermoleculesmeansthatittakesmuchheat(onecalorie)toraisethetemperatureof1mLofwaterjust1°C.Thehydrogenbondinghastobegivenalotofenergytogetthemtovibrateandgeneratethetemperaturechange.Thispropertyofwateriscalledspecificheat.Itmeansthatthisliquidcanabsorbmuchheatfromthevariouschemicalreactionsoccurringincellswithouttemperaturechange;itisaheatbuffer.Ithelpsmaintainanevenbodytemperature.

3)Waterhasahighlatentheatofvaporization(高汽化热)Amongliquids,waterhasthehighestlatentheatofvaporization(44kJ·mol-1).Thismeansthatwhenwatergoesfromliquidtogasittakesalotofenergy.Thispropertycanobviouslybetraceddirectlytohydrogenbondingagain.Asthehighestenergymoleculesintheliquidachievewhatittakestomoveawayasagas,theirenergyisremovedfromtheliquidanditgetscooler.Wesometimesrefertothisasevaporativecooling.Thisisacriticalpropertyinmaintainingthetemperatureofdarkgreenleavesessentially"parked"insunshine.3)Waterdemonstratescohesionandadhesion(内聚力和粘附力)内聚力(Cohesion):液体状况下同类分子间的吸引力叫内聚力，水的内聚力可达30MPa.粘附力(adhesion):液相与固相间的吸引力叫粘附力或附着力原因：水是极性分子，与其它极性分子形成氢鍵。若水与某物质的粘附力大于水的内聚力，则水可吸附在该物质上，该物质即为可湿的，或可浸润的；反之则不可湿或不可浸润。如水可浸润土壤(SiO32-)、滤纸、CO32-，SO42-等，而不可浸润石碏，石墨等。4)cohesion,adhesionandsurfacetensionleadstocapillarity(内聚力、粘附力和表面张力导致毛细现象)h=2Tcos/rg=14.9×10-6/r毛细作用对植物的重要性：土壤下层的水分可通过毛细作用上升，供植物利用。细胞壁的纤维素微纤丝间有空隙，形成很多小而弯曲的毛细管，这样可使植物的细胞壁通过毛细作用保持湿润。植物导管是一种水分可湿的毛细管，如把一根干树杆插入水中，水可上升一段距离(导管r=20~40um,水可上升0.37~0.74m，但不要希望导管的毛细作用给植物供水，因毛细作用产生的前提是存在空气—水界面，而导管和上下都是完整的细胞，无空气—水界面。)蒸腾时气孔下腔细胞间的毛细管产生的蒸腾拉力是水分上升的主要动力。5)Waterisnotcompressible

(水是不可压缩的)Whilegasescanbecompressedintosmallerandsmallerspaces,liquidwaterisnotsocompressible.Thuscompressionofwaterintoaspacesurroundedbyacellwallproducesturgorpressure.Thisformofhydraulicpressureiscriticalforcellgrowth,fortheopeningandclosingofstomata,flowprocessesintranslocationinthephloem,exchangeofmaterialswithinandbetweencellcompartments,andfortherigidityandsupportforherbaceous(notsupportedbylignininwood)plants.Turgorkeepspetalsandleavesextendedintotheairandpreventswilting.6)Waterhashightensilestrength(高抗张强度)抗张强度即为物体在断裂前所能经受的最大张力(拉力)。由于水分子间氢鍵的存在，水可忍受很高的张力，20℃时可忍受30MPa的拉力，相当于同样粗铜丝的10%。这可保证导管中的水不会被轻易拉断。7)Waterisanexcellentsolvent

（水是极好的溶剂）Thepolarityofthewatermoleculegivesittheabilitytodissolvepolarmoleculesaswellasless-polarmolecules.Infact,amongknownliquids,waterdissolvesthewidestrangeofchemicalsolutes.Thismakeswateramediumforchemicaltransportandexchange.Forplants,waterdissolvessoilmineralsandcarriesthemuptheplantinthetranspirationstreaminthexylem;photosynthesisproducescarbohydrateswhicharedissolvedinwaterandcarriedfromtheleaftotherestoftheplantinthetranslocationstreaminthephloem.8)Watercanformhydrationlayeraroundpolarmolecules(水合作用)Watercanformhydrationlayeraroundpolarmolecules，andthusmaintainthestabilityofthemoleculesandinthesametimeincreasestheirsolubility.9)Waterishighlytransparent(透光性强)VisiblelightandUVcan