水的特性與生物學(xué)意義

1、Chapter 1 Plant Water Metabolism Contents in brief: Absorption of water Transport of water Loss of water(transpiration) Irrigation1 水的結(jié)構(gòu)與特性結(jié)構(gòu)：水是極性分子，能形成分子間氫鍵2. 水的特性1) High boiling point(高沸點(diǎn)) Compare: CH4 16 CH3CH3 30 CH3CH2CH3 44 CH3CH2CH2CH3 58 CH3(CH2)3CH3 72 boiling point 36 but : water 18 boilin

2、g point 100 reason: n H2O(H2O) n +heat gas2)Water has high specific heat(高比熱高比熱) The polarity of water and the resulting hydrogen bonding among water molecules means that it takes much heat (one calorie) to raise the temperature of 1 mL of water just 1C. The hydrogen bonding has to be given a lot of

3、 energy to get them to vibrate and generate the temperature change. This property of water is called specific heat. It means that this liquid can absorb much heat from the various chemical reactions occurring in cells without temperature change; it is a heat buffer. It helps maintain an even body te

4、mperature. 3) Water has a high latent heat of vaporization(高汽化熱高汽化熱) Among liquids, water has the highest latent heat of vaporization (44 kJmol-1). This means that when water goes from liquid to gas it takes a lot of energy. This property can obviously be traced directly to hydrogen bonding again. A

5、s the highest energy molecules in the liquid achieve what it takes to move away as a gas, their energy is removed from the liquid and it gets cooler. We sometimes refer to this as evaporative cooling. This is a critical property in maintaining the temperature of dark green leaves essentially parked

6、in sunshine. 3) Water demonstrates cohesion and adhesion(內(nèi)聚力和粘附力內(nèi)聚力和粘附力) 內(nèi)聚力(Cohesion):液體狀況下同類分子間的吸引力叫內(nèi)聚力 ，水的內(nèi)聚力可達(dá)30MPa. 粘附力(adhesion):液相與固相間的吸引力叫粘附力或附著力 原因：水是極性分子，與其它極性分子形成氫鍵。 若水與某物質(zhì)的粘附力大于水的內(nèi)聚力，則水可吸附在該物質(zhì)上，該物質(zhì)即為可濕的，或可浸潤的；反之則不可濕或不可浸潤。如水可浸潤土壤(SiO32-)、濾紙、CO32-，SO42-等，而不可浸潤石碏，石墨等。4) cohesion, adhesion a

7、nd surface tension leads to capillarity(內(nèi)聚力、粘附力和表面張力導(dǎo)致毛細(xì)現(xiàn)象)NoImageh=2Tcos/rg=14.910-6/r 毛細(xì)作用對植物的重要性：土壤下層的水分可通過毛細(xì)作用上升，供植物利用。細(xì)胞壁的纖維素微纖絲間有空隙，形成很多小而彎曲的毛細(xì)管，這樣可使植物的細(xì)胞壁通過毛細(xì)作用保持濕潤。植物導(dǎo)管是一種水分可濕的毛細(xì)管，如把一根干樹桿插入水中，水可上升一段距離(導(dǎo)管r =2040um,水可上升0.37 0.74m，但不要希望導(dǎo)管的毛細(xì)作用給植物供水，因毛細(xì)作用產(chǎn)生的前提是存在空氣水界面，而導(dǎo)管和上下都 是完整的細(xì)胞，無空氣水界面。)1. 蒸

8、騰時氣孔下腔細(xì)胞間的毛細(xì)管產(chǎn)生的蒸騰拉力是水分上升的主要動力。5) Water is not compressible(水是不可壓縮的水是不可壓縮的) While gases can be compressed into smaller and smaller spaces, liquid water is not so compressible. Thus compression of water into a space surrounded by a cell wall produces turgor pressure. This form of hydraulic pressure i

9、s critical for cell growth, for the opening and closing of stomata, flow processes in translocation in the phloem, exchange of materials within and between cell compartments, and for the rigidity and support for herbaceous (not supported by lignin in wood) plants. Turgor keeps petals and leaves exte

10、nded into the air and prevents wilting.6)Water has high tensile strength(高抗張強(qiáng)度) 抗張強(qiáng)度即為物體在斷裂前所能經(jīng)受的最大張力(拉力)。由于水分子間氫鍵的存在，水可忍受很高的張力，20時可忍受30MPa的拉力，相當(dāng)于同樣粗銅絲的10%。這可保證導(dǎo)管中的水不會被輕易拉斷。7) Water is an excellent solvent（水是極好的溶劑）水是極好的溶劑） The polarity of the water molecule gives it the ability to dissolve polar mol

11、ecules as well as less-polar molecules. In fact, among known liquids, water dissolves the widest range of chemical solutes. This makes water a medium for chemical transport and exchange. For plants, water dissolves soil minerals and carries them up the plant in the transpiration stream in the xylem;

12、 photosynthesis produces carbohydrates which are dissolved in water and carried from the leaf to the rest of the plant in the translocation stream in the phloem. 8) Water can form hydration layer around polar molecules(水合作用水合作用) Water can form hydration layer around polar molecules，and thus maintain

13、 the stability of the molecules and in the same time increases their solubility.9) Water is highly transparent(透光性強(qiáng)透光性強(qiáng)) Visible light and UV can penetrate through water, thus submerged plants can survive; Furthermore, the inner mesophyll cells can absorb the penetrated light to photosynthesize as w

14、ell.3 水分的遷移方式擴(kuò)散(diffusion)集流(bulk flow)滲透(osmosis) 1) 擴(kuò)散 定義：物質(zhì)從高濃度(高化學(xué)勢)的區(qū)域向低濃度(低化學(xué)勢)區(qū)域自發(fā)的轉(zhuǎn)移稱為擴(kuò)散。 原理：分子隨機(jī)熱運(yùn)動的結(jié)果，高濃度區(qū)分子密集其相互碰撞的機(jī)會多，因而向相反方面移動。 擴(kuò)散速率：JB=-D C/ tc= ( 2/D)K 對于一般植物細(xì)胞而言，其直徑為50m,水的擴(kuò)散系數(shù)D=10-9m2S-1,則t=(5010-6m)2/ 10-9m2S-1 =2.5S 對于1m的距離來說，t= 1m2/ 10-9m2S-1 = 109 S24年 故：擴(kuò)散僅適應(yīng)于短距離水的遷移2)集流 定義：指液體

15、中成群的分子在壓力梯度下共同的移動 特點(diǎn)：是植物體內(nèi)水經(jīng)木質(zhì)部做長距離遷移的主要機(jī)制，集流只與水柱兩端的壓力差有關(guān)，而與濃度梯度無關(guān)。 流速：=r4(8)-1 -1 其中r為管道半徑， 為水的粘度系數(shù)，為兩端的壓力差， 為距離。3)滲透作用 滲透作用滲透作用是指水透過半透膜的一種遷移方式，事實(shí)上是一種特殊的擴(kuò)散，但也與集流(膜兩側(cè)的壓力差)有關(guān)。由于細(xì)胞膜的脂雙分子層對水是可通透的，所以水分子可直接通過脂雙分子層擴(kuò)散進(jìn)入；另外，在膜上有內(nèi)在的水孔蛋白 和水通道蛋白，水分子可以不通過脂雙分子層而由后者跨膜遷移。 水孔蛋白(aquaporin)是一種位于質(zhì)膜、液泡膜和某些細(xì)胞器膜上的主要內(nèi)在蛋白(

16、MIP)，MW2630KD，它由6個-helix跨膜而成通道，允許水分通過。水分通過水孔蛋白遷移的速度遠(yuǎn)遠(yuǎn)大于通過脂雙分子層的速度。 滲透作用是由膜兩側(cè)的水勢差所驅(qū)動的。2. 植物體中水分的作用一 植物的含水量含水量的指標(biāo)： 含水量=(鮮重-干重)/鮮重 100% WC=(FW-DW)/FW 100% 相對含水量=(鮮重-干重)/(飽和鮮重-干重) 100% RWC=(FW-DW)/(SFW-DW) 100%一些經(jīng)驗(yàn)數(shù)值(以含水量計)： 水生植物90%；旱生地衣6%一般植物7090% 生長旺盛部位如根尖、芽、幼葉7090% 生長緩慢部位如主干3570%；體眠種子515%二 植物體內(nèi)水分存在的狀態(tài)水分在植物體內(nèi)的作用，不僅與含水量有關(guān)，還與其存在狀態(tài)有關(guān)，水分在植物體內(nèi)以束縛水和自由水兩種狀態(tài)存在。束縛水(bound water):是指牢固地與細(xì)胞內(nèi)的膠體顆粒吸附而不易流動的水，自由水(free water):距膠粒較遠(yuǎn)而可以自由移動的水。自由水可參加細(xì)胞的各種代謝活動而束縛水不能。自由水與束縛水的比例決定了原生質(zhì)膠體所處的狀態(tài)：比例大時，原生質(zhì)顆粒均勻地做好事在水中，顆粒之間聯(lián)系弱，膠體呈溶液狀態(tài)，這種膠體稱為溶膠sol;反之，原生質(zhì)膠粒相互聯(lián)結(jié)成網(wǎng)狀而水分子分布于網(wǎng)眼中，膠體失去其流動性而凝結(jié)為挖固體的狀態(tài)