水的特性及生物学意义

1、Chapter 1 Plant Water Metabolism Contents in brief: Absorption of water Transport of water Loss of water(transpiration) Irrigation1 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) High boiling point(高沸点) 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(高比热高比热) 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(高汽化热高汽化热) 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(内聚力和粘附力内聚力和粘附力) 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, adhesion a

7、nd surface tension leads to capillarity(内聚力、粘附力和表面张力导致毛细现象)NoImageh=2Tcos/rg=14.910-6/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 =2040um,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。)1. 蒸

8、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) Water is not compressible(水是不可压缩的水是不可压缩的) 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) Water is an excellent solvent（水是极好的溶剂）水是极好的溶剂） 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(透光性强透光性强) 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 水分的迁移方式扩散(diffusion)集流(bulk flow)渗透(osmosis) 1) 扩散 定义：物质从高浓度(高化学势)的区域向低浓度(低化学势)区域自发的转移称为扩散。 原理：分子随机热运动的结果，高浓度区分子密集其相互碰撞的机会多，因而向相反方面移动。 扩散速率：JB=-D C/ tc= ( 2/D)K 对于一般植物细胞而言，其直径为50m,水的扩散系数D=10-9m2S-1,则t=(5010-6m)2/ 10-9m2S-1 =2.5S 对于1m的距离来说，t= 1m2/ 10-9m2S-1 = 109 S24年 故：扩散仅适应于短距离水的迁移2)集流 定义：指液体

15、中成群的分子在压力梯度下共同的移动 特点：是植物体内水经木质部做长距离迁移的主要机制，集流只与水柱两端的压力差有关，而与浓度梯度无关。 流速：=r4(8)-1 -1 其中r为管道半径， 为水的粘度系数，为两端的压力差， 为距离。3)渗透作用 渗透作用渗透作用是指水透过半透膜的一种迁移方式，事实上是一种特殊的扩散，但也与集流(膜两侧的压力差)有关。由于细胞膜的脂双分子层对水是可通透的，所以水分子可直接通过脂双分子层扩散进入；另外，在膜上有内在的水孔蛋白 和水通道蛋白，水分子可以不通过脂双分子层而由后者跨膜迁移。 水孔蛋白(aquaporin)是一种位于质膜、液泡膜和某些细胞器膜上的主要内在蛋白(

16、MIP)，MW2630KD，它由6个-helix跨膜而成通道，允许水分通过。水分通过水孔蛋白迁移的速度远远大于通过脂双分子层的速度。 渗透作用是由膜两侧的水势差所驱动的。2. 植物体中水分的作用一 植物的含水量含水量的指标： 含水量=(鲜重-干重)/鲜重 100% WC=(FW-DW)/FW 100% 相对含水量=(鲜重-干重)/(饱和鲜重-干重) 100% RWC=(FW-DW)/(SFW-DW) 100%一些经验数值(以含水量计)： 水生植物90%；旱生地衣6%一般植物7090% 生长旺盛部位如根尖、芽、幼叶7090% 生长缓慢部位如主干3570%；体眠种子515%二 植物体内水分存在的状态水分在植物体内的作用，不仅与含水量有关，还与其存在状态有关，水分在植物体内以束缚水和自由水两种状态存在。束缚水(bound water):是指牢固地与细胞内的胶体颗粒吸附而不易流动的水，自由水(free water):距胶粒较远而可以自由移动的水。自由水可参加细胞的各种代谢活动而束缚水不能。自由水与束缚水的比例决定了原生质胶体所处的状态：比例大时，原生质颗粒均匀地做好事在水中，颗粒之间联系弱，胶体呈溶液状态，这种胶体称为溶胶sol;反之，原生质胶粒相互联结成网状而水分子分布于网眼中，胶体失去其流动性而凝结为挖固体的状态