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,二、植物吸收矿质营养的机理序言 (一). 植物细胞吸收矿质元素的机理(二).根系吸收矿质营养的特点,Plant cannot live on CO2 and water alone. There are at least 13 other elements required to sustain plant health, vigor and normal development.,(一)植物细胞对矿质元素的吸收,Given the importance of membrane transport, cells utilize a wide range of transport mechanisms. The mechanisms fall into one of three categories: simple diffusion, facilitated diffusion, and active transport.,Inorganic ions commonly referred to as macronutrients” (Marschner, 1995) play central roles in of plant cell biology as substrates for metabolic assimilation (inorganic phosphate Pi, NH+4, NO3- as a generator of intracellular osmotic pressure component of signaling pathways (Ca2+).,一.被动吸收: (passive absorption) 由于扩散作用而进行的吸收,这种不需要代谢提供能量的吸收矿质的过程称为被动吸收。,特点: 沿浓度(化学)梯度(或电化学梯度)扩散; 不需要提供能量; 没有或有膜蛋白的协助。,被动 吸收,扩散作用,协助扩散,离子通道,载体蛋白,(一)扩散作用(diffusion) :简单扩散/自由扩散(free diffusing). 特点是:没有膜蛋白的协助。 扩散作用是指分子或离子沿着化学势或电化学势梯度转移的现象。,(一)扩散作用,化学势:化学浓度差。分子一般是按化学势扩散。K+:0.2M0.5M之间就是化学势。 电化学势梯度:电化学势梯度包括化学势梯度和电势梯度(膜电势)两方面 。细胞内外的离子扩散决定于这两种梯度的大小 。,电化学势梯度的构成: 植物细胞, 膜内侧具有较高的负电荷, 膜外侧具有较高的正电荷。细胞从环境中吸收了较多的阳离子,致使细胞内该离子浓度较高。按照化学势梯度,细胞内的阳离子应向外扩散;而按电势梯度,由于细胞内有较高的负电荷,则这种阳离子又应该从细胞外向内扩散。,正常细胞:在未受到刺激时存在于细胞内外两侧的电位差,称为静息电位差( resting potential,RP)。在所有被研究过的动植物细胞中(少数植物细胞例外),静息电位都表现为膜内较膜外为负(内负外正);如规定膜外电位为0,则膜内电位大都在10100mV之间。,不带电的中性分子和离子的扩散取决于其化学势梯度或浓度梯度。 带电离子的扩散取决于其电化学势的梯度.,Nernst方程:离子j在膜内外被动转运(扩散)达到平衡时膜内外电势差与化学势差之间的关系。 En = En:Nernst势(mV);R:气体常数8.314J/mol.K;T:绝对温度;F-法拉第常数为96.500J/mV.mol,zj:离子电荷, Cjout:膜外浓度,Cjin:膜内浓度,在25摄氏度下,对于一价阳离子(如K+)而言,上述公式可简化为: En = 59 log10(Cjout/Cjin) 这意味着:59mV的电化学势可以使膜内外被动扩散的K+的浓度保持在10倍的差异 (Cjout/Cjin = 10/1,log1010 = 1)! 换言之,当膜内外钾离子的浓度差在10倍时,则这些离子沿浓度差的被动扩散将导致膜内外产生59mV的电化学势差。,2.3(273+25)K8.314J/96.5J/V.mol=59.2 实际测定的细胞膜的电化学势常为-130-110mV,而经计算而得的值应为-80-50mV。 其原因在于,由H+ATP酶可以向膜外排出H+。,细胞的膜电势可用由微电极、参比电极和高灵敏度的电位计组成的测定装置测定 。见下图,用微电极测定植物细胞膜电势的示意图。一个玻璃微电极插入到要研究的细胞(通常是液泡或细胞质)中,另外一个插入电解液作为参比。微电极和电压计相连,此电压计能记录细胞和溶液间的电势差。典型细胞的膜电势通常在-60mV和-240mV之间。放大部分的图示微电极如何和细胞相联接。 微电极中有导电的盐溶液。,非亲水,自 由 扩 散,(二)协助扩散(facilitated diffusion) 特点:有膜蛋白介导。 I.定义:是离子或小分子物质经膜转运蛋白顺浓度梯度或电化学梯度跨膜的转运,不消耗细胞能量。,II 膜蛋白类型(进行协助扩散) 1.离子通道 2. 载体蛋白,1.离子通道: (channel)(1)定义:由膜内在蛋白构成的孔道。横跨膜两侧,是离子顺着电化学势差(梯度)、被动的单方向地跨膜运输的通道。一种通道通常只允许适宜大小的分子和离子通过。 通道蛋白中有感受蛋白(sensor protein)或感受器,它可能通过改变其构象对适当刺激作出反应,并引起“孔”的开闭。,配体门通道,Embed-插入,accessory-附加,(2). 类型 根据孔开闭的机制,可将通道分为两类: 一类对跨膜电势梯度产生响应, 一类对外界刺激(如光,激素等)产生反应。 现已观察到原生质膜中有、Cl、Ca2+通道。原生质膜中也可能存在着供有机离子通过的通道。从保卫细胞中已鉴定出两种通道,一种是允许外流的通道,另一种是吸收的内流通道,两种通道都受膜电位控制。,一个开放的离子通道,每秒可运输107108个离子,(3)离子通道的测定:膜片钳技术。,A.测定原理与装置:a.测定原理图,在玻璃微电极尖端截取的膜片上,如有开放的离子通道时,离子通过通道进入微电极,产生的电流经放大器放大后, 由监视器显示或由记录仪记录;b.测定装置,示安装玻璃微电极的装置,有吸引接口和信号 输出接口; B.信道开闭时的电流输出记录图,示仅通过一个离子通道时的膜电流情况,只 有在通道开时才能测到电流。,膜片钳技术测定离子通道示意图,The patch-clamp method was originally developed to record single channels in the membrane underneath the pipette(吸液管) following the establishment of a tight seal between the glass and the membrane. The method is much more useful than this, however. The tight electrical seal is also mechanically stable, allowing the experimenter to pull a patch of membrane from the cell and record,the activity of channels in either an inside-out (cytosolic side exposed to the bathing solution) or an outside-out configuration(结构). Establishing an inside-out patch requires pulling the pipette away from the cell following the formation of a tight seal.,To form an outside-out patch, one first ruptures the membrane patch by applying a strong pulse of suction. Pulling the pipette from the cell then results, in many cases, in the cell membrane lateral to the pipette collapsing back on itself and reforming as an outside-out patch. Such a configuration is useful, for example, when one is recording the activity of a ligand-gated channel that requires application of an agonist to activate the channel. In addition to the various patch configurations, it is also possible to record whole-cell,currents or voltages in much the same way as was done previously with sharp electrodes. However, there are two distinct advantages of the whole-cell method. First, it is possible to make recordings by rupturing the patch membrane (allowing one to exchange the cells normal intracellular constituents with those in the pipette). This is not feasible with sharp electrodes because the geometry makes the time required for exchange,via diffusion unacceptably long. Second, one can leave the patch membrane intact and include certain pore-forming antibiotics(抗生素) in the pipette to make “holes” in the membrane. These pores are large enough to permit good electrical access to the cell in order to voltage-clamp it, but too small to allow the myriad(无数的) intracellular components (critical to the cells normal functioning) to escape.,2.载体蛋白:载体蛋白又称为载体、透过酶或运输酶,是一类跨膜运输物质的内在蛋白。在跨膜区域不形成明显的孔道结构。当运输物质时,首先在膜一侧有选择的与离子(分子)结合,形成载体-离子(分子)复合物,载体蛋白构象发生变化,透过膜把离子(分子)释放到膜的另一侧。,载体蛋白对被转运物质的结合及释放,与酶促反应中酶与底物的结合及对产物的释放情况相似。,Carrier proteins(also known as permeases or transporters) bind a specific type of solute and are thereby induced to undergo a series of conformational changes which has the effect of carrying the solute to the other side of the membrane. The carrier then discharges the solute and, through another conformational change, reorients in the membrane to its original state. Typically, a given carrier will transport only a small group of related molecules.,
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