📚 The Transport of Water in Plants | 植物体内水分的运输
Water is essential for plant life, not only as a solvent for biochemical reactions but also as a means of transporting dissolved minerals, maintaining turgor pressure, and cooling the plant through transpiration. Understanding how water moves from the soil, through the root system, up the stem, and out into the atmosphere via the leaves is a central topic in A-Level Biology. This journey relies on physical processes such as osmosis, cohesion, adhesion, and evaporation, and is facilitated by specialised tissues like xylem vessels. This article provides a detailed exploration of the pathway and the forces that drive water transport in flowering plants.
水对植物生命至关重要，它不仅是生化反应的溶剂，还能运输溶解的矿物质、维持细胞膨压并通过蒸腾作用为植物降温。理解水如何从土壤进入根系，沿茎向上运输，再经叶片散失到大气中，是A-Level生物学的核心主题。这一旅程依赖于渗透作用、内聚力、附着力和蒸发等物理过程，并由木质部导管等特化组织介导。本文将详细探索开花植物体内水分运输的路径与驱动力。
1. The Significance of Water in Plants | 水在植物体内的意义
Water is the most abundant molecule in plant cells, accounting for up to 95% of the fresh weight of herbaceous tissues. It serves as the medium in which mineral ions dissolved in soil water enter the roots. Inside cells, it participates in photolysis during the light-dependent stage of photosynthesis, releasing electrons, protons, and oxygen. Water is also vital for maintaining cell turgidity; the pressure exerted by the vacuole against the cell wall keeps non-woody plants upright and drives cell expansion. Moreover, evaporative cooling during transpiration prevents overheating in sunlight.
水是植物细胞中含量最丰富的分子，约占草本组织鲜重的95%。它是土壤水中溶解的矿物质离子进入根部的介质。在细胞内，水参与光合作用光反应阶段的光解，释放电子、质子和氧。水对于维持细胞膨压也至关重要；液泡对细胞壁施加的压力使草本植物保持直立并驱动细胞伸展。此外，蒸腾过程中的蒸发冷却作用可防止植物在阳光下过热。
2. The Pathway from Soil to Root Xylem | 从土壤到根木质部的途径
Water enters the plant mainly through the root hairs, which are elongated epidermal cells that greatly increase surface area for absorption. From there, it must cross the root cortex, endodermis, and pericycle before reaching the xylem vessels in the stele. The movement can be described as radial, following the water potential gradient from the soil (higher water potential) to the inside of the root (lower water potential). Water molecules move along three possible routes: the apoplast pathway, the symplast pathway, and the transmembrane pathway.
水主要通过根毛进入植物体，根毛是伸长的表皮细胞，极大地增加了吸收表面积。从根毛出发，水必须穿过根皮层、内皮层和中柱鞘，才能到达中柱内的木质部导管。这种径向移动遵循水势梯度，即从土壤（较高水势）向根内部（较低水势）运动。水分子可沿着三条可能的路线移动：质外体途径、共质体途径和跨细胞途径。
The apoplast pathway is the movement of water through the interconnected cell walls and intercellular spaces, without crossing any plasma membranes. This route offers the least resistance and allows rapid bulk flow. The symplast pathway takes water through the cytoplasm of cells, connected from one cell to the next by plasmodesmata, living bridges that allow the continuous movement of cytoplasmic fluid. The transmembrane pathway involves water entering and exiting each cell by crossing both the plasma membrane and the tonoplast, often regulated by aquaporins. In reality, the three routes operate simultaneously, but the apoplastic flow dominates until it is blocked by the Casparian strip.
质外体途径是指水通过相连的细胞壁和细胞间隙移动，不穿过任何质膜。该途径阻力最小，可实现快速集流。共质体途径使水穿过细胞质，通过胞间连丝在细胞间连续移动，胞间连丝是允许细胞质液连续流动的活桥。跨细胞途径则涉及水通过穿越质膜和液泡膜进出每个细胞，通常受水通道蛋白调控。事实上，三条路线同时运作，但质外体流动占主导地位，直到被凯氏带阻断。
3. The Role of the Casparian Strip | 凯氏带的作用
The Casparian strip is a band of suberin and lignin impregnated into the radial and transverse walls of endodermal cells. It is impermeable to water and dissolved ions. As water and dissolved minerals travel through the apoplast of the cortex, they are forced at the endodermis to cross the plasma membrane of an endodermal cell to enter the symplast. This barrier ensures that all substances must pass through a selectively permeable membrane, allowing the plant to control which ions enter the vascular tissue. It also prevents the backflow of ions from the xylem into the cortex.
凯氏带是浸润在内皮层细胞径向壁和横向壁上的一圈木栓质和木质素。它不透水，也不透溶解的离子。当水和溶解的矿物质通过皮层质外体运输时，在内皮层处被迫穿过内皮层细胞的质膜进入共质体。这一屏障确保所有物质都必须通过选择性透膜，使植物能够控制哪些离子进入维管组织。它还能阻止离子从木质部倒流回皮层。
4. Structure of Xylem Vessels | 木质部导管的结构
Xylem tissue is composed of several cell types, the most important being the tracheary elements: tracheids and vessel elements. In angiosperms, vessel elements are wide, dead cells arranged end-to-end, with their end walls largely broken down to form continuous tubes called xylem vessels. Their side walls are thickened with lignin in annular, spiral, or reticulate patterns, which provide mechanical strength and prevent collapse under tension. Adjacent vessel elements and tracheids communicate through pits, thin areas in the lignified wall that allow lateral water movement. The absence of cytoplasm and organelles in mature xylem vessels means there is no resistance to water flow, making them highly efficient conduits.
木质部组织由多种细胞类型组成，最重要的是管状分子：管胞和导管分子。在被子植物中，导管分子是宽大的死细胞，首尾相连，其端壁大部分被分解，形成连续的长管即木质部导管。它们的侧壁以环纹、螺纹或网纹样式增厚木质素，提供机械强度并防止在张应力下塌陷。相邻的导管分子和管胞通过纹孔相连，纹孔是木质化壁上的薄区域，允许水的横向移动。成熟木质部导管中不含细胞质和细胞器，因此对水流的阻力为零，使其成为高效的输水管道。
5. Transpiration and the Transpiration Stream | 蒸腾作用与蒸腾流
Transpiration is the loss of water vapour from the aerial parts of a plant, primarily through the stomata in the leaves. When stomata open to allow carbon dioxide entry for photosynthesis, water vapour diffuses out down a concentration gradient. This loss of water generates a water potential gradient between the leaf mesophyll cells and the atmosphere. The tension created pulls water from the xylem vessels in the veins, setting up the transpiration stream — a continuous column of water from the roots to the leaves. Transpiration is essentially an unavoidable consequence of gas exchange, but it also drives the upward movement of water and dissolved minerals.
蒸腾作用是水蒸气从植物地上部分散失的过程，主要经由叶片上的气孔。当气孔张开以允许二氧化碳进入进行光合作用时，水蒸气便沿浓度梯度向外扩散。水分的散失在叶肉细胞与大气之间形成了水势梯度。由此产生的张力拉动叶脉木质部导管中的水，建立起蒸腾流——一条从根部到叶片的连续水柱。蒸腾作用本质上是气体交换不可避免的后果，但它同时也驱动了水和溶解矿物质的上行运输。
6. The Cohesion-Tension Theory | 内聚力-张力理论
The cohesion-tension theory is the most widely accepted explanation for the ascent of sap in xylem. As water evaporates from the mesophyll cell walls into the air spaces of the leaf, a meniscus forms at the air-water interface, creating a negative pressure (tension). Because water molecules are strongly cohesive due to hydrogen bonding, this tension is transmitted all the way down the continuous water column in the xylem to the roots. Adhesion of water molecules to the xylem walls also helps counteract gravity by pulling the column upward. No metabolic energy is required to move water through the xylem; it is a purely physical, passive process. The strong cohesion of water supports a column that can withstand tensions up to -2 MPa, ensuring no breakage under normal physiological conditions.
内聚力-张力理论是关于木质部内汁液上升最广为接受的解释。当水从叶肉细胞壁蒸发进入叶片空气间隙时，在气-水界面形成弯月面，产生负压（张力）。由于水分子之间通过氢键具有很强的内聚力，这种张力会沿木质部内连续的水柱一直向下传递到根部。水分子对木质部管壁的附着力也有助于向上拉动水柱，从而抵消重力的影响。水在木质部中的移动不需要代谢能量；它是一种纯粹的物理被动过程。水的强内聚力使得水柱能够承受高达-2 MPa的张力，确保在正常生理条件下不会断裂。
7. Evidence for the Cohesion-Tension Mechanism | 内聚力-张力机制的实验证据
Several lines of evidence support the cohesion-tension theory. Measurements of the diameter of tree trunks show diurnal changes: during the day, when transpiration rates are high, tension increases and the trunk shrinks slightly; at night, tension decreases and the trunk expands. If a xylem vessel is punctured, air enters instantly, breaking the water column and preventing further upward water movement — a phenomenon called cavitation. Furthermore, the pressure in the xylem can be measured using a Scholander pressure chamber and is found to be strongly negative during transpiration, confirming the existence of tension. These observations align with the cohesion-tension model.
几类证据支持内聚力-张力理论。对树干直径的测量显示其具有昼夜变化：白天蒸腾速率高时，张力增加，树干略微收缩；夜晚张力降低，树干则舒展。若将木质部导管刺穿，空气会立即进入，使水柱断裂并阻止水继续上移——这一现象称为空穴化。此外，使用 Scholander 压力室可以测得木质部内的压力，发现在蒸腾期间其值为强负压，证实了张力的存在。这些观察均与内聚力-张力模型相符。
8. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
Transpiration rate is influenced by several environmental factors that alter the water potential gradient between the leaf and the atmosphere. An increase in temperature raises the kinetic energy of water molecules, so evaporation rate increases and the water vapour concentration that can be held in saturated air rises, steepening the gradient. Higher light intensity stimulates stomatal opening, allowing more water vapour to escape. Increased air movement (wind) removes the boundary layer of humid air from the leaf surface, maintaining a steep concentration gradient. Conversely, high humidity reduces the gradient, slowing transpiration. These factors can be investigated using a potometer, which measures the rate of water uptake by a leafy shoot.
蒸腾速率受多种环境因素影响，它们会改变叶片与大气之间的水势梯度。温度升高会增加水分子的动能，蒸发速率加快，同时饱和空气中的水蒸气浓度上升，加大了梯度。光强度增强会刺激气孔开放，使更多水蒸气逸出。空气流动（风）能带走叶面上的湿空气边界层，维持蒸腾的浓度梯度。相反，高湿度会减小梯度，减缓蒸腾。这些因素可通过蒸腾计（potometer）进行探究，该仪器可测量带叶枝条的吸水速率。
| Factor / 因素 | Effect on Transpiration Rate / 对蒸腾速率的影响 | Explanation / 解释 |
|---|---|---|
| Temperature / 温度 | Increase / 升高 | Faster evaporation, increased water-holding capacity of air / 蒸发加速，空气持水能力增强 |
| Light intensity / 光强度 | Increase / 升高 | Stomata open wider / 气孔张开更大 |
| Wind speed / 风速 | Increase / 升高 | Removes humid air layer, maintaining gradient / 移走潮湿空气层，维持梯度 |
| Humidity / 湿度 | Decrease / 降低 | Reduces water potential gradient / 减小水势梯度 |
9. Root Pressure and Guttation | 根压与吐水现象
Root pressure is a positive pressure that can develop in the xylem of roots when the soil is well watered and transpiration is low, such as at night. Active transport of mineral ions into the xylem lowers the water potential, causing water to enter by osmosis from the cortex, generating a hydrostatic pressure that pushes water upward. Root pressure can cause guttation, the exudation of liquid water from special pores called hydathodes at the leaf margins of some herbaceous plants. Although root pressure can contribute to water movement in small plants, it is insufficient to push water to the tops of tall trees, and the cohesion-tension mechanism remains the primary driver.
根压是当土壤水分充足且蒸腾速率较低时（如夜间），根部木质部中能够产生的一种正压。矿物离子通过主动运输进入木质部，降低了水势，使水通过渗透作用从皮层进入，由此产生的静水压力推动水向上移动。根压可引起吐水现象，即液态水从一些草本植物叶缘特化的排水器泌出。虽然根压在小植物中对水分移动有所贡献，但它不足以将水推至高大乔木的顶端，内聚力-张力机制仍是主要驱动力。
10. Xerophytic Adaptations to Reduce Water Loss | 旱生植物减少水分散失的适应性
Xerophytes are plants adapted to survive in arid environments. They exhibit a range of structural features that reduce the transpiration rate. Thick, waxy cuticles on leaves and stems minimise cuticular transpiration. Sunken stomata in pits trap moist air, reducing the diffusion gradient. Rolled leaves create a microclimate of high humidity around the stomata. A dense covering of trichomes (leaf hairs) also traps moisture. Reduced leaf area, with leaves reduced to spines in cacti, decreases the total transpiring surface. These adaptations allow xerophytes to conserve water while still permitting gas exchange for photosynthesis.
旱生植物是适应干旱环境生存的植物。它们表现出一系列降低蒸腾速率的结构特征。叶片和茎上厚厚的蜡质角质层可最小化角质层蒸腾。凹陷在气孔窝内的气孔能截留湿空气，减弱扩散梯度。卷曲的叶片能够在气孔周围形成高湿度微气候。浓密的表皮毛（叶毛）亦可截留水汽。叶面积减小，如仙人掌的叶退化为刺，减少了总蒸发表面积。这些适应特征使旱生植物能在节约用水的同时，仍然允许气体交换以进行光合作用。
11. Water Transport and Mineral Ion Uptake | 水分运输与矿物质离子吸收
Water transport is tightly linked to mineral ion absorption. Dissolved nitrate, phosphate, potassium, and other essential ions enter the root along with water. Some mineral ions are absorbed by facilitated diffusion or active transport across the plasma membrane of root epidermal cells. Once inside the symplast, they lower the water potential inside the root cells, promoting water entry by osmosis. The flow of water through the apoplast carries dissolved ions up to the endodermis, where the Casparian strip ensures selective uptake. Thus, the transpiration stream not only delivers water but also distributes essential nutrients to all parts of the plant.
水分运输与矿质离子吸收紧密相关。溶解态的硝酸盐、磷酸盐、钾离子及其他必需离子随水一起进入根部。一些矿质离子通过协助扩散或主动运输穿越根表皮细胞的质膜被吸收。一旦进入共质体，它们会降低根细胞内部的水势，促进水通过渗透作用进入。水在质外体中的流动将溶解离子携带至内皮层，凯氏带则确保对其进行选择性吸收。因此，蒸腾流不仅输送水分，还将必需养分分配至植物体各处。
12. Summary and Biological Significance | 总结与生物学意义
The transport of water in higher plants is a remarkable integration of physical principles and biological structures. Water moves down a water potential gradient from the soil into roots, ascends through the xylem under cohesion-tension, and is evaporated via transpiration. This passive, energy-efficient system supplies water for photosynthesis, nutrient transport, cell turgidity, and temperature regulation. The evolution of lignified xylem vessels and waterproofing features such as the Casparian strip and cuticle has allowed plants to colonise terrestrial habitats and grow to great heights. Understanding these processes is fundamental to plant physiology and helps explain how plants respond to their environment.
高等植物体内水分的运输是物理原理与生物结构巧妙结合的体现。水沿水势梯度从土壤进入根部，在内聚力-张力作用下通过木质部上升，并经由蒸腾作用蒸发。这一被动且能效高的系统为光合作用、养分运输、细胞膨压及温度调节提供水分。木质化导管以及凯氏带、角质层等防水结构的进化使植物得以占领陆地生境并长到很高的高度。理解这些过程是植物生理学的基础，有助于解释植物如何响应其环境。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply