The Transport of Water in Plants | 植物体内水分的运输

📚 The Transport of Water in Plants | 植物体内水分的运输

Water transport in plants is a central topic in Cambridge A-Level Biology. It explains how water moves from the soil into roots, across the root cortex, up the xylem and finally out of the leaves as water vapour. This movement is driven by water potential gradients and the cohesion-tension mechanism, and is tightly linked to transpiration and stomatal control.

植物体内的水分运输是剑桥 A-Level 生物学的核心主题。它解释了水如何从土壤进入根部、穿过根皮层、沿木质部向上运输，最终以水蒸气形式从叶片散失。这一过程由水势梯度和内聚力-张力机制驱动，并与蒸腾作用及气孔调控密切相关。

1. Why Water Transport Matters | 水分运输的重要性

In land plants, water is the main solvent for mineral ions and the medium in which most biochemical reactions take place. Water is also a reactant in photosynthesis, maintains turgor pressure for cell expansion and support, and helps cool the leaf through evaporation. Without a continuous water transport system, leaves far from the soil would dry out and photosynthesis would stop.

在陆生植物中，水是矿质离子的主要溶剂，也是大多数生化反应发生的介质。水还是光合作用的反应物，能维持细胞膨压以支持细胞伸展和植物体支撑，并通过蒸发帮助叶片降温。如果没有连续的水分运输系统，远离土壤的叶片会干枯，光合作用也会停止。

2. Water Potential and Osmosis | 水势与渗透

Water movement is best explained using water potential, symbolised by the Greek letter ψ. Pure water at standard temperature and pressure has a water potential of 0 MPa. Adding solute particles lowers the water potential, making it more negative, while applying pressure raises it. Water always moves from a region of higher water potential to a region of lower water potential.

水分运动最好用水势来解释，水势用希腊字母 ψ 表示。在标准温度和压力下，纯水的水势为 0 MPa。加入溶质会降低水势，使其变得更负；施加压力则会升高水势。水总是从水势较高的区域向水势较低的区域移动。

ψ = ψₛ + ψₚ

where ψₛ is the solute potential and ψₚ is the pressure potential.

其中 ψₛ 是溶质势，ψₚ 是压力势。

3. Water Uptake by Roots | 根系对水分的吸收

Root hairs are long, thin extensions of epidermal cells that greatly increase the surface area for water uptake. Soil water usually has a higher water potential than the cytoplasm of root hair cells, so water enters by osmosis. Root cells also actively transport mineral ions from the soil into the root, which lowers their water potential and maintains a steep water potential gradient for continued water entry.

根毛是表皮细胞的细长突起，能大大增加吸收水分的表面积。土壤水的水势通常高于根毛细胞细胞质的水势，因此水通过渗透作用进入根毛细胞。根细胞还会主动将土壤中的矿质离子运入根内，从而降低自身水势，维持较陡的水势梯度，使水分持续进入。

4. Pathways Across the Root | 水分穿过根部的途径

Once inside the root, water can travel through the apoplast pathway, which consists of cell walls and intercellular spaces, or through the symplast pathway, which passes through the cytoplasm and plasmodesmata. A third vacuolar pathway crosses cell vacuoles. The apoplast is the fastest route until water reaches the endodermis, where the waterproof Casparian strip blocks further apoplastic movement. Water is then forced into the symplast, allowing the plant to control which ions enter the xylem.

进入根部后，水可以沿质外体途径（由细胞壁和细胞间隙组成）运输，也可以沿共质体途径（通过细胞质和胞间连丝）运输。此外还有穿过液泡的液泡途径。质外体是最快的路线，但当水到达内皮层时，防水的凯氏带会阻止质外体继续通行。这时水被迫进入共质体，使植物能够控制哪些离子进入木质部。

5. Entry into Xylem and Root Pressure | 水分进入木质部与根压

Endodermal cells actively transport salts into the xylem vessels, lowering the water potential of the xylem fluid. Water follows by osmosis from the endodermal cells and surrounding tissues into the xylem. This active ion secretion can generate a small positive hydrostatic pressure called root pressure. Root pressure may cause guttation, where water droplets are forced out of leaf margins at night, but it is not sufficient to lift water to the tops of tall trees.

内皮层细胞主动将盐类运入木质部导管，降低木质部液体的水势。水随后通过渗透从内皮层细胞和周围组织进入木质部。这种主动离子分泌可以产生较小的正静水压，称为根压。根压可能引起吐水现象，即夜间水珠从叶缘被压出，但根压不足以把水提升到高大树木的顶端。

6. Structure of Xylem Vessels | 木质部导管的结构

Xylem vessels are formed from cells that die and lose their end walls, producing continuous hollow tubes. Their walls are thickened with lignin, which can be deposited in annular, spiral or reticulate patterns. Lignin is strong and waterproof, preventing the vessel from collapsing under tension. Xylem vessels have no cytoplasm or organelles, so they offer very little resistance to water flow. Bordered pits allow water to move sideways between vessels.

木质部导管由死亡并失去端壁的细胞连接而成，形成连续的中空管道。其细胞壁由木质素加厚，木质素可呈环纹、螺纹或网纹沉积。木质素坚硬且防水，能防止导管在张力下塌陷。木质部导管没有细胞质和细胞器，因此对水流阻力很小。具缘纹孔允许水在导管之间横向移动。

7. Cohesion-Tension Theory | 内聚力-张力学说

The cohesion-tension theory is the accepted explanation for the ascent of water in xylem. Transpiration from leaves creates a tension, or negative pressure, at the top of the xylem. Because water molecules are strongly cohesive due to hydrogen bonding, they form a continuous column and are pulled upward as a whole. Water molecules also adhere to the hydrophilic walls of the xylem vessels, which helps prevent the column from breaking. The energy driving this process ultimately comes from the Sun, which evaporates water from the leaf.

内聚力-张力学说是解释木质部中水分上升的公认理论。叶片蒸腾在木质部顶端产生张力（负压）。由于水分子之间因氢键具有很强的内聚力，它们形成连续的水柱，并被整体向上拉动。水分子还会黏附在亲水的木质部导管壁上，这有助于防止水柱断裂。驱动这一过程的能量最终来自太阳，因为太阳使叶片中的水蒸发。

8. Transpiration Stream and Leaf Exit | 蒸腾流与叶片散失

In the leaf, water moves from xylem vessels in the veins into the cell walls of mesophyll cells. It then evaporates into the air spaces of the spongy mesophyll and diffuses out through open stomata down a water vapour potential gradient. This loss of water from the leaf is called trans

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