Water Transport Pathways in Plants | 水分在植物体内的运输路径

📚 Water Transport Pathways in Plants | 水分在植物体内的运输路径

Water is essential for plant life. It acts as a solvent, a transport medium, a temperature regulator, and a raw material for photosynthesis. Understanding how water moves from the soil into the roots, through the stem, and out into the atmosphere is a core topic in A-Level Biology, especially for CIE candidates.

水对于植物生命至关重要。它是溶剂、运输介质、温度调节剂，也是光合作用的原料。理解水分如何从土壤进入根部、穿过茎干并最终散逸到大气中，是A-Level生物学的核心主题，尤其对CIE考生而言。

1. Water Uptake at the Root Hair Cells | 根毛细胞处的水分吸收

Water enters the plant primarily through root hair cells, which are thin-walled extensions of epidermal cells in the root maturation zone. These cells have a huge surface area, often increasing the root’s absorptive surface by up to 20-fold. Root hairs are in close contact with soil water and mineral ions.

水分主要通过根毛细胞进入植物体。根毛是根成熟区表皮细胞的薄壁延伸，具有巨大的表面积，通常能将根的吸收面积提高至20倍。根毛与土壤水分和矿质离子紧密接触。

The soil solution normally has a higher water potential than the root hair cell cytoplasm and vacuole. Water therefore enters by osmosis, moving across the partially permeable cell surface membrane. Mineral ions are actively transported into root cells, lowering their water potential further and maintaining a steep water potential gradient.

土壤溶液的水势通常高于根毛细胞的细胞质和液泡。因此，水分通过渗透作用穿过部分通透的细胞膜进入根毛。矿质离子被主动转运进入根细胞，进一步降低其水势，维持陡峭的水势梯度。

This initial uptake is a passive process driven by the water potential gradient, but it depends on active ion transport to maintain that gradient.

这一初始吸收是被动过程，由水势梯度驱动，但依赖主动离子运输来维持该梯度。

2. Symplast and Apoplast Pathways | 共质体途径与质外体途径

Once water enters the root cortex, it travels towards the stele (vascular cylinder) by two main routes: the symplast pathway and the apoplast pathway.

水分进入根皮层后，主要通过两条路线向中柱（维管柱）运输：共质体途径和质外体途径。

Symplast pathway: Water moves through the cytoplasm of adjacent cells via plasmodesmata. This route involves crossing cell membranes only at the initial entry point, so it is partially regulated.
共质体途径：水分通过胞间连丝在相邻细胞的细胞质中移动。这条路线仅在初始进入点穿过细胞膜，因此部分受调控。
Apoplast pathway: Water moves through the continuous network of cell walls and intercellular spaces, without crossing any membranes. This is the fastest route and carries the majority of water in most plants.
质外体途径：水分沿细胞壁和细胞间隙的连续网络移动，不穿过任何膜。这是最快的路线，在大多数植物中承担大部分水分的运输。

Because the apoplast pathway bypasses membranes, it does not offer selectivity. The symplast pathway, however, allows cells to control which solutes move along with water.

由于质外体途径绕过了细胞膜，因此不具有选择性。而共质体途径允许细胞控制哪些溶质随水分移动。

3. The Endodermis and the Casparian Strip | 内皮层与凯氏带

Before water can enter the xylem, it must pass through the endodermis, the innermost layer of the cortex. The endodermis is a single layer of cells surrounding the vascular cylinder.

在水分进入木质部之前，必须穿过内皮层，即皮层最内层。内皮层是围绕维管柱的单层细胞。

The radial and transverse walls of endodermal cells contain a band of suberin and lignin called the Casparian strip. This hydrophobic band blocks the apoplast pathway, forcing water and dissolved ions to cross the selectively permeable cell membranes of the endodermal cells and enter the symplast pathway.

内皮层细胞的径向壁和横向壁含有由木栓质和木质素组成的带状结构，称为凯氏带。这一疏水带阻断了质外体途径，迫使水分和溶解离子穿过内皮层细胞的选择性通透膜，进入共质体途径。

The Casparian strip therefore acts as a checkpoint, ensuring that harmful solutes are excluded and essential minerals are actively transported into the stele. This is a key adaptation for controlling ion uptake.

因此，凯氏带充当检查站，确保有害溶质被排除，必需矿物质被主动转运进入中柱。这是控制离子吸收的关键适应结构。

4. Xylem Structure: Vessels and Tracheids | 木质部结构：导管与管胞

Once inside the stele, water is loaded into the xylem, the main water-conducting tissue. Xylem consists of two types of conducting cells: tracheids and vessel elements.

进入中柱后，水分被装载到木质部中，木质部是主要的水分输导组织。木质部由两类输导细胞组成：管胞和导管分子。

Tracheids: Long, slender cells with tapered ends. They have bordered pits and conduct water slowly through overlapping walls. They are present in all vascular plants.
管胞：长而细的细胞，末端渐尖。具有具缘纹孔，通过重叠的细胞壁缓慢输导水分。它们存在于所有维管植物中。
Vessel elements: Wider, shorter cells joined end-to-end forming continuous tubes. Their end walls are perforated, allowing rapid water flow. They are more efficient but more vulnerable to embolism.
导管分子：较宽、较短的细胞，首尾相连形成连续管道。端壁具有穿孔，允许快速的水流。效率更高，但更容易发生栓塞。

Both cell types are dead at maturity. Their walls are thickened with lignin, which provides mechanical support and prevents the collapse of vessels under tension.

这两类细胞成熟时均为死细胞。细胞壁因木质素沉积而加厚，提供机械支撑并防止导管在张力下塌陷。

Lignin is a complex hydrophobic polymer that makes the walls rigid, waterproof, and resistant to compression.

木质素是一种复杂的疏水聚合物，使细胞壁坚硬、防水且抗压缩。

5. The Cohesion–Tension Theory | 内聚力–张力学说

The most widely accepted explanation for the upward movement of water in tall plants is the cohesion–tension theory. It involves three key principles: transpiration pull, cohesion, and adhesion.

解释高大植物中水分向上移动最广泛接受的理论是内聚力–张力学说。它涉及三个关键原理：蒸腾拉力、内聚力和附着力。

1. Transpiration pull: Water evaporates from the surfaces of mesophyll cells in leaves into intercellular air spaces, then diffuses out through stomata. This creates a water potential gradient between the leaf cells and the atmosphere, causing water to leave the leaf cell walls.

1. 蒸腾拉力：水分从叶片叶肉细胞表面蒸发进入细胞间隙，然后通过气孔扩散出去。这会在叶片细胞和大气之间形成水势梯度，导致水分离开叶肉细胞壁。

2. Cohesion: Water molecules are attracted to each other by hydrogen bonds. This strong cohesive force allows a continuous column of water to be pulled upward through the xylem without breaking.

2. 内聚力：水分子通过氢键相互吸引。这种强大的内聚力使得连续的液柱能够在木质部中被向上拉动而不断裂。

3. Adhesion: Water molecules also adhere to the hydrophilic walls of xylem vessels. This adhesion, combined with cohesion, helps maintain the water column and reduces the risk of cavitation.

3. 附着力：水分子还附着在木质部导管亲水的细胞壁上。附着力与内聚力共同维持液柱，并降低空穴化风险。

Transpiration pull → Tension in xylem → Cohesion pulls water column upward

蒸腾拉力 → 木质部中产生张力 → 内聚力将水柱向上拉动

6. Transpiration: The Driving Force | 蒸腾作用：驱动力

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. It is sometimes described as a ‘necessary evil’ because it drives water transport but also causes water loss.

蒸腾作用是指植物地上部分水汽的散失，主要通过气孔进行。它有时被描述为“必要的恶”，因为它驱动水分运输，同时也导致水分损失。

The rate of transpiration is influenced by several environmental factors:

蒸腾速率受多种环境因素影响：

Factor	Effect on transpiration
Light intensity	Increases stomatal opening → increases transpiration
Temperature	Increases evaporation rate and water-holding capacity of air
Humidity	High humidity reduces the water potential gradient → decreases transpiration
Wind speed	Removes water vapour around leaf → increases transpiration
Water availability	Low soil water → stomatal closure → decreases transpiration

Transpiration rate ∝ Water potential gradient between leaf and air

蒸腾速率 ∝ 叶片与空气之间的水势梯度

7. Stomatal Opening and Closing | 气孔的开闭机制

Stomata are microscopic pores in the leaf epidermis, each surrounded by a pair of guard cells. The opening and closing of stomata regulate gas exchange and water loss.

气孔是叶片表皮上的微小孔隙，由一对保卫细胞环绕。气孔的开闭调节气体交换和水分损失。

The mechanism is based on changes in turgor pressure of guard cells, primarily driven by potassium ion movement:

该机制基于保卫细胞膨压的变化，主要由钾离子运动驱动：

Opening: In light, guard cells actively pump K⁺ ions into them, often accompanied by Cl⁻ and the synthesis of malate. This lowers the water potential of guard cells, causing water to enter by osmosis. The guard cells become turgid and bow outward, creating a pore.
开启：在光照下，保卫细胞主动泵入K⁺离子，常伴随Cl⁻摄入和苹果酸合成。这降低了保卫细胞的水势，使水分通过渗透进入。保卫细胞膨胀并向外弯曲，形成孔隙。
Closing: When water is scarce or at night, K⁺ ions move out of guard cells. Water follows by osmosis, guard cells become flaccid, and the pore closes.
关闭：当缺水或夜晚时，K⁺离子移出保卫细胞。水分通过渗透随之流出，保卫细胞变得松弛，孔隙关闭。

Abscisic acid (ABA) is a plant hormone that triggers stomatal closure during drought stress by stimulating K⁺ loss from guard cells.

脱落酸（ABA）是一种植物激素，在干旱胁迫下通过促进K⁺从保卫细胞中流出而触发气孔关闭。

8. Water Movement within the Leaf | 水分在叶片内的移动

Water reaching the leaf travels through the xylem of leaf veins, which branch into increasingly fine networks. From the terminal veins, water moves into the apoplast of mesophyll cells.

到达叶片的水分通过叶脉中的木质部运输，叶脉分支成越来越细的网络。从末端叶脉起，水分进入叶肉细胞的质外体。

The walls of mesophyll cells are moist and exposed to intercellular air spaces. Water evaporates from these cell wall surfaces into the air spaces, a process driven by the heat energy of the leaf.

叶肉细胞的细胞壁湿润，暴露于细胞间隙。水分从这些细胞壁表面蒸发进入气隙，这一过程由叶片的热能驱动。

As water evaporates, the water potential of the mesophyll cell walls decreases. Water then moves from the xylem into the walls, maintaining a continuous flow. This is why the tension generated at the leaf surface is transmitted all the way down to the roots.

随着水分蒸发，叶肉细胞壁的水势降低。于是水分从木质部移向细胞壁，维持连续流动。这就是为什么叶片表面产生的张力能一直传递到根部。

The intimate contact between the xylem and mesophyll ensures that the transpiration pull is efficiently propagated through the plant.

木质部与叶肉之间的紧密接触确保了蒸腾拉力在植物体内高效传递。

9. Supporting Evidence and Root Pressure | 支持证据与根压

Several observations support the cohesion–tension theory, although some limitations exist.

一些观察结果支持内聚力–张力学说，尽管该理论存在一定局限性。

Continuous water columns: Xylem vessels contain continuous columns of water that break when the stem is cut, indicating tension.
连续水柱：木质部导管含有连续的水柱，切断茎时水柱会断裂，表明存在张力。
Trunk diameter changes: Trees shrink slightly during the day when transpiration is high and expand at night, consistent with changes in xylem tension.
树干直径变化：白天蒸腾旺盛时，树木会略微收缩；夜间则膨胀，这与木质部张力的变化一致。
Negative pressure in xylem: A pressure probe attached to xylem can measure negative pressure, confirming the pull mechanism.
木质部负压：连接在木质部上的压力探针可测量到负压，证实了拉力机制。

Root pressure is a minor force that pushes water upward. It is caused by active transport of mineral ions into the xylem, which lowers the water potential and draws water from the cortex. Root pressure can cause guttation, the exudation of water droplets from leaf margins in some plants, especially at night.

根压是一种推动水分上升的次要力量。它是由矿质离子被主动转运进入木质部引起的，这降低了木质部的水势，将水分从皮层吸入。根压可导致吐水现象，即在某些植物（尤其是夜间）叶片边缘渗出小水滴。

However, root pressure is generally too weak to explain water movement in tall trees, so transpiration pull remains the dominant mechanism.

然而，根压通常太弱，无法解释高大树木中的水分上升，因此蒸腾拉力仍然是主要机制。

10. Summary and Exam Focus | 总结与考试重点

For CIE A-Level Biology, you should be able to describe and explain the pathway of water from soil to atmosphere, with reference to the structures and processes involved.

对于CIE A-Level生物学，你应该能够描述并解释水分从土壤到大气的路径，并涉及相关的结构和过程。

Key points to remember:

需记忆的关键要点：

Water enters root hairs by osmosis, driven by a water potential gradient maintained by active ion transport.
水分通过渗透作用进入根毛，由主动离子运输维持的水势梯度驱动。
Movement through the cortex can be symplastic or apoplastic; the Casparian strip forces the apoplastic water into the symplast.
穿越皮层可以是共质体或质外体途径；凯氏带强制质外体水分进入共质体途径。
Xylem vessels are dead, lignified tubes with perforated end walls that provide a low-resistance pathway.
木质部导管是死亡、木质化的管道，具有穿孔端壁，提供低阻力路径。
The cohesion–tension theory explains long-distance transport: transpiration pull generates tension, and cohesion prevents column breakage.
内聚力–张力学说解释了长距离运输：蒸腾拉力产生张力，内聚力防止水柱断裂。
Stomatal opening and closing are controlled by guard cell turgor, driven by K⁺ ion movements.
气孔的开闭由保卫细胞的膨压控制，由K⁺离子的移动驱动。

In exams, always use precise vocabulary such as ‘water potential’, ‘cohesion’, ‘tension’, ‘Casparian strip’, and ‘transpiration pull’ to gain full marks.

在考试中，务必使用精确的术语，如“水势”、“内聚力”、“张力”、“凯氏带”和“蒸腾拉力”，以获得满分。

Practice drawing labelled diagrams of a root hair cell, endodermis, and xylem vessel to strengthen your understanding.

练习绘制根毛细胞、内皮层和木质部导管的标注图，以增强理解。

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Water Transport Pathways in Plants | 水分在植物体内的运输路径