Transport in Plants | 3.3 植物体内运输考点突破

📚 Transport in Plants | 3.3 植物体内运输考点突破

Efficient transport systems are essential for multicellular plants to move water, minerals, and organic compounds over long distances. This article breaks down the key mechanisms of xylem and phloem transport, focusing on water potential, transpiration, cohesion-tension theory, and translocation processes. Master these concepts to excel in your A-level Biology exam.

高效运输系统对于多细胞植物长距离运输水分、矿物质和有机化合物至关重要。本文深度解析木质部和韧皮部运输的关键机制，重点涵盖水势、蒸腾作用、内聚力-张力理论以及运输过程，助你在A-level生物考试中取得突破。

1. Introduction to Transport Systems in Plants | 植物运输系统导论

Plants lack a pumping heart, yet they sustain long-distance transport via passive physical forces and carefully maintained gradients. The xylem carries water and dissolved minerals unidirectionally from roots to leaves, while the phloem distributes sucrose and other assimilates from sources to sinks in any direction required.

植物虽然没有泵血心脏，却通过被动物理力和精细维持的梯度实现长距离运输。木质部将水和溶解的矿物质从根部单向输送到叶片，而韧皮部则按需将蔗糖等同化物从源分配到库，方向可上可下。

Xylem consists of dead, lignified cells that form hollow tubes under tension. Phloem sieve tubes are unique living cells that lack nuclei but are supported by companion cells. Appreciating these structural differences is the foundation for understanding the whole transport system.

木质部由死细胞和木质化细胞构成，在张力下形成中空管道。韧皮部筛管是特化的活细胞，没有细胞核但依靠伴胞支持。理解这些结构差异是掌握整个运输系统的基础。

2. Water Potential and Its Components | 水势及其组成

Water potential (Ψ) quantifies the free energy of water and predicts the direction of water movement. It is measured in megapascals (MPa). Pure water at standard temperature and atmospheric pressure is assigned a Ψ of 0 MPa. Dissolving solutes lowers the solute potential (Ψₛ), making Ψ more negative; physical pressure can increase or decrease the pressure potential (Ψₚ).

水势(Ψ)量化了水的自由能，可预测水分移动方向，单位是兆帕(MPa)。标准温度和气压下的纯水水势定为0 MPa。溶解溶质会降低溶质势(Ψₛ)，使Ψ变得更负；物理压力可增加或减小压力势(Ψₚ)。

The relationship is given by:

Ψ = Ψₛ + Ψₚ

关系式为：

Ψ = Ψₛ + Ψₚ

Water always moves from a region of higher Ψ to a region of lower Ψ. When a flaccid plant cell is placed in pure water, water enters by osmosis because the cell’s more negative Ψₛ draws water in; this generates turgor pressure, increasing Ψₚ, and eventually the cell reaches equilibrium.

水分总是从较高Ψ的区域流向较低Ψ的区域。将萎蔫的植物细胞放入纯水时，水会因渗透作用进入，因为细胞较负的Ψₛ吸引水进入；这产生了膨压，使Ψₚ增加，最终细胞达到平衡。

3. Water Uptake and Movement in Roots | 水分吸收与根内移动

Water enters root hairs by osmosis and moves radially through one or more pathways: the apoplast (cell walls), symplast (cytoplasm connected via plasmodesmata), or the transcellular (vacuolar) route. The endodermis, with its Casparian strip made of suberin, blocks uncontrolled apoplastic flow, forcing water and dissolved ions to pass through the selectively permeable plasma membrane.

水分通过渗透进入根毛，并经由一种或多种途径径向移动：质外体途径（细胞壁）、共质体途径（经胞间连丝连通的细胞质）或跨细胞途径。具有木栓质凯氏带的内皮层阻滞了不受控制的质外体流，迫使水和溶质穿越选择性透过膜。

This filtration step enables selective uptake of mineral ions such as K⁺ and NO₃⁻ while excluding unwanted substances. Root pressure can generate a slight upward push, visible as guttation in small plants, but it cannot account for lifting water to the tops of tall trees.

该过滤步骤可实现K⁺和NO₃⁻等离子的选择性吸收，同时排除有害物质。根压可产生微弱的向上推力，在小植物中表现为吐水现象，但不足以把水提升到大树的顶端。

4. The Cohesion-Tension Theory | 内聚力-张力理论

Transpiration from mesophyll cells creates a negative pressure (tension) that pulls water up through the xylem. Strong hydrogen bonds between water molecules generate cohesion, maintaining a continuous water column from roots to leaves. Simultaneously, adhesion of water to the hydrophilic walls of xylem vessels helps to resist gravitational pull.

叶肉细胞的蒸腾作用产生负压（张力），把水向上拉动穿过木质部。水分子间强大的氢键产生内聚力，维持了从根到叶的连续水柱。同时，水与木质部管壁亲水表面的附着力有助于对抗重力。

Evidence for this theory includes daily shrinking of tree trunks during peak transpiration, and the observation that when a xylem vessel is cut, the water column snaps away from the cut surface. These phenomena can only be explained if the water is under tension.

这一理论的证据包括蒸腾高峰时树干直径的日间收缩，以及切断木质部导管时水柱从切面弹开的现象。这些现象只有在水分处于张力下才能解释。

5. Factors Affecting Transpiration | 影响蒸腾作用的因素

Light intensity: Stomata usually open in the light, allowing CO₂ to enter and water vapour to exit. Higher light intensity increases stomatal opening and thus transpiration rate, up to a saturation point.

光照强度：气孔通常在光照下开放，使CO₂进入、水蒸气逸出。较高的光照强度增加气孔开度，从而提高蒸腾速率，直到饱和点。

Temperature: A rise in temperature increases the kinetic energy of water molecules, elevating the rate of evaporation from cell surfaces and the water vapour concentration gradient, which accelerates transpiration. Extremely high temperatures can cause stomatal closure to conserve water.

温度：温度升高增加了水分子的动能，加快了从细胞表面蒸发的速率和水汽浓度梯度，从而加速蒸腾。极端高温可导致气孔关闭以保存水分。

Humidity: High atmospheric humidity reduces the water vapour concentration gradient between leaf air spaces and the external environment, slowing down transpiration. Dry, windy conditions greatly enhance the gradient and promote water loss.

湿度：高大气湿度减小了叶片内部气隙与外界之间的水汽浓度梯度，使蒸腾减慢。干燥多风的环境大大增强梯度，促进水分散失。

Wind speed: Moving air sweeps away the boundary layer of humid air near the leaf surface, steepening the diffusion gradient and increasing transpiration. Stomata may close in very strong wind to prevent excessive water loss.

风速：流动的空气把叶面附近的湿空气边界层吹走，使扩散梯度变陡，增加蒸腾。强风下气孔可能关闭以防止过度失水。

6. Xylem Structure and Function | 木质部结构与功能

Xylem tissue contains tracheids, vessel elements, fibres and parenchyma. Vessel elements are wide, dead cells with end perforation plates, forming continuous open tubes for efficient water conduction. Tracheids are narrower, with tapering ends and pits, allowing both vertical and lateral water movement.

木质部组织包含管胞、导管分子、纤维和薄壁细胞。导管分子是宽大的死细胞，端部有穿孔板，形成连续开放管道以高效输导水分。管胞较窄，末端渐尖并具有纹孔，允许水分的纵向和横向移动。

Lignin is deposited in the secondary walls in various patterns such as annular, spiral, scalariform and pitted. This rigid polymer provides mechanical strength, prevents collapse under tension, and also waterproofs the cell wall, keeping the conduction channels open.

木质素以环纹、螺纹、梯纹和孔纹等多种模式沉积在次生壁中。这种刚性聚合物提供机械强度，防止在张力下塌陷，同时使细胞壁防水，保持输导通道畅通。

7. Phloem Structure and Translocation | 韧皮部结构与运输

Phloem sieve tube elements are elongated living cells aligned end-to-end, connected by sieve plates with large pores. They lose their nuclei and most organelles during development, minimising resistance to mass flow. Companion cells, connected by numerous plasmodesmata, supply ATP and proteins essential for sieve tube survival.

韧皮部筛管分子是长形活细胞首尾相连，由具大孔的筛板连接。它们在发育过程中失去细胞核和大部分细胞器，以尽量减少对集流的阻力。伴胞通过大量胞间连丝相连，提供ATP和筛管生存所需的蛋白质。

Translocation distributes assimilates, mainly sucrose, from source tissues (e.g., photosynthesising leaves) to sink tissues (e.g., roots, developing fruits). The direction of flow is determined by the relative strengths and locations of sources and sinks, allowing both upward and downward transport simultaneously in different sieve tubes.

运输将同化物（主要是蔗糖）从源组织（如光合叶）分配到库组织（如根、发育中的果实）。流动方向由源和库的相对强度和位置决定，可在不同筛管中同时向上和向下运输。

8. Pressure-Flow (Mass Flow) Hypothesis | 压力流动假说

At the source, sucrose is actively loaded into companion cells and then into sieve tubes, lowering the water potential inside the phloem. Water enters from the adjacent xylem by osmosis, generating a high hydrostatic pressure. At the sink, sucrose is unloaded actively or passively, raising the water potential, so water diffuses back to the xylem, reducing the pressure.

在源端，蔗糖被主动装载入伴胞继而进入筛管，降低韧皮部内的水势。水通过渗透从邻近木质部进入，产生高静水压。在库端，蔗糖被主动或被动卸载，水势升高，水分扩散回木质部，压力下降。

This pressure gradient from source to sink drives a bulk flow of phloem sap through sieve pores. Aphid stylet experiments, where sap exudes due to high pressure, and radioactive tracer studies showing directional movement of labelled assimilates, support the mass flow hypothesis.

这种从源到库的压力梯度驱动韧皮部汁液通过筛孔的整体流动。蚜虫口针实验（汁液因高压而渗出）和放射性示踪研究（显示标记同化物的定向移动）均支持集流假说。

9. Evidence for Transport Mechanisms | 运输机制的实验证据

Girdling (ringing) experiments involve removing a ring of bark containing phloem. Swelling and sugar accumulation above the ring confirm that the phloem is responsible for downward translocation of sugars, while the xylem remains functional for water transport.

环割实验是指剥去一圈包含韧皮部的树皮。环割上方肿胀和糖分积累证实韧皮部负责糖类的向下运输，而木质部仍可维持水分运输。

Radioactive labelling using ¹⁴CO₂ generates ¹⁴C-sucrose that can be detected by autoradiography in sink tissues. Dye infusion experiments with eosin or safranin colour the xylem vessels, revealing the pathway of water movement without affecting living phloem.

使用¹⁴CO₂进行放射性标记生成¹⁴C-蔗糖，可通过放射自显影在库组织中检测到。注入伊红或番红的染色实验着色木质部导管，揭示水分移动的路径，而不影响活韧皮部。

10. Uptake and Transport of Minerals | 矿物质的吸收与运输

Mineral ions such as K⁺

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Transport in Plants | 3.3 植物体内运输 考点突破