Transport in Plants | 植物运输

📚 Transport in Plants | 植物运输

Transport in plants is a fundamental topic in AQA A Level Biology, covering how water, minerals, and organic solutes are moved throughout the plant. Understanding the mechanisms of water transport in xylem and sugar transport in phloem is essential for explaining how plants obtain resources, maintain turgor, and distribute energy. This article distils key exam points, from root uptake to source–sink relationships, helping you consolidate the essential knowledge.

植物运输是AQA A Level生物学的基础主题，涵盖水、无机盐及有机溶质在植物体内的运输。理解木质部中水分运输和韧皮部中糖类运输的机制，对解释植物如何获取资源、维持膨压以及分配能量至关重要。本文凝练关键考点，从根部吸收到源库关系，帮助你夯实必备知识。

1. Introduction to Plant Transport Systems | 植物运输系统简介

Plants, especially tall trees, cannot rely solely on diffusion to distribute water and nutrients. They possess a sophisticated vascular system composed of xylem and phloem, organised into vascular bundles. Xylem transports water and dissolved mineral ions upwards from the roots, while phloem transports organic solutes, mainly sucrose, from sources (where they are produced) to sinks (where they are used or stored).

植物，特别是高大的树木，无法仅靠扩散来分配水分和养分。它们拥有由木质部和韧皮部构成的精密维管系统，组织成维管束。木质部将水和溶解的矿物离子从根部向上运输，而韧皮部则负责将有机溶质（主要是蔗糖）从源（产生部位）运输到库（利用或贮藏部位）。

2. Xylem Structure and Function | 木质部的结构与功能

Xylem tissue is composed of dead cells, primarily vessel elements and tracheids. Vessel elements are aligned end-to-end, with the end walls largely broken down, forming long continuous tubes. Their lignified walls provide structural support and prevent collapse under the tension created by transpiration. Pits in the walls allow lateral movement of water between adjacent vessels. Tracheids are elongated cells with tapered ends and also have lignified walls, but they lack open ends; water moves through pits.

木质部组织由死细胞构成，主要是导管分子和管胞。导管分子端端相连，端壁大部分降解，形成长而连续的管道。木质化壁提供结构支撑，并防止因蒸腾产生的张力导致塌陷。壁上的纹孔允许水在相邻导管间横向运输。管胞是细长细胞，两端渐尖，同样具有木质化壁，但没有开放的端部；水通过纹孔移动。

3. Phloem Structure and Function | 韧皮部的结构与功能

Phloem consists of sieve tube elements and companion cells. Sieve tube elements are living cells that lack a nucleus and most organelles, reducing resistance to flow. Their end walls are perforated to form sieve plates, through which phloem sap flows. Each sieve tube element is closely associated with a companion cell, which contains a full set of organelles and provides ATP and metabolic support via numerous plasmodesmata. This close relationship is vital for active loading and unloading of sugars.

韧皮部由筛管分子和伴胞组成。筛管分子是活细胞，但缺乏细胞核和大多数细胞器，以降低对流动的阻力。它们端壁穿孔形成筛板，韧皮部汁液由此流过。每个筛管分子与一个伴胞紧密相连，伴胞含有完整的细胞器，通过大量胞间连丝提供ATP和代谢支持。这种紧密联系对糖类的主动装载和卸出至关重要。

4. Water Uptake by Roots | 根部吸水

Water enters the root through root hair cells, which greatly increase the surface area for absorption. The soil solution generally has a higher water potential than the root hair cytoplasm, so water moves into the root hairs by osmosis. Once inside, dissolved mineral ions are actively transported into the xylem by endodermal cells, generating a lower water potential in the stele that maintains the inward flow of water across the root. Root pressure, though detectable, is a relatively minor contributor to long‑distance water transport.

水通过根毛细胞进入根部，根毛极大地增加了吸收表面积。土壤溶液的水势通常高于根毛细胞质的水势，因此水通过渗透作用进入根毛。进入后，溶解的矿物离子被内皮层细胞主动运输进木质部，在中柱内产生更低的水势，维持水向根内的持续流入。虽然可检测到根压，但它对长距离水分运输的贡献相对较小。

5. Water Movement Across the Root: Apoplast, Symplast, and Vacuolar Pathways | 水分跨越根的途径：质外体、共质体与液泡途径

Once inside the root cortex, water can travel via three routes: the apoplast pathway (through cell walls and intercellular spaces), the symplast pathway (through the continuous cytoplasm connected by plasmodesmata), and the vacuolar pathway (through vacuoles). However, at the endodermis, the hydrophobic Casparian strip — a band of suberin — blocks the apoplast pathway, forcing water and dissolved ions to cross the plasma membrane into the symplast. This selective barrier allows the plant to regulate ion uptake and prevents toxic substances from entering the vascular system.

进入根皮层后，水可通过三条途径移动：质外体途径（经细胞壁和细胞间隙）、共质体途径（经由胞间连丝连接起来的连续细胞质）和液泡途径（穿过液泡）。然而，在内皮层处，疏水的凯氏带——一条木栓质带——阻断质外体途径，迫使水和溶解的离子穿过细胞膜进入共质体。这一选择性屏障使植物得以调控离子吸收，并阻止有害物质进入维管系统。

6. Transpiration and the Transpiration Stream | 蒸腾作用与蒸腾流

Transpiration is the evaporation of water vapour from the aerial parts of the plant, primarily through stomata in the leaves. As water evaporates from the moist cell walls of mesophyll cells into air spaces and subsequently diffuses out of the stomata, it generates a water potential gradient between the leaf interior and the atmosphere. This gradient draws water out of xylem vessels in the leaf, creating a tension that pulls the entire water column upwards from roots to leaves — the transpiration stream.

蒸腾作用是水蒸气从植物地上部（主要通过叶片气孔）蒸发散失的过程。当水从叶肉细胞湿润的细胞壁蒸发进入气室，再扩散出气孔时，便在叶片内部与大气之间产生了水势梯度。这一梯度将叶内木质部导管中的水抽出，形成张力，将整个水柱从根向上拉至叶——这就是蒸腾流。

7. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素

The rate of transpiration is influenced by several environmental factors:

Light intensity: Stomata open in light, increasing transpiration.
Temperature: Higher temperature increases the rate of evaporation and increases the water vapour pressure deficit.
Humidity: Low relative humidity increases the concentration gradient, speeding up diffusion of water vapour out of the leaf.
Air movement (wind): Wind removes the boundary layer of humid air, maintaining a steep diffusion gradient.
Soil water availability: Limited water supply can cause stomatal closure and reduce transpiration.

蒸腾速率受以下环境因素影响：

光照强度：光下气孔打开，蒸腾加快。
温度：温度升高加速蒸发，增大水蒸气压差。
湿度：相对湿度低增大浓度梯度，加快水蒸气扩散出叶。
空气流动（风）：风移除潮湿空气的边界层，保持陡峭的扩散梯度。
土壤水分可用性：供水不足可导致气孔关闭，降低蒸腾。

8. Cohesion-Tension Theory | 内聚力—张力理论

The cohesion-tension theory is the widely accepted mechanism explaining how water rises in the xylem. Water molecules are polar and form hydrogen bonds, creating strong cohesive forces between them. They also adhere to the hydrophilic xylem walls (adhesion). As transpiration pulls water out of the leaf xylem, it generates a negative pressure (tension) at the top of the water column. Because the column is continuous from root to leaf, this tension is transmitted downwards, pulling up water from the roots. Lignified xylem walls prevent vessel collapse under tension. Root pressure may provide a small upward push, especially at night, but it is insufficient to drive the transpiration stream in tall plants.

内聚力—张力理论是被广泛接受的解释木质部水分上升的机制。水分子具有极性，形成氢键，由此产生强内聚力。它们还附着于亲水的木质部壁（附着力）。当蒸腾将叶片木质部中的水抽出时，在水柱顶端产生负压（张力）。由于水柱从根至叶连续不断，这一张力向下传递，将水从根部拉上。木质化的木质部壁防止导管在张力下塌陷。根压可能提供微小的向上推力，特别在夜间，但不足以驱动高大植物的蒸腾流。

9. Translocation in the Phloem: Mass Flow Hypothesis | 韧皮部的转运：集流假说

The mass flow hypothesis (also called pressure flow hypothesis) describes the movement of sucrose and other solutes in the phloem:

Active loading: At the source (e.g., mature leaf), sucrose is actively loaded into the sieve tubes via companion cells, lowering the water potential in the phloem.
Water entry: Water moves from the adjacent xylem into the phloem by osmosis, raising the hydrostatic pressure at the source.
Pressure-driven flow: At the sink (e.g., root tips, developing fruits), sucrose is actively unloaded, causing water to leave the phloem and reducing pressure. The resulting pressure gradient drives a bulk flow of phloem sap from source to sink.

集流假说（也称压力流动假说）描述了蔗糖等溶质在韧皮部的运输：

主动装载：在源（如成熟叶片），蔗糖通过伴胞被主动装载至筛管，降低韧皮部的水势。
水进入：水从相邻木质部通过渗透进入韧皮部，升高源端的静水压力。
压力驱动的流动：在库（如根尖、发育中的果实），蔗糖被主动卸出，导致水离开韧皮部，压力下降。由此产生的压力梯度驱动韧皮部汁液从源向库的集流。

10. Sources and Sinks | 源与库

A source is any plant region that produces or releases more organic solutes than it uses. Photosynthesising mature leaves are primary sources. Storage organs (e.g., potato tubers, tap roots) can act as sources when they mobilise stored reserves for growing shoots. A sink is any region that utilises or stores more carbohydrates than it produces, such as meristems, developing fruits, seeds, and young leaves. The same organ can switch between sink and source depending on developmental stage and season.

源是指净产出或释放有机溶质的任何植物区域。进行光合作用的成熟叶是主要源。贮藏器官（如马铃薯块茎、主根）在调动贮备物质供给新生枝条时可作为源。库是指净利用或贮藏碳水化合物的区域，如分生组织、发育中的果实、种子和幼叶。同一器官根据发育阶段和季节可在库与源之间转换。

11. Experimental Evidence and Evaluation of Mass Flow | 集流的实验证据与评价

Evidence supporting the mass flow hypothesis includes:

Aphid stylet experiments: When aphid stylets are severed, phloem sap continues to exude, demonstrating positive pressure.
Ringing experiments: Removing a ring of bark (and thus phloem) leads to swelling above the ring due to accumulated sugars, showing that sugars are transported in the phloem.
Radioactive labelling: Tracking ¹⁴C-sucrose confirms its movement from source to sink and the speed of translocation.
Correlation: The rate of translocation correlates positively with source activity and sink demand.

However, the hypothesis does not fully explain how bidirectional movement can occur in the same sieve tube, or the precise role of electrical potentials and protein filaments in regulating flow.

支持集流假说的证据包括：

蚜虫口针实验：切断蚜虫口针后，韧皮部汁液继续渗出，证明存在正压。
环割实验：剥去一圈树皮（因而除去韧皮部）导致环上方因糖分累积而膨大，表明糖类由韧皮部运输。
放射性标记：追踪¹⁴C标记的蔗糖，证实其从源到库的移动及转运速度。
相关性：转运速率与源活性和库需求呈正相关。

然而，该假说未能完全解释同一筛管内如何发生双向运输，亦未阐明电位和蛋白质丝在调节流动中的确切作用。

12. Practical Investigations and Potometer Use | 实验探究与蒸腾计的使用

Students are expected to design and interpret experiments investigating transport. A potometer measures the rate of water uptake by a cut shoot, which is an indirect measure of transpiration rate (assuming the water taken up is mostly lost by transpiration). Key practical considerations include: cutting the shoot under water to prevent air bubbles entering xylem, ensuring all connections are airtight, controlling one variable at a time (e.g., light, wind, humidity), and calculating the rate from the distance travelled by an air bubble per unit time. Ringing experiments and aphid stylectomy provide direct insights into phloem transport.

学生应能够设计和解释探讨运输的实验。蒸腾计测量剪枝的吸水速率，这是蒸腾速率的间接测量（假设吸收的水大部分通过蒸腾散失）。关键实验注意点包括：在水下剪枝以防气泡进入木质部，确保连接处气密，每次控制一个变量（如光、风、湿度），并根据气泡单位时间移动的距离计算速率。环割实验和蚜虫口针技术则为韧皮部运输提供直接证据。

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Transport in Plants | 植物运输