📚 Transport in Plants | 植物运输精讲
Plants require efficient transport systems to move water, dissolved minerals and organic nutrients between their roots, stems and leaves. Unlike animals, plants do not have a heart to pump fluids; instead they rely on physical processes such as transpiration and osmosis. In the IGCSE Edexcel Biology specification, you need to understand the roles of xylem and phloem, the mechanisms of water and sugar transport, and how experimental evidence supports these ideas.
植物需要高效的运输系统在根、茎和叶之间输送水分、溶解的矿物质和有机养分。与动物不同，植物没有像心脏那样的泵，而是依赖蒸腾作用和渗透等物理过程。在 IGCSE Edexcel 生物考试中，你需要理解木质部和韧皮部的作用、水分和糖类运输的机制，以及实验证据如何支持这些观点。
1. Overview of Plant Transport Systems | 植物运输系统概述
Vascular plants contain two main transport tissues: xylem and phloem. Xylem tissue transports water and dissolved mineral ions from the roots up through the stem to the leaves. Phloem tissue transports the products of photosynthesis — mainly sucrose and amino acids — from sources (e.g. leaves) to sinks (e.g. growing tips, roots and developing fruits). These tissues are arranged in vascular bundles, which are found in different patterns in roots and stems. In roots the bundles form a central stele, whereas in stems they are arranged in a ring toward the outside.
维管植物含有两种主要的运输组织：木质部和韧皮部。木质部组织将水和溶解的矿物质离子从根部经茎向上输送到叶片。韧皮部组织将光合作用的产物——主要是蔗糖和氨基酸——从源（如叶片）运输到库（如生长点、根和发育中的果实）。这些组织分布成维管束，在根和茎中的排列形式不同：根中维管束形成中央的中柱，而茎中维管束在外围成环状排列。
2. Structure of Xylem and Phloem | 木质部与韧皮部的结构
Xylem vessels are long, hollow tubes formed from dead cells. Their cell walls are thickened with lignin, which provides mechanical strength and makes the walls waterproof. The end walls between xylem cells break down completely, so the tubes are continuous with no cross-walls. Lignin can be deposited in rings, spirals or as a full lining, leaving small unlignified areas called pits where water can move laterally. There is no cytoplasm or nuclei, so there is no obstruction to water flow. Phloem tissue consists of sieve tube elements and companion cells. Sieve tube elements are living cells with perforated end walls called sieve plates, which allow the flow of cell sap. They have a thin layer of cytoplasm but no nucleus. Each sieve tube element has an associated companion cell that carries out the metabolic functions needed to keep the sieve tube alive.
木质部导管是由死细胞构成的长而中空的管。它们的细胞壁由木质素（lignin）加厚，既提供了机械强度又使壁防水。木质部细胞之间的端壁完全消失，因此导管连续且无横隔。木质素可沉积成环状、螺旋状或形成完整内衬，留下未木质化的小区域——纹孔（pits），水分可在此横向移动。导管内没有细胞质或细胞核，因此水流不受阻碍。韧皮部组织由筛管分子和伴胞组成。筛管分子是活细胞，端壁上有筛孔形成筛板（sieve plates），允许细胞液流动。筛管分子有一薄层细胞质但不含细胞核。每个筛管分子都有一个相伴的伴胞，负责维持筛管生命所需的代谢功能。
3. Water Absorption by Root Hair Cells | 根毛细胞对水分的吸收
Root hair cells are epidermal cells with long, thin extensions that increase the surface area for absorption of water and mineral ions. The cytoplasm of root hair cells contains a high concentration of solutes — maintained by active transport of mineral ions into the cell — making the water potential inside the cell lower (more negative) than that of the soil water. Water therefore enters the root hair by osmosis. Mineral ions such as nitrates (NO₃⁻) and magnesium (Mg²⁺) are taken up by active transport using energy from respiration.
根毛细胞是表皮细胞，具有长而细的突起，增大了吸收水分和矿物质离子的表面积。根毛细胞质中含有高浓度的溶质——通过主动运输将矿物质离子泵入细胞来维持——使得细胞内的水势低于（更负于）土壤水。因此，水分通过渗透作用进入根毛。硝酸根（NO₃⁻）和镁离子（Mg²⁺）等矿物质离子则通过需要呼吸作用的主动运输被吸收。
4. Pathways of Water through the Root | 水分通过根的途径
Once inside a root hair cell, water travels across the root to reach the xylem in the centre. There are three pathways: the apoplast pathway (through the cell walls and intercellular spaces without crossing membranes), the symplast pathway (through the cytoplasm and plasmodesmata connecting neighbouring cells), and the vacuolar pathway (through the vacuoles and cytoplasm). The apoplast pathway is blocked at the endodermis by the Casparian strip, a band of suberin that is impermeable to water and ions. This forces water and dissolved minerals to enter the cytoplasm of endodermal cells, allowing the plant to control which ions enter the xylem.
水分进入根毛细胞后，会横穿根部到达中央的木质部。水分通过的途径有三种：质外体途径（通过细胞壁和胞间隙，不需要越过细胞膜）、共质体途径（通过细胞质和连接相邻细胞的胞间连丝）和液泡途径（穿过液泡和细胞质）。质外体途径在内皮层被凯氏带（Casparian strip）阻断，凯氏带是一条由木栓质（suberin）构成的防水带，迫使水分和溶解的矿物质进入内皮层细胞的细胞质，从而使植物能够控制哪些离子进入木质部。
5. Transpiration and the Transpiration Stream | 蒸腾作用与蒸腾流
Transpiration is the evaporation of water from the internal surfaces of leaves through stomata. As water vapour exits the leaf, it creates a water potential gradient that pulls more water up through the xylem from the roots — this continuous column of water moving upward is called the transpiration stream. Transpiration cools the plant, supplies water for photosynthesis and enables the transport of mineral ions dissolved in water. Wilting occurs when the rate of transpiration exceeds the rate of water uptake, causing cells to lose turgor pressure.
蒸腾作用是指水分从叶片内部表面通过气孔蒸发的过程。水蒸气离开叶片时，会形成水势梯度，将更多水分通过木质部从根部向上拉——这股持续上行的水柱称为蒸腾流。蒸腾作用可以降温、为光合作用提供水分，并使溶解在水中的矿物质离子得以运输。当蒸腾速率超过吸水速率时，细胞失去膨压，植物就会发生萎蔫。
6. The Cohesion–Tension Theory | 内聚力–张力理论
The cohesion–tension theory explains how water is pulled up through the plant. The main forces are: (1) Cohesion — water molecules are attracted to each other by hydrogen bonds, forming a continuous, unbroken column in the narrow xylem vessels. (2) Adhesion — water molecules are also attracted to the lignin walls of xylem vessels, helping to counteract gravity. (3) Tension — as water evaporates from leaves (transpiration), tension (negative pressure) is created at the top of the water column, pulling the entire column upward. This pull is transmitted downwards because of the high tensile strength of the water column. Root pressure may provide a small upward push, but most tall plants rely almost entirely on the transpiration pull.
内聚力–张力理论解释了水分如何被向上拉入植物体。主要作用力包括：(1) 内聚力——水分子之间通过氢键相互吸引，在窄细的木质部导管中形成连续的、无断裂的水柱；(2) 附着力——水分子也被木质部导管的木质化管壁吸引，有助于抵消重力的影响；(3) 张力——水分从叶片蒸发（蒸腾）时，在水柱顶部产生张力（负压），将整条水柱向上拉。由于水柱具有很高的抗拉强度，这种拉力可以向下传递。根压可能提供微小的向上的推力，但绝大多数高大植物几乎完全依赖蒸腾拉力。
7. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
The rate of transpiration is influenced by four main environmental factors, all by altering the water potential gradient between the leaf and the surrounding air or by changing stomatal opening: (a) Light intensity — bright light stimulates stomata to open, increasing transpiration. (b) Temperature — higher temperatures increase the kinetic energy of water molecules and increase the rate of evaporation, and also lower the external water vapour concentration. (c) Humidity — high humidity reduces the water potential gradient between leaf and air, so transpiration decreases. (d) Wind speed — wind removes the layer of humid air around the leaf, steepening the concentration gradient and increasing transpiration. A potometer is used to measure the rate of water uptake, which can be used as an estimate of the transpiration rate under controlled conditions.
蒸腾速率受到四个主要环境因素的影响，这些因素都会改变叶片与周围空气之间的水势梯度或影响气孔开闭：(a) 光照强度——强光刺激气孔开放，蒸腾速率增大；(b) 温度——较高的温度增加水分子的动能，加快蒸发速度，同时也降低了外界水蒸气浓度；(c) 湿度——高湿度减小了叶片与空气之间的水势梯度，因此蒸腾速率下降；(d) 风速——风可将叶片周围潮湿的空气层吹走，使浓度梯度更陡，从而加快蒸腾。使用蒸腾计（potometer）可以测量吸水速率，在控制条件下可用以估算蒸腾速率。
8. Translocation in the Phloem | 韧皮部的输导作用
Translocation is the movement of organic solutes — mainly sucrose and amino acids — through the phloem from sources to sinks. Sources are parts of the plant that produce or release sugars, such as mature leaves (photosynthesis) or storage organs during germination. Sinks are areas where sugars are used or stored, such as developing roots, shoot tips and fruits. The process is active, requiring energy from respiration. The exact mechanism is not fully required at IGCSE, but the pressure-flow hypothesis states that sugars are actively loaded into the phloem at the source, lowering the water potential and causing water to enter by osmosis. This builds up hydrostatic pressure, pushing the sap toward regions of lower pressure at the sink where sugars are unloaded.
输导作用（translocation）指的是有机溶质——主要是蔗糖和氨基酸——通过韧皮部从源运输到库。源是产生或释放糖分的部位，如成熟叶片（光合作用）或萌发时的贮藏器官。库是糖被利用或储存的部位，如发育中的根、茎尖和果实。这一过程需要呼吸作用提供的能量，是主动过程。IGCSE 对运输的具体机制不做深入要求，但压力流假说认为：在源端，糖被主动装载到韧皮部中，降低了筛管的水势，导致水分通过渗透进入，产生静水压力，将细胞液推向压力较低的库端，在那里糖被卸出。
9. Evidence for Transport Mechanisms | 运输机制的证据
Two classic experiments are often used to demonstrate the function of phloem and xylem. (1) Ringing experiment — removing a complete ring of bark (which contains the phloem) from a woody stem. Over time, the region above the ring swells as sugars accumulate because translocation downwards is interrupted, while the region below shows reduced growth. Water transport in the xylem remains unaffected, showing that xylem and phloem have separate roles. (2) Aphid stylet technique — aphids insert their mouthparts into phloem sieve tubes. The stylet can be severed and left in place, allowing the contents of the phloem to be collected and analysed, confirming that sucrose and amino acids are transported. Radioactive tracers such as ¹⁴CO₂ can also be supplied to leaves, and autoradiography then shows the labelled carbon moving through the phloem to sinks.
两个经典实验通常用来证明韧皮部和木质部的功能。(1) 环割实验——从木本茎上剥去一整圈树皮（内含韧皮部）。一段时间后，环割处上方部位因糖分积累而膨大，因为向下的输导作用被阻断，而下方则生长减缓。木质部中的水分运输不受影响，表明木质部和韧皮部各自执行不同的功能。(2) 蚜虫口针技术——蚜虫将其口器插入韧皮部的筛管中。口针可以被切断并保留在原处，这样就可以收集韧皮部的内容物进行分析，证实运输的是蔗糖和氨基酸。也可将放射性示踪剂如 ¹⁴CO₂ 供应给叶片，然后通过放射自显影显示标记的碳经韧皮部向库移动。
10. Mineral Ions and Deficiency Symptoms | 矿物质离子与缺乏症状
Plants require specific mineral ions for healthy growth, and a deficiency leads to visible symptoms. Magnesium ions (Mg²⁺) are a central component of chlorophyll; a deficiency causes chlorosis — yellowing of leaves, especially between veins — because the plant cannot make enough chlorophyll. Nitrate ions (NO₃⁻) are needed to synthesise amino acids and proteins; a shortage results in stunted growth and older leaves turning yellow due to protein breakdown. Other essential ions include phosphate (for ATP and DNA), potassium (for stomatal opening) and calcium (for cell wall formation), but magnesium and nitrate effects are the key ones for IGCSE.
植物需要特定的矿物质离子来保持健康生长，缺乏时会出现明显症状。镁离子（Mg²⁺）是叶绿素分子的核心组成成分；缺镁会导致缺绿病——叶片变黄，尤其是叶脉之间的部位黄化——因为植物无法合成足够的叶绿素。硝酸根离子（NO₃⁻）是合成氨基酸和蛋白质所必需的；缺乏硝酸盐会导致植株矮小，老叶因蛋白质分解而变黄。其他必需离子还包括磷酸根（用于合成 ATP 和 DNA）、钾离子（参与气孔开闭）和钙离子（参与细胞壁形成），但镁和硝酸盐的作用是 IGCSE 的重点。
11. Comparison of Xylem and Phloem | 木质部与韧皮部的比较
The table below summarises the key differences between xylem and phloem tissues, which are commonly examined.
下表总结了木质部与韧皮部之间的主要区别，这些是考试中常见的知识点。
| Feature / 特征 | Xylem / 木质部 | Phloem / 韧皮部 |
|---|---|---|
| Main transport material / 主要运输物 | Water and mineral ions / 水和矿物质离子 | Sucrose and amino acids / 蔗糖与氨基酸 |
| Direction of flow / 流动方向 | Upwards only (roots to leaves) / 只向上（根到叶） | Up and down (sources to sinks) / 可上可下（源到库） |
| Cell vitality / 细胞死活 | Dead at maturity / 成熟时死细胞 | Living cells / 活细胞 |
| Cell wall / 细胞壁 | Thick, lignified / 厚壁、木质化 | Thin, cellulose; not lignified / 薄壁、纤维素；未木质化 |
| End walls / 端壁 | Absent / 消失 | Present as sieve plates / 有筛板 |
| Driving force / 驱动力 | Transpiration pull (passive) / 蒸腾拉力（被动） | Active loading; pressure flow / 主动装载；压力流 |
12. Exam Tips and Common Pitfalls | 考试技巧与常见陷阱
When answering questions about plant transport, be precise with terminology. Use 'transpiration' for water loss from leaves, not simply 'evaporation'; use 'translocation' for movement in phloem, not just 'transport'. Do not confuse xylem and phloem functions — xylem carries water and mineral ions, not sucrose. In describing the cohesion–tension theory, always mention cohesion, adhesion and the continuous water column. For experiments, always state what is being measured (e.g. bubble movement in a potometer) and what it represents (water uptake as an estimate of transpiration). When discussing ringing, make it clear that removing bark removes the phloem while xylem remains intact. Also, never say that water is 'sucked up' — it is pulled by tension generated at the leaf surface.
回答植物运输相关问题时，请务必使用准确的术语。叶片水分散失要用“蒸腾作用（transpiration）”，不单用“蒸发”；韧皮部内的移动要用“输导作用（translocation）”，而不仅仅是“运输”。不要混淆木质部和韧皮部的功能——木质部运输水和矿物质离子，而不是蔗糖。在描述内聚力–张力理论时，一定要提到内聚力、附着力以及连续的水柱。对于实验，要说明测量的是什么（如蒸腾计中气泡的移动）及其代表的意义（吸水速率作为蒸腾速率的估算）。讨论环割实验时，要明确切除树皮移除的是韧皮部，而木质部保持完整。此外，绝不要说水分是被“吸上去”的——它是由叶片表面产生的张力拉上去的。
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