📚 Plant Transport: Xylem, Phloem and Water Movement | 植物运输：木质部、韧皮部与水分运输
Understanding how plants move water, minerals and sugars is a core topic in both IB and OCR Biology. Transport systems in plants rely on specialised vascular tissues, physical forces and cellular energy to distribute essential substances against gravity, over long distances, without a pumping heart.
理解植物如何运输水分、矿物质和糖类是IB和OCR生物学的核心主题。植物体内的运输系统依靠特化的维管组织、物理力和细胞能量，克服重力、跨越长距离，在没有泵血心脏的情况下完成必需物质的分配。
1. Introduction to Plant Transport Systems | 植物运输系统简介
Plants require a constant supply of water, mineral ions and the products of photosynthesis. Water and ions are absorbed from the soil by roots and transported upwards through the xylem to all aerial parts. Organic solutes, mainly sucrose, are produced in source tissues (mostly leaves) and moved to sinks (growing points, roots, storage organs) via the phloem. These two vascular tissues form a continuous transport network throughout stems and roots, often arranged in vascular bundles.
植物需要持续的水分、矿质离子和光合产物。水分和离子由根从土壤中吸收，通过木质部向上运输到所有地上部分。有机溶质，主要是蔗糖，在源组织（主要是叶）中产生，并通过韧皮部移动到库（生长点、根、贮藏器官）。这两种维管组织在茎和根中形成连续的运输网络，通常排列成维管束。
2. Xylem Structure and Function | 木质部的结构与功能
Xylem tissue transports water and dissolved mineral ions from roots to shoots. Its functional cells are tracheids and vessel elements, both of which are dead at maturity and hollow, forming continuous tubes. Vessel elements are stacked end-to-end; their end walls become perforated or disappear entirely, creating uninterrupted pipelines. Their cell walls are thickened with lignin, which provides mechanical strength and prevents collapse under the tension generated during transpiration. Lignin may form spiral, annular or pitted patterns, allowing some flexibility and lateral water movement through pits.
木质部组织将水及溶解的矿质离子从根部运输到地上部分。其功能细胞是管胞和导管分子，二者在成熟时都是死亡的、中空的，形成连续的管道。导管分子首尾相连，端壁穿孔或完全消失，形成不间断的通道。细胞壁由木质素加厚，提供机械强度，并防止在蒸腾作用产生的张力下塌陷。木质素可形成螺旋状、环状或纹孔状的花纹，保留一定的柔韧性，并通过纹孔实现横向水分移动。
3. Phloem Structure and Function | 韧皮部的结构与功能
Phloem transports organic products, primarily sucrose and amino acids, from sources to sinks. Its key cell types are sieve tube elements and companion cells. Sieve tube elements are living cells arranged end-to-end, with sieve plates – perforated end walls – allowing cytoplasmic continuity and solute flow. These cells lack a nucleus, ribosomes and a large vacuole at maturity to minimise resistance. Companion cells are metabolically active and connected to sieve tube elements by numerous plasmodesmata; they provide ATP and proteins needed to maintain the sieve tube and load/unload solutes.
韧皮部运输有机产物，主要是蔗糖和氨基酸，从源到库。其关键细胞类型是筛管分子和伴胞。筛管分子是活细胞，首尾相连，端壁为筛板——具有穿孔的端壁——保证细胞质延续和溶质流动。成熟时，这些细胞失去细胞核、核糖体和中央大液泡，以降低阻力。伴胞代谢活跃，通过大量胞间连丝与筛管分子相连，为维持筛管功能以及溶质装载/卸载提供ATP和蛋白质。
4. Water Uptake from Roots | 根部吸水
Water enters the root hair cells from the soil by osmosis because the root hair cytoplasm has a lower water potential than the soil solution (more negative solute potential due to mineral ions and organic solutes). The water then moves along the apoplast pathway (through cell walls and intercellular spaces), the symplast pathway (through the cytoplasm and plasmodesmata) or the vacuolar pathway. At the endodermis, the Casparian strip – a waxy, impermeable band of suberin – blocks the apoplast pathway, forcing water and dissolved minerals to cross the cell membrane via the symplast. This selective barrier controls which ions enter the vascular cylinder, allowing the plant to actively transport essential ions into the xylem and generate a root pressure that can force water upwards under low transpiration conditions.
水分通过渗透作用从土壤进入根毛细胞，因为根毛细胞质的渗透势低于土壤溶液（由于矿质离子和有机溶质使渗透势更负）。水随后通过质外体途径（经细胞壁和细胞间隙）、共质体途径（经细胞质和胞间连丝）或液泡途径移动。在内皮层，凯氏带——一种木栓质的蜡质不透水带——阻断质外体途径，迫使水和溶解的矿物质通过共质体途径跨越细胞膜。这个选择性的屏障控制哪些离子进入维管柱，使植物能够将必需的离子主动运输到木质部，并在低蒸腾条件下产生根压，将水向上推动。
5. The Transpiration Stream | 蒸腾流
Transpiration is the evaporation of water from the aerial parts of the plant, mainly through stomata in leaves. The loss of water vapour from the moist cell walls of mesophyll cells into the intercellular air spaces, and then out through stomata, creates a water potential gradient. This gradient draws water from the xylem in leaf veins into the mesophyll cells. The removal of water from leaf xylem reduces pressure at the top of the xylem column, generating a tension (negative pressure) that pulls water up through the entire xylem network in a continuous stream – the transpiration stream. The stream is passive, driven by solar energy, and can reach heights of over 100 metres in tall trees.
蒸腾作用是水分从植物地上部分蒸发的过程，主要通过叶片上的气孔进行。水蒸气从叶肉细胞湿润的细胞壁散失到细胞间隙，再经气孔扩散出去，形成水势梯度。这个梯度使水分从叶脉的木质部进入叶肉细胞。叶片木质部水分的流失降低木质部柱顶端的压力，产生张力（负压），通过连续的蒸腾流将水沿整个木质部网络向上拉。蒸腾流是被动的，由太阳能驱动，在高大树木中可将水提升到100米以上。
6. Cohesion-Tension Theory | 内聚力-张力理论
The cohesion-tension theory explains how water rises in xylem against gravity. It relies on three key properties of water: cohesion (hydrogen bonding between water molecules), adhesion (attraction of water to xylem walls) and tension generated by transpiration. As water evaporates from leaves, tension is transmitted down the continuous water column. Cohesion holds the water molecules together as a chain, preventing them from separating, while adhesion helps the column stay attached to the vessel walls, counteracting gravitational pull. The narrow diameter of xylem vessels also supports capillary action, but the primary driving force is the transpiration pull from leaves. This theory is supported by evidence such as diurnal changes in trunk diameter (shrinking during the day as tension increases) and direct measurement of negative pressure.
内聚力-张力理论解释了水如何克服重力在木质部中上升。它依赖于水的三个关键性质：内聚力（水分子间的氢键）、附着力（水对木质部壁的吸引力）以及由蒸腾作用产生的张力。当水从叶片蒸发，张力沿连续水柱向下传递。内聚力使水分子像链条一样连接在一起，防止断开，而附着力帮助水柱附着在导管壁上，抵消重力牵引。木质部导管较窄的直径也支持毛细作用，但主要驱动力是来自叶片的蒸腾拉力。这一理论得到一些现象的支持，例如树干直径的日变化（白天张力增大时树干收缩）以及负压的直接测量。
7. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
Transpiration rate is influenced by environmental conditions that affect the water vapour concentration gradient and stomatal aperture. The four main factors are:
蒸腾速率受环境条件影响，这些条件影响水蒸气浓度梯度和气孔开度。四个主要因素是：
- Light intensity – light stimulates stomatal opening, so higher intensity increases transpiration.
- 光照强度 – 光刺激气孔张开，所以光照越强，蒸腾越强。
- Temperature – higher temperature increases the kinetic energy of water molecules and the rate of evaporation; warm air can hold more water vapour, steepening the gradient.
- 温度 – 较高的温度增加水分子的动能和蒸发速率；暖空气能容纳更多水蒸气，增大浓度梯度。
- Humidity – high humidity reduces the vapour pressure deficit, slowing transpiration; dry air accelerates water loss.
- 湿度 – 高湿度减小水气压差，降低蒸腾速率；干燥空气加速水分丧失。
- Wind/air movement – wind removes the boundary layer of humid air around the leaf surface, maintaining a steep diffusion gradient.
- 风/空气流动 – 风带走叶片表面湿润的边界层，维持陡峭的扩散梯度。
Plants can also regulate transpiration by closing stomata, for example under water stress using the hormone abscisic acid (ABA).
植物还可以通过关闭气孔来调节蒸腾，例如在缺水胁迫下利用激素脱落酸（ABA）。
8. Translocation in the Phloem | 韧皮部中的运输
Translocation is the movement of organic solutes (mainly sucrose) from sources to sinks through the phloem. Sources are usually photosynthesising tissues (mature leaves) or storage organs releasing reserves (e.g., germinating seeds). Sinks are regions of active growth or storage, such as root tips, developing fruits and tubers. The direction of flow can be upwards or downwards and depends on relative source–sink relationships, which can change seasonally.
运输是有机溶质（主要是蔗糖）通过韧皮部从源到库的移动。源通常是进行光合作用的组织（成熟叶片）或释放贮藏物的贮藏器官（如萌发中的种子）。库是活跃生长或贮藏的区域，如根尖、发育中的果实和块茎。流动方向可向上或向下，取决于相对的源-库关系，这种关系可随季节改变。
9. Pressure-Flow Hypothesis | 压力流动假说
The pressure-flow (or mass flow) hypothesis is the most widely accepted mechanism for phloem transport. At the source, sucrose is actively loaded into companion cells and sieve tube elements, lowering the water potential inside the phloem. Water then enters from the adjacent xylem by osmosis, increasing hydrostatic pressure. At the sink, sucrose is actively unloaded, raising the water potential; water leaves the phloem and returns to the xylem, lowering pressure. The difference in hydrostatic pressure between source and sink drives a bulk flow of phloem sap, carrying solutes along the gradient. This process requires metabolic energy (ATP) for active loading/unloading.
压力流动假说（或以质流假说）是韧皮部运输最广泛接受的机制。在源端，蔗糖被主动装载进伴胞和筛管分子，降低韧皮部内的水势。随后，水通过渗透作用从相邻的木质部进入，增加静水压力。在库端，蔗糖被主动卸载，提高水势；水离开韧皮部回到木质部，压力下降。源端和库端之间的静水压力差异驱动韧皮部汁液的群体流动，将溶质顺梯度携带。这一过程需要代谢能量（ATP）用于主动装载/卸载。
10. Adaptations of Plants to Water Availability | 植物对水分有效性的适应
Plants show a range of adaptations to cope with water scarcity (xerophytes) or waterlogged environments (hydrophytes). Xerophytes, such as marram grass and cacti, may possess thick cuticles, sunken stomata, rolled leaves, reduced leaf area (spines), extensive root systems and the ability to store water (succulence). Some use Crassulacean Acid Metabolism (CAM) to open stomata at night. Hydrophytes, like water lilies, have adaptations for a buoyant life: aerenchyma (air spaces for buoyancy and oxygen transport), stomata only on the upper leaf surface and poorly developed xylem because water is abundant.
植物表现出一系列适应，以应对缺水（旱生植物）或淹水环境（水生植物）。旱生植物，如滨草和仙人掌，可能具有厚的角质层、下陷气孔、卷曲叶片、缩小叶面积（刺）、庞大的根系和储水能力（多肉性）。有些植物利用景天酸代谢（CAM）在夜间打开气孔。水生植物，如睡莲，具有适应浮水生活的特征：通气组织（用于浮力和氧气运输的气腔）、仅在上表皮分布的气孔，以及因水分充裕而发育不良的木质部。
11. Investigating Plant Transport | 植物运输实验研究
Common practical investigations include:
常见的实验研究包括：
- Potometer – a device that measures water uptake by a leafy shoot, used as an estimate of transpiration rate. The rate of movement of an air bubble along a capillary tube is recorded under different conditions (light, wind, humidity). Assumptions: water uptake ≈ transpiration rate, no photosynthesis-driven weight gain.
- 蒸腾计（Potometer） – 测量带叶枝条吸水量的装置，用于估算蒸腾速率。记录不同条件（光、风、湿度）下毛细管中气泡移动的速率。前提假设：吸水量≈蒸腾速率，没有光合作用引起的增重。
- Ringing experiments – removing a ring of bark (containing phloem) from a woody stem results in swelling above the ring due to accumulation of solutes, demonstrating that phloem transports organic substances downwards.
- 环割实验 – 从木本茎干上剥去一圈树皮（包含韧皮部），会导致环割上方肿胀，因为溶质积累，证明韧皮部向下运输有机物质。
- Use of radioactive tracers – 14C-labelled carbon dioxide is supplied to a leaf; subsequent autoradiography shows translocation of labelled sucrose through phloem to sinks.
- 放射性示踪剂的应用 – 将14C标记的二氧化碳供应给叶片，随后自显影显示标记蔗糖通过韧皮部运输到库。
12. Comparison of Xylem and Phloem | 木质部与韧皮部的比较
| Feature / 特征 | Xylem / 木质部 | Phloem / 韧皮部 |
|---|---|---|
| Main substance transported / 主要运输物质 | Water and mineral ions / 水和矿质离子 | Sucrose, amino acids / 蔗糖、氨基酸 |
| Direction of flow / 流动方向 | Upwards (roots → shoots) / 向上（根→地上部） | Up and down (source → sink) / 向上和向下（源→库） |
| Key conducting cells / 关键输导细胞 | Trachery elements (dead) / 管状分子（死细胞） | Sieve tube elements (living) / 筛管分子（活细胞） |
| Wall features / 细胞壁特征 | Lignified, thick / 木质化、厚 | Cellulosic, thin / 纤维素、薄 |
| Mechanism / 机制 | Cohesion-tension (passive) / 内聚力-张力（被动） | Pressure-flow (active loading) / 压力流动（主动装载） |
| Associated cells / 伴随细胞 | Parenchyma, fibres / 薄壁细胞、纤维 | Companion cells / 伴胞 |
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