A-Level生物 植物运输 木质部 韧皮部

A-Level生物 植物运输 木质部 韧皮部

1. 植物运输系统概述 Overview of Plant Transport Systems

Plants require efficient transport systems to move water, mineral ions, and organic solutes between their organs. Unlike animals, plants lack a centralised pumping organ such as a heart, so they rely on physical processes including osmosis, transpiration, and active transport. The two specialised vascular tissues responsible for transport are xylem, which conducts water and dissolved minerals from roots to shoots, and phloem, which translocates organic assimilates such as sucrose and amino acids from sources to sinks.

植物需要高效的运输系统来将水、矿物质离子和有机溶质在器官之间运输。与动物不同，植物没有像心脏这样的中央泵送器官，因此它们依赖渗透作用、蒸腾作用和主动运输等物理过程。负责运输的两种特化维管组织是木质部（将水和溶解的矿物质从根部输送到地上部分）和韧皮部（将蔗糖和氨基酸等有机同化物从源输送到库）。

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

Xylem tissue is composed of several cell types, the most important of which are tracheids and xylem vessel elements. Both are dead at functional maturity: their cell contents, including the cytoplasm and end walls, are broken down by programmed cell death and autolysis, leaving hollow tubes reinforced with lignin for structural support. Vessel elements are wider than tracheids, lack end walls entirely (forming continuous tubes called vessels), and have pits in their side walls to allow lateral water movement. Tracheids are narrower, spindle-shaped cells with tapered ends and extensive pitting on all walls; they overlap extensively and water moves between them through bordered pits.

木质部组织由多种细胞类型组成，其中最重要的是管胞和导管分子。两者在功能成熟时均为死细胞：它们的细胞内容物（包括细胞质和端壁）通过程序性细胞死亡和自溶被分解，留下被木质素加固的中空管道以提供结构支撑。导管分子比管胞更宽，完全没有端壁（形成称为导管的连续管道），侧壁上有纹孔以允许横向水分运动。管胞是较窄的纺锤形细胞，末端渐尖，所有壁上都有广泛纹孔；它们大量重叠，水通过具缘纹孔在它们之间流动。

3. 内聚力-张力理论 The Cohesion-Tension Theory

The cohesion-tension theory explains how water is pulled up through the xylem from roots to leaves against gravity, sometimes to heights exceeding 100 metres. The driving force is transpiration: the evaporation of water from mesophyll cell walls into the air spaces of the leaf, which diffuses out through stomata. This creates a low water potential (very negative pressure or tension) at the top of the xylem column. Because water molecules are strongly cohesive due to hydrogen bonding, the entire water column is pulled upward as a continuous, unbroken chain. Adhesion of water molecules to the lignin-lined xylem walls helps counteract gravity and prevents the column from falling back down.

内聚力-张力理论解释了水如何通过木质部从根部被拉到叶片，逆重力而行，有时可达100米以上的高度。驱动力是蒸腾作用：水分从叶肉细胞壁蒸发进入叶片空气间隙，然后通过气孔扩散出去。这在木质部柱的顶部产生低水势（非常负的压力或张力）。由于水分子通过氢键具有强大的内聚力，整个水柱作为一个连续不断的链条被向上拉。水分子对木质素衬里的木质部壁的附着力有助于抵消重力并防止水柱回落。

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

Transpiration rate is influenced by four main environmental factors. Light intensity increases transpiration because stomata open wider in bright light to allow CO2 entry for photosynthesis, providing a larger exit pathway for water vapour. Temperature increases transpiration because warmer air holds more water vapour, steepening the water vapour concentration gradient between the leaf interior and the external atmosphere, and because water molecules have greater kinetic energy to evaporate. Humidity decreases transpiration as the external air becomes more saturated: a shallower concentration gradient reduces the rate of diffusion. Wind or air movement removes the boundary layer of humid air that accumulates around the leaf surface, maintaining a steep concentration gradient and increasing transpiration rate. A potometer can be used to measure the rate of water uptake by a cut shoot, providing an indirect estimate of transpiration rate.

蒸腾速率受四个主要环境因素影响。光照强度增加蒸腾作用，因为在强光下气孔开得更大以允许CO2进入进行光合作用，为水蒸气提供更大的出口通道。温度升高增加蒸腾作用，因为较暖的空气能容纳更多水蒸气，使叶片内部与外部大气之间的水蒸气浓度梯度更陡，而且水分子具有更大的动能来蒸发。湿度增加时蒸腾作用下降，因为外部空气变得更饱和：较浅的浓度梯度降低了扩散速率。风或空气流动移除了在叶片表面积聚的潮湿空气边界层，维持陡峭的浓度梯度并增加蒸腾速率。气泡蒸腾计可用于测量切枝的水分吸收速率，提供蒸腾速率的间接估计。

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

Phloem tissue contains sieve tube elements, companion cells, parenchyma, and sclerenchyma fibres. Sieve tube elements are elongated living cells that form the conducting channels for assimilate transport. Unlike xylem vessel elements, they retain a modified cytoplasm but lose their nucleus, ribosomes, and vacuole at maturity, reducing resistance to flow. The end walls between adjacent sieve tube elements are perforated to form sieve plates, through which cytoplasmic strands (phloem protein, or P-protein) connect adjacent cells. Each sieve tube element is intimately associated with one or more companion cells: small, metabolically active cells with a dense cytoplasm, large nucleus, and numerous mitochondria. Companion cells provide ATP and metabolic support to the sieve tube elements via numerous plasmodesmata connections.

韧皮部组织包含筛管分子、伴胞、薄壁组织和厚壁纤维。筛管分子是细长的活细胞，形成同化物运输的传导通道。与木质部导管分子不同，它们保留了经过修饰的细胞质，但在成熟时失去了细胞核、核糖体和液泡，减少了流动阻力。相邻筛管分子之间的端壁穿孔形成筛板，细胞质丝（韧皮蛋白，或P蛋白）通过筛板连接相邻细胞。每个筛管分子与一个或多个伴胞紧密相连：伴胞是小而代谢活跃的细胞，具有致密的细胞质、大细胞核和大量线粒体。伴胞通过大量胞间连丝连接为筛管分子提供ATP和代谢支持。

6. 集流（压力流）假说 The Mass Flow (Pressure Flow) Hypothesis

The mass flow hypothesis, also known as the pressure flow model, was proposed by Ernst Munch in 1930 to explain the mechanism of phloem translocation. At the source (typically mature leaves during the day or storage organs during mobilisation), sucrose is actively loaded into the sieve tube elements at companion cells. This active loading lowers the water potential in the sieve tube, causing water to enter by osmosis from the adjacent xylem. The influx of water generates a high hydrostatic pressure at the source end. At the sink (growing regions, roots, developing fruits, or storage organs), sucrose is actively unloaded from the phloem and either used in respiration or converted to starch for storage. The removal of solutes raises the water potential, so water leaves the sieve tube by osmosis, returning to the xylem. This reduces the hydrostatic pressure at the sink end. The pressure gradient between source and sink drives a bulk flow of phloem sap, carrying solutes along the sieve tubes at rates of 0.2 to 1.0 metre per hour.

集流假说，也称为压力流模型，由Ernst Munch于1930年提出，用于解释韧皮部输导的机制。在源端（通常是白天的成熟叶片或动员期间的储存器官），蔗糖在伴胞处被主动装载到筛管分子中。这种主动装载降低了筛管中的水势，导致水通过渗透作用从相邻的木质部进入。水的流入在源端产生高静水压力。在库端（生长区域、根部、发育中的果实或储存器官），蔗糖从韧皮部被主动卸载，要么用于呼吸作用，要么转化为淀粉储存。溶质的移除提高了水势，因此水通过渗透作用离开筛管返回木质部。这降低了库端的静水压力。源和库之间的压力梯度驱动韧皮部汁液的集流，以每小时0.2至1.0米的速度沿筛管携带溶质。

7. 输导理论的证据 Evidence for and Against Translocation Theories

Several lines of evidence support the mass flow hypothesis. Aphid stylectomy experiments show that when an aphid’s stylet is severed while feeding on a sieve tube, phloem sap continues to exude under pressure for hours, confirming the existence of positive hydrostatic pressure. Radioactive tracer studies using carbon-14 labelled CO2 demonstrate that photosynthates move from source leaves to sink tissues at rates consistent with mass flow predictions. The composition of phloem sap (primarily sucrose with smaller amounts of amino acids, hormones, and inorganic ions) is consistent across plant species, suggesting a common transport mechanism. However, the hypothesis has limitations. It cannot explain bidirectional movement in the same sieve tube, which has been observed in some species. It does not account for the differential transport rates of different solutes, nor does it explain how sink tissues selectively extract specific solutes from the phloem stream. The electro-osmotic theory and the cytoplasmic streaming hypothesis have been proposed as alternative or complementary mechanisms, though they lack the broad experimental support that the mass flow hypothesis enjoys.

多条证据支持集流假说。蚜虫口针切割实验表明，当蚜虫口针在取食筛管时被切断，韧皮部汁液在压力下持续渗出数小时，证实了正静水压力的存在。使用碳-14标记的CO2进行的放射性示踪研究证明，光合产物以与集流预测一致的速率从源叶移动到库组织。韧皮部汁液的组成（主要是蔗糖，少量氨基酸、激素和无机离子）在不同植物物种中是一致的，表明存在共同的运输机制。然而，该假说也有局限性。它无法解释在某些物种中观察到的同一筛管中的双向运输。它无法解释不同溶质的不同运输速率，也无法解释库组织如何从韧皮部流中选择性地提取特定溶质。电渗理论和细胞质环流假说已被提出作为替代或补充机制，尽管它们缺乏集流假说所享有的广泛实验支持。

8. 木质部与韧皮部的比较 Comparison of Xylem and Phloem

Xylem and phloem differ fundamentally in structure, transport direction, driving force, and the nature of the transported substances. Xylem transports water and mineral ions unidirectionally from roots to shoots; phloem transports organic assimilates bidirectionally from sources to sinks, which can change with the plant’s developmental stage. Xylem transport is a passive, physical process driven by the transpiration pull, requiring no metabolic energy from the plant. Phloem transport, by contrast, requires active loading and unloading of sucrose at source and sink ends, making it an active, energy-dependent process at those specific sites, although the bulk flow through the sieve tubes themselves is passive. Xylem vessel elements are dead at maturity with lignified, waterproof walls and no cytoplasm, minimising resistance. Sieve tube elements are living cells with modified cytoplasm and sieve plates, enabling the controlled passage of solutes.

木质部和韧皮部在结构、运输方向、驱动力和运输物质的性质方面存在根本差异。木质部从根部到地上部分单向运输水和矿物质离子；韧皮部从源到库双向运输有机同化物，源库关系可随植物的发育阶段而变化。木质部运输是一个被动的物理过程，由蒸腾拉力驱动，不需要植物的代谢能量。相比之下，韧皮部运输需要在源端和库端主动装载和卸载蔗糖，使其在这些特定位置成为一个主动的、依赖能量的过程，尽管通过筛管本身的集流是被动的。木质部导管分子在成熟时是死细胞，具有木质化、防水的细胞壁且没有细胞质，将阻力降至最低。筛管分子是具有经过修饰的细胞质和筛板的活细胞，能够控制溶质的通过。

9. 备考技巧 Exam Tips

When labelling diagrams of transverse sections, always identify xylem and phloem by their relative positions: in roots, xylem is central and star-shaped with phloem between the arms; in stems, vascular bundles are arranged around the periphery with xylem towards the inside and phloem towards the outside; in leaves, xylem is on the upper side of the midrib vein and phloem is on the lower side. Be specific when describing the cohesion-tension theory: mention hydrogen bonding explicitly, state that the water column is under tension (negative pressure), and explain the role of stomata in controlling water loss. For phloem translocation questions, always link sucrose loading to water potential changes and the resulting hydrostatic pressure gradient. Use the terms: “source,” “sink,” “active loading,” “water potential,” “hydrostatic pressure,” and “mass flow” precisely in your answers.

在标注横切面图时，始终根据木质部和韧皮部的相对位置来识别它们：在根部，木质部居中呈星形，韧皮部位于臂之间；在茎中，维管束排列在周围，木质部朝向内侧，韧皮部朝向外侧；在叶片中，木质部位于中脉的上侧，韧皮部位于下侧。在描述内聚力-张力理论时要具体：明确提到氢键，说明水柱处于张力（负压）状态，并解释气孔在控制水分流失中的作用。对于韧皮部输导问题，始终将蔗糖装载与水势变化及由此产生的静水压力梯度联系起来。在答案中准确使用以下术语：”源”、”库”、”主动装载”、”水势”、”静水压力”和”集流”。

10. 总结 Summary

Transport in flowering plants depends on the complementary functions of xylem and phloem. Xylem uses the cohesion-tension mechanism, driven by transpiration from the leaves, to move water and dissolved minerals upward from the soil through dead, lignified vessels and tracheids. Phloem uses the mass flow mechanism, driven by active sucrose loading at sources and unloading at sinks, to distribute organic assimilates to all parts of the plant through living sieve tube elements and companion cells. Understanding both systems requires familiarity with water potential terminology, the structure-function relationships of vascular tissues, and the experimental evidence supporting each transport model. Mastery of these concepts is essential for high marks on A-Level exam questions concerning plant physiology and transport.

开花植物的运输依赖于木质部和韧皮部的互补功能。木质部利用内聚力-张力机制，由叶片的蒸腾作用驱动，通过死去的木质化导管和管胞将水和溶解的矿物质从土壤向上运输。韧皮部利用集流机制，由源端的主动蔗糖装载和库端的卸载驱动，通过活的筛管分子和伴胞将有机同化物分配到植物的各个部分。理解这两个系统需要熟悉水势术语、维管组织的结构-功能关系以及支持每种运输模型的实验证据。掌握这些概念对于在A-Level考试中关于植物生理和运输的问题获得高分至关重要。