📚 Plant Transport in IB Edexcel Biology | IB Edexcel 生物：植物运输 考点精讲
Plant transport systems are essential for moving water, mineral ions and organic nutrients around the plant body. In IB and Edexcel Biology, this topic explores how xylem and phloem tissues coordinate the long-distance transport of substances without a heart or pump. Understanding the cohesion-tension mechanism in xylem and the mass flow hypothesis in phloem is crucial for exam success.
植物运输系统对于将水分、矿质离子和有机养分输送到植物体各处至关重要。在 IB 和 Edexcel 生物课程中，这一专题探讨木质部和韧皮部组织如何在没有心脏或泵的情况下协调物质的长距离运输。理解木质部中的凝聚-张力机制以及韧皮部中的集流假说是考试成功的关键。
1. Overview of Plant Transport Systems | 植物运输系统概述
Vascular plants possess two specialised transport tissues: xylem, which conducts water and dissolved minerals from roots to shoots, and phloem, which distributes organic assimilates, mainly sucrose and amino acids, from sources to sinks.
维管植物拥有两种特化的运输组织：木质部负责将水分和溶解的矿物质从根部输送到地上部分，韧皮部则将有机同化物（主要是蔗糖和氨基酸）从源分配到库。
These tissues are arranged in vascular bundles. In roots, xylem is central and star-shaped; in stems, vascular bundles are arranged around the periphery; in leaves, they form veins with xylem on the upper side and phloem on the lower side.
这些组织排列在维管束中。在根部，木质部位于中心且呈星状；在茎中，维管束排列在周边；在叶片中，它们构成叶脉，木质部在叶片上侧，韧皮部在下侧。
2. Xylem Structure and Function | 木质部结构与功能
Xylem vessels are long, continuous, hollow tubes formed from dead cells. Their walls are thickened with lignin, which provides mechanical strength and waterproofing. Lignin may be deposited in annular, spiral or pitted patterns.
木质部导管是由死细胞构成的长而连续的中空管道。其细胞壁因木质素而加厚，提供机械强度和防水性。木质素可以呈环纹、螺纹或孔纹沉积。
The end walls between vessel elements break down to form a continuous lumen, reducing resistance to water flow. Bordered pits allow lateral movement of water between adjacent vessels.
导管分子之间的端壁分解形成连续的内腔，减小水流阻力。具缘纹孔允许水分在相邻导管之间横向移动。
Xylem also contains tracheids (in conifers and ferns) and fibres. Tracheids are elongated with tapered ends and conduct water through pits, while fibres provide extra support.
木质部还包含管胞（在针叶树和蕨类中）和纤维。管胞细长，末端尖削，通过纹孔输导水分，而纤维提供额外支撑。
3. Cohesion-Tension Theory of Transpiration | 蒸腾作用的凝聚-张力理论
Transpiration is the loss of water vapour from aerial parts of the plant, mainly through stomata. The cohesion-tension theory explains how water moves up the xylem against gravity.
蒸腾作用是水分以水蒸气形式从植株地上部分（主要通过气孔）散失的过程。凝聚-张力理论解释了水分如何克服重力沿木质部向上运输。
Evaporation from mesophyll cell walls creates a negative pressure (tension) at the leaf surface. This tension pulls water out of xylem veins and into the leaf air spaces.
叶肉细胞壁的水分蒸发在叶片表面形成负压（张力）。这种张力将水分从叶脉木质部中拉入叶片气室。
Water molecules are cohesive: they stick to each other by hydrogen bonds. This cohesion transmits the tension from leaf to root, pulling a continuous column of water. Adhesion of water molecules to xylem walls also aids ascent.
水分子具有内聚力：它们通过氢键彼此粘附。这种内聚力将张力从叶传递到根，牵拉连续的水柱。水分子对木质部壁的附着力也有助于水分上升。
4. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
Light intensity: brighter light stimulates stomatal opening, increasing transpiration. Temperature: higher temperatures raise the kinetic energy of water molecules, speeding evaporation. Humidity: lower humidity increases the water vapour concentration gradient, enhancing transpiration. Wind: moving air removes the boundary layer of humid air, steepening the diffusion gradient.
光照强度：较强的光照刺激气孔开放，增加蒸腾作用。温度：较高温度提高水分子的动能，加快蒸发。湿度：较低湿度增加水蒸气浓度梯度，增强蒸腾作用。风：流动的空气移走潮湿的空气边界层，使扩散梯度变陡。
Carbon dioxide concentration also influences stomatal aperture; elevated CO₂ can cause partial closure, reducing transpiration. Soil water availability is a limiting factor – if water is scarce, stomata close and transpiration decreases.
二氧化碳浓度也影响气孔开度；升高的 CO₂ 可导致气孔部分关闭，减少蒸腾作用。土壤水分可用性是限制因子——如果水分缺乏，气孔关闭，蒸腾速率下降。
5. Phloem Structure and Function | 韧皮部结构与功能
Phloem transports organic solutes, particularly sucrose and amino acids, from photosynthetic tissues (sources) to non-photosynthetic sinks such as roots, fruits and developing leaves. It consists of sieve tube elements and companion cells.
韧皮部将有机溶质，特别是蔗糖和氨基酸，从光合组织（源）运输到非光合作用的库，如根、果实和发育中的叶片。它由筛管分子和伴胞构成。
Sieve tube elements are living cells arranged end-to-end, with perforated end walls called sieve plates. They lack a nucleus, vacuole and ribosomes at maturity, reducing resistance to flow.
筛管分子是活细胞，首尾相连，端壁上有穿孔的筛板。成熟时它们缺少细胞核、液泡和核糖体，从而降低流动阻力。
Each sieve tube element is closely associated with one or more companion cells, which carry out metabolic functions for both cells. Companion cells have numerous mitochondria to provide ATP for active loading of sucrose.
每个筛管分子与一个或多个伴胞紧密相连，伴胞为二者执行代谢功能。伴胞含有大量线粒体，为蔗糖的主动装载提供 ATP。
6. Translocation of Organic Solutes | 有机溶质的转运
Translocation is the movement of assimilates via the phloem from source to sink. Sources are net producers of photoassimilates (e.g., mature leaves), while sinks are net consumers or storage organs (roots, meristems, fruits).
转运是同化物通过韧皮部从源到库的移动。源是光合产物的净生产者（如成熟叶片），而库是净消耗者或储存器官（根、分生组织、果实）。
The direction of transport is not predetermined; it can be bidirectional in the phloem depending on the plant's developmental stage and nutrient demands. The same sieve tube can carry solutes in opposite directions at different times.
运输方向不是预先确定的；在韧皮部中可以是双向的，取决于植物的发育阶段和养分需求。同一筛管可在不同时间将溶质向相反方向运输。
7. The Pressure-Flow (Mass Flow) Hypothesis | 压力流动（集流）假说
The mass flow hypothesis proposes that phloem transport is driven by a hydrostatic pressure gradient between source and sink. At the source, sucrose is actively loaded into sieve tubes, lowering water potential. This causes water to enter by osmosis from adjacent xylem, generating high hydrostatic pressure.
集流假说提出韧皮部运输是由源和库之间的静水压力梯度驱动的。在源端，蔗糖被主动装载到筛管中，降低水势。这导致水分通过渗透作用从邻近的木质部进入，产生高静水压力。
At the sink, sucrose is actively unloaded or metabolised, raising water potential in sieve tubes. Water then moves out by osmosis into sink cells, lowering hydrostatic pressure. The pressure difference drives a bulk flow of phloem sap from source to sink.
在库端，蔗糖被主动卸载或被代谢，提高筛管中的水势。水分子随后通过渗透作用进入库细胞，降低静水压力。压力差驱动韧皮部汁液从源到库的集流。
8. Sucrose Loading and Unloading | 蔗糖的装载与卸载
Active loading of sucrose into companion cells and sieve tubes occurs via proton co-transport. A proton pump uses ATP to transport H⁺ out of companion cells, creating a proton gradient. H⁺ returns with sucrose via a symport protein.
蔗糖到伴胞和筛管的主动装载通过质子共转运进行。质子泵利用 ATP 将 H⁺ 泵出伴胞，形成质子梯度。H⁺ 通过同向转运蛋白与蔗糖一同返回。
In some plants, much of the path from mesophyll to sieve elements occurs through plasmodesmata, following a symplastic route. Unloading at sinks can be symplastic or apoplastic, depending on the sink type; developing seeds often require apoplastic unloading.
在某些植物中，从叶肉到筛管分子的大部分途径通过胞间连丝进行，遵循共质体途径。库端的卸载可以是共质体或质外体途径，取决于库的类型；发育中的种子通常需要质外体卸载。
9. Evidence for the Mass Flow Hypothesis | 压力流动假说的证据
Ringing (girdling) experiments: removing a ring of bark containing phloem leads to accumulation of sugars above the cut and swelling, while tissues below the ring eventually starve. This demonstrates that phloem transports organic nutrients.
环割实验：移除一圈含有韧皮部的树皮会导致切口上方糖分积累和肿胀，而切口下方的组织最终饥饿。这证明韧皮部运输有机养分。
Aphid stylet technique: aphids insert their stylets directly into sieve tubes. Cutting the stylet leaves the tip in the phloem, allowing sap to exude for analysis. Sap contains sucrose and amino acids, and pressure measurements show a gradient between source and sink.
蚜虫吻针技术：蚜虫将吻针直接插入筛管。切断吻针使针尖留在韧皮部中，让汁液渗出以供分析。汁液含有蔗糖和氨基酸，压力测量显示源与库之间存在梯度。
Radioactive tracers: supplying ¹⁴CO₂ to a leaf shows radioactive sucrose appearing rapidly in the phloem and moving to sinks, consistent with mass flow. Metabolic inhibitors that stop ATP production also block translocation, confirming the role of active loading.
放射性示踪剂：供给一片叶 ¹⁴CO₂ 显示放射性蔗糖迅速出现在韧皮部中并移动到库，与集流一致。阻止 ATP 产生的代谢抑制剂也会阻断转运，证实主动装载的作用。
10. Xylem and Phloem Comparison | 木质部与韧皮部比较
| Feature | Xylem | Phloem |
|---|---|---|
| Direction of flow | Upwards (mainly) | Bidirectional |
| Substances transported | Water and mineral ions | Sucrose, amino acids, hormones |
| Maturity of cells | Dead (except xylem parenchyma) | Living (sieve tubes and companion cells) |
| Driving force | Transpiration pull + cohesion-tension | Pressure gradient from active loading |
| Walls | Lignified, thick | Unlignified, relatively thin |
This table summarises the key contrasting features that are frequently assessed in both IB and Edexcel exams.
此表总结了 IB 和 Edexcel 考试中经常考察的关键对比特征。
11. Measuring Transpiration with a Potometer | 使用蒸腾计测量蒸腾作用
A potometer measures the rate of water uptake by a leafy shoot, which is used as an indirect estimate of transpiration rate. The apparatus typically consists of a capillary tube with an air bubble, connected to a reservoir.
蒸腾计测量带叶枝条的吸水速率，用于间接估算蒸腾速率。该装置通常由带气泡的毛细管和一个储水器连接组成。
The movement of the air bubble over time allows calculation of the volume of water absorbed. It is assumed that water uptake is roughly equal to water lost by transpiration, although some water may be used in photosynthesis or retained in cells.
气泡随时间的移动可计算所吸收的水体积。我们假设水分吸收量大致等于蒸腾失水量，尽管部分水分可能用于光合作用或保留在细胞内。
When using a potometer, it is important to cut the shoot under water to prevent air locks, ensure all connections are airtight, and allow the shoot to acclimatise before recording. Variables such as light, temperature, humidity and wind can be manipulated to investigate their effects.
使用蒸腾计时，必须在水下剪切枝条以防止气泡栓塞，确保所有连接处密闭，并在记录前让枝条适应环境。可以操控光照、温度、湿度和风等变量来探究其影响。
12. Adaptations of Plants to Water Availability | 植物对水分可用性的适应
Xerophytes are plants adapted to arid conditions. Adaptations include thick cuticles, sunken stomata, reduced leaf area (spines), hairy leaves, rolled leaves, succulent tissues for water storage, and extensive root systems. These reduce water loss and maximise water uptake.
旱生植物是适应干旱条件的植物。适应特征包括厚角质层、下陷气孔、减少的叶面积（刺）、叶片有毛、卷叶、用于储水的肉质组织以及广阔的根系。这些特征减少水分流失并最大化水分吸收。
Hydrophytes are plants adapted to aquatic environments. Adaptations include thin or absent cuticles, stomata on the upper epidermis, large air spaces (aerenchyma) for buoyancy and gas exchange, and reduced xylem because water support is high.
水生植物是适应水生环境的植物。适应特征包括角质层薄或无、气孔位于上表皮、用于浮力和气体交换的大气腔（通气组织），以及因水分支撑力强而退化的木质部。
Both IB and Edexcel specifications require learners to link specific features to environmental challenges, often in data-response or long-answer questions.
IB 和 Edexcel 大纲都要求学习者将特定特征与环境挑战联系起来，常见于数据分析或长答题中。
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