📚 GCSE WJEC Biology: Plant Transport Key Points | GCSE WJEC 生物：植物运输 考点精讲
This article summarises the essential concepts of plant transport for the GCSE WJEC Biology specification. It covers the structure and function of xylem and phloem, transpiration, translocation, and practical skills. Each key point is presented in English followed by its Chinese translation to help bilingual learners grasp the core material efficiently.
本文总结了GCSE WJEC生物课程中植物运输的关键概念。涵盖了木质部和韧皮部的结构与功能、蒸腾作用、转运作用以及实验技能。每个要点均以英文和中文配对呈现，帮助双语学习者高效掌握核心内容。
1. Introduction to Plant Transport Systems | 植物运输系统介绍
Plants need to transport water, mineral ions, and products of photosynthesis throughout their bodies. The two main transport tissues are xylem and phloem, which together form vascular bundles. Xylem transports water and dissolved minerals from the roots to the shoots, while phloem transports sugars and amino acids (assimilates) from sources (e.g., leaves) to sinks (e.g., roots, fruits). This article covers the key exam points for GCSE WJEC Biology.
植物需要在整个体内运输水、矿质离子和光合作用产物。两种主要的运输组织是木质部和韧皮部，它们共同组成维管束。木质部将水和溶解的矿物质从根部运输到茎叶，韧皮部将糖和氨基酸（同化物）从源（如叶）运输到库（如根、果实）。本文涵盖了GCSE WJEC生物的考试重点。
2. Xylem Structure and Function | 木质部结构与功能
Xylem vessels are made of dead cells arranged end-to-end, with no end walls, forming continuous hollow tubes. The cell walls are strengthened with lignin, which provides structural support and makes them waterproof. Lignin can be deposited in rings, spirals, or pitted patterns. Xylem transport is unidirectional (upward from roots). Water moves by transpiration pull and cohesion-tension. There are no living contents, such as cytoplasm or nucleus, to allow free flow. Xylem parenchyma and fibres provide additional support.
木质部导管由死细胞首尾相连形成，端壁消失，形成连续的空心管。细胞壁由木质素加固，提供结构支撑并防水。木质素可以呈环状、螺旋纹或纹孔状沉积。木质部的运输是单向的（从根部向上）。水通过蒸腾拉力和内聚力-张力机制运动。导管内没有活细胞内容物，如细胞质或细胞核，以利于自由流动。木质部薄壁细胞和纤维提供额外支持。
In stems, xylem is usually located on the inner side of vascular bundles, towards the centre. In roots, it is central and often arranged in a star shape.
在茎中，木质部通常位于维管束内侧，朝向中心。在根中，木质部位于中央，常排列成星形。
3. Phloem Structure and Function | 韧皮部结构与功能
Phloem consists of living cells: sieve tube elements and companion cells. Sieve tube elements are elongated cells with perforated end walls called sieve plates, allowing flow of phloem sap. They lack a nucleus and many organelles but remain alive. Companion cells lie next to sieve tube elements, providing metabolic support, as they have a nucleus, many mitochondria, and ribosomes. Phloem transports sugars (mainly sucrose) and amino acids in both directions, from sources to sinks, a process called translocation.
韧皮部由活细胞组成：筛管分子和伴胞。筛管分子是细长的细胞，端壁穿孔称为筛板，允许韧皮部汁液流动。它们没有细胞核和许多细胞器，但保持活性。伴胞位于筛管分子旁边，提供代谢支持，因为它们有细胞核、大量线粒体和核糖体。韧皮部双向运输糖（主要是蔗糖）和氨基酸，从源到库，这一过程称为转运。
4. Root Hair Cells and Water Uptake | 根毛细胞与水分吸收
Root hair cells are specialised epidermal cells with long projections that increase surface area for absorption. Water enters root hairs by osmosis, moving from a higher water potential in the soil to a lower water potential in the cell. Mineral ions (e.g., nitrate, magnesium) are taken up by active transport, which uses energy from respiration to move ions against their concentration gradient. This active uptake lowers the water potential inside the root cells, ensuring a constant water potential gradient driving water entry. Root hair cells have many mitochondria to supply ATP for active transport.
根毛细胞是特化的表皮细胞，具有长突起，增加了吸收的表面积。水通过渗透作用进入根毛，从土壤中较高的水势向细胞内较低的水势移动。矿质离子（如硝酸盐、镁）通过主动运输吸收，利用呼吸作用产生的能量逆浓度梯度移动离子。这种主动吸收降低了根细胞内的水势，确保恒定的水势梯度驱动水分进入。根毛细胞含有许多线粒体，为主动运输供应ATP。
Once water enters the root, it travels across the root cortex via the apoplast pathway (through cell walls), symplast pathway (through cytoplasm and plasmodesmata), or vacuolar pathway. The Casparian strip in the endodermis blocks the apoplast pathway, forcing water and minerals to pass through the selectively permeable cell membrane, controlling entry into the xylem.
一旦水进入根系，它通过质外体途径（经细胞壁）、共质体途径（经细胞质和胞间连丝）或液泡途径穿越根皮层。内皮层中的凯氏带阻断质外体途径，迫使水和矿物质通过选择性透过的细胞膜，控制进入木质部。
5. Transpiration: The Driving Force | 蒸腾作用：驱动力
Transpiration is the evaporation of water vapour from the surfaces of mesophyll cells inside the leaf, followed by diffusion out of stomata. This water loss creates a negative pressure (tension) at the top of the xylem, which pulls water up from the roots. The process is driven by solar energy (heat). Transpiration is not just a 'leakage'; it is a vital mechanism for transporting water and dissolved minerals, cooling the leaf, and maintaining turgidity.
蒸腾作用是水蒸气从叶内叶肉细胞表面蒸发，然后通过气孔扩散出去。这种水分损失在木质部顶端产生负压（张力），将水从根部向上拉。这个过程由太阳能（热）驱动。蒸腾作用不仅仅是“渗漏”，它是运输水和溶解矿物质、冷却叶片以及保持膨压的重要机制。
Stomata are surrounded by guard cells. When guard cells are turgid, stomata open; when flaccid, they close. This regulates gas exchange and water loss.
气孔由保卫细胞围绕。当保卫细胞膨压高时，气孔打开；当松弛时，气孔关闭。这调节了气体交换和水分损失。
6. The Cohesion-Tension Theory | 内聚力-张力理论
The cohesion-tension theory explains how water moves up the xylem against gravity. Water molecules are polar and form hydrogen bonds with each other – this is cohesion. They also adhere to the lignin-lined walls of xylem vessels – this is adhesion. As water evaporates from the leaf, tension is transmitted down the continuous column of water because of cohesion. The column does not break because of the high tensile strength of water. This creates a 'pull' that draws water upwards from the roots.
内聚力-张力理论解释了水如何克服重力沿木质部向上运动。水分子是极性的，彼此之间形成氢键——这就是内聚力。它们还附着在木质部导管壁的木质素层上——这就是附着力。当水从叶片蒸发时，由于内聚力，张力通过连续的水柱向下传递。由于水的抗张强度高，水柱不会断裂。这就产生一种“拉力”，将水从根部向上吸。
There is no need for a pump; transpiration provides the solar-powered pulling force. This theory is supported by evidence such as changes in xylem diameter during the day (tension narrows the vessel) and the ability to pull mercury up a tube in experiments.
不需要泵；蒸腾作用提供了太阳能驱动的拉力。该理论有证据支持，例如白天木质部直径的变化（张力使导管变窄），以及实验中水银能被拉上管子的现象。
7. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
Several environmental factors affect
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