📚 IGCSE WJEC Biology: Plant Transport Key Points | IGCSE WJEC 生物：植物运输 考点精讲
Plants need efficient transport systems to distribute water, mineral ions and the products of photosynthesis throughout their bodies. In IGCSE WJEC Biology, you must understand the structure and function of xylem and phloem, the mechanisms of transpiration and translocation, and how these processes are affected by environmental factors. This revision guide covers all the key points, supported by experiments and clear explanations.
植物需要高效的运输系统将水分、矿物质离子和光合作用产物分布到全身。在 IGCSE WJEC 生物考试中，你必须理解木质部和韧皮部的结构与功能、蒸腾作用和转运作用的机制，以及环境因素如何影响这些过程。本复习指南涵盖所有考点，并辅以实验与清晰解释。
1. Plant Transport Tissue Overview | 植物运输组织概述
Plants possess two main transport tissues: xylem and phloem. These tissues are usually found together in vascular bundles running through the roots, stems and leaves. Xylem is responsible for carrying water and dissolved mineral ions from the roots upwards, while phloem transports organic molecules such as sucrose and amino acids from the leaves to other parts of the plant.
植物拥有两种主要的运输组织：木质部和韧皮部。这些组织通常共同存在于贯穿根、茎、叶的维管束中。木质部负责将水和溶解的矿物质离子从根部向上运输，而韧皮部则将有机分子如蔗糖和氨基酸从叶片运送到植物其他部位。
2. Xylem Vessels and Water Transport | 木质部导管与水分运输
Xylem vessels are made of dead, elongated cells arranged end to end. The end walls between these cells break down, forming continuous hollow tubes. The cell walls are thickened and reinforced with lignin, which provides mechanical strength and prevents the vessels from collapsing under tension. Lignin can be deposited in rings, spirals or as a complete covering.
木质部导管由死去的长形细胞首尾相接而成。细胞之间的端壁消失，形成连续的空心管道。细胞壁因木质素而加厚和强化，这既能提供机械强度，又能防止导管在张力下塌陷。木质素可以呈环状、螺旋状或完全覆盖状沉积。
Water moves up the xylem in a continuous column due to the cohesion-tension theory. As water evaporates from the leaves during transpiration, it creates a negative pressure (tension) at the top of the xylem. Cohesion between water molecules ensures they stick together, pulling the entire water column upwards from the roots. Adhesion of water molecules to the xylem walls also helps to combat gravity.
根据内聚力-张力理论，水以连续水柱的形式在木质部中向上移动。在蒸腾作用中，水从叶片蒸发时，会在木质部顶端产生负压（张力）。水分子之间的内聚力使它们粘在一起，从而将整个水柱从根部向上拉。水分子与木质部壁之间的附着力也有助于对抗重力。
3. Phloem and Translocation | 韧皮部与转运作用
Phloem tissue consists of living cells: sieve tube elements and companion cells. Sieve tube elements form long tubes joined by sieve plates with many pores, allowing dissolved substances to flow through. These cells have no nucleus and very little cytoplasm to reduce resistance. Each sieve tube element is associated with a companion cell, which contains a nucleus and many mitochondria to provide the energy needed for active transport.
韧皮部组织由活细胞组成：筛管分子和伴胞。筛管分子通过带有许多筛孔的筛板连接成长管，使溶解的物质能够流过。这些细胞没有细胞核，细胞质极少，以减少阻力。每个筛管分子都与一个伴胞相连，伴胞含有细胞核和大量线粒体，为主动运输提供所需能量。
Translocation is the movement of sugars (mostly sucrose) and amino acids from source regions, where they are produced or stored, to sink regions, where they are used for growth or storage. For example, mature green leaves act as sources during photosynthesis, while roots, developing fruits and young leaves act as sinks. The flow occurs in both directions within phloem and requires energy, so it is an active process.
转运作用是指糖类（主要是蔗糖）和氨基酸从源区域（产生或储存这些物质的地方）运输到库区域（用于生长或储存的地方）的过程。例如，成熟的绿叶在光合作用时充当源，而根、发育中的果实和嫩叶充当库。该流动在韧皮部中可双向进行，且需要能量，因此是一个主动过程。
4. Transpiration: Definition and Mechanism | 蒸腾作用：定义与机制
Transpiration is the evaporation of water from the aerial parts of a plant, mainly through the stomata on the lower surface of leaves. This loss of water vapour generates a suction force known as the transpiration pull, which is the primary driver of water uptake and long-distance transport in the xylem.
蒸腾作用是水分从植物地上部分（主要通过叶片背面气孔）蒸发的过程。这种水蒸气的散失会产生一种吸力，称为蒸腾拉力，这是水分吸收和木质部长距离运输的主要驱动力。
Apart from driving water upward, transpiration also helps to cool the leaf surface and delivers dissolved mineral ions to the leaves, where they are needed for synthesising essential molecules. The opening and closing of stomata are regulated by guard cells, which swell or shrink in response to changes in light intensity, CO₂ concentration and water availability.
除了驱动水分向上运输，蒸腾作用还有助于冷却叶片表面，并将溶解的矿物质离子输送到叶片，满足合成必需分子的需求。气孔的开闭由保卫细胞调节，这些细胞根据光照强度、CO₂浓度和水分供应情况膨胀或收缩。
5. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
Several environmental factors influence the rate of transpiration. Understanding these helps explain how plants cope with different conditions.
若干环境因素影响蒸腾速率。理解这些因素有助于解释植物如何应对不同环境条件。
Temperature: An increase in temperature raises the kinetic energy of water molecules, causing faster evaporation. It also lowers the relative humidity inside the leaf, increasing the water vapour concentration gradient between the leaf and the outside air. This leads to a higher transpiration rate.
温度：温度升高会增加水分子的动能，导致蒸发加快。它还会降低叶片内部相对湿度，增大叶片与外界空气之间的水蒸气浓度梯度，从而使蒸腾速率上升。
Humidity: High external humidity reduces the concentration gradient of water vapour between the stomatal cavities and the surrounding atmosphere. As a result, diffusion of water vapour out of the leaf slows down, and transpiration decreases.
湿度：外界高湿度会减小气孔下腔与外界大气之间的水蒸气浓度梯度。结果，水蒸气从叶片向外扩散的速度减慢，蒸腾作用下降。
Wind speed: Moving air sweeps away water vapour that accumulates around stomata, maintaining a steep concentration gradient. Stronger winds therefore increase the rate of transpiration. In still air, the gradient becomes shallower, reducing water loss.
风速：流动的空气会将气孔周围累积的水蒸气带走，维持一个陡峭的浓度梯度。因此，较强的风会增加蒸腾速率。在静止空气中，梯度变平缓，水分损失减少。
Light intensity: Stomata generally open in the light to allow CO₂ to enter for photosynthesis. When stomata are open, water vapour can escape more easily. Hence, higher light intensity normally increases transpiration, while darkness reduces it.
光照强度：气孔通常在光照下开放，以便CO₂进入进行光合作用。当气孔开放时，水蒸气更容易散失。因此，较高的光照强度通常会提高蒸腾速率，而黑暗则会降低它。
6. Measuring Transpiration: Using a Potometer | 测量蒸腾作用：使用蒸腾计
A potometer is a device used to estimate the rate of transpiration by measuring the rate of water uptake by a cut shoot. It does not measure transpiration directly, because some water may be used in photosynthesis and by cells for turgidity, but under normal conditions water uptake is closely related to water loss.
蒸腾计是一种通过测量切枝吸水速率来估算蒸腾速率的装置。它不能直接测量蒸腾作用，因为部分水可能用于光合作用和维持细胞膨压，但在正常条件下，吸水量与水分散失密切相关。
To set up a potometer, a leafy shoot is cut under water to prevent air from entering the xylem and blocking water flow. The apparatus is assembled underwater or with extreme care to ensure it is airtight. An air bubble is introduced into the capillary tube, and the distance the bubble moves over a set time is recorded. Rate can be expressed in mm min⁻¹ or cm³ per unit time after calibration.
安装蒸腾计时，需在水下剪取带叶枝条，以防止空气进入木质部堵塞水流。装置在水下组装或极其小心地操作以确保气密性。将一个小气泡引入毛细管中，记录气泡在一定时间内移动的距离。速率可以表示为 mm min⁻¹ 或校准后的单位时间体积。
Ensure a reservoir of water is attached to push the bubble back to the start for repeat readings. The investigation can be modified using a fan (wind), a heater (temperature) or a lamp (light intensity) to show how different factors affect transpiration.
确保连接一个储水器，能将气泡推回起点以便重复读数。实验可改用风扇（风）、加热器（温度）或灯（光照强度）来显示不同因素如何影响蒸腾作用。
7. Absorption of Water and Minerals by Roots | 根系对水分和矿物质的吸收
Root hair cells, located in the zone of maturation near root tips, greatly increase the surface area for absorption. Water enters these cells by osmosis because the soil water has a higher water potential than the cytoplasm and vacuole sap of the root hair cell. From the root hairs, water moves radially inward through the root cortex towards the central vascular cylinder, either through cell walls (apoplast pathway) or through the cytoplasm connected by plasmodesmata (symplast pathway).
根毛细胞位于根尖附近的成熟区，极大地增加了吸收表面积。水通过渗透作用进入这些细胞，因为土壤水势高于根毛细胞的细胞质和液泡液。水从根毛经根皮层径向向内朝中央维管柱移动，可通过细胞壁（质外体途径）或通过胞间连丝相连的细胞质（共质体途径）。
Mineral ions such as nitrates, phosphates and potassium are usually taken up against a concentration gradient by active transport. This process requires energy from respiration, which is why roots need oxygen and can be damaged in waterlogged soils. Once in the root cells, ions move to the xylem and are carried up with the transpiration stream.
矿物质离子如硝酸盐、磷酸盐和钾通常通过主动运输逆浓度梯度被吸收。此过程需要呼吸作用提供的能量，这正是根需要氧气并且在涝渍土壤中会受损的原因。离子进入根细胞后，便转移到木质部中并随蒸腾流向上运送。
8. Evidence for Transport in Xylem and Phloem | 木质部和韧皮部运输的证据
Staining experiment: If a plant stem is placed in a solution of a dye such as eosin or methylene blue, the coloured water rises only in the xylem vessels, staining them. A cross-section reveals coloured areas in the vascular bundles corresponding to xylem, proving that xylem carries water.
染色实验：如果将植物茎放入曙红或亚甲蓝等染料溶液中，有色水只会在木质部导管中上升并将其染色。横切面可观察到维管束中对应木质部的染色区域，证明木质部运输水。
Ringing experiment: When a complete ring of bark (including the phloem but leaving the xylem intact) is removed from a woody stem, the tissue above the ring swells after a few weeks. Analysis shows an accumulation of sugars and amino acids above the ring, while the part below does not swell. This indicates that phloem transports organic substances downwards from the leaves.
环割实验：如果从木本茎上剥去一圈完整的树皮（包括韧皮部但保留木质部），几周后环割处上方的组织会肿胀。分析显示环割上方有糖和氨基酸积累，而下方不肿胀。这表明韧皮部将有机物质从叶片向下运输。
Radioactive tracers: Carbon-14 in CO₂ supplied to a leaf can be incorporated into sugars by photosynthesis. Using autoradiography, the radioactive sugars can be traced moving out of the leaf and into other plant parts via the phloem, confirming translocation.
放射性示踪：提供给叶片的CO₂中的碳-14可通过光合作用掺入糖类中。通过放射自显影，可以追踪到放射性糖类离开叶片并通过韧皮部进入植物其他部分，从而证实了转运作用。
9. Wilting and Its Causes | 萎蔫及其原因
Wilting occurs when the rate of water loss from transpiration exceeds the rate of water absorption from the soil. Cells lose their turgor pressure, and the plant becomes flaccid. Leaves and soft stems droop, and stomata close to conserve water. This reduces photosynthesis but helps the plant survive temporary water shortage.
当蒸腾失水速率超过从土壤中吸水的速率时，就会发生萎蔫。细胞失去膨压，植株变得萎软。叶片和软茎下垂，气孔关闭以保存水分。这会减少光合作用，但有助于植物在暂时缺水时存活。
Prolonged wilting can be caused by drought, high temperatures, strong winds or root damage. If water is not replenished, permanent wilting leads to cell death. Understanding wilting highlights the importance of maintaining a balance between water uptake and transpiration.
长期萎蔫可能由干旱、高温、强风或根系损伤引起。如果不能及时补水，永久萎蔫将导致细胞死亡。了解萎蔫凸显了维持水分吸收与蒸腾之间平衡的重要性。
10. Comparison of Xylem and Phloem | 木质部与韧皮部的比较
Xylem and phloem differ markedly in structure, composition and function. Here is a direct comparison to help you recall the key points for the exam.
木质部和韧皮部在结构、组成和功能上明显不同。以下直接比较有助于你记忆考试要点。
Xylem transports water and mineral ions unidirectionally upwards from the roots. Xylem vessels are composed of dead, hollow cells with lignified walls; they contain no cytoplasm or organelles and rely on physical forces (transpiration pull) for mass flow. Xylem provides mechanical support to the plant.
木质部从根部向上单向运输水和矿物质离子。木质部导管由死的中空细胞组成，细胞壁木质化；不含细胞质或细胞器，依赖物理作用力（蒸腾拉力）进行集流。木质部为植物提供机械支撑。
Phloem transports sugars and amino acids bidirectionally, moving from sources to sinks according to the plant’s needs. Phloem sieve tube elements are living cells, though they lack a nucleus, and are closely associated with companion cells that provide metabolic energy. Translocation is an active process, using energy from
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