📚 Plant Transport in IB and WJEC Biology | IB WJEC 生物：植物运输 考点精讲
This article provides a comprehensive review of plant transport systems, covering both xylem and phloem function, water and sugar movement, and key experimental approaches. It is tailored to the core requirements of IB Biology (Topics 9.1 and 9.2) and aligns closely with WJEC A-level specifications, ensuring that learners gain a thorough understanding of transpiration, translocation, and regulatory mechanisms.
本文系统梳理了植物运输系统的核心考点，涵盖木质部与韧皮部的功能、水分与糖类的运输以及重要实验方法。内容严格对应 IB 生物学（9.1 和 9.2 节）的核心要求，并与 WJEC A-level 考纲紧密衔接，帮助学习者深入掌握蒸腾作用、转运过程及相关调控机制。
1. Introduction to Plant Transport Systems | 植物运输系统概述
Plants lack circulatory organs but rely on two long-distance transport tissues: xylem and phloem. Xylem conducts water and dissolved minerals upwards from roots to shoots, while phloem transports organic solutes, primarily sucrose, from sources to sinks. Their combined activity maintains cellular hydration, delivers nutrients, and enables growth and reproduction.
植物虽无循环器官，但依赖两种长距离运输组织：木质部和韧皮部。木质部将水分及溶解于水中的无机盐从根部向上运送至茎和叶，韧皮部则将有机物（主要是蔗糖）从源输送到库。两者的协同作用维持了细胞的水分平衡，完成养分分配，支持生长与繁殖。
- Xylem is dead at maturity; tracheids and vessel elements are hollow tubes.
- 木质部成熟时为死细胞；管胞和导管分子构成中空管道。
- Phloem consists of living sieve tube elements and companion cells.
- 韧皮部由活细胞的筛管分子和伴胞组成。
2. Water Uptake by Roots | 根部对水分的吸收
Most water enters plants through root hairs, elongated extensions of epidermal cells that increase surface area. Water moves down a water potential gradient from the soil into root cells. The movement is passive and can follow both apoplastic and symplastic routes. Root pressure, generated by active ion uptake into the vascular cylinder, can force water upward, but it is insufficient for tall trees.
大部分水分通过根毛进入植物体，根毛是表皮细胞的突起，大大增加了吸收表面积。水分沿水势梯度由土壤被动进入根细胞，可经质外体或共质体途径。根系通过向维管柱主动吸收离子产生根压，可将水上推，但对高大乔木而言并不足够。
3. The Apoplast, Symplast, and Vacuolar Pathways | 质外体、共质体和液泡途径
Water moving through cell walls and intercellular spaces follows the apoplast pathway without crossing membranes. In the symplast pathway, water enters the cytoplasm and passes via plasmodesmata. A vacuolar pathway crosses the tonoplast and vacuoles, though it contributes minimally to bulk flow. The apoplast is fast and dominates until the endodermis.
沿细胞壁和胞间空隙运动的水分属于质外体途径，无需跨膜；共质体途径则要求水进入细胞质，经胞间连丝传送。液泡途径穿越液泡膜和液泡，但对大流量输运贡献很小。质外体途径速度快，在内皮层之前占主导地位。
4. Casparian Strip and Selective Transport | 凯氏带与选择性运输
The Casparian strip is a suberin-impregnated band in the radial and transverse walls of endodermal cells. It blocks apoplastic flow, forcing water and ions to enter endodermal cells through membrane transporters. This enables selective uptake, prevents backflow, and helps maintain ion gradients essential for root pressure and mineral nutrition.
凯氏带是内皮层细胞径向壁和横向壁上一道木栓质浸渍的带状结构，阻断了质外体通道，迫使水和离子必须经膜转运蛋白进入内皮层细胞。这实现了选择性吸收，防止回流，帮助维持离子梯度，对根压形成及矿质营养至关重要。
5. Cohesion-Tension Theory of Xylem Transport | 木质部运输的凝聚力-张力理论
Water rises through xylem under tension generated by transpiration at the leaf surface. Evaporation from mesophyll cell walls creates negative pressure (tension) that pulls the continuous water column upward. Cohesion between water molecules (hydrogen bonding) and adhesion to xylem cell walls maintain an unbroken chain of water. The theory explains how tall trees overcome gravity without a pump.
叶片蒸腾作用产生的拉力是木质部水分上升的主要驱动力。叶肉细胞壁蒸发水分时产生负压（张力），牵引连续水柱向上移动。水分子间的凝聚力（氢键）及其对木质部细胞壁的附着力保证了水柱不断裂。这一理论解释了高大树木如何无需泵就能克服重力。
Ψleaf < Ψstem < Ψroot < Ψsoil
水势梯度：叶 < 茎 < 根 < 土壤
6. Transpiration and Factors Affecting It | 蒸腾作用及其影响因素
Transpiration is the loss of water vapor from aerial plant parts, mainly through stomata. The rate depends on light intensity, temperature, humidity, and air movement. A potometer can measure water uptake as an indirect estimate of transpiration rate. High light through guard cell activity, increased temperature, low humidity, and wind all raise transpiration.
蒸腾作用是植物地上部分（主要通过气孔）散失水蒸气的过程。其速率取决于光照强度、温度、湿度和空气流动。蒸腾计可通过测量水分吸收来间接推算蒸腾速率。光照通过保卫细胞活动、升温、低湿度和风均加速蒸腾。
| Factor 因素 | Effect on transpiration 对蒸腾的影响 |
|---|---|
| Light 光照 | Increases – stomata open 加快 – 气孔打开 |
| Temperature 温度 | Increases – more evaporation 加快 – 蒸发增强 |
| Humidity 湿度 | Decreases at high humidity 高湿下减慢 |
| Wind 风 | Increases – removes boundary layer 加快 – 带走界面层 |
7. Stomatal Regulation | 气孔调节
Guard cells control stomatal aperture by changes in turgor. During the day, active accumulation of K⁺ ions lowers the water potential, causing water influx and cell swelling, which opens the pore. At night or under water stress, K⁺ efflux leads to water loss and closure. The hormone abscisic acid (ABA) promotes closure by triggering ion efflux.
保卫细胞通过膨压变化调控气孔开度。白天主动积累 K⁺，降低水势，水分内流使细胞膨胀，气孔张开；夜间或干旱胁迫下 K⁺ 外流，水分丧失，气孔关闭。激素脱落酸（ABA）通过引发离子外流加速气孔关闭。
8. Translocation in Phloem: Pressure-Flow Hypothesis | 韧皮部的转运：压力流动假说
The pressure-flow (or mass flow) hypothesis accounts for phloem transport. At sources (e.g., mature leaves), sucrose is actively loaded into sieve tubes, decreasing the water potential. Water enters from adjacent xylem, raising hydrostatic pressure. At sinks (e.g., roots, fruits), sucrose is unloaded, causing water to leave and pressure to drop. The pressure gradient drives a bulk flow of phloem sap from source to sink.
压力流动假说（或称集流假说）解释了韧皮部转运机制。在源（如成熟叶片）端，蔗糖被主动载入筛管，导致水势下降，水分自邻近木质部进入，静水压升高；在库（如根、果实）端，蔗糖卸出，水分随之流出，压力下降。压力梯度驱动韧皮部汁液从源向库流动。
Source (high pressure) → Sink (low pressure)
源（高压力）→ 库（低压力）
9. Sources and Sinks: Loading and Unloading | 源和库：装载与卸载
Phloem loading at sources can be apoplastic (active transport of sucrose into sieve element–companion cell complex via H⁺-sucrose cotransporters) or symplastic (diffusion through plasmodesmata into intermediary cells). Unloading at sinks occurs passively or actively, depending on the organ. Developing seeds often require active unloading, while growing roots may rely on symplastic pathways. The direction of translocation depends on the plant’s developmental stage and local source-sink relationships.
源端装载可分为质外体途径（通过 H⁺-蔗糖共转运蛋白主动泵入筛管-伴胞复合体）和共质体途径（经胞间连丝扩散进入中间细胞）。库端卸出依器官不同可为被动或主动：发育中的种子需主动卸出，而生长中的根可借共质体途径。转运方向取决于植物发育阶段和局部源-库关系。
10. Comparing Xylem and Phloem Structure and Function | 木质部与韧皮部结构和功能的比较
| Feature 特征 | Xylem 木质部 | Phloem 韧皮部 |
|---|---|---|
| Materials transported 运输物质 | Water + mineral ions 水及无机离子 | Sucrose, amino acids, hormones 蔗糖、氨基酸、激素 |
| Direction 方向 | Upwards (root to shoot) 向上 | Bidirectional (source to sink) 双向 |
| Cell viability 细胞死活 | Dead at maturity 成熟时死亡 | Living (sieve tubes lack nucleus) 活细胞（筛管无核） |
| Driving force 驱动力 | Tension (negative pressure) 张力 | Hydrostatic pressure gradient 静水压差 |
| Lignification 木质化 | Lignified for strength 木质化增强 | Not lignified 无木质化 |
Xylem tissue comprises tracheids and vessel elements, both reinforced by lignin rings or spirals to withstand negative pressure. Phloem contains sieve plates with pores that allow continuous cytoplasm and rapid flow of sap.
木质部包含管胞和导管分子，均由木质素环或螺旋增厚加固，可耐受负压。韧皮部具有筛板，筛孔使细胞质相连，利于汁液快速流动。
11. Experimental Investigations: Potometer and Ringing Experiments | 实验研究：蒸腾计与环割实验
A potometer measures the rate of water uptake by a leafy shoot, used as a proxy for transpiration rate. Precautions include cutting the shoot under water, ensuring an air-tight seal, and drying leaves before assembly. Variables can be manipulated to test light, wind, or humidity effects. Ringing (removal of a bark ring including phloem) demonstrates phloem transport: sugars accumulate above the ring, causing swelling, while tissues below starve.
蒸腾计测量带叶枝条的吸水速率，以此间接反映蒸腾作用强度。操作时需在水下剪切枝条、确保气密密封、并擦干叶片。可调整光照、风或湿度等变量观察影响。环割实验（剥去包含韧皮部的一圈树皮）能证明韧皮部转运：环割上方蔗糖积累膨大，下方组织则饥饿萎缩。
- Aphid stylet technique can sample pure phloem sap and measure pressure.
- 蚜虫吻针技术可获取纯净韧皮部汁液并测量压力。
- Radioactive tracers (e.g., ¹⁴C-sucrose) trace translocation pathways.
- 放射性示踪剂（如 ¹⁴C-蔗糖）可追踪转运途径。
12. Adaptations of Xerophytes and Hydrophytes | 旱生植物和湿生植物的适应
Xerophytes, adapted to dry environments, possess thick cuticles, sunken stomata, rolled leaves, and succulence to reduce transpiration. Hydrophytes live in water-rich habitats; they show reduced xylem, large air spaces (aerenchyma) for buoyancy and gas exchange, and floating leaves with stomata on the upper epidermis. These adaptations reflect trade-offs between water conservation and efficient photosynthesis.
旱生植物适应干燥环境，具有厚角质层、凹陷气孔、叶片卷曲和多汁组织等保水结构。湿生植物生长在富水环境，木质部简化，具有发达的气腔（通气组织）以助浮力和气体交换，浮水叶气孔多分布于上表皮。这些适应体现了保水与高效光合作用之间的权衡。
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