📚 Plant Transport: Exam Essentials for IB & AQA Biology | 植物运输：IB 与 AQA 生物考点精讲
Understanding how water and solutes move through plants is fundamental to both IB Biology and AQA A-level Biology. This article covers xylem transport, transpiration, cohesion-tension theory, phloem translocation, and the pressure-flow hypothesis, with exam-focused tips.
理解水分和溶质如何在植物体内运输是 IB 生物和 AQA A-level 生物的基础。本文涵盖了木质部运输、蒸腾作用、内聚力-张力理论、韧皮部转运和压力流动假说，并提供聚焦考试的技巧。
1. Introduction to Plant Transport | 植物运输简介
Plants require a transport system to deliver water, minerals, and sugars to all cells. Unlike animals, plants lack a pumping heart; they rely on physical forces and cellular processes. Two vascular tissues—xylem and phloem—perform this role. Water and minerals move upward through xylem, while organic nutrients (mainly sucrose) are translocated in phloem.
植物需要运输系统向所有细胞输送水、矿物质和糖类。与动物不同，植物没有泵血心脏；它们依赖物理力和细胞过程。两种维管组织——木质部和韧皮部——承担这一角色。水和矿物质通过木质部向上运输，而有机养分（主要是蔗糖）则在韧皮部中转运。
2. Xylem Structure and Function | 木质部的结构与功能
Xylem vessels are long, continuous hollow tubes formed from dead cells arranged end-to-end. Their walls are strengthened with lignin, which provides rigidity and waterproofing. Lignification patterns include annular, spiral, and reticulate thickenings. Xylem also contains tracheids and fibres. The primary function is the transport of water and dissolved mineral ions from roots to leaves. Adaptations include: no cytoplasm or organelles to impede flow; pits in walls that allow lateral movement; and narrow diameter to facilitate capillarity.
木质部导管是由死细胞首尾相连形成的长而连续的空心管。其细胞壁因木质素而增强，提供刚性并防水。木质化型式包括环纹、螺纹和网纹加厚。木质部还含有管胞和纤维。其主要功能是将水和溶解的矿物离子从根部运输到叶片。适应特征包括：无细胞质或细胞器阻碍液流；壁上有纹孔允许侧向运输；直径狭窄以利于毛细作用。
3. Transpiration and the Transpiration Stream | 蒸腾作用与蒸腾流
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. It creates a negative pressure (tension) in the leaf, which pulls water up the xylem in a continuous transpiration stream. This process is a passive transport mechanism driven by solar energy. The rate of transpiration depends on environmental conditions.
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。它在叶片中产生负压（张力），将水连续不断地沿蒸腾流拉上木质部。该过程是由太阳能驱动的被动运输机制。蒸腾速率取决于环境条件。
4. The Cohesion-Tension Theory | 内聚力-张力理论
The cohesion-tension theory explains water ascent in xylem. Cohesion: water molecules are polar and form hydrogen bonds with each other, creating a strong cohesive force. Tension: transpiration from leaves generates a negative pressure at the top of the xylem, pulling the water column upward. Adhesion: water molecules also adhere to the hydrophilic xylem walls (capillarity). This combined mechanism can lift water many metres against gravity. Evidence includes: diameter of tree trunks decreases during the day when tension is high; cutting a xylem vessel allows water to recede; and stable isotopic studies confirm continuous columns.
内聚力-张力理论解释了木质部中水分的上升。内聚力：水分子具有极性，彼此间形成氢键，产生强大的内聚力。张力：叶片蒸腾在木质部顶端产生负压，拉拽水柱向上。附着力：水分子还会附着在亲水性木质部管壁上（毛细作用）。这种综合机制可将水提升数米以对抗重力。证据包括：白天张力高时树干直径减小；切断木质部导管水会缩回；稳定同位素研究证实水柱的连续性。
5. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
The main factors are light intensity, temperature, humidity, and air movement (wind). Increased light causes stomata to open, enhancing transpiration. Higher temperature increases the kinetic energy of water molecules, speeding up evaporation and diffusion. Lower humidity increases the water vapour concentration gradient, raising transpiration. Wind removes humid air from around stomata, maintaining a steep gradient. Use a potometer to measure water uptake as a proxy for transpiration rate.
主要因素有光照强度、温度、湿度和空气流动（风）。光照增强导致气孔开放，促进蒸腾。温度升高会增加水分子的动能，加速蒸发和扩散。湿度降低增大了水蒸气浓度梯度，提高蒸腾速率。风将气孔周围的湿空气带走，维持较陡的梯度。可使用蒸腾计测量吸水量作为蒸腾速率的指标。
Transpiration rate = Water uptake volume ÷ Time
蒸腾速率 = 吸水量 ÷ 时间
6. Measuring Transpiration: Potometers | 测量蒸腾作用：蒸腾计
A potometer measures the rate of water uptake by a cut shoot, which approximates transpiration rate when conditions are controlled. The bubble potometer tracks an air bubble’s movement in a capillary tube. The mass potometer measures weight loss due to evaporation. Students must record distance moved, cross-sectional area, and time to calculate rate. Key precautions: cut shoot underwater to prevent air embolism; ensure airtight seals; allow time for acclimatisation; and keep all environmental variables constant except the one being tested.
蒸腾计测量切离枝条的吸水速率，在控制条件下可近似代表蒸腾速率。气泡蒸腾计通过毛细管中气泡的移动来测量。质量蒸腾计测量因蒸发导致的质量减少。学生必须记录移动距离、横截面积和时间以计算速率。关键注意事项：在水下剪取枝条以防空气栓塞；确保密封；给予适应时间；除测试变量外，保持所有环境条件恒定。
7. Phloem: Structure and Translocation | 韧皮部：结构与转运
Phloem transports organic solutes, primarily sucrose, from sources (e.g. mature leaves) to sinks (e.g. roots, fruits). It consists of sieve tube elements and companion cells. Sieve tubes are living cells with reduced cytoplasm, lacking nucleus and ribosomes, connected end-to-end via sieve plates with large pores. Companion cells are metabolically active, providing ATP for active loading of sucrose into sieve tubes. Plasmodesmata link companion cells to sieve tube elements.
韧皮部将有机溶质（主要是蔗糖）从源（如成熟叶片）运输到库（如根、果实）。它由筛管分子和伴胞组成。筛管是活细胞，细胞质减少，缺少细胞核和核糖体，通过筛板及其大孔首尾相连。伴胞代谢活跃，为蔗糖主动装载进入筛管提供 ATP。胞间连丝连接伴胞与筛管分子。
8. The Pressure-Flow (Mass Flow) Hypothesis | 压力流动（集流）假说
Also known as the mass flow hypothesis, it proposes: (1) active loading of sucrose from source cells into sieve tubes at the source reduces water potential (ψ). (2) Water enters by osmosis from adjacent xylem, increasing hydrostatic pressure. (3) At the sink, sucrose is actively unloaded, raising water potential. (4) Water leaves the phloem by osmosis, decreasing pressure. The pressure gradient drives bulk flow of phloem sap from source to sink. Evidence: aphid stylets exude sap when inserted into sieve tubes; radioactive tracers follow sucrose movement; the process requires metabolic energy for loading/unloading.
也称为集流假说，其提出：（1）在源端，蔗糖从源细胞主动装载到筛管，降低水势 (ψ)。（2）水分通过渗透从邻近的木质部进入，使静水压力升高。（3）在库端，蔗糖被主动卸载，水势升高。（4）水分因渗透离开韧皮部，压力下降。压力梯度驱动韧皮部汁液从源向库的集流。证据：蚜虫口针刺入筛管后会渗出汁液；放射性示踪剂跟踪蔗糖移动；该过程需要代谢能量进行装载和卸载。
Pressure gradient (ΔP) → Bulk flow from source to sink
压力梯度 (ΔP) → 从源到库的集流
9. Source-Sink Relationships and Examples | 源-库关系与例子
A source is any plant organ that produces or releases sugars (e.g. mature leaf via photosynthesis, storage organ during germination). A sink is any organ that consumes or stores sugars (e.g. growing root tip, developing fruit). The relationship can change: a young leaf acts as sink, becoming source as it matures. In spring, storage roots (e.g. carrot) are sources providing sugars for new shoot growth; in autumn, roots become sinks storing carbohydrates. Companion cell transport proteins facilitate symplastic or apoplastic loading.
源是任何产生或释放糖类的植物器官（如通过光合作用的成熟叶、萌发期间的储存器官）。库是任何消耗或储存糖类的器官（如生长根尖、发育中的果实）。关系可以变化：幼叶作为库，成熟后变为源。春季，储存根（如胡萝卜）作为源为新生枝条提供糖分；秋季，根又变为库储存碳水化合物。伴胞转运蛋白协助共质体或质外体装载。
10. Comparison of Xylem and Phloem | 木质部与韧皮部比较
The table below compares key features of xylem and phloem.
| Feature | Xylem | Phloem |
|---|---|---|
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