📚 IB CIE Biology: Plant Transport – Key Exam Points | IB CIE 生物：植物运输考点精讲
Plant transport systems are essential for moving water, mineral ions, and the products of photosynthesis between different organs. In IB and CIE examinations, understanding the structure-function relationships in xylem and phloem, as well as the mechanisms driving long-distance transport, forms a core part of the syllabus. This article breaks down the most important concepts, theories, and common exam pitfalls to help you consolidate your revision effectively.
植物运输系统对水、矿物质离子以及光合作用产物在不同器官之间的运输至关重要。在IB和CIE考试中，理解木质部和韧皮部的结构-功能关系，以及驱动长距离运输的机制，是课程的核心部分。本文梳理了最重要的概念、理论和常见考试陷阱，帮助你高效巩固复习。
1. Introduction to Plant Transport Systems | 植物运输系统概述
Plants possess two major vascular tissues: xylem and phloem. Together they form vascular bundles that run through roots, stems, and leaves. The xylem transports water and dissolved mineral ions from the roots upward, while the phloem distributes organic assimilates, mainly sucrose and amino acids, from sources to sinks. These tissues are adapted for mass flow over long distances without the need for a pumping heart, relying instead on physical and osmotic forces.
植物拥有两种主要的维管组织：木质部和韧皮部。它们共同构成贯穿根、茎、叶的维管束。木质部将水分和溶解的矿物质离子由根部向上运输，而韧皮部则将有机同化物（主要是蔗糖和氨基酸）从源运送到库。这些组织适应于长距离的集流运输，无需类似心脏的泵，而是依赖物理和渗透力量。
2. Xylem Structure and Function | 木质部结构与功能
Xylem tissue contains tracheids and vessel elements, both of which are dead at maturity and have lignified cell walls. This lignin provides strength and prevents collapse under the negative pressure generated during transpiration. Vessel elements are aligned end-to-end, with perforated end walls, forming continuous tubes. Bordered pits in the lateral walls allow sideways movement of water. The absence of cytoplasm and organelles means there is no resistance to water flow.
木质部组织包含管胞和导管分子，两者在成熟时均为死细胞，并具有木质化的细胞壁。木质素提供强度并防止在蒸腾作用产生的负压下坍塌。导管分子首尾相连，端壁穿孔，形成连续的管道。侧壁上的具缘纹孔允许水分的侧向移动。没有细胞质和细胞器意味着水流无阻力。
In stems and roots, xylem is usually found closer to the centre, providing structural support as well as transport. In leaves, xylem forms part of the leaf veins, bringing water close to mesophyll cells for photosynthesis. The adhesion of water to the xylem walls, combined with the narrow lumen of the vessels, is critical for maintaining continuous water columns.
在茎和根中，木质部通常位于更靠近中心的位置，同时提供结构支撑和运输。在叶片中，木质部构成叶脉的一部分，将水带到叶肉细胞附近以供光合作用。水对木质部管壁的附着力，以及导管狭窄的管腔，对于维持连续水柱至关重要。
3. The Cohesion-Tension Theory | 内聚力-张力理论
The cohesion-tension theory explains how water moves up a plant against gravity. Transpiration from leaf mesophyll cells creates a water potential gradient that pulls water out of xylem veins. This pull generates a negative pressure (tension) at the top of the water column. Because water molecules are cohesive due to hydrogen bonding, the entire column from root to leaf is dragged upward as a continuous chain. There is no break in the column under normal conditions.
内聚力-张力理论解释了水如何克服重力向上运输。叶肉细胞的蒸腾作用产生水势梯度，将水从叶脉的木质部中拉出。这种拉力在水柱顶端产生负压（张力）。由于水分子间因氢键而具有内聚力，从根到叶的整个水柱便像一根连续的链条被向上拖拽。正常情况下水柱不会断裂。
The theory is supported by experimental evidence: diurnal changes in trunk diameter (shrinking during the day, expanding at night), and the fact that cutting a stem underwater prevents air entry and maintains flow. The tensile strength of water is sufficient to withstand the forces involved in tall trees.
该理论得到实验证据的支持：树干直径的昼夜变化（白天收缩、夜间膨胀），以及在水下切割茎部可防止空气进入并维持流动。水的抗张强度足以承受高大树木中涉及的力量。
4. Transpiration and Its Regulation | 蒸腾作用及其调节
Transpiration is the evaporation of water from the aerial parts of a plant, primarily through stomata in leaves. It is both a necessary evil—it drives the transpiration stream and cools the leaf, but can lead to excessive water loss. Plants control transpiration mainly by opening and closing stomata, regulated by guard cells. When guard cells are turgid, stomata open; when flaccid, they close.
蒸腾作用是水分从植物地上部分（主要通过叶片气孔）蒸发的过程。它是一件必需的麻烦事——它驱动蒸腾流并冷却叶片，但可能导致过度失水。植物主要通过保卫细胞调控气孔的开闭来控制蒸腾作用。保卫细胞膨压高时，气孔张开；萎蔫时，气孔关闭。
Abscisic acid (ABA) is a key hormone involved in stomatal closure during water stress. Other factors such as light, carbon dioxide concentration, and circadian rhythms also influence stomatal aperture. In many xerophytes, adaptations like sunken stomata, thick cuticles, and rolled leaves reduce transpiration.
脱落酸（ABA）是水分胁迫下参与气孔关闭的关键激素。其他因素如光照、二氧化碳浓度和昼夜节律也影响气孔开度。在许多旱生植物中，凹陷的气孔、厚的角质层和卷曲的叶片等适应性特征可减少蒸腾作用。
5. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
Four main environmental factors influence the rate of transpiration: light intensity, temperature, humidity, and air movement. Increasing light usually opens stomata, raising transpiration. Higher temperature increases the kinetic energy of water molecules and the water-holding capacity of air, so evaporation accelerates. High humidity reduces the water potential gradient between leaf and atmosphere, slowing down loss. Wind removes saturated air near the leaf surface, maintaining a steep diffusion gradient.
四个主要环境因素影响蒸腾速率：光照强度、温度、湿度和空气流动。增加光照通常使气孔张开，提高蒸腾作用。温度升高增加水分子的动能和空气的持水能力，因此蒸发加速。高湿度降低了叶片与大气之间的水势梯度，减缓水分损失。风带走叶片表面的饱和空气，维持陡峭的扩散梯度。
In exam questions, you may be asked to interpret graphs from a potometer. Remember that a potometer measures water uptake, not transpiration rate directly, but under most conditions the two are proportional. Constant volume of water lost corresponds to the slope of the air bubble movement.
在考试问题中，你可能需要解释蒸腾计图表。记住蒸腾计测量的是水分吸收量，而非直接测量蒸腾速率，但在大多数条件下两者成正比。恒定失水量对应气泡移动的斜率。
6. Water Uptake by Roots | 根系吸水
Water enters root hair cells by osmosis because the soil water has a higher water potential than the root hair cytoplasm. Once inside the root, water moves across the cortex to the xylem via three pathways: the apoplast pathway (through cell walls), the symplast pathway (through cytoplasm and plasmodesmata), and the vacuolar pathway (through vacuoles). At the endodermis, the Casparian strip blocks the apoplast pathway, forcing water and mineral ions to cross the plasma membrane, controlled by the symplast route.
水因土壤水势高于根毛细胞质的水势而通过渗透作用进入根毛细胞。进入根部后，水通过三条途径穿过皮层到达木质部：质外体途径（通过细胞壁）、共质体途径（通过细胞质和胞间连丝）和液泡途径（通过液泡）。在内皮层，凯氏带阻断质外体途径，迫使水和矿物离子通过细胞膜，受共质体途径控制。
Root pressure may push water up the xylem when transpiration is low, but it is not the main driving force in tall plants. Active transport of ions into the xylem generates a low water potential that draws water in. Guttation, the exudation of liquid droplets at leaf margins, is evidence of root pressure.
当蒸腾作用低时，根压可能推动水在木质部中上升，但它不是高大植物中的主要驱动力。离子向木质部的主动运输产生低水势，将水吸入。吐水，即叶缘渗出液滴，是根压存在的证据。
7. Phloem Structure and Function | 韧皮部结构与功能
Phloem is composed of sieve tube elements and companion cells. Sieve tube elements are living cells that lose most organelles but retain a functional plasma membrane. They are connected by sieve plates with large pores to form a continuous cytoplasmic tube. Each sieve tube element is closely associated with one or more companion cells, which contain many mitochondria to provide ATP for active loading of solutes.
韧皮部由筛管分子和伴胞组成。筛管分子是活细胞，失去大多数细胞器但保留功能性细胞膜。它们通过具大孔的筛板相连，形成连续的细胞质管道。每个筛管分子与一个或多个伴胞紧密相连，伴胞含有大量线粒体，为溶质的主动装载提供ATP。
In some species, transfer cells with wall ingrowths increase the surface area for transport. The phloem also contains fibres and parenchyma. The arrangement of phloem relative to xylem varies between organs: in roots they alternate, in stems they are often external to xylem, and in leaves they lie below xylem in veins.
在某些物种中，转移细胞具有细胞壁内突，增加了运输的表面积。韧皮部还包含纤维和薄壁组织。韧皮部与木质部的相对排列因器官而异：在根中它们交替排列，在茎中常位于木质部外侧，在叶中则位于叶脉木质部下方。
8. The Pressure-Flow Hypothesis | 压力流动假说
The pressure-flow (or mass flow) hypothesis explains the movement of sugars in the phloem. At the source (e.g., mature leaf), sucrose is actively loaded into sieve tubes by companion cells. This decreases water potential, causing water to enter from xylem by osmosis. The increase in hydrostatic pressure forces the phloem sap to flow toward regions of lower pressure—the sinks (e.g., roots, fruits, meristems). At the sink, sucrose is unloaded actively or passively, and water follows, reducing pressure.
压力流动（或集流）假说解释了糖类在韧皮部中的移动。在源（如成熟叶片），蔗糖通过伴胞被主动装载到筛管中。这降低了水势，导致水分通过渗透作用从木质部进入。静水压力增加迫使韧皮部汁液流向低压区域——库（如根、果实、分生组织）。在库端，蔗糖被主动或被动卸出，水分随之移动，压力降低。
Experiments with aphid stylets have confirmed the presence of high turgor pressure in sieve tubes and the composition of phloem sap. Radioactive carbon-14 labelling can trace the movement of assimilates. The hypothesis requires that sieve plates do not obstruct flow significantly, and the bidirectional movement sometimes observed can be explained by different sources supplying different sinks.
利用蚜虫口针的实验证实了筛管内高膨压的存在以及韧皮部汁液的成分。放射性碳-14标记可追踪同化物的移动。该假说要求筛板不会显著阻碍流动，而有时观察到的双向流动可用不同源供应不同库来解释。
9. Translocation of Organic Solutes | 有机溶质的运输
Translocation refers to the transport of soluble organic compounds, primarily sucrose, in the phloem. Sucrose is the main transport sugar because it is non-reducing and relatively unreactive, so it does not interfere with metabolic processes. In some plants, other carbohydrates like raffinose or sugar alcohols are translocated. Amino acids and plant hormones are also moved through the phloem.
运输是指可溶性有机化合物（主要是蔗糖）在韧皮部中的转运。蔗糖是主要的运输糖，因为它非还原性且相对不活泼，因此不会干扰代谢过程。在一些植物中，其他碳水化合物如棉子糖或糖醇也被运输。氨基酸和植物激素也通过韧皮部移动。
Phloem unloading can be symplastic or apoplastic. In developing seeds, a specialized transfer tissue often mediates unloading. The direction of translocation changes seasonally: in spring, stored starch in roots is converted to sucrose and moved to developing buds, while in summer, photosynthates move from leaves to storage organs.
韧皮部卸出可以是共质体或质外体途径。在发育中的种子中，常有专门的转移组织介导卸出。运输方向随季节变化：春季，根中储存的淀粉转化为蔗糖运往发育中的芽，而夏季则是由叶片向储藏器官运输光合产物。
10. Mineral Ion Uptake and Transport | 矿质离子的吸收与运输
Mineral ions such as nitrates (NO₃⁻), phosphates (PO₄³⁻), potassium (K⁺), and magnesium (Mg²⁺) are absorbed from the soil by active transport into root hairs, often using proton pumps and co-transporters. Once in the symplast, they diffuse toward the xylem, where they are actively secreted into the xylem vessels. This lowers the water potential in the xylem, aiding water uptake. The flow of water then carries the ions upward in the transpiration stream.
矿质离子如硝酸盐（NO₃⁻）、磷酸盐（PO₄³⁻）、钾离子（K⁺）和镁离子（Mg²⁺）通过主动运输从土壤进入根毛细胞，常利用质子泵和共转运蛋白。进入共质体后，它们扩散向木质部，在那里被主动分泌到木质部导管中。这降低了木质部的水势，帮助吸水。水流随后在蒸腾流中将这些离子向上携带。
Some mineral deficiencies cause distinctive symptoms: magnesium deficiency leads to chlorosis because it is a component of chlorophyll, while nitrate deficiency causes stunted growth and yellowing due to reduced protein synthesis. The transport of ions is selective, and root cells can control the uptake through proton gradients.
某些矿物质缺乏症引起特征性症状：缺镁导致黄化，因为它是叶绿素的组分；缺氮则由于蛋白质合成减少导致生长受阻和叶片变黄。离子的运输具有选择性，根细胞可通过质子梯度控制吸收。
11. Comparison of Xylem and Phloem | 木质部与韧皮部比较
It is critical to distinguish the two vascular tissues in exam questions. The table below summarises the key differences.
考试中区分两种维管组织至关重要。下表总结了关键区别。
| Feature 特征 | Xylem 木质部 | Phloem 韧皮部 |
|---|---|---|
| Main conducting cells 主要输导细胞 | Tracheids, vessel elements (dead) 管胞、导管分子（死细胞） | Sieve tube elements (living) 筛管分子（活细胞） |
| Direction of flow 流动方向 | Upward mainly (roots to shoots) 主要向上（根到茎） | Bidirectional (source to sink) 双向（源到库） |
| Transported substances 运输物质 | Water, mineral ions 水、矿质离子 | Sucrose, amino acids, hormones 蔗糖、氨基酸、激素 |
| Driving force 驱动力 | Cohesion-tension (transpiration pull) 内聚力-张力（蒸腾拉力） | Pressure flow (hydrostatic pressure gradient) 压力流动（静水压梯度） |
| Cell wall lignification 胞壁木质化 | Extensive (strong support) 广泛（强支撑） | No lignification (flexible) 无木质化（有弹性） |
| Companion cells 伴胞 | Absent 无 | Present, metabolically active 有，代谢活跃 |
Being able to annotate diagrams of stem cross-sections showing the position of xylem and phloem is a common exam requirement. Remember that in roots the xylem is often star-shaped and phloem lies between the arms; in stems, vascular bundles are arranged in a ring in dicots, with xylem interior to phloem.
能够标注显示木质部和韧皮部位置的茎横截面图是常见的考试要求。记住在根中木质部常呈星状，韧皮部位于臂间；在茎中，双子叶植物的维管束呈环状排列，木质部在内，韧皮部在外。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Many students confuse the direction of transport in xylem and phloem. Always refer to xylem as ‘unidirectional from roots to leaves’ and phloem as ‘bidirectional from sources to sinks’. Do not say water is pumped by the root under normal conditions; the main driving force is transpiration pull. Use precise terminology: water potential, hydrostatic pressure, cohesion, adhesion, stomatal conductance.
许多学生混淆了木质部和韧皮部的运输方向。始终将木质部描述为‘从根到叶的单向运输’，韧皮部为‘从源到库的双向运输’。不要说水在正常条件下由根部泵出；主要驱动力是蒸腾拉力。使用精确术语：水势、静水压力、内聚力、附着力、气孔导度。
When explaining the cohesion-tension theory, link the evaporation from mesophyll cell walls to the curvature of water menisci in cellulose microfibrils, which generates the low water potential needed to pull water from xylem. State that the water column is under tension, not pressure. In pressure-flow questions, explain loading at the source, osmotic movement of water, and unloading at the sink in a logical sequence.
解释内聚力-张力理论时，将叶肉细胞壁的蒸发与纤维素微纤丝中水弯月面的曲率联系起来，这产生从木质部拉水所需的低水势。说明水柱处于张力而非压力之下。在压力流动问题中，按逻辑顺序解释源端的装载、水的渗透移动以及库端的卸出。
Don’t forget to mention that symplastic loading may involve polymer traps in some species, and that apoplastic loading requires sucrose-H⁺ cotransporters. Also, be ready to interpret experimental data from ringing or girdling experiments—these show that removal of bark (phloem) causes swelling above the cut due to accumulation of sugars, confirming phloem’s role in downward translocation.
不要忘记提到在某些物种中共质体装载可能涉及多聚体陷阱，质外体装载需要蔗糖-H⁺共转运蛋白。同时，准备好解释环剥实验的数据——这些实验表明去除树皮（韧皮部）导致切口上方因糖类积累而肿胀，证实了韧皮部在向下运输中的作用。
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