A-Level生物植物运输系统蒸腾韧皮部

1. 为什么植物需要运输系统 Why Plants Need Transport Systems

植物作为多细胞生物，无法仅靠扩散来满足所有细胞的物质需求。高大的树木从根部吸收水分和矿物质，但这些物质需要被输送到数十米高的叶片中进行光合作用：同样，叶片制造的糖类也必须被分配到根、茎、果实等不能进行光合作用的部位。运输系统（维管系统）解决了这一问题。

As multicellular organisms, plants cannot rely on diffusion alone to meet the material needs of all their cells. Tall trees absorb water and minerals through their roots, but these substances must be transported tens of metres upward to the leaves for photosynthesis; similarly, sugars produced in the leaves must be distributed to roots, stems, fruits, and other non-photosynthetic tissues. The transport system (vascular system) solves this problem.

2. 木质部的结构与功能 Xylem Structure and Function

木质部负责将水分和溶解的矿物质从根部向上运输到植物的地上部分。木质部导管由死细胞构成，细胞壁加厚并木质化（含有木质素），端壁完全消失形成连续的管道。木质素沉积形成环状、螺旋状或网状图案，既提供结构支撑防止导管塌陷，又保持一定的柔韧性。

Xylem is responsible for transporting water and dissolved minerals upward from the roots to the aerial parts of the plant. Xylem vessels are composed of dead cells with thickened, lignified cell walls (containing lignin), and the end walls are completely broken down to form continuous tubes. Lignin deposition forms ring, spiral, or reticulate patterns, which provide structural support to prevent vessel collapse while maintaining some flexibility.

3. 蒸腾作用与内聚力-张力理论 Transpiration and the Cohesion-Tension Theory

蒸腾作用是水分以水蒸气的形式从叶片气孔散失的过程，为木质部中水分的向上运输提供了驱动力。当水分从叶肉细胞表面蒸发时，细胞的水势降低，从邻近细胞中吸取水分，这种拉力通过木质部水柱的连续性一直传递到根部。水分子的内聚力（氢键）使水柱在张力下不会断裂。

Transpiration is the loss of water vapour from leaf stomata, and it provides the driving force for the upward movement of water in the xylem. As water evaporates from the surfaces of mesophyll cells, the water potential of those cells drops, drawing water from neighbouring cells; this tension is transmitted all the way down to the roots through the continuous column of water in the xylem. The cohesion of water molecules (hydrogen bonding) prevents the water column from breaking under tension.

4. 影响蒸腾速率的因素 Factors Affecting Transpiration Rate

光照强度是影响蒸腾速率的主要因素：光促使气孔开放，增加气体交换，加速水分蒸发。温度升高会增加叶片内部的水蒸气浓度梯度（因为温暖空气能容纳更多水蒸气），同时加快水分子的动能，两者都加速蒸腾。风速带走叶片周围的潮湿空气层，维持陡峭的浓度梯度。湿度则相反：高湿度降低浓度梯度，减缓蒸腾。

Light intensity is a major factor affecting transpiration rate: light stimulates stomatal opening, increases gas exchange, and accelerates water evaporation. Higher temperatures increase the water vapour concentration gradient between the leaf interior and the external air (because warm air can hold more water vapour) while also increasing the kinetic energy of water molecules ： both accelerate transpiration. Wind removes the humid air layer surrounding the leaf, maintaining a steep concentration gradient. Humidity has the opposite effect: high humidity reduces the concentration gradient and slows transpiration.

5. 气孔开闭机制 Stomatal Opening and Closing Mechanism

气孔由一对保卫细胞包围。在光照条件下，保卫细胞通过主动运输（使用ATP）积累钾离子（K⁺），降低细胞的水势，使水分通过渗透作用进入保卫细胞。保卫细胞膨胀后，由于其细胞壁不均匀加厚（内侧较厚），细胞向外弯曲，打开气孔。在黑暗或水分胁迫条件下，钾离子被泵出，水分流失，保卫细胞松弛，气孔关闭。

Stomata are surrounded by a pair of guard cells. Under light conditions, guard cells actively transport potassium ions (K⁺) inward using ATP, lowering the water potential of the cells so that water enters by osmosis. As guard cells become turgid, they bend outward due to uneven cell wall thickening (thicker on the inner side), opening the stomatal pore. In darkness or under water stress, potassium ions are pumped out, water is lost, guard cells become flaccid, and the stomata close.

6. 韧皮部的结构与功能 Phloem Structure and Function

韧皮部负责将光合作用产物（主要是蔗糖）从”源”（source，如成熟叶片）运输到”库”（sink，如根尖、发育中的果实、储存器官）。韧皮部由筛管和伴胞组成：筛管是由活细胞连接而成的管道，端壁上有筛孔允许物质通过；伴胞紧邻筛管，含有大量线粒体，为蔗糖的主动装载提供ATP能量。

Phloem is responsible for transporting the products of photosynthesis (mainly sucrose) from “sources” (e.g., mature leaves) to “sinks” (e.g., root tips, developing fruits, storage organs). Phloem consists of sieve tubes and companion cells: sieve tubes are pipelines formed by living cells connected end to end, with sieve plates (perforated end walls) that allow substances to pass through; companion cells sit adjacent to sieve tubes, contain abundant mitochondria, and provide the ATP energy needed for active loading of sucrose.

7. 韧皮部运输的压力流动假说 Pressure Flow Hypothesis for Phloem Transport

在”源”端（如叶片），蔗糖通过主动运输被装载到筛管中，降低了筛管的水势，使水分从邻近的木质部通过渗透作用进入筛管，产生高静水压力。蔗糖的装载涉及质子共转运（蔗糖-H⁺共转运蛋白），伴胞利用ATP通过质子泵建立质子梯度，驱动蔗糖逆浓度梯度进入筛管。在”库”端，蔗糖被主动卸载并迅速转化为淀粉或用于呼吸，筛管的水势升高，水分通过渗透作用回到木质部，静水压力降低。源端和库端之间的压力梯度驱动筛管内的溶液从高压区流向低压区，这一过程称为压力流动。

At the source (e.g., leaf), sucrose is actively loaded into the sieve tube, lowering its water potential so that water enters from the adjacent xylem by osmosis, generating a high hydrostatic pressure. Sucrose loading involves proton co-transport (sucrose-H⁺ symporters), where companion cells use ATP to pump protons out, establishing a proton gradient that drives sucrose into the sieve tube against its concentration gradient. At the sink, sucrose is actively unloaded and rapidly converted to starch or used in respiration; the water potential of the sieve tube rises, water moves back into the xylem by osmosis, and the hydrostatic pressure drops. The pressure gradient between source and sink drives the bulk flow of solution through the sieve tubes from the high-pressure zone to the low-pressure zone ： this is the pressure flow mechanism.

8. 放射性示踪物与环割实验证据 Evidence from Radioactive Tracers and Ringing Experiments

使用碳-14（¹⁴C）标记的二氧化碳进行的示踪实验为韧皮部运输提供了直接证据：植物在含有¹⁴CO₂的环境中光合作用后，放射性自显影显示标记的蔗糖出现在韧皮部而非木质部中。环割实验（剥去一圈树皮，移除韧皮部但保留木质部）导致环割上方膨大，因为糖类无法向下运输而在切口上方积累，证明韧皮部负责有机物的运输。

Tracer experiments using carbon-14 (¹⁴C) labelled carbon dioxide provide direct evidence for phloem transport: after a plant photosynthesises in an atmosphere containing ¹⁴CO₂, autoradiography reveals that labelled sucrose appears in the phloem, not the xylem. Ringing experiments (removing a ring of bark, which strips away the phloem while leaving the xylem intact) cause swelling above the ring because sugars cannot be transported downward and accumulate above the cut ： demonstrating that phloem is responsible for organic solute transport.

9. 木质部与韧皮部的比较 Comparing Xylem and Phloem

木质部运输水分和矿物质，方向为单向（从根向上），由死细胞（导管和管胞）组成，细胞壁含木质素，运输机制是被动的（依赖蒸腾拉力），导管直径较宽（15-200μm），流速较快。韧皮部运输蔗糖和氨基酸，方向为双向（从源到库），由活细胞（筛管和伴胞）组成，运输机制需要能量（主动装载和卸载），筛管直径较窄（10-30μm），流速较慢但由压力梯度驱动。两个系统共同构成维管束，在茎和根中通常排列在一起，其中木质部位于内侧，韧皮部位于外侧。

Xylem transports water and minerals, direction is unidirectional (upward from roots), composed of dead cells (vessels and tracheids), cell walls contain lignin, and the transport mechanism is passive (driven by transpiration pull). Vessel diameter is relatively wide (15-200 μm), allowing faster flow rates. Phloem transports sucrose and amino acids, direction is bidirectional (from source to sink), composed of living cells (sieve tubes and companion cells), and the transport mechanism requires energy (active loading and unloading). Sieve tube diameter is narrower (10-30 μm), flow is slower but pressure-driven. Together, these two systems form vascular bundles, which are typically arranged together in stems and roots, with xylem positioned on the inner side and phloem on the outer side.

10. 旱生植物与水生植物的适应性 Xerophyte and Hydrophyte Adaptations

旱生植物（如仙人掌、马兰草）演化出多种适应性以减少水分流失：叶片退化为刺（减少表面积）、角质层特别厚、气孔深陷于凹坑中并常被毛状体覆盖以捕捉潮湿空气、以及发达的储水组织。某些旱生植物具有景天酸代谢（CAM），夜间开放气孔固定CO₂，白天关闭气孔以减少蒸腾。这些适应性使旱生植物能在干旱环境中生存。

Xerophytes (e.g., cacti, marram grass) have evolved multiple adaptations to reduce water loss: leaves reduced to spines (minimising surface area), particularly thick cuticles, stomata sunken into pits often covered by trichomes (hairs) that trap humid air, and well-developed water storage tissues. Some xerophytes use Crassulacean Acid Metabolism (CAM), opening stomata at night to fix CO₂ and closing them during the day to minimise transpiration. These adaptations enable xerophytes to survive in arid environments.

11. 水生植物的适应性 Hydrophyte Adaptations

水生植物（如睡莲、金鱼藻）面临相反的问题：需要确保气体交换并保持浮力。它们的叶片宽大而薄，具有发达的气室（通气组织）提供浮力并储存氧气，气孔仅分布在上表皮（因为下表皮浸没在水中），角质层很薄或无角质层（因为不需要防止水分流失），根系通常退化，因为水分和矿物质可直接从周围水体中吸收。

Hydrophytes (e.g., water lilies, hornwort) face the opposite challenge: they must ensure gas exchange and maintain buoyancy. Their leaves are broad and thin, with well-developed air spaces (aerenchyma) providing buoyancy and storing oxygen, stomata are restricted to the upper epidermis (since the lower epidermis is submerged), the cuticle is thin or absent (no need to prevent water loss), and roots are often reduced because water and minerals can be absorbed directly from the surrounding water.

Exam Tips 考试技巧

在解释内聚力-张力理论时，一定要按顺序提及三个关键概念：蒸腾作用产生拉力、水分子的内聚力防止水柱断裂、以及木质部导管的小直径对毛细作用的贡献。描述韧皮部运输时，避免说”糖类向下运输”：应使用”从源到库”这一正确的双向概念，并准确使用”蔗糖”而非笼统的”糖”。

When explaining the cohesion-tension theory, always mention the three key concepts in order: transpiration generates the pulling force, cohesion of water molecules prevents the column from breaking, and the narrow diameter of xylem vessels contributes to capillarity. When describing phloem transport, avoid saying “sugars move downward” ： use the correct bidirectional concept of “source to sink” and be precise with the term “sucrose” rather than the generic “sugar.”

Key Bilingual Terms 关键双语术语

A-Level生物 植物运输系统 蒸腾 韧皮部