A-Level OCR Biology: Transport in Plants | A-Level OCR 生物：植物运输 考点精讲

📚 A-Level OCR Biology: Transport in Plants | A-Level OCR 生物：植物运输 考点精讲

Plants, like all multicellular organisms, require efficient transport systems to move water, mineral ions, and the products of photosynthesis between different parts of the organism. Unlike animals, plants have two distinct vascular tissues: xylem and phloem. This revision guide covers the key OCR A-Level Biology concepts for transport in plants, including the structure and function of xylem and phloem, the mechanisms of water movement (transpiration, cohesion-tension theory), and the translocation of sugars (mass flow hypothesis). Understanding these processes is essential for success in both the AS and A-level examinations.

植物与所有多细胞生物一样，需要高效的运输系统在不同部位之间输送水分、矿质离子和光合作用产物。与动物不同，植物拥有两种截然不同的维管组织：木质部和韧皮部。本考点精讲涵盖了 A-Level OCR 生物学植物运输的关键概念，包括木质部和韧皮部的结构与功能、水分运输机制（蒸腾作用、内聚力-张力理论）以及糖类的转运（压力流动假说）。理解这些过程对于在 AS 和 A-level 考试中取得好成绩至关重要。

1. Introduction to Transport Systems in Plants | 植物运输系统简介

In plants, xylem transports water and dissolved minerals from the roots to the leaves, while phloem transports organic solutes (mainly sucrose) from sources (e.g., leaves) to sinks (e.g., roots, growing tips). These vascular tissues are arranged in vascular bundles, which differ in distribution between stems and roots. In herbaceous dicots, vascular bundles in the stem are arranged in a ring, whereas in roots they form a central cylinder. The need for transport systems arises because diffusion alone cannot move substances over large distances within a multicellular body. Additionally, plants lose water continuously through stomata during gas exchange for photosynthesis, requiring a constant upward supply.

植物中，木质部将水和溶解矿物质从根部输送到叶片，而韧皮部将有机溶质（主要是蔗糖）从源（例如叶片）运送到库（例如根和生长点）。这些维管组织排列成维管束，在茎和根中的分布不同。在草本双子叶植物中，茎的维管束呈环状排列，而在根中则形成中央柱。运输系统的出现是因为单靠扩散无法在多细胞生物体内长距离运输物质。此外，植物在通过气孔进行光合作用气体交换时会不断散失水分，这需要持续向上的水分供应。

2. Structure of Xylem Vessels | 木质部导管的结构

Xylem vessels are long, hollow tubes made of dead cells arranged end-to-end. The cell walls are thickened with lignin, which provides structural support and prevents collapse under tension. Lignification can be spiral, annular, or reticulate, allowing flexibility in young parts. There are no end walls between vessel elements, forming a continuous tube. Pit openings remain in the lignified walls to allow lateral movement of water between adjacent vessels. Xylem also contains tracheids – long, thin cells with tapered ends and bordered pits, which also conduct water but are less efficient. Both vessel elements and tracheids have no cytoplasm or organelles at maturity, reducing resistance to water flow.

木质部导管是由死细胞首尾相连形成的长中空管。细胞壁因木质素的沉积而加厚，这不仅提供结构支持，还能防止导管在张力下塌陷。木质化的模式有螺旋状、环状和网状，使幼嫩部分具有弹性。导管分子之间没有端壁，形成连续的管道。木质化壁上的纹孔允许水分在相邻导管间横向移动。木质部还含有管胞——末端渐尖、具缘纹孔的长薄细胞，也能输导水分但效率较低。导管分子和管胞在成熟后均无细胞质或细胞器，从而降低水流阻力。

3. Structure of Phloem Sieve Tubes | 韧皮部筛管的结构

Phloem is composed of sieve tube elements and companion cells. Sieve tube elements are living cells that form long tubes for solute transport. They lack a nucleus and many organelles to reduce resistance to flow, but remain alive. Their end walls are perforated to form sieve plates, allowing cytoplasmic continuity. Each sieve tube element is closely associated with one or more companion cells, which are metabolically active, contain a nucleus and many mitochondria, and provide ATP for the loading of solutes into the sieve tubes. Plasmodesmata connect companion cells to sieve tube elements, facilitating exchange of materials.

韧皮部由筛管分子和伴胞组成。筛管分子是活细胞，形成长管用于溶质运输。它们缺乏细胞核和许多细胞器以减少流动阻力，但仍保持生命活性。其端壁穿孔形成筛板，保持细胞质连续性。每个筛管分子都与一个或多个伴胞紧密相连，伴胞代谢活跃，含有细胞核和大量线粒体，为溶质装载入筛管提供 ATP。胞间连丝将伴胞与筛管分子连接起来，便于物质交换。

4. Water Transport Pathways: Apoplast, Symplast, and Vacuolar | 水分运输途径：质外体、共质体和液泡途径

Once water enters the root hair by osmosis, it travels across the root cortex to the xylem via three pathways. The apoplast pathway moves water through the cell walls and intercellular spaces, bypassing the cytoplasm. It is the faster route as it offers little resistance, until it reaches the Casparian strip in the endodermis, which blocks the apoplastic flow and forces water into the symplast. The symplast pathway moves water through the cytoplasm and plasmodesmata of cells. The vacuolar pathway is similar to the symplast but also crosses the tonoplast into vacuoles. The Casparian strip, a band of suberin in the radial and transverse walls of endodermal cells, ensures that all water and ions must pass through the selectively permeable plasma membrane of endodermal cells, regulating what enters the xylem.

水分通过渗透进入根毛后，通过三种途径穿过根皮层到达木质部。质外体途径使水分沿细胞壁和细胞间隙移动，绕过细胞质。这是较快的路径，因为阻力很小，直到在凯氏带处受阻——内皮层的凯氏带阻断质外体流，迫使水分进入共质体。共质体途径使水分通过细胞的细胞质和胞间连丝移动。液泡途径与共质体途径类似，但还会穿过液泡膜进入液泡。凯氏带是内皮层细胞径向壁和横向壁上的一条木栓质带，确保所有水分和离子必须通过内皮层细胞的选择性通透质膜，从而调控进入木质部的物质。

5. Root Pressure and Water Uptake | 根压与水分吸收

Water uptake from the soil into root hair cells occurs by osmosis, driven by a lower water potential inside the root hairs due to active transport of mineral ions. Once in the xylem, an upward push known as root pressure can be generated when water moves into the xylem by osmosis, increasing hydrostatic pressure. Root pressure is minor and can force water up short distances, evident as guttation (water droplets on leaf margins) in some plants during the night when transpiration is low. However, it is not sufficient to explain water rising to the tops of tall trees. The main driver is transpiration pull.

土壤中的水分通过渗透进入根毛细胞，驱动力是根毛内因矿物质离子主动运输导致的水势降低。进入木质部后，当水分通过渗透进入木质部时，会产生向上的推力即根压，增加静水压力。根压作用较小，只能推动水分上升短距离，在一些植物中当夜间蒸腾作用减弱时可见为吐水（叶片边缘的水珠）。然而，这不足以解释水分上升到高大乔木顶部的原因。主要的驱动力是蒸腾拉力。

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

Transpiration is the evaporation of water from the internal leaf surfaces through stomata into the atmosphere. The loss of water lowers the water potential in the leaf air spaces, causing water to move from the xylem in the leaf veins into the mesophyll cells. This creates a tension (negative pressure) in the xylem, which pulls water up the entire column from the roots. The cohesion-tension theory explains how this works: water molecules are cohesive (attracted to each other by hydrogen bonds) and adhesive (attracted to the walls of xylem vessels). These properties maintain an unbroken column of water under tension. The transpiration pull is strong enough to lift water hundreds of metres in tall trees like giant sequoias.

蒸腾作用是水分从叶片内部表面通过气孔蒸发到大气中的过程。水分散失降低了叶内空气空间的水势，使水分从叶脉中的木质部进入叶肉细胞。这会在木质部中产生张力（负压），将水柱从根部向上拉动。内聚力-张力理论解释了这一机制：水分子具有内聚力（通过氢键相互吸引）和粘附力（附着在木质部导管壁上）。这些特性维持了一个连续的、处于张力状态的水柱。蒸腾拉力强大到足以将水提升到如巨杉等高大乔木数百米的高度。

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

Several environmental factors affect the rate of transpiration. Light intensity increases transpiration because stomata open wider in light, allowing more water vapour to diffuse out. Temperature increases the kinetic energy of water molecules, speeding evaporation, and also decreases the relative humidity of the air outside the leaf. Humidity directly affects the water vapour potential gradient; high humidity reduces transpiration while low dry air accelerates it. Air movement (wind) removes the layer of moist air around the leaf, maintaining a steep water vapour gradient. Under very windy conditions, stomata may close to prevent excessive water loss. The table below summarises these effects.

多种环境因素影响蒸腾速率。光照强度增加蒸腾作用，因为在光下气孔张开更大，允许更多水蒸气扩散出去。温度增加水分子的动能，加速蒸发，同时也降低叶片外部空气的相对湿度。湿度直接影响水蒸气压梯度；高湿度降低蒸腾，而低干燥空气加速蒸腾。空气流动（风）带走叶片周围的湿润空气层，维持陡峭的水蒸气梯度。在强风条件下，气孔可能关闭以防止过度失水。下表总结了这些影响。

Factor / 因素	Effect on transpiration rate / 对蒸腾速率的影响	Explanation / 解释
Light intensity / 光照强度	Increase / 增加	Stomata open, allowing gas exchange / 气孔张开，允许气体交换
Temperature / 温度	Increase / 增加	Faster evaporation, lower external humidity / 蒸发加快，外部湿度降低
Humidity / 湿度	Decrease (if humidity rises) / 降低（若湿度上升）	Reduced water vapour potential gradient / 水蒸气压梯度减小
Air movement / 空气流动	Increase (to a point) / 增加（至某一限度）	Maintains steep diffusion gradient / 维持陡峭的扩散梯度

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8. Measuring Transpiration: The Potometer | 测量蒸腾作用：蒸腾计

A potometer is a device used to estimate transpiration rate by measuring the water uptake by a leafy shoot. It usually consists of a capillary tube with an air bubble, connected to a reservoir. As the plant takes up water, the bubble moves along the scale, giving a measure of uptake rate. It is important to note that a potometer measures water absorption, which is not exactly equal to transpiration rate (some water is used in photosynthesis or cell expansion), but under steady conditions it provides a reliable estimate. All joints must be airtight, and the shoot should be cut under water to avoid air embolisms. Variables such as light, wind, and temperature can be manipulated to see their effects.

蒸腾计是一种通过测量叶片枝条吸水速率来估算蒸腾速率的装置。它通常包含一个带有气泡的毛细管，连接到一个储水器。当植物吸水时，气泡沿刻度移动，给出吸水速率的测量值。需注意，蒸腾计测量的是水分吸收量，并不完全等于蒸腾速率（一些水分用于光合作用或细胞扩增），但在稳定条件下可提供可靠估计。所有接头必须气密，且枝条应于水中剪取以避免空气栓塞。可操控光照、风和温度等变量观察其影响。

9. Translocation in Phloem: Source to Sink | 韧皮部的运输：源到库

Translocation is the movement of assimilates, primarily sucrose and amino acids, through the phloem from sources to sinks. A source is any region that produces or stores more assimilates than it consumes, such as mature leaves (photosynthesising) or storage organs during mobilisation. A sink is any region that consumes or stores assimilates, such as roots, developing fruits, and apical meristems. The direction of flow in phloem can be bidirectional depending on the needs of the plant, but an individual sieve tube transports materials in one direction at a given time. Sucrose is actively loaded into the sieve tubes at the source, lowering the water potential and drawing water from xylem by osmosis, generating high hydrostatic pressure. At the sink, sucrose is unloaded, water follows back to xylem, and pressure decreases.

转运是指同化物（主要是蔗糖和氨基酸）通过韧皮部从源到库的移动。源是任何产生或储存的同化物多于自身消耗的区域，如成熟叶片（进行光合作用）或处于动员期的贮藏器官。库是任何消耗或储存同化物的区域，如根、发育中的果实和顶端分生组织。韧皮部中的流动方向可根据植物需求双向进行，但单个筛管在特定时刻只在一个方向上运输物质。蔗糖在源处被主动装载到筛管中，降低水势并借渗透作用从木质部中吸收水分，产生高静水压力。在库处，蔗糖被卸载，水分随之回到木质部，压力降低。

10. Pressure Flow (Mass Flow) Hypothesis | 压力流动假说

The pressure flow hypothesis (also known as mass flow) is the most widely accepted model for phloem translocation. It involves three main stages: loading of sucrose at the source – sucrose is actively transported into the sieve tube–companion cell complex using ATP, reducing water potential; water entry – water moves from the adjacent xylem into the phloem by osmosis, increasing hydrostatic pressure at the source; unloading at the sink – sucrose is actively or passively unloaded, water follows out by osmosis, lowering the pressure. The pressure difference between source and sink drives a bulk flow of phloem sap along the sieve tubes. This flow can be up to 1 metre per hour. The process requires metabolic energy only for loading and unloading.

压力流动假说（也称为集流）是被广泛接受的韧皮部转运模型。它包含三个主要阶段：在源处装载蔗糖——蔗糖利用 ATP 被主动运输到筛管-伴胞复合体中，降低水势；水分进入——水分通过渗透作用从相邻木质部进入韧皮部，增加源处的静水压力；在库处卸载——蔗糖被主动或被动卸载，水分随之借渗透作用流出，压力降低。源库之间的压力差驱动韧皮部液沿筛管集流移动。这一流动速率可达每小时 1 米。该过程仅在装载和卸载时需要代谢能量。

11. Evidence for the Mass Flow Hypothesis | 支持压力流动假说的证据

Several pieces of evidence support the mass flow hypothesis. Aphid stylets – when aphids are severed from their stylet inserted into a sieve tube, phloem sap continues to exude, showing positive pressure inside. pH gradients – there is a higher pH in companion cells than in neighbouring phloem, consistent with proton pump activity for active loading. Radioactive tracers – when ¹⁴CO₂ is supplied to a leaf, radioactive sucrose appears in the phloem and moves toward the sink at rates consistent with mass flow. Metabolic inhibitors – blocking ATP production stops translocation, confirming the need for active transport. However, some opposing evidence, such as the bidirectional movement in the same sieve tube, suggests the process may be more complex.

多项证据支持压力流动假说。蚜虫口针——将蚜虫从其插入筛管的口针处切断后，韧皮部液继续渗出，表明内部为正压。pH 梯度——伴胞的 pH 值高于邻近韧皮部，这与主动装载的质子泵活动一致。放射性示踪——向叶片提供 ¹⁴CO₂ 后，放射性蔗糖出现在韧皮部中，并以与集流一致的速率向库移动。代谢抑制剂——阻断 ATP 生成会停止转运，证实需要主动运输。然而，一些相反的证据，如同一个筛管中的双向运输，表明这一过程可能更加复杂。

12. Comparing Xylem and Phloem Transport | 木质部与韧皮部运输的比较

The table below provides a concise comparison between the two vascular tissues in plants, which is a common examination topic. Understanding the differences in cell types, substances transported, direction, driving force, and tissue vitality helps reinforce the key concepts for A-level Biology.

下表简明对比了植物的两种维管组织，这是常见的考试主题。理解细胞类型、运输物质、方向、驱动力和组织活力等方面的差异，有助于巩固 A-level 生物学的关键概念。

Feature / 特征	Xylem / 木质部	Phloem / 韧皮部
Main cells / 主要细胞	Dead vessel elements and tracheids / 死导管分子和管胞	Living sieve tube elements and companion cells / 活筛管分子和伴胞
Substances transported / 运输物质	Water and mineral ions / 水和矿质离子	Sucrose and amino acids / 蔗糖和氨基酸
Direction of flow / 流动方向	Upwards (roots to leaves) / 向上（根到叶）	Bidirectional (source to sink) / 双向（源到库）
Driving force / 驱动力	Transpiration pull (cohesion-tension) / 蒸腾拉力（内聚力-张力）	Pressure difference (mass flow) / 压力差（集流）
Metabolic energy / 代谢能量	Not required for transport (passive) / 运输不需能量（被动）	Required for loading/unloading / 装载和卸载需要能量
Walls / 壁	Lignified and thickened / 木质化加厚	Cellulose, not lignified / 纤维素，无木质化

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