Plant Transport in IGCSE AQA Biology | IGCSE AQA 生物：植物运输考点精讲

📚 Plant Transport in IGCSE AQA Biology | IGCSE AQA 生物：植物运输考点精讲

Plants need to move water, mineral ions and sugars over long distances from the roots to the leaves and from the leaves to the rest of the plant. This transport system relies on two specialised tissues: xylem and phloem. Understanding how these tissues are structured and how they work together is essential for the IGCSE AQA Biology specification. In this article, we break down every key concept, from water uptake by root hair cells to the factors that control transpiration, so you can feel confident approaching any exam question on plant transport.

植物需要将水分、矿质离子和糖类从根部输送到叶片，再从叶片运往植物体各处，其间距离往往很长。这套运输系统依赖两种特化的组织：木质部和韧皮部。理解这两种组织是如何构建的、如何协同工作，正是 IGCSE AQA 生物考纲中的核心内容。本文将逐一拆解从根毛细胞吸水到影响蒸腾作用的各个关键概念，帮助你在面对植物运输的任何考题时都胸有成竹。

1. Why Plants Need Transport Systems | 植物为何需要运输系统

Unlike single-celled organisms, multicellular plants have a small surface area to volume ratio. Many of their cells are deep inside the body and cannot rely on diffusion alone to obtain water, mineral ions, and sugars. A specialised transport system is therefore necessary to move substances over long distances efficiently.

与单细胞生物不同，多细胞植物的表面积与体积之比较小。体内许多细胞位于组织深处，单靠扩散无法及时获取水分、矿质离子和糖类。因此，需要一套专门的运输系统高效地长距离运输物质。

The transport needs of a plant fall into two main categories: the upward movement of water and dissolved minerals from roots to stems and leaves, and the movement of sugars (produced in photosynthesis) from leaves to other parts of the plant, including storage organs and growing regions.

植物的运输需求主要分为两类：一是水和溶解矿物质从根部向上运输到茎和叶片；二是光合作用产生的糖类从叶片运往植物的其他部位，包括储存器官和生长区域。

2. Xylem and Phloem: The Two Transport Tissues | 木质部和韧皮部：两大运输组织

Land plants possess two distinct vascular tissues. Xylem transports water and mineral ions from the roots to the leaves. Phloem transports dissolved sugars, such as sucrose and amino acids, from the leaves to the rest of the plant. This dual system is known as the vascular bundle, and its arrangement differs between roots and stems.

陆生植物拥有两种不同的维管组织。木质部将水和矿物质离子从根部输送到叶片。韧皮部则将溶解的糖类（如蔗糖）和氨基酸从叶片运往植物其他部位。这种双系统合称为维管束，在根和茎中的排列方式有所不同。

In a root cross-section, the xylem is typically located in the centre, forming a star-shaped arrangement, with phloem found between the arms of the xylem. In a young stem, the vascular bundles are arranged in a ring near the outer edge, giving the stem flexibility and strength.

在根的横切面中，木质部通常位于中央，呈星形排列，韧皮部则分布在木质部的“星臂”之间。在幼茎中，维管束排成环状，靠近茎的外缘，既赋予茎柔韧性又提供支撑。

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

Xylem vessels are made up of dead cells aligned end to end, forming continuous hollow tubes. The end walls between these cells break down, so there is no cytoplasm or nucleus to obstruct the flow. The cell walls are strengthened with a waterproof substance called lignin, which can form rings, spirals, or pits. Lignin prevents collapse under the tension created by transpiration and also helps plants grow tall.

木质部导管由死细胞端对端衔接而成，形成连续的中空管道。细胞间的端壁已经瓦解，因此没有细胞质和细胞核阻碍水流。细胞壁由一种防水的木质素加固，木质素的沉积可呈环纹、螺纹或具纹孔。木质素能防止导管在蒸腾拉力下塌陷，也能帮助植物增高。

There are no end walls in the mature xylem vessels, allowing an uninterrupted column of water to move upwards. The narrow diameter of xylem vessels also aids cohesion of water molecules, which is vital for the transpiration stream.

成熟的木质部导管中没有端壁，水柱得以不间断地向上移动。木质部导管口径窄小，也有助于水分子的内聚力，这对蒸腾流至关重要。

4. Water Uptake by Root Hair Cells | 根毛细胞如何吸收水分

Root hair cells are specialised epidermal cells with long, thin projections that enormously increase the surface area for absorption. Water enters root hair cells by osmosis because the water potential inside the cell is lower than in the surrounding soil water. This is due to the active transport of mineral ions into the root hair cells, making the cell sap more concentrated.

根毛细胞是特化的表皮细胞，长而细的突起极大地增加了吸收的表面积。由于根毛细胞内的水势低于周围土壤水的水势，水分通过渗透作用进入根毛细胞。这是因为矿质离子通过主动运输进入根毛细胞，使得细胞液浓度更高。

Once inside the root hair cell, water moves from cell to cell through the root cortex either via the cell walls (apoplast pathway) or through the cytoplasm and plasmodesmata (symplast pathway), until it reaches the xylem in the centre of the root.

进入根毛细胞后，水分通过根皮层细胞间传递，既可以沿细胞壁移动（质外体途径），也可以通过细胞质和胞间连丝（共质体途径），最终到达根中央的木质部。

5. Transpiration: Definition and Process | 蒸腾作用：定义与过程

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata in the leaves. It is an inevitable consequence of gas exchange. When stomata open to let carbon dioxide in for photosynthesis, water vapour diffuses out down a water potential gradient.

蒸腾作用是指水分以水蒸气的形式从植物地上部分散失的过程，主要通过叶片上的气孔进行。这是气体交换不可避免的结果。气孔开启让二氧化碳进入用于光合作用时，水蒸气便沿着水势梯度向外扩散。

Transpiration is not a waste; it creates the transpiration pull that lifts water up through the xylem from the roots. As water molecules evaporate from the mesophyll cells, more water is pulled up due to the cohesive forces between water molecules. This continuous column of water is known as the transpiration stream.

蒸腾作用并非单纯的浪费，它产生的蒸腾拉力将水分从根部沿木质部向上提升。随着水分从叶肉细胞蒸发，由于水分子之间的内聚力，更多的水被向上牵引。这条连续的水柱称为蒸腾流。

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

Several environmental factors influence the rate of transpiration. An increase in temperature raises the kinetic energy of water molecules, increasing the rate of evaporation and diffusion through the stomata. Higher light intensity causes stomata to open wider, which also increases transpiration. Increased air movement (wind) sweeps away the water vapour surrounding the leaf, maintaining a steep diffusion gradient. Conversely, high humidity reduces the diffusion gradient and lowers the rate of transpiration.

多种环境因素影响蒸腾速率。温度升高增加水分子的动能，加快蒸发和通过气孔的扩散速率。光照强度增大使气孔开得更大，同样增加蒸腾。空气流动（风）吹走叶周围的水蒸气，维持较陡的扩散梯度。相反，湿度高会降低扩散梯度，减缓蒸腾速率。

Students can investigate these effects using a potometer, which measures the rate of water uptake by a leafy shoot. The distance moved by an air bubble in a capillary tube over a set time gives an indirect measure of transpiration rate.

学生可用蒸腾计探究这些影响。蒸腾计测量带叶枝条的吸水速率，毛细管中气泡在一定时间内移动的距离可作为蒸腾速率的间接量度。

7. Cohesion-Tension Theory | 内聚力-张力理论

The cohesion–tension theory explains how water is pulled up through the xylem. Water molecules are polar and form hydrogen bonds with one another, creating strong cohesive forces. As water evaporates from the leaf mesophyll during transpiration, tension (negative pressure) is created at the top of the water column. This tension is transmitted down the column, and because water molecules cohere strongly, the entire column moves upwards in one continuous mass.

内聚力–张力理论解释了水分如何通过木质部被向上牵引。水分子具有极性，彼此间形成氢键，产生强大的内聚力。蒸腾作用中水分从叶肉细胞蒸发时，在水柱顶部产生张力（负压）。这一张力沿水柱向下传递，由于水分子之间紧密结合，整个水柱便作为一个连续整体向上移动。

The lignified walls of xylem vessels prevent them from collapsing under this tension. Adhesion of water molecules to the walls of the vessels also assists the upward movement.

木质部导管因木质化的壁而不会被此张力拉瘪。水分子与导管壁的附着力也有助于上升运动。

8. Structure of Phloem Tubes | 韧皮部的结构

Unlike xylem, phloem is made of living cells. The main conducting cells are sieve tube elements, which are arranged end to end to form sieve tubes. The sieve plates at the end walls have pores that allow the passage of phloem sap. Sieve tube elements have no nucleus and very little cytoplasm, so they rely on companion cells that lie adjacent to them. Companion cells have many mitochondria and provide the energy needed for active transport of sugars into the sieve tubes.

与木质部不同，韧皮部由活细胞组成。主要的输导细胞是筛管分子，它们端对端连接形成筛管。端壁上的筛板有孔，允许韧皮部汁液通过。筛管分子没有细胞核，细胞质也很少，因此依赖紧邻的伴胞。伴胞含有大量线粒体，为糖类主动运入筛管提供能量。

Phloem transports sucrose and amino acids from sources (areas of production or storage) to sinks (areas where they are used or stored). This process is called translocation and requires energy.

韧皮部将蔗糖和氨基酸从“源”（生产或储存的部位）运往“库”（使用或储存的部位），这一过程称为转运，需要能量。

9. Translocation and the Mass Flow Hypothesis | 转运与压力流动假说

The most widely accepted explanation for translocation is the mass flow hypothesis. At the source (e.g., a mature leaf), sucrose is actively loaded into the phloem sieve tubes, lowering the water potential. Water then enters by osmosis from the adjacent xylem, increasing the hydrostatic pressure. At the sink (e.g., a growing root tip), sucrose is actively unloaded, causing water to leave the phloem by osmosis, thus reducing the pressure. This pressure difference drives a bulk flow of phloem sap from source to sink.

目前最广为接受的转运解释是压力流动假说。在“源”（如成熟叶片），蔗糖被主动装载到韧皮部筛管中，使水势降低。水分随后通过渗透作用由邻近的木质部进入，使静水压力升高。在“库”（如生长的根尖），蔗糖被主动卸载，导致水分通过渗透离开韧皮部，压力随之下降。这一压力差推动韧皮部汁液从“源”向“库”整体流动。

Evidence for this mechanism comes from aphid stylet experiments and from the observation that phloem sap flows faster when the pressure gradient is steeper. The process depends on active transport and therefore on metabolic energy; if respiration is inhibited, translocation slows or stops.

支持这一机制的证据来自蚜虫口针实验，以及观察到压力梯度越陡时韧皮部汁液流速越快。该过程依赖主动运输，因而需要代谢能量；若抑制呼吸作用，转运便会减慢甚至停止。

10. Ringing Experiments | 环割实验

Ringing experiments provide simple yet powerful evidence for the role of phloem in transporting sugars. When a complete ring of bark (containing phloem) is removed from a woody stem, the tissues above the ring swell due to the accumulation of sugars, while tissues below the ring may starve and eventually die. Water transport through the xylem is not affected because the inner xylem remains intact. This demonstrates that sugars are transported in the phloem and that the movement is bidirectional, from leaves downward and from storage organs upward.

环割实验为韧皮部运输糖类的作用提供了简单而有力的证据。从木本茎上完整地割除一圈树皮（含韧皮部），环割上方的组织因糖类积累而膨大，而环割下方的组织则会因缺乏养分而逐渐死亡。通过木质部的水分运输不受影响，因为内部的木质部保持完整。这表明糖类在韧皮部中运输，且该运输是双向的——可从叶片向下，也可从储存器官向上。

11. Mineral Ion Uptake and Transport | 矿质离子的吸收与运输

Plants require mineral ions such as nitrates (for amino acids and proteins), magnesium (for chlorophyll), and phosphates (for DNA and ATP). These ions are absorbed from the soil by root hair cells through active transport, often against a concentration gradient. The energy required comes from aerobic respiration, so well-aerated, warm soils promote better mineral uptake.

植物需要矿质离子，如硝酸盐（用于合成氨基酸和蛋白质）、镁（用于合成叶绿素）和磷酸盐（用于DNA和ATP）。这些离子通过主动运输由根毛细胞从土壤中吸收，通常是逆浓度梯度进行的。所需能量来自有氧呼吸，因此通气良好、温暖的土壤有利于矿物质的吸收。

Once inside the root, mineral ions move through the symplast pathway to the xylem and travel upward dissolved in the transpiration stream. The rate of mineral transport is thus linked to the rate of transpiration, although some selective transport also occurs.

进入根部后，矿质离子通过共质体途径到达木质部，并溶解在蒸腾流中向上运送。矿物质的运输速率因此与蒸腾速率相关，但也存在一些选择性运输。

12. Adaptations to Reduce Water Loss | 减少水分流失的适应

Plants living in dry habitats (xerophytes) have evolved adaptations to minimise water loss by transpiration. Thick, waxy cuticles on leaves reduce evaporation. Stomata may be sunken into pits or located mainly on the lower epidermis, where they are sheltered from wind. Some plants have rolled leaves or hairy surfaces to trap a layer of moist air, thus reducing the water potential gradient.

生活在干燥生境中的植物（旱生植物）演化出了减少蒸腾水分流失的适应结构。叶片表面的厚蜡质角质层可减少蒸发。气孔可能下陷成窝，或主要分布在下表皮，以避开风的影响。有些植物的叶片卷曲或表面有毛，可形成一层潮湿空气，从而降低水势梯度。

Cacti and other succulents store water in their fleshy stems and have leaves modified into spines, drastically reducing the surface area for transpiration. These adaptations allow them to survive in environments where water is scarce.

仙人掌和其他多肉植物将水分储存在肉质茎中，叶片退化成刺，大大降低了蒸腾的表面积。这些适应帮助它们在缺水的环境中生存。

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Plant Transport in IGCSE AQA Biology | IGCSE AQA 生物：植物运输 考点精讲