📚 Edexcel Biology 3.3 Transport in Plants | 爱德思生物 3.3 植物运输
Transport in plants is a core topic in Edexcel Biology. It explains how water, mineral ions and assimilates move through xylem and phloem, and how transpiration and translocation link the roots, stem and leaves into one functioning system.
植物运输是爱德思生物学的核心主题。它解释了水分、矿质离子和同化产物如何在木质部和韧皮部中移动，以及蒸腾作用和运输作用如何将根、茎、叶联系成一个整体功能系统。
1. Introduction to Plant Transport Systems | 植物运输系统简介
Plants need transport systems because they are multicellular and have relatively low surface area to volume ratios. Simple diffusion cannot supply all cells with water, minerals and sugars over long distances.
植物需要运输系统，因为它们是复杂的多细胞生物，表面积与体积之比相对较小。简单扩散无法在长距离内为所有细胞提供水分、矿质和糖类。
Two main transport tissues are involved: xylem transports water and mineral ions from roots to shoots, while phloem transports organic solutes such as sucrose from sources to sinks.
植物主要依靠两种运输组织：木质部将水分和矿质离子从根部运送到地上部分，韧皮部则将蔗糖等有机溶质从源运输到库。
Vascular bundles in roots, stems and leaves are arranged differently, but all contain both xylem and phloem. In roots, xylem is often central and star-shaped; in stems, vascular bundles lie around the outside; in leaves, they form the network of veins.
根、茎、叶中的维管束排列方式不同，但都含有木质部和韧皮部。根中木质部通常位于中央并呈星形；茎中维管束分布在周围；叶中它们形成叶脉网络。
2. Xylem and Phloem Structure | 木质部和韧皮部结构
Xylem vessels are formed from dead cells arranged end to end. The end walls break down to form a continuous hollow tube, and the cell walls are thickened with lignin, which provides strength and waterproofing.
木质部导管由死细胞首尾相连形成。端壁分解后形成连续的空心管道，细胞壁因木质素加厚，具有强度和不透水性。
Lignin may be deposited in rings, spirals or bordered pits. Bordered pits allow water to move sideways between vessels and into surrounding tissues.
木质素可以呈环状、螺旋状沉积，也可以形成具缘纹孔。具缘纹孔允许水分在导管之间以及向周围组织侧向移动。
Phloem consists of sieve tube elements and companion cells. Sieve tube elements are living cells with perforated sieve plates and little cytoplasm; companion cells carry out metabolic functions and load sucrose into the sieve tubes.
韧皮部由筛管分子和伴随细胞组成。筛管分子是活细胞，具有穿孔的筛板，细胞质很少；伴随细胞执行代谢功能，并将蔗糖装入筛管。
| Feature | Xylem | 木质部 | Phloem | 韧皮部 |
|---|---|---|
| Main transport | Water and mineral ions | 水分和矿质离子 | Sucrose and organic solutes | 蔗糖和有机溶质 |
| Cell state | Dead, hollow tubes | 死细胞，中空管道 | Living cells | 活细胞 |
| Cell wall | Lignified | 木质化 | Cellulose, not lignified | 纤维素，不木质化 |
| Direction | Mostly upward | 主要向上 | Source to sink, up and down | 从源到库，可上可下 |
3. Water Uptake by Roots | 根系对水分的吸收
Root hair cells are the main site of water uptake. They have long extensions that increase surface area, thin cellulose walls, and many mitochondria to provide ATP for active transport of mineral ions.
根毛细胞是吸收水分的主要部位。它们具有细长突起以增大表面积、薄的纤维素细胞壁以及大量线粒体，为矿质离子的主动运输提供 ATP。
Mineral ions are actively transported into root hair cells, lowering the water potential inside the cell. Water then enters by osmosis from the soil solution down a water potential gradient.
矿质离子通过主动运输进入根毛细胞，降低细胞内的水势。随后水沿水势梯度通过渗透作用从土壤溶液进入细胞。
Water then moves across the root by three pathways: the apoplast pathway through cell walls, the symplast pathway through cytoplasm and plasmodesmata, and the vacuolar pathway through vacuoles. The Casparian strip in the endodermis blocks the apoplast pathway and forces water through cell membranes, allowing the plant to control entry into the xylem.
水随后通过三条途径穿过根部：通过细胞壁的质外体途径、通过细胞质和胞间连丝的共质体途径，以及通过液泡的液泡途径。内皮层中的凯氏带阻断质外体途径，迫使水穿过细胞膜，使植物能够控制水分进入木质部。
4. Transpiration and the Transpiration Stream | 蒸腾作用与蒸腾流
Transpiration is the loss of water vapour from the leaves, mainly through stomata. It is an unavoidable consequence of opening stomata for gas exchange during photosynthesis.
蒸腾作用是叶片水蒸气散失的过程，主要经由气孔进行。这是光合作用中气孔开放以进行气体交换的必然结果。
The transpiration stream is the continuous movement of water from the roots, through the xylem, and out of the leaf. Water evaporates from mesophyll cell walls, moves into the air spaces, and diffuses out of the leaf down a water vapour potential gradient.
蒸腾流是指水从根部经木质部持续向上运输并散失到叶片外的过程。水从叶肉细胞壁蒸发，进入细胞间隙，然后沿水蒸气浓度梯度扩散出叶片。
Transpiration is important because it pulls water and dissolved mineral ions up the plant, cools leaves by evaporation, and keeps cells turgid for support and growth.
蒸腾作用很重要，因为它拉动水分和溶解的矿质离子在植物体内上运，通过蒸发冷却叶片，并保持细胞膨压以维持支撑和生长。
5. Cohesion-Tension Theory | 内聚力-张力理论
The cohesion-tension theory explains how water rises to the top of tall trees. Water molecules form hydrogen bonds with one another, creating cohesion, and they also adhere to the walls of xylem vessels, creating adhesion.
内聚力-张力理论解释了水如何上升到高大树木的顶端。水分子之间通过氢键相互吸引，产生内聚力；同时它们也附着在木质部导管壁上，产生附着力。
As water evaporates from leaf mesophyll cells during transpiration, tension is created in the leaf. This negative pressure is transmitted down the continuous water column in the xylem, pulling water upward from the roots.
蒸腾作用期间，水分从叶肉细胞蒸发，在叶片中产生张力。这种负压沿木质部中连续的水柱向下传递，把水从根部向上拉。
Evidence for the cohesion-tension theory includes the observation that tree trunk diameter shrinks during the day when transpiration rates are high, and the fact that air bubbles can break the water column and stop flow.
支持内聚力-张力理论的证据包括：白天蒸腾速率高时树干直径会收缩；气泡会打断水柱并阻断水分运输。
Ψ(total) = Ψ(solute) + Ψ(pressure)
总水势 = 溶质势 + 压力势
6. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素
Light intensity affects transpiration: bright light causes stomata to open wider, increasing water loss and also providing energy for photosynthesis. Darkness closes stomata and reduces transpiration.
光照强度影响蒸腾作用：强光使气孔开得更大，增加水分散失，同时为光合作用提供能量。黑暗中气孔关闭，蒸腾作用降低。
Temperature increases transpiration because warmer air holds more water vapour, and water molecules have more kinetic energy to evaporate. High humidity decreases the water vapour potential gradient between leaf and air, so transpiration slows.
温度升高加快蒸腾，因为暖空气能容纳更多水蒸气，水分子也有更多动能用于蒸发。高湿度降低了叶片与空气之间的水蒸气梯度，因此蒸腾减慢。
Wind removes saturated air from around the leaf, maintaining a steep water vapour gradient and increasing transpiration. A potometer can be used to investigate these factors by measuring water uptake.
风会带走叶片周围饱和的空气，维持较陡的水蒸气梯度，从而加快蒸腾。可以使用蒸腾计通过测量吸水量来研究这些因素。
- Light, temperature and wind increase transpiration. 光照、温度和风加快蒸腾。
- High humidity decreases transpiration. 高湿度减慢蒸腾。
- Water uptake in a potometer is an estimate of transpiration rate. 蒸腾计中的吸水量是蒸腾速率的估算值。
7. Translocation of Sucrose | 蔗糖的运输
Translocation is the movement of organic solutes such as sucrose and amino acids through the phloem from source to sink. Sources include photosynthesising leaves and storage organs during mobilisation; sinks include growing roots, fruits and developing seeds.
运输作用是指蔗糖和氨基酸等有机溶质通过韧皮部从源向库移动的过程。源包括进行光合作用的叶片和储存在动员中的器官；库包括生长中的根、果实和发育中的种子。
Sucrose is loaded into sieve tube elements at the source by active transport, often with the help of companion cells. This loading reduces the water potential inside the phloem.
在源端，蔗糖通过主动运输被装入筛管分子，通常有伴随细胞协助。这种装载会降低韧皮部内部的水势。
Water then enters the phloem from the xylem by osmosis, increasing hydrostatic pressure near the source. At the sink, sucrose is unloaded and water leaves the phloem, reducing pressure. The pressure difference drives mass flow through the sieve tubes.
随后水通过渗透作用从木质部进入韧皮部，增加源端的静水压力。在库端，蔗糖被卸出，水离开韧皮部，压力降低。压力差驱动筛管内的质量流动。
8. Mass Flow Hypothesis | 压力流动假说
The mass flow hypothesis describes how phloem sap moves from source to sink by a pressure gradient. It has key steps: phloem loading, water entry by osmosis, bulk flow under pressure, and unloading at the sink.
压力流动假说描述了韧皮部汁液如何依靠压力梯度从源移动到库。其关键步骤包括：韧皮部装载、水渗透进入、在压力下的集流运输以及库端卸载。
At the source, companion cells actively transport sucrose into sieve tube elements. The water potential falls, so water enters from xylem by osmosis. This creates high hydrostatic pressure.
在源端，伴随细胞将蔗糖主动运输到筛管分子中。水势下降，水通过渗透从木质部进入。这产生较高的静水压力。
At the sink, sucrose is removed by active transport or converted into starch, increasing the water potential. Water leaves the phloem, lowering hydrostatic pressure. The resulting pressure gradient pushes phloem sap from source to sink.
在库端，蔗糖通过主动运输被移走或转化为淀粉，水势升高。水离开韧皮部，静水压力降低。由此产生的压力梯度推动韧皮部汁液从源流向库。
9. Evidence for Transport Mechanisms | 运输机制的证据
Ringing experiments remove a ring of bark and phloem from a stem. After a time, swelling occurs above the ring because sugars cannot pass down, while the region below the ring receives little sugar and may stop growing.
环割实验是剥除茎上的一圈树皮和韧皮部。一段时间后，环的上方出现肿胀，因为糖类无法向下运输；环的下方几乎得不到糖，可能停止生长。
Aphid stylet experiments show that phloem sap is under positive pressure. If an aphid is cut from its stylet, phloem sap continues to flow out, proving that translocation is driven by pressure rather than simple diffusion.
蚜虫口针实验表明韧皮部汁液处于正压下。如果将蚜虫从口针处切断，韧皮部汁液会继续流出，证明运输作用由压力驱动，而不是简单扩散。
Radioactive tracers such as carbon-14 can be supplied to leaves as carbon dioxide. The labelled carbon appears in phloem sap and later in sinks, confirming the direction and pathway of translocation.
放射性示踪剂如碳-14 可以以二氧化碳形式提供给叶片。标记碳出现在韧皮部汁液中，之后出现在库中，证实了运输作用的方向和途径。
10. Xerophytic Plant Adaptations | 旱生植物适应
Xerophytes are plants adapted to dry conditions. They reduce water loss while still allowing gas exchange, and they often store water in specialised tissues.
旱生植物是适应干燥环境的植物。它们在保持气体交换的同时减少水分损失，并常在特化组织中储存水分。
- Thick waxy cuticle reduces evaporation. 厚蜡质角质层减少蒸发。
- Sunken stomata trap moist air and reduce the diffusion gradient. 凹陷气孔截留湿润空气，降低扩散梯度。
- Rolled leaves and leaf hairs trap water vapour. 卷曲叶片和叶毛截留水蒸气。
- Low surface area to volume ratio, such as needle-like leaves, reduces water loss. 低表面积与体积比，如针状叶，减少水分损失。
- Shallow or deep extensive roots maximise water uptake. 浅层或深广的根系最大化吸水。
11. The Potometer Practical | 蒸腾计实验
A potometer measures water uptake by a cut shoot, which is used to estimate transpiration rate. The shoot is placed in a sealed tube, and an air bubble in a capillary tube moves as water is taken up.
蒸腾计用于测量切枝的吸水量，以此估算蒸腾速率。将枝条放入密封管中，当水被吸收时，毛细管中的气泡会移动。
Key precautions include cutting the shoot underwater to prevent air bubbles from entering xylem, keeping leaves dry, and allowing the apparatus to equilibrate before taking readings. The distance moved by the bubble over time is used to calculate rate.
关键注意事项包括：在水下切割枝条，防止气泡进入木质部；保持叶片干燥；在读数前让装置平衡。通过气泡在一段时间内移动的距离可以计算速率。
Changing one factor at a time, such as light, wind or humidity, allows comparison of transpiration rates. The potometer measures water uptake, not actual water loss, because some water is used in photosynthesis and cell expansion.
每次只改变一个因素，如光照、风或湿度，可以比较蒸腾速率。蒸腾计测量的是吸水量，而不是实际失水量，因为部分水用于光合作用和细胞扩张。
12. Exam Tips and Common Misconceptions | 考试提示与常见误区
Do not say that xylem transports food or that phloem transports water. Xylem carries water and mineral ions; phloem carries sucrose and amino acids.
不要错误地说木质部运输食物或韧皮部运输水分。木质部运输水分和矿质离子；韧皮部运输蔗糖和氨基酸。
Transpiration is the evaporation of water from leaves, while translocation is the movement of solutes in phloem. These terms are not interchangeable.
蒸腾作用是叶片水分的蒸发，而运输作用是韧皮部中溶质的移动。这两个术语不能互换。
Plants do not pump water like a heart; water movement depends on osmosis, cohesion, adhesion and evaporation. Active transport is used for mineral uptake in roots and sucrose loading in phloem, but not for moving water in xylem.
植物不像心脏那样泵送水分；水分移动依赖渗透、内聚力、附着力和蒸发。主动运输用于根部吸收矿质和韧皮部装载蔗糖，而不是用于木质部中水分的移动。
When describing results, remember that water moves from high water potential to low water potential, and that turgor pressure is essential for mass flow in phloem.
在描述结果时，请记住水从高水势向低水势移动，而膨压对于韧皮部的质量流动至关重要。
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