📚 Transport in Plants | 植物体内的运输
Plants are multicellular organisms that require efficient transport systems to move water, mineral ions and the products of photosynthesis throughout their bodies. Unlike animals, plants have no heart to pump a fluid around the body, yet they can raise water from the roots to the leaves against gravity and distribute sugars from sources to sinks. Understanding water movement in the xylem and sugar translocation in the phloem is central to A-Level Biology, because it shows how structure and function are intimately linked.
植物是多细胞生物，需要高效的运输系统把水分、矿物质离子和光合作用产物运送到全身。与动物不同，植物没有心脏来泵送体液，却能克服重力把水从根运送到叶，并把糖类从源分配到库。理解木质部中的水分运动和韧皮部中的糖类易位，是 A-Level 生物学的核心内容，因为它揭示了结构与功能之间的紧密联系。
1. The Need for Transport Systems | 运输系统的必要性
All living cells require a constant supply of oxygen and nutrients and must remove metabolic waste. In small organisms such as unicellular algae, simple diffusion across the cell surface is sufficient because the surface area to volume ratio (SA:V) is very large and no part of the cell is far from the outside. As an organism increases in size, however, its volume rises much faster than its surface area, so the SA:V ratio falls sharply and diffusion alone cannot meet the metabolic demands of the internal tissues.
所有活细胞都需要持续的氧气和营养供应，并必须排出代谢废物。在单细胞藻类等微小生物中，仅靠体表扩散就足够了，因为其表面积与体积之比（SA:V）非常大，细胞内任何部位都离外界很近。但当生物体积增大时，体积的增长远快于表面积，SA:V 比值急剧下降，单靠扩散已无法满足内部组织的代谢需求。
Vascular plants overcame this limitation by evolving two specialised transport tissues: xylem, which carries water and dissolved mineral ions from the roots to the leaves, and phloem, which distributes organic solutes (mainly sucrose) from sources such as mature leaves to sinks such as roots, stems, fruits and growing regions. Plants also need to move water and minerals absorbed from the soil to the leaves for photosynthesis, and to deliver sugars to non-photosynthetic parts that cannot make their own.
维管植物通过演化出两种专门的运输组织突破了这一限制：木质部把水和溶解的矿物质离子从根运到叶；韧皮部则把有机溶质（主要是蔗糖）从成熟叶片等"源"分配到根、茎、果实和生长区等"库"。植物还需要把从土壤吸收的水和矿物质运到叶以供光合作用，并把糖类送到不能自行制造有机物的非光合部位。
SA:V = surface area ÷ volume
Because leaves are thin and flat, they keep a large surface area for gas exchange and light capture, while the transport tissues inside the plant provide the internal delivery network that diffusion alone could never supply.
由于叶片薄而扁平，它们保持了很大的表面积用于气体交换和捕光；而植物体内的运输组织则提供了单靠扩散永远无法完成的内部输送网络。
2. An Overview of the Transport Tissues | 运输组织概览
Xylem and phloem are arranged together in vascular bundles. In a root, the vascular tissue is found in the centre: the xylem forms a cross or X shape with the phloem in the gaps between the arms. This central position helps the root resist the pulling force (tension) transmitted down the stem when water is drawn up. In a stem, the bundles are arranged in a ring near the outside, which gives strength and flexibility. In a leaf, the bundles form the veins of the midrib and branching network, with xylem usually above the phloem. Between the two tissues lies a layer of cambium, a meristem whose cell divisions produce new xylem and phloem, allowing the plant to grow thicker.
木质部和韧皮部共同排列成维管束。在根中，维管组织位于中央：木质部呈十字形或 X 形，韧皮部位于木质部臂之间的空隙里。这种中央位置有助于根部抵抗水分上升时沿茎传下来的拉力。在茎中，维管束呈环状靠近外围排列，既提供强度又保持柔韧性。在叶中，维管束构成主脉和分枝的叶脉网络，通常木质部在上、韧皮部在下。两种组织之间是形成层——一种分生组织，其细胞分裂产生新的木质部和韧皮部，使植物能够加粗生长。
Placing the tissues side by side is significant. Water arriving in the xylem is close to the phloem, so water can move osmotically between them, which is essential for the pressure-driven flow of sap in the phloem. At the same time, the two tissues carry completely different cargoes in different directions, and they must never be confused in an exam answer.
两种组织并排排列意义重大：到达木质部的水与韧皮部相邻，水可以依靠渗透在两者之间移动，这对韧皮部汁液的压力驱动流动至关重要。同时，这两种组织运输的货物完全不同、方向也不同，在答题时绝不可混淆。
3. Xylem: Structure and Function | 木质部：结构与功能
Xylem tissue contains vessels, tracheids, fibres and parenchyma. Xylem vessels are dead at maturity: they have no cytoplasm, no nucleus and no end walls, so they form continuous hollow tubes running from root to leaf. Their walls are thickened with lignin, which makes them strong and waterproof and prevents them from collapsing under the tension generated by transpiration. Lignin is laid down in different patterns – annular (rings), spiral, reticulate (net) and pitted – and these patterns allow the vessel to stretch as the plant grows.
木质部含有导管、管胞、纤维和薄壁细胞。导管在成熟时已经死亡：没有细胞质、没有细胞核、也没有端壁，因此形成从根到叶连续的中空管道。其细胞壁被木质素加厚，使管道坚固、防水，并能防止其在蒸腾产生的拉力下塌陷。木质素以不同方式沉积——环纹、螺旋纹、网纹和孔纹——这些方式使导管在植物生长时仍能伸长。
- Narrow lumen – gives a large capillary effect and helps water columns resist breaking. | 管腔狭窄——产生较强的毛细作用，有助于水柱保持连续不断裂。
- Pits in the wall – allow lateral movement of water between neighbouring vessels and into living cells. | 壁上的纹孔——允许水在相邻导管之间以及进入活细胞进行横向移动。
- No organelles or end walls – reduces resistance so the transpiration stream can flow quickly. | 无细胞器和端壁——减小阻力，使蒸腾流能够快速流动。
- Dead, empty tubes – the water column is continuous and held together by cohesion. | 死亡的空心管道——水柱连续，靠内聚力维持。
Tracheids are narrower, tapering cells with bordered pits that overlap one another. They are less efficient than vessels but are found in all vascular plants, including conifers, whose wood is made mainly of tracheids.
管胞较窄、两端渐尖，壁上带有具缘纹孔，彼此重叠连接。其输水效率低于导管，但存在于所有维管植物中，针叶树的木材就主要由管胞构成。
4. Water Uptake by the Roots | 根对水分的吸收
Water enters a plant mainly through root hair cells. Each root hair is a long, thin extension of an epidermal cell, so a single root system has an enormous surface area for absorption. Root hair cells also have thin walls and a cytoplasm and vacuole rich in dissolved solutes, so the water potential inside the cell is lower than that of the soil solution, and water moves in by osmosis down the water potential gradient.
水主要通过根毛细胞进入植物。每根根毛都是表皮细胞细长的突起，因此整个根系拥有巨大的吸收表面积。根毛细胞壁薄，细胞质和液泡中溶解的溶质浓度高，所以细胞内水势低于土壤溶液，水便沿水势梯度通过渗透进入细胞。
Ψ_soil > Ψ_root hair cell > Ψ_cortex > Ψ_xylem
From the root hair, water travels across the cortex to the endodermis by three routes:
从根毛出发，水通过三条途径穿过皮层到达内皮层：
- Apoplast pathway – through the cellulose cell walls and the spaces between cells, without crossing any membrane. | 质外体途径——沿纤维素细胞壁和细胞间隙移动，不穿过任何膜。
- Symplast pathway – through the cytoplasm of cells connected by plasmodesmata. | 共质体途径——通过由胞间连丝相连的细胞质移动。
- Vacuolar pathway – through the cytoplasm and vacuoles of successive cells. | 液泡途径——依次穿过连续细胞的细胞质和液泡。
The endodermis contains a Casparian strip, a band of the waterproof substance suberin in the radial and transverse walls. It blocks the apoplast pathway, forcing water and dissolved ions to pass through a selectively permeable membrane before entering the xylem. This lets the root control which ions enter the xylem and prevents harmful solutes from leaking in.
内皮层的径向壁和横向壁上有一条由防水物质木栓质构成的凯氏带。它阻断质外体途径，迫使水和溶解的离子在进入木质部之前必须穿过选择透过性膜。这样根就能控制哪些离子进入木质部，并阻止有害溶质渗入。
Mineral ions such as nitrate, phosphate, potassium, calcium and magnesium are absorbed by active transport using carrier proteins and energy from ATP, because their concentration in the soil is usually much lower than inside the root. This active uptake lowers the water potential in the xylem, so water follows by osmosis. The resulting hydrostatic pressure is called root pressure, and it can push water a short distance up the stem, causing guttation – droplets of water exuding from the tips of leaves at night when transpiration is low.
硝酸根、磷酸根、钾、钙和镁等矿物质离子通过载体蛋白和 ATP 提供的能量进行主动运输被吸收，因为它们在土壤中的浓度通常远低于根内。这种主动吸收降低了木质部的水势，水便随之渗透进入。由此产生的静水压称为根压，它能把水向上推动一小段距离，并引起吐水现象——夜间蒸腾很弱时，叶尖渗出小水滴。
5. Moving Water Up the Stem | 水分在茎中的上升
The main mechanism that raises water in tall plants is the cohesion-tension theory, proposed by Dixon and Joly. Transpiration from the leaves creates a negative pressure, or tension, in the water films of the leaf mesophyll. Because water molecules are held together by hydrogen bonds (cohesion), the whole column of water in a xylem vessel is pulled upwards as one continuous thread. Water molecules also stick to the hydrophilic walls of the vessel (adhesion), which helps to support the column, and the narrow lumen increases capillary action.
高大植物中水分上升的主要机制是内聚力学说（Dixon 与 Joly 提出）。叶片的蒸腾在叶肉的水膜中产生负压，即拉力。由于水分子之间以氢键相互吸引（内聚力），导管中的整条水柱就被作为一个连续的水线向上拉动。水分子还黏附在导管的亲水壁上（附着力），有助于支撑水柱，而狭窄的管腔增强了毛细作用。
Pressure gradient needed ≈ 10 kPa per metre of height
To raise water to the top of a 100 m tree, a pressure difference of roughly 1 MPa is required, and measurements of xylem sap under tension confirm that living trees do generate such forces, especially on warm, dry, windy days.
要把水送到 100 m 高树冠的顶部，需要大约 1 MPa 的压力差；对处于张力下木质部汁液的测定证实，活树确实能产生这样的力量，尤其是在温暖、干燥、有风的日子。
Several observations support the theory. The diameter of a tree trunk is slightly smaller during the day, when tension is greatest, and slightly larger at night. If the water column breaks, an air bubble (embolism) forms and the vessel can no longer conduct water, which is why wide vessels are more vulnerable to drought-induced cavitation than narrow ones. Root pressure contributes only a small amount and cannot explain water movement in the tallest trees.
有几项观察支持该学说：白天张力最大时树干直径略小，夜间则略大；水柱一旦断裂，就会形成气泡（气栓），该导管便失去输水能力，因此宽导管比窄导管更易在干旱时发生空穴化。根压的贡献很小，无法解释最高大树的水分上升。
6. Transpiration and Its Measurement | 蒸腾作用及其测量
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves. Inside a leaf, water evaporates from the moist surfaces of the mesophyll cells into the air spaces of the spongy mesophyll. Because the air spaces are saturated with water vapour while the outside air is usually drier, water vapour diffuses out through the stomatal pores down a diffusion gradient. A small amount is also lost through the waxy cuticle (cuticular transpiration) and through lenticels in woody stems.
蒸腾作用是水分以水蒸气形式从植物地上部分散失，主要通过叶片的气孔进行。叶内水从叶肉细胞湿润的表面蒸发到海绵组织的细胞间隙中。由于细胞间隙的水蒸气接近饱和，而外界空气通常更干燥，水蒸气便沿扩散梯度经气孔扩散出去。少量水分也通过蜡质角质层（角质层蒸腾）和木质茎上的皮孔散失。
A typical plant loses about 99% of the water it absorbs by transpiration, and only about 1% is used in photosynthesis and other metabolic reactions. Although some students assume transpiration is wasteful, the transpiration stream is essential because it delivers mineral ions in solution to the leaves and cools the leaf by evaporation.
典型植物吸收的水约有 99% 通过蒸腾散失，只有约 1% 用于光合作用等代谢反应。虽然有些学生认为蒸腾是浪费，但蒸腾流其实不可或缺：它把溶解的矿物质离子送到叶片，并通过蒸发为叶片降温。
Transpiration rate can be estimated with a potometer. A leafy shoot is sealed into a tube of water and an air bubble is introduced; as the shoot transpires, the bubble moves along the capillary tube and the distance moved per unit time is recorded. The rate is calculated as:
蒸腾速率可用蒸腾计测定。把带叶的枝条密封在盛水的管中，引入一个气泡；枝条蒸腾时气泡沿毛细管移动，记录单位时间移动的距离。速率按下式计算：
rate of transpiration = distance moved by the air bubble ÷ time taken
When using a potometer it is important to seal all joints to prevent leaks, to cut the shoot under water to avoid air locks, and to allow the shoot to equilibrate before starting. A potometer measures water uptake, which is only an approximation of water loss.
使用蒸腾计时必须密封所有接口以防漏气，在水中剪取枝条以避免气栓，并在开始前让枝条达到平衡。蒸腾计实际测量的是吸水量，它只是水分损失量的近似值。
7. Factors Affecting the Rate of Transpiration | 影响蒸腾速率的因素
The rate of transpiration depends on the strength of the diffusion gradient between the air spaces inside the leaf and the air outside, and on how widely the stomata are open. Any factor that increases this gradient, or increases the number of open stomata, raises the rate.
蒸腾速率取决于叶片内部细胞间隙与外界空气之间的扩散梯度强度，以及气孔开放的程度。任何增大该梯度或使更多气孔开放的因素都会提高蒸腾速率。
| Factor | 因素 | Effect on rate | 对速率的影响 | Explanation | 解释 |
| Light intensity | 光照强度 | Increases | 提高 | Stomata open in light for CO₂ uptake; also warms the leaf. | 气孔在光下为吸收 CO₂ 而开放；光也会使叶片升温。 |
| Temperature | 温度 | Increases | 提高 | More kinetic energy, faster evaporation and diffusion; warmer air holds more vapour. | 分子动能增大，蒸发和扩散加快；暖空气能容纳更多水蒸气。 |
| Humidity | 湿度 | Decreases | 降低 | A smaller water vapour gradient between the leaf air spaces and the outside air. | 叶片细胞间隙与外界空气之间的水蒸气梯度变小。 |
| Wind speed | 风速 | Increases | 提高 | Removes the boundary layer of saturated air around the leaf, keeping the gradient steep. | 带走叶片周围饱和的边界层空气，使梯度保持陡峭。 |
| Soil water availability | 土壤水分 | Decreases if limiting | 缺水时降低 | Less water available; stomata close as the plant wilts. | 可供水减少；植物萎蔫时气孔关闭。 |
| Leaf area and stomatal density | 叶面积与气孔密度 | Increases | 提高 | More evaporating surface and more pores to diffuse through. | 蒸发面积更大，可扩散的气孔更多。 |
In an exam, always link the factor to the water vapour gradient or to stomatal aperture rather than simply saying "it gets faster because it is hot". A controlled potometer investigation should keep other variables constant, for example by using the same shoot, the same light source distance and the same time interval for each measurement.
考试作答时，务必把该因素与"水蒸气梯度"或"气孔开度"联系起来，而不是简单写"因为热所以更快"。用蒸腾计做对照实验时，应保持其他变量不变，例如使用同一枝条、灯距相同、每次测量时间相同。
8. Adaptations of Xerophytes and Hydrophytes | 旱生植物与水生植物的适应
Xerophytes live in habitats where water is in short supply, and they show adaptations that reduce water loss or increase water storage. A thick, waxy cuticle reduces cuticular transpiration. Rolling the leaf, often helped by hinge cells, traps a layer of moist air around the stomata. Sunken stomata in pits serve the same purpose, and a covering of hairs further slows air movement. Reducing leaf surface area – for example to spines in cacti – lowers the evaporating area but also lowers photosynthesis, so some plants instead carry out CAM photosynthesis, opening their stomata at night to fix carbon dioxide. Deep tap roots reach water far below ground, and succulent stems store water for long dry periods.
旱生植物生活在缺水环境中，具有减少失水或增加储水的适应特征。厚而具蜡质的角质层减少角质层蒸腾；叶片卷曲（常由铰链细胞完成）在气孔周围困住一层湿润空气；气孔下陷成气孔窝有
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