📚 The Transport Needs of Plants | 植物的运输需求
Plants are multicellular photoautotrophs. Although they produce their own food by photosynthesis, they still need internal transport systems to move water, mineral ions, sugars and signalling molecules between organs. In small plants, diffusion may be enough; in larger vascular plants, bulk flow through xylem and phloem is essential.
植物是多细胞光合自养生物。尽管植物通过光合作用自己制造有机物，它们仍需要内部运输系统，在器官之间运送水分、矿质离子、糖类和信号分子。在小型植物中扩散可能足够；在较大的维管植物中，通过木质部和韧皮部的集流是必不可少的。
1. Why Plants Need a Transport System | 为什么植物需要运输系统
A single-celled alga can exchange materials directly with its surroundings by diffusion and active transport. A flowering plant cannot do this because many of its cells are far from the external environment and from photosynthetic tissues.
单细胞藻类可以通过扩散和主动运输直接与环境交换物质。开花植物却无法这样做，因为它的许多细胞远离外部环境和光合组织。
Vascular plants have roots in the soil, leaves in the air, and stems in between. Transport systems link source tissues, which produce or absorb materials, with sink tissues, which use or store them.
维管植物的根在土壤中，叶在空气中，茎在两者之间。运输系统将产生或吸收物质的源组织与利用或储存物质的库组织连接起来。
2. Surface Area to Volume Ratio | 表面积与体积之比
Large multicellular plants have a small surface area to volume ratio compared with unicellular organisms. Diffusion alone is too slow to supply all living cells with water, mineral ions and sugars over long distances.
与单细胞生物相比，大型多细胞植物的表面积与体积之比较小。仅靠扩散太慢，无法长距离为所有活细胞提供水分、矿质离子和糖类。
Leaves increase the surface area for gas exchange, but this does not solve the problem of moving water from roots to shoots or moving assimilates from leaves to roots and fruits.
叶片增加了气体交换的表面积，但这并不能解决将水分从根部运往地上部分，或将同化物从叶片运往根和果实的问题。
3. Metabolic Demands of Plants | 植物的代谢需求
Photosynthesis, respiration, growth and storage create high metabolic demands. For example, a large tree can lose hundreds of litres of water per day by transpiration, and this water must be replaced from the soil.
光合作用、呼吸作用、生长和储存产生了高代谢需求。例如，一棵大树每天可通过蒸腾作用散失数百升水，这些水必须从土壤中得到补充。
Meristems, developing fruits and underground storage organs often cannot photosynthesise enough to meet their own energy and carbon needs, so they must import assimilates from mature leaves.
分生组织、发育中的果实和地下储存器官通常无法通过自身光合作用满足能量和碳需求，因此必须从成熟叶片输入同化物。
4. Materials That Must Be Transported | 必须运输的物质
Plants need to move several types of material. Water and dissolved mineral ions move in the xylem from roots to shoots. Sucrose and amino acids move in the phloem from sources to sinks. Hormones move through vascular tissues or by cell-to-cell pathways.
植物需要运输多种物质。水和溶解的矿质离子在木质部中从根部向地上部分运输。蔗糖和氨基酸在韧皮部中从源向库运输。激素通过维管组织或细胞间途径运输。
| Substance 物质 | Main tissue 主要组织 | Direction 方向 |
|---|---|---|
| Water and mineral ions 水与矿质离子 | Xylem 木质部 | Roots to shoots 根到地上部 |
| Sucrose and amino acids 蔗糖与氨基酸 | Phloem 韧皮部 | Sources to sinks 源到库 |
| Hormones 激素 | Vascular tissues or cell-to-cell 维管组织或细胞间 | Various 多样 |
The table summarises the major transported materials, the tissues involved and the main direction of movement in a flowering plant.
上表总结了开花植物中主要运输物质、涉及的组织和主要运输方向。
5. Water Uptake and Upward Transport | 水分吸收与向上运输
Root hairs absorb water from the soil by osmosis. Water then crosses the root cortex through the apoplast, symplast or vacuolar pathways and enters the xylem vessels.
根毛通过渗透作用从土壤中吸收水分。水随后通过质外体、共质体或液泡途径穿过根皮层，进入木质部导管。
Xylem vessels are hollow dead tubes with lignified walls. They carry water and dissolved minerals upwards under tension created by transpiration from the leaves.
木质部导管是中空的死细胞管，具有木质化细胞壁。它们在叶片蒸腾作用产生的张力下向上运输水和溶解的矿物质。
6. Mineral Ion Uptake and Xylem Loading | 矿质离子吸收与木质部装载
Mineral ions such as nitrate and magnesium are absorbed by active transport, often against a concentration gradient. This uptake lowers the water potential in root cells and helps water enter by osmosis.
硝酸盐和镁等矿质离子通过主动运输被吸收，通常逆浓度梯度进行。这种吸收降低了根细胞的水势，有助于水通过渗透进入。
After uptake, ions are loaded into the xylem and transported to leaves. They are used in processes such as chlorophyll synthesis, protein formation and enzyme activation.
吸收后，离子被装载到木质部并运输到叶片。它们用于叶绿素合成、蛋白质形成和酶激活等过程。
7. Translocation of Assimilates in Phloem | 韧皮部中同化物的运输
Phloem transports assimilates, mainly sucrose, from photosynthetic sources to non-photosynthetic sinks. Sieve tube elements and companion cells are the main cell types involved in this transport.
韧皮部运输同化物，主要是蔗糖，从光合作用源到非光合作用库。筛管分子和伴胞是参与这一运输的主要细胞类型。
Mass flow in the phloem is driven by differences in hydrostatic pressure between the source end and the sink end of the sieve tube system.
韧皮部中的集流由筛管系统源端和库端之间的静水压差驱动。
8. Source-Sink Relationships | 源-库关系
A source is any tissue that exports more sugar than it uses, such as a mature photosynthesising leaf. A sink is any tissue that imports sugar, such as growing roots, developing fruits or storage tubers.
源是输出糖类多于消耗的组织，例如成熟的光合叶片。库是输入糖类的组织，例如生长中的根、发育中的果实或储存块茎。
The direction of phloem transport can change with the season. For example, a potato tuber acts as a sink in summer when it accumulates starch, but becomes a source in spring when stored starch is mobilised for new shoot growth.
韧皮部运输的方向可随季节变化。例如，马铃薯块茎在夏季积累淀粉时是库，但在春季储存的淀粉被动员用于新芽生长时则成为源。
9. Gas Exchange: Diffusion Is Sufficient | 气体交换：扩散已足够
Plants do not need a circulatory system for oxygen and carbon dioxide. These gases diffuse through stomata, lenticels and intercellular air spaces directly to and from respiring or photosynthesising cells.
植物不需要循环系统来运输氧气和二氧化碳。这些气体通过气孔、皮孔和细胞间隙直接扩散到呼吸或光合作用细胞或从这些细胞扩散出去。
The network of air spaces in leaves and stems keeps diffusion distances short. Therefore, gas exchange can occur efficiently by diffusion even in large plants.
叶片和茎中的气腔网络使扩散距离保持较短。因此，即使在大型植物中，气体交换也可以通过扩散高效进行。
10. Coordination by Plant Hormones | 植物激素的协调运输
Plant hormones such as auxin move from shoot tips to roots and coordinate tropisms, apical dominance and root development. This transport is often polar and cell-to-cell in young tissues.
生长素等植物激素从茎尖向根部运输，协调向性、顶端优势和根发育。这种运输在幼嫩组织中通常是极性且细胞间的。
Cytokinins and gibberellins are also transported, although often over shorter distances or through the xylem and phloem. Hormone transport allows different parts of the plant to respond to environmental and developmental signals.
细胞分裂素和赤霉素也可运输，但通常距离较短或通过木质部和韧皮部进行。激素运输使植物不同部位能够对环境信号和发育信号作出反应。
11. Comparing Plant and Mammalian Transport Systems | 植物与哺乳动物运输系统的比较
Plants use two separate vascular tissues rather than a single circulatory fluid. They have no central pump; xylem transport is driven by transpiration and cohesion-tension, while phloem translocation uses pressure flow.
植物使用两种独立的维管组织，而不是单一的循环流体。它们没有中央泵；木质部运输由蒸腾作用和内聚力-张力驱动，而韧皮部运输利用压力流。
| Feature 特征 | Plants 植物 | Mammals 哺乳动物 |
|---|---|---|
| Main transport fluids 主要运输流体 | Xylem sap, phloem sap 木质部汁液、韧皮部汁液 | Blood, lymph 血液、淋巴 |
| Pump 泵 | None; transpiration and pressure gradients 无；蒸腾作用和压力梯度 | Heart 心脏 |
| Vessels 管道 | Xylem and phloem 木质部和韧皮部 | Arteries, veins, capillaries 动脉、静脉、毛细血管 |
| Oxygen transport 氧气运输 | Mostly by diffusion in air spaces 主要在气腔中扩散 | Bound to haemoglobin in red blood cells 与红细胞中的血红蛋白结合 |
This comparison shows that the transport needs of plants are different because plant cells are supported by cell walls, have lower metabolic rates and can rely on diffusion for gas exchange.
这一比较表明，植物的运输需求不同，因为植物细胞有细胞壁支持，代谢率较低，并且气体交换可以依赖扩散。
12. Exam-Style Summary of Transport Needs | 运输需求的考试型总结
In an exam, define a transport need as the requirement to move water, mineral ions, assimilates and hormones between different plant organs. Link each substance to the correct vascular tissue and the main mechanism involved.
在考试中，将运输需求定义为在植物不同器官之间运输水、矿质离子、同化物和激素的需求。将每种物质与正确的维管组织和主要机制联系起来。
You should be able to explain why diffusion is sufficient for oxygen and carbon dioxide but not for water, mineral ions and sugars over long distances. Use surface area to volume ratio, metabolic demand and the separation of roots and leaves as key arguments.
你应该能够解释为什么扩散对氧气和二氧化碳足够，但对长距离运输水、矿质离子和糖类不够。使用表面积与体积比、代谢需求以及根与叶的分离作为关键论据。
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