📚 IGCSE CIE Biology: Plant Transport Key Points | IGCSE CIE 生物：植物运输考点精讲
Plants require a transport system to move water, mineral ions, and organic nutrients throughout their bodies. Unlike animals, plants have two distinct vascular tissues—xylem and phloem—that form the plant's transport system. Understanding how these tissues are structured and how they function is essential for IGCSE CIE Biology. This article breaks down every key concept, from root hair absorption to transpiration and translocation, ensuring you are fully prepared for the exam.
植物需要一个运输系统来输送水、矿物质离子和有机营养物质到全身。与动物不同，植物有两种不同的维管组织——木质部和韧皮部——它们构成了植物的运输系统。理解这些组织的结构和功能对于 IGCSE CIE 生物学至关重要。本文分解从根毛吸收到蒸腾作用和转运的每一个关键概念，确保你为考试做好充分准备。
1. Introduction to Plant Transport Systems | 植物运输系统简介
Multicellular plants have a relatively large body and a low surface area to volume ratio, so diffusion alone cannot supply all cells with water and nutrients. Therefore, they possess specialised transport tissues: xylem and phloem. These form vascular bundles running through roots, stems, and leaves, providing efficient transport over long distances.
多细胞植物体型较大，表面积与体积之比较低，因此仅靠扩散无法为所有细胞提供水分和营养。因此，它们拥有专门的输导组织：木质部和韧皮部。这些组织形成贯穿根、茎、叶的维管束，实现远距离的高效运输。
In roots, vascular tissue is central; in stems, it forms a ring of bundles; in leaves, xylem is on the upper side of veins and phloem on the lower. This arrangement supports the distinct functions of each tissue.
在根中，维管组织位于中央；在茎中，它形成一圈维管束；在叶中，木质部位于叶脉的上侧，韧皮部在下侧。这种排列支持了每种组织的不同功能。
2. Xylem: Structure and Function | 木质部：结构与功能
Xylem tissue transports water and dissolved mineral ions from the roots up to the stems and leaves. The flow in xylem is unidirectional (upwards). Xylem vessels are formed from dead cells aligned end-to-end, with no cytoplasm, no nucleus, and no end walls, creating a continuous hollow tube.
木质部组织将水和溶解的矿物质离子从根部向上运输到茎和叶。木质部中的流动是单向的（向上）。木质部导管由首尾相接的死细胞形成，没有细胞质，没有细胞核，也没有端壁，从而形成连续的空心管道。
The walls are strengthened with lignin, a waterproof substance that provides structural support and prevents collapse. Lignin can be deposited in rings, spirals, or solid patterns. The presence of pits (small unlignified areas) allows lateral movement of water between adjacent vessels.
细胞壁由木质素加固，木质素是一种防水物质，可提供结构支撑并防止塌陷。木质素可以沉积成环状、螺旋状或实心图案。纹孔（小的未木质化区域）的存在允许水分在相邻导管间横向移动。
Key adaptations of xylem for transport: (i) hollow tubes with no cytoplasm offer low resistance to water flow; (ii) lignin strengthens the walls, preventing collapse under tension; (iii) pits permit water to move sideways if a vessel is blocked.
木质部运输的关键适应：(i) 空管无细胞质，对水流阻力小；(ii) 木质素加固管壁，防止在张力下塌陷；(iii) 纹孔允许导管受阻时水横向移动。
3. Phloem: Structure and Function | 韧皮部：结构与功能
Phloem transports organic nutrients, mainly sucrose and amino acids, from sources to sinks. This process is called translocation and the flow can be bidirectional. Phloem is composed of sieve tube elements and companion cells.
韧皮部将有机营养物质，主要是蔗糖和氨基酸，从源输送到库。这个过程称为转运，流动是双向的。韧皮部由筛管分子和伴胞组成。
Sieve tube elements are living cells but lack a nucleus and many organelles; they have perforated end walls called sieve plates, allowing cytoplasmic connections and mass flow of sap. Companion cells are metabolically active, containing a nucleus, many mitochondria, and ribosomes, and they provide energy (ATP) to load sucrose into sieve tubes.
筛管分子是活细胞，但缺少细胞核和许多细胞器；它们有穿孔的端壁，称为筛板，允许细胞质连接和汁液的整体流动。伴胞代谢活跃，含有细胞核、大量线粒体和核糖体，它们提供能量（ATP）将蔗糖装载到筛管中。
ls via plasmodesmata ensures efficient transport. Remember: xylem = dead, phloem = living.
筛管与伴胞通过胞间连丝紧密联系，确保了高效运输。记住：木质部 = 死细胞，韧皮部 = 活细胞。
4. Water Uptake by Roots | 根部水分吸收
Water is absorbed from the soil through root hair cells, which are long, thin extensions of epidermal cells, increasing surface area for absorption. Water enters root hairs by osmosis, as the cell sap has a lower water potential than the soil solution (due to dissolved solutes like mineral ions).
水分通过根毛细胞从土壤中吸收。根毛是表皮细胞的长而薄的延伸物，增加了吸收表面积。水分通过渗透作用进入根毛，因为细胞液的水势低于土壤溶液（由于溶解的溶质如矿物质离子）。
Mineral ions, such as nitrates and magnesium, are taken up by active transport, requiring energy from respiration. This increases the solute concentration inside the root, lowering water potential and further enhancing water uptake. From the root hair, water moves across the cortex via the apoplast and symplast pathways, eventually reaching the xylem in the vascular cylinder.
矿物质离子（如硝酸盐和镁）通过主动运输吸收，需要呼吸作用提供的能量。这增加了根部内部的溶质浓度，降低了水势，进一步促进水分吸收。水从根毛穿过皮层，通过质外体和共质体途径，最终到达维管柱中的木质部。
Apoplast pathway: water moves through cell walls and intercellular spaces, not entering the cytoplasm. Symplast pathway: water moves through the cytoplasm and plasmodesmata. At the endodermis, the Casparian strip (a waxy, waterproof band) blocks the apoplast route, forcing water into the symplast, allowing selective control of mineral uptake.
质外体途径： 水分通过细胞壁和细胞间隙移动，不进入细胞质。共质体途径： 水分通过细胞质和胞间连丝移动。在内皮层，凯氏带（一种蜡质防水带）阻断质外体途径，迫使水进入共质体，从而实现对矿物质吸收的选择性控制。
5. Transpiration and the Transpiration Stream | 蒸腾作用与蒸腾流
Transpiration is the loss of water vapour from the above-ground parts of a plant, mainly through stomata in the leaves. This evaporation creates a negative pressure (tension) at the top of the xylem, pulling water up in a continuous stream – the transpiration stream.
蒸腾作用是水分以水蒸气形式从植物地上部分（主要通过叶片上的气孔）散失的过程。这种蒸发在木质部顶部产生负压（张力），以连续的水流——蒸腾流——将水向上拉。
Transpiration serves several functions: it transports mineral ions dissolved in water, cools the plant, and provides water for photosynthesis and cell turgidity. However, the plant must balance water loss with CO₂ uptake, as stomata open for gas exchange.
蒸腾作用具有多种功能：运输溶解在水中的矿物质离子，冷却植物，为光合作用和细胞膨压提供水。
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