Transport in Plants | 植物体内的运输

📚 Transport in Plants | 植物体内的运输

In plants, transport systems move water, mineral ions, sugars and other solutes over distances that simple diffusion cannot cover. This article explains how xylem and phloem are adapted for long-distance transport, and how transpiration and translocation work.

植物体内的运输系统负责将水分、矿质离子、糖类和其他溶质运送到简单扩散无法到达的距离。本文将解释木质部和韧皮部如何适应长距离运输，以及蒸腾作用和韧皮部运输是如何进行的。

1. Why Plants Need Transport Systems | 植物为什么需要运输系统

Multicellular plants have many organs that are large and compact, so their surface area to volume ratio is often small. Diffusion alone is too slow to supply all cells with water, mineral ions and organic solutes, especially over long distances from roots to leaves.

多细胞植物有许多器官较大且结构紧密，因此它们的表面积与体积之比通常较小。仅靠扩散太慢，无法为所有细胞提供水分、矿质离子和有机溶质，尤其是从根到叶的长距离运输。

Diffusion is only efficient over very short distances because the time taken is proportional to the square of the distance. A molecule may cross a cell in milliseconds, but moving one metre by diffusion could take years. Plants therefore need bulk flow systems in xylem and phloem.

扩散只在很短的距离内高效，因为所需时间与距离的平方成正比。一个分子穿过一个细胞可能只需几毫秒，但靠扩散移动一米可能需要数年。因此，植物需要木质部和韧皮部中的集流运输系统。

Roots absorb water and mineral ions from the soil, while leaves produce sugars during photosynthesis. Transport systems link these source and sink regions and maintain water balance throughout the plant.

根从土壤中吸收水分和矿质离子，而叶片在光合作用中产生糖类。运输系统将这些源区和库区连接起来，并维持整个植物的水分平衡。

2. Xylem and Phloem: Two Transport Tissues | 木质部与韧皮部：两种运输组织

Xylem and phloem are vascular tissues found together in vascular bundles. Xylem is responsible for transporting water and dissolved mineral ions upwards from roots to shoots. Phloem transports organic solutes, mainly sucrose, and amino acids from sources to sinks.

木质部和韧皮部是共同存在于维管束中的维管组织。木质部负责将水分和溶解的矿物质从根向上运输到地上部分。韧皮部运输有机溶质，主要是蔗糖，以及氨基酸，从源运往库。

The arrangement of vascular bundles differs between organs. In roots, xylem and phloem alternate around the centre of the root, while in stems they form a ring. In leaves, vascular bundles form the veins and midrib, bringing xylem close to photosynthesising mesophyll cells.

维管束的排列方式因器官不同而不同。在根中，木质部和韧皮部在根中心周围交替排列；在茎中，它们形成一个环。在叶片中，维管束形成叶脉和中脉，使木质部靠近进行光合作用的叶肉细胞。

The two tissues are structurally different because they carry different fluids and operate under different pressures. Xylem mainly carries water under tension, while phloem carries sucrose-rich sap under positive pressure.

两种组织在结构上不同，因为它们运输的液体不同，工作压力也不同。木质部主要在张力下运输水分，而韧皮部在正压下运输富含蔗糖的汁液。

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

Xylem vessels are long continuous tubes formed from dead cells arranged end to end. The end walls between cells break down, leaving a hollow lumen with no cytoplasm, nucleus or organelles to resist water flow.

木质部导管是由

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