Transport in Plants: Xylem, Phloem and Transpiration | 植物运输：木质部、韧皮部与蒸腾作用

📚 Transport in Plants: Xylem, Phloem and Transpiration | 植物运输：木质部、韧皮部与蒸腾作用

In A-Level Biology, transport in plants brings together water relations, cell structure, and whole-plant physiology. You must be able to explain how xylem transports water and mineral ions from roots to leaves, how phloem moves sucrose from sources to sinks, and how experiments such as ringing, aphid tapping, and potometry provide evidence for these processes.

在 A-Level 生物中，植物运输将水分关系、细胞结构和整株生理学联系在一起。你必须能够解释木质部如何将水和矿质离子从根部运送到叶片，韧皮部如何将蔗糖从源运输到库，以及环割、蚜虫取食和蒸腾计等实验如何为这些过程提供证据。

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

Small aquatic plants can exchange gases and solutes by diffusion alone because every cell is close to the surrounding medium. In larger land plants, diffusion is far too slow: oxygen, carbon dioxide, water and mineral ions must travel distances of many metres between roots and shoots. Therefore plants need a bulk transport system made of xylem and phloem.

小型水生植物仅靠扩散就能进行气体和溶质交换，因为每个细胞都靠近周围介质。在较大的陆生植物中，扩散太慢：氧气、二氧化碳、水和矿质离子必须在根与茎叶之间运输数米以上的距离。因此，植物需要由木质部和韧皮部组成的集流运输系统。

As a plant grows, its surface area to volume ratio decreases, so simple diffusion cannot supply all cells. The vascular tissues provide a mass flow system that moves substances quickly over long distances.

随着植物生长，其表面积与体积之比下降，因此简单的扩散无法供应所有细胞。维管组织提供了集流系统，可以快速地将物质运输到较远的距离。

2. Xylem and Phloem: The Two Transport Tissues | 木质部与韧皮部：两大运输组织

Xylem is the water-conducting tissue. It contains tracheids and vessel elements. Vessel elements form continuous hollow tubes after their end walls break down. They are dead at maturity and have lignified cell walls, which prevent collapse under tension.

木质部是输导水分的组织。它包含管胞和导管分子。导管分子的端壁消失后形成连续的空心管道。它们在成熟时是死细胞，细胞壁木质化，可防止在张力下坍塌。

Phloem is the assimilate-conducting tissue. Sieve tube elements remain alive but lose their nucleus and most organelles. Each sieve tube element has a companion cell with a nucleus and many mitochondria that supplies ATP for active loading of sucrose.

韧皮部是输导同化产物的组织。筛管分子保持存活，但失去了细胞核和大部分细胞器。每个筛管分子旁边都有一个伴胞，伴胞含有细胞核和许多线粒体，为蔗糖的主动装载提供 ATP。

3. Water Uptake into Roots | 水分进入根部

Root hair cells are long extensions of epidermal cells that increase surface area for absorption. Water enters by osmosis because the soil solution normally has a higher water potential than the root hair cytoplasm. Mineral ions such as nitrate and potassium are absorbed by active transport and by facilitated diffusion down electrochemical gradients.

根毛细胞是表皮细胞的细长突起，可增加吸收表面积。水分通过渗透进入，因为土壤溶液的水势通常高于根毛细胞质。硝酸根和钾等矿质离子通过主动运输和沿电化学梯度的协助扩散被吸收。

Ψ = Ψₛ + Ψₚ

In this equation, Ψ is the total water potential, Ψₛ is the solute potential, and Ψₚ is the pressure potential. Active ion uptake lowers Ψₛ inside root cells, so water moves in by osmosis.

在该公式中，Ψ 为总水势，Ψₛ 为溶质势，Ψₚ 为压力势。根细胞通过主动吸收离子降低内部 Ψₛ，使水通过渗透进入。

The absorbed water then travels through the root by two pathways: the apoplast pathway through cell walls and intercellular spaces, and the symplast pathway through the cytoplasm connected by plasmodesmata. At the endodermis, the Casparian strip blocks the apoplast and forces water and ions through the plasma membrane, allowing the plant to control which solutes enter the xylem.

吸收的水分随后通过两条途径穿过根部：质外体途径沿细胞壁和细胞间隙移动，共质体途径通过由胞间连丝相连的细胞质移动。在内皮层，凯氏带阻断质外体途径，迫使水和离子穿过质膜，从而使植物能够控制哪些溶质进入木质部。

4. Root Pressure and Capillarity (Limited Roles) | 根压与毛细作用（有限作用）

Root pressure is a small positive pressure generated when endodermal cells actively pump ions into the xylem. This lowers the water potential in the xylem, so water enters by osmosis and pushes the water column upward. Guttation, the appearance of water droplets on leaf margins at night, is evidence of root pressure.

根压是一种较小的正压力，由内皮层细胞主动将离子泵入木质部而产生。这降低了木质部内的水势，使水通过渗透进入并向上推动水柱。吐水现象——夜间叶缘出现水滴——是根压存在的证据。

However, root pressure rarely raises water more than a few metres and is absent in many tall trees. Capillarity also contributes only a small rise because xylem vessels are too wide to generate sufficient adhesive force. These mechanisms cannot explain transport to the top of a 100 m tree.

然而，根压通常只能使水上升几米，而且在许多高大树木中并不存在。毛细作用也只能产生很小的上升高度，因为木质部导管太宽，无法产生足够的附着力。这些机制无法解释水分如何被运输到 100 米高的树顶。

5. The Cohesion-Tension Theory | 内聚力-张力理论

The cohesion-tension theory is the accepted explanation for water movement in xylem. Transpiration from leaves generates tension, a negative pressure, at the top of the plant. Because water molecules are cohesive due to hydrogen bonding, the whole column of water is pulled up the xylem as one continuous stream.

内聚力-张力理论是解释木质部中水分运输的公认理论。叶片蒸腾在植物顶部产生张力，即负压。由于水分子之间因氢键而具有很强的内聚力，整条水柱被作为一个连续的水流沿木质部向上拉动。

Published by TutorHao | A-Level Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）