📚 Plant Transport in IB Biology – Key Points | IB 生物：植物运输考点精讲
Plants rely on specialised vascular systems to move water, mineral ions, and organic solutes across sometimes great distances. Without xylem and phloem, roots could not supply leaves with water, and photosynthetic products would never reach non-photosynthetic tissues. This revision guide unpacks the essential concepts of plant transport that frequently appear in IB Biology examinations, from water potential to pressure-flow.
植物依赖特化的维管系统将水、矿质离子和有机溶质运输到有时很远的距离。如果没有木质部和韧皮部，根系就无法为叶片供水，光合产物也永远无法到达非光合组织。本篇复习指南梳理了 IB 生物考试中频频出现的植物运输核心概念，从水势到压力流。
1. Overview of Plant Transport Systems | 植物运输系统概述
Vascular plants possess two long-distance transport tissues: xylem and phloem. Xylem conducts water and dissolved minerals from the roots to the stems and leaves in a mostly upward direction. Phloem transports organic compounds, primarily sucrose, from sources (e.g., mature leaves) to sinks (e.g., roots, developing fruits) and can move materials both upwards and downwards.
维管植物拥有两种长距离运输组织：木质部和韧皮部。木质部将水分和溶解的矿物质以基本上向上的方向从根运输到茎和叶。韧皮部则将有机化合物（主要是蔗糖）从源（如成熟叶片）运输到库（如根、发育中的果实），可以向上或向下移动。
The need for transport systems arises because diffusion alone is too slow to meet the metabolic demands of large multicellular plants. The vascular bundles, arranged differently in roots and stems, provide efficient conduits and mechanical support.
之所以需要运输系统，是因为单凭扩散太慢，无法满足大型多细胞植物的代谢需求。维管束在根和茎中排列方式不同，既提供了高效的管道，也提供了机械支撑。
2. Water Absorption by Roots | 根系吸水
Most water enters the plant through root hairs, which are extensions of epidermal cells located just behind the root tip. These thin-walled projections greatly increase the surface area for osmosis. Water moves from the soil into root cells down a water potential gradient; the soil solution usually has a higher water potential than the root hair cytoplasm.
大部分水通过根毛进入植物体，根毛是位于根尖后方的表皮细胞突起。这些薄壁突起大大增加了渗透作用的表面积。水沿着水势梯度从土壤进入根部细胞；土壤溶液的水势通常高于根毛细胞质的水势。
Once inside the root, water travels radially across the root cortex towards the xylem in the stele. Dissolved mineral ions are taken up by active transport and facilitated diffusion, which helps to lower the water potential inside root cells, drawing in more water by osmosis.
进入根部后，水沿根部皮层径向移动，朝着位于中柱的木质部前进。溶解的矿质离子通过主动运输和协助扩散被吸收，这有助于降低根部细胞内的水势，从而通过渗透作用吸收更多水分。
3. Pathways of Water Movement in the Root | 根中水分运输途径
Three pathways are available for water and solutes moving across the root cortex: the apoplast pathway (through cell walls and intercellular spaces), the symplast pathway (through the cytoplasm and plasmodesmata), and the vacuolar pathway (across vacuoles). In the apoplast route, water moves freely until it reaches the endodermis.
水分和溶质穿过根皮层时有三条途径：质外体途径（通过细胞壁和细胞间隙）、共质体途径（通过细胞质和胞间连丝）和液泡途径（穿过液泡）。在质外体途径中，水自由移动，直到抵达内皮层。
The endodermis contains a waterproof Casparian strip made of suberin that blocks the apoplastic flow, forcing water and dissolved ions to cross the plasma membrane into the symplast. This selective barrier allows the plant to control which ions enter the xylem and prevents backflow.
内皮层含有由木栓质构成的防水凯氏带，它会阻断质外体流动，迫使水分和溶解的离子穿过质膜进入共质体。这道选择性屏障使植物能够控制哪些离子进入木质部，并防止倒流。
4. Transpiration and the Cohesion-Tension Theory | 蒸腾作用与内聚力-张力理论
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves. This evaporation creates a negative pressure (tension) at the leaf surface, which pulls water up the xylem in a continuous column.
蒸腾作用是水蒸气从植物地上部分散失的过程，主要通过叶片上的气孔。这种蒸发在叶片表面产生负压（张力），将木质部中的水柱不断向上拉。
The cohesion-tension theory explains how water rises against gravity. Cohesion (hydrogen bonds between water molecules) keeps the water column intact, while adhesion of water molecules to xylem vessel walls helps to counteract gravity. The strong tensile strength of water prevents the column from breaking under transpirational pull.
内聚力-张力理论解释了水如何克服重力上升。内聚力（水分子之间的氢键）使水柱保持连续，而水分子与木质部导管壁的附着力则帮助抵消重力。水强大的抗张强度使得水柱在蒸腾拉力下不会断裂。
Root pressure, resulting from active ion uptake into the xylem, can generate a small upward push, but it is not the main driving force for water transport in tall trees. Guttation in some small plants is evidence of root pressure, but transpirational pull is far more significant.
根压是由于离子被主动泵入木质部而产生的一种微弱上推力，但它并不是高大乔木水分运输的主要驱动力。一些小植株出现的吐水现象是根压的证据，但蒸腾拉力要重要得多。
5. Factors Affecting Transpiration | 影响蒸腾作用的因素
Transpiration rate is influenced by several environmental factors. Light stimulates stomatal opening and raises leaf temperature, increasing evaporation. Higher temperatures raise the kinetic energy of water molecules and increase the water vapour pressure deficit between the leaf and the air.
蒸腾速率受多种环境因素影响。光照促进气孔开放并使叶片升温，从而加快蒸发。较高的温度提高了水分子的动能，增大了叶片与空气之间的水蒸气压差。
Humidity exerts an inverse effect: as the air becomes more saturated with water vapour, the gradient for diffusion from the leaf interior decreases, so transpiration slows. Wind removes the boundary layer of humid air around the stomata, steepening the diffusion gradient and increasing transpiration rate, though very strong wind can cause stomatal closure.
湿度具有相反的作用：当空气水蒸气较为饱和时，从叶片内部向外扩散的梯度减小，蒸腾变慢。风会将气孔周围湿润的空气边界层带走，使扩散梯度变陡，从而提高蒸腾速率，但强风可能导致气孔关闭。
Stomatal aperture is regulated by guard cells. When guard cells accumulate K? ions, water follows by osmosis, cells become turgid and the stoma opens. Loss of K? causes the guard cells to lose turgor and the stoma closes. Abscisic acid (ABA) signals stomatal closure under water stress.
气孔开度由保卫细胞调控。当保卫细胞积累 K? 离子时，水分因渗透作用进入，细胞变得膨大，气孔张开。K? 外流会导致保卫细胞失水萎蔫，气孔关闭。在水分胁迫下，脱落酸 (ABA) 会触发气孔关闭。
6. Xylem Structure and the Transpiration Stream | 木质部结构与蒸腾流
Xylem vessels are formed from dead cells aligned end-to-end, with their end walls broken down to create long, hollow tubes. Their walls are reinforced with lignin, which may be deposited in annular, spiral, or pitted patterns, providing mechanical strength and preventing collapse under negative pressure.
木质部导管由首尾相接的死细胞构成，其端壁消失，形成长长的中空管道。管壁上沉积着木质素，可呈现环纹、螺纹或孔纹，提供机械强度并防止在负压下塌陷。
Tracheids, also lignified, are elongated cells with tapered ends and pits in their walls. Water moves between adjacent tracheids through pits. In flowering plants, xylem vessels are the main water conduits, but tracheids still occur in many species and are the only water-conducting cells in conifers.
管胞也是木质化的长形细胞，末端渐尖，壁上有具缘纹孔。水分通过纹孔在相邻管胞间流动。在开花植物中，导管是主要的输水通道，但管胞仍存在于许多物种中，且是针叶树唯一的输导细胞。
In exams, you may be asked to draw and label a transverse section of a dicotyledonous stem or root, showing the position of xylem and phloem. Remember that in roots the xylem is centrally located and often star-shaped, while in stems the vascular bundles are arranged in a ring with xylem closer to the pith.
考试中可能要求绘制并标注双子叶植物茎或根的横切面，显示木质部和韧皮部的位置。请记住，在根部木质部位于中央，常呈星状；而在茎中，维管束成环状排列，木质部靠近髓部。
7. Phloem Structure and Translocation | 韧皮部结构与运输
Phloem is composed of sieve tube elements and companion cells. Sieve tube elements are living cells that lack a nucleus, ribosomes, and a vacuole at maturity, but retain a functional plasma membrane and thin cytoplasm. Their end walls form sieve plates with large pores that allow the mass flow of phloem sap.
韧皮部由筛管分子和伴胞组成。筛管分子是生活的细胞，成熟后失去细胞核、核糖体和液泡，但保留功能性质膜和薄层细胞质。其端壁形成筛板，具有大孔，允许韧皮部汁液的整体流动。
Each sieve tube element is closely associated with at least one companion cell via numerous plasmodesmata. The companion cell contains a nucleus and many mitochondria, providing ATP for active loading of sucrose into the sieve tube.
每个筛管分子通过大量胞间连丝与至少一个伴胞紧密联系。伴胞含有细胞核和大量线粒体，为蔗糖主动装载进入筛管提供 ATP。
Translocation is the movement of organic solutes, mainly sucrose, from sources to sinks. Unlike xylem transport, phloem transport requires metabolic energy and can occur in either direction depending on the location of sources and sinks.
运输作用是指有机溶质（主要是蔗糖）从源向库的移动。与木质部运输不同，韧皮部运输需要代谢能量，并可根据源和库的位置双向进行。
8. Phloem Loading: From Source to Sink | 韧皮部装载：由源至库
In source tissues such as mature leaves, sucrose produced by photosynthesis is actively loaded into the sieve tubes. At the companion cell membrane, a proton pump (H?-ATPase) creates a proton gradient. A sucrose-H? cotransporter then uses this gradient to move sucrose into the companion cells against its concentration gradient.
在成熟叶片等源组织中，光合作用产生的蔗糖被主动装载到筛管中。在伴胞膜上，质子泵（H?-ATPase）建立质子梯度。然后，蔗糖-H? 共转运蛋白利用这一梯度将蔗糖逆浓度梯度运入伴胞。
In some plants, sucrose moves symplastically through plasmodesmata from mesophyll cells into companion cells, but active loading is still required to maintain the steep concentration difference that drives the pressure-flow mechanism. Once inside the sieve tubes, the high solute concentration lowers water potential.
在有些植物中，蔗糖通过胞间连丝以共质体途径从叶肉细胞进入伴胞，但仍需主动装载以维持驱动压力流机制所需的陡峭浓度差。一旦进入筛管，高溶质浓度会降低水势。
9. The Pressure-Flow Hypothesis | 压力流假说
The pressure-flow mechanism explains how phloem sap moves from source to sink. At the source, active loading of sucrose into the sieve tubes lowers the water potential, causing water to enter from the adjacent xylem by osmosis. This influx of water generates a high hydrostatic pressure.
压力流机制解释了韧皮部汁液如何从源流向库。在源端，蔗糖主动装载进入筛管降低了水势，促使水分通过渗透从邻近木质部进入。这股水流的涌入产生了较高的静水压力。
At the sink, sucrose is actively or passively unloaded into cells that use or store it. This raises the water potential in the sieve tube, so water leaves the phloem and re-enters the xylem, resulting in a lower hydrostatic pressure. The pressure gradient between source and sink drives a bulk flow of sap through the sieve pores.
在库端，蔗糖被主动或被动地卸载到利用或储存它的细胞中。这使筛管内的水势升高，水分离开韧皮部重新进入木质部，导致静水压力下降。源库之间的压力梯度驱动汁液通过筛孔进行整体流动。
Evidence supporting the pressure-flow hypothesis includes the exudation of phloem sap from cut aphid stylets, the measurement of pressure gradients along the phloem, and the correlation between solute concentration and flow rate. Students should be able to relate the water potential equation, Ψ = Ψₛ + Ψₚ, to this process.
支持压力流假说的证据包括：切断蚜虫口器后会渗出韧皮部汁液、在韧皮部中测得压力梯度，以及溶质浓度与流速的相关性。学生应能运用水势方程 Ψ = Ψₛ + Ψₚ 解释该过程。
10. Experimental Evidence for Plant Transport | 植物运输的实验证据
A potometer measures the rate of water uptake by a leafy shoot. Although it does not measure transpiration directly, it provides a close estimate under controlled conditions. Students should be able to design investigations to examine how light intensity, wind speed, or humidity affect the rate of water uptake.
蒸腾计测量带叶枝条的吸水速率。虽然它不能直接测量蒸腾作用，但在控制条件下可以给出近似的估算值。学生应能设计实验探究光照强度、风速或湿度如何影响吸水速率。
Ringing experiments, in which a ring of bark and phloem is removed from a woody stem, cause swelling above the ring because sugars accumulate at the source side. This demonstrates that phloem is responsible for downward translocation of organic nutrients, while xylem transport continues unaffected.
环剥实验将木本茎的一圈树皮和韧皮部剥去，之后环剥上方会膨大，原因是糖类在源侧积聚。这证明韧皮部负责有机养分的向下运输，而木质部运输不受影响。
Radioactive tracers such as ?⁴C? labelled CO₂ can be supplied to a leaf; autoradiography then reveals the movement of labelled assimilates through the phloem. Aphids feeding on phloem can be used to collect pure phloem sap for analysis.
放射性示踪剂（如 ?⁴C? 标记的 CO₂）可供给叶片，随后通过放射自显影显示标记同化产物在韧皮部中的移动。取食韧皮部的蚜虫可用于收集纯净的韧皮部汁液进行分析。
11. Adaptations of Plants to Water Stress | 植物对水分胁迫的适应
Xerophytes, such as marram grass and cacti, possess adaptations that minimise water loss. These include a thick waxy cuticle, sunken stomata in pits that trap humid air, rolled leaves that reduce the surface area exposed to wind, and extensive shallow or deep roots to maximise water uptake.
旱生植物（如马兰草和仙人掌）具有最大限度减少水分损失的适应特征，包括厚厚的蜡质角质层、陷在凹坑中的气孔（可锁住湿润空气）、卷曲的叶片以减少受风面积，以及广布的浅根系或深根系以最大化吸水。
Many xerophytes also use Crassulacean Acid Metabolism (CAM), in which stomata open at night to fix CO₂ into organic acids, then close during the day. This temporal separation of gas exchange drastically reduces water loss while still allowing photosynthesis to occur.
许多旱生植物还采用景天酸代谢 (CAM)，夜间气孔张开将 CO₂ 固定为有机酸，白天则关闭气孔。这种气体交换的时间分离大幅减少了水分损失，同时仍能进行光合作用。
Halophytes tolerate high-salinity environments by accumulating solutes in their roots to maintain a favourable water potential gradient, secreting salt through salt glands, or compartmentalising salt in vacuoles. Though less commonly examined, their adaptations illustrate the flexibility of plant transport physiology.
盐生植物耐受高盐环境的方式包括：在根部积累溶质以维持有利的水势梯度、通过盐腺分泌盐分，或将盐分隔在液泡中。虽然考察较少，但这些适应特征体现了植物运输生理的灵活性。
Published by TutorHao | IB Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导