Plant Transport in IGCSE OCR Biology – Key Concepts Explained | IGCSE OCR 生物：植物运输考点精讲

📚 Plant Transport in IGCSE OCR Biology – Key Concepts Explained | IGCSE OCR 生物：植物运输考点精讲

Plants need effective transport systems to move water, mineral ions and organic nutrients over long distances between roots and leaves. In IGCSE OCR Biology, understanding how xylem and phloem tissues work, the mechanisms driving transpiration and translocation, and the factors affecting these processes is essential for exam success. This article breaks down every key concept with clear explanations, diagrams in words, and exam-style tips.

植物需要高效的运输系统将水分、矿质离子和有机养分在根与叶之间长距离运输。在 IGCSE OCR 生物学中，掌握木质部和韧皮部的功能、驱动蒸腾作用和易位作用的机制，以及影响这些过程的因素是考试成功的关键。本文用清晰解释、文字图示和应考技巧，拆解每一个核心概念。

1. Why Plants Need Transport Systems | 植物为何需要运输系统

Multicellular plants have a small surface-area-to-volume ratio compared to unicellular organisms. Simple diffusion would be far too slow to supply all cells with water and nutrients. Vascular tissues — xylem and phloem — form a specialised transport network linking roots, stems and leaves. This system also provides structural support.

与单细胞生物相比，多细胞植物的表面积与体积之比较小。简单的扩散速度太慢，无法为所有细胞供应水和养分。维管组织——木质部和韧皮部——构成了连接根、茎、叶的专门运输网络。该系统还提供了结构支撑。

In the OCR specification, you must be able to describe the position and function of xylem and phloem in roots, stems and leaves. You should also link the need for transport to the size and metabolic demands of plants.

在 OCR 考纲中，你必须能描述木质部和韧皮部在根、茎、叶中的位置与功能。你还需将运输的需求与植物的大小及代谢需求联系起来。

2. Xylem Vessels – Structure and Function | 木质部导管——结构与功能

Xylem tissue transports water and dissolved mineral ions from the roots to the shoots. It consists of dead, hollow cells arranged end-to-end to form continuous tubes called xylem vessels. The end walls between vessel elements break down completely, leaving an uninterrupted column of water. Lignin deposited in the cell walls strengthens the walls and makes them waterproof.

木质部组织将水和溶解的矿质离子从根运输到茎叶。它由死亡的、中空的细胞首尾相连形成的连续管道——木质部导管——组成。导管分子之间的端壁完全消失，留下连续不断的水柱。细胞壁中的木质素增强了壁的强度并使其防水。

Key adaptations: No cytoplasm or nuclei (no obstruction to flow), lignin rings or spirals prevent collapse under tension, and pits (unlignified areas) allow sideways movement of water. In roots, the xylem is located centrally in the vascular cylinder; in stems, it is usually inside vascular bundles; in leaves, xylem is found on the upper side of veins.

关键适应特征：没有细胞质或细胞核（水流无阻碍），木质素环或螺旋防止在张力下塌陷，纹孔（未木质化区域）允许水分的侧向移动。在根中，木质部位于维管柱的中央；在茎中，通常位于维管束内侧；在叶中，木质部在叶脉的上侧。

3. Phloem – Structure and Function | 韧皮部——结构与功能

Phloem transports the products of photosynthesis (mainly sucrose and amino acids) from sources (e.g., leaves) to sinks (e.g., growing roots, fruits). Phloem is composed of living cells: sieve tube elements and companion cells. Sieve tubes are elongated cells arranged end-to-end; their end walls form sieve plates with pores that allow solutes to pass. These cells lose their nucleus and most organelles to reduce resistance to flow.

韧皮部将光合作用产物（主要是蔗糖和氨基酸）从“源”（如叶片）运输到“库”（如生长的根、果实）。韧皮部由活细胞组成：筛管分子和伴胞。筛管是首尾相连的长形细胞；它们的端壁形成筛板，上面有孔允许溶质通过。这些细胞失去了细胞核和大多数细胞器，以减少流动阻力。

Companion cells lie next to sieve tubes and retain a nucleus and many mitochondria. They provide ATP for active loading of sucrose into the sieve tubes. In vascular bundles, phloem is located on the outer side. In leaves, phloem is found on the lower side of veins.

伴胞紧邻筛管，保留细胞核和大量线粒体。它们为蔗糖主动装载进入筛管提供 ATP。在维管束中，韧皮部位于外侧。在叶中，韧皮部在叶脉的下侧。

4. Water Uptake and the Pathway into the Xylem | 水分吸收及进入木质部的路径

Water enters root hair cells by osmosis because the soil water has a higher water potential than the cell sap. Root hairs increase surface area. Water then moves across the root cortex via the apoplast pathway (through cell walls), symplast pathway (through cytoplasm and plasmodesmata), and vacuolar pathway. At the endodermis, the Casparian strip — a band of waterproof suberin — blocks the apoplast pathway, forcing water into the symplast, allowing selective mineral ion uptake.

水以渗透作用进入根毛细胞，因为土壤水的水势高于细胞液。根毛增加表面积。然后水通过质外体途径（经过细胞壁）、共质体途径（经过细胞质和胞间连丝）以及液泡途径穿过根皮层。在内皮层中，凯氏带——一条防水木栓质带——阻断质外体途径，迫使水进入共质体，从而允许选择性地吸收矿质离子。

Finally, water moves into the xylem vessels. This process does not require energy from the plant; it is driven by the water potential gradient. You must be able to label a diagram of a root cross-section showing the position of the xylem and the Casparian strip for the OCR exam.

最终，水进入木质部导管。该过程不需要植物的能量，由水势梯度驱动。在 OCR 考试中，你必须能够标注根横切面图中木质部和凯氏带的位置。

5. Transpiration – Definition and Process | 蒸腾作用——定义与过程

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves. It is a consequence of gas exchange: stomata open to allow carbon dioxide in for photosynthesis, and water vapour diffuses out. Transpiration creates a water potential gradient between the leaf cells and the atmosphere, which pulls water up the xylem.

蒸腾作用是指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进行。它是气体交换的结果：气孔打开让二氧化碳进入进行光合作用，同时水蒸气扩散出去。蒸腾作用在叶细胞与大气之间形成水势梯度，将水向上拉动通过木质部。

Transpiration is not simply “evaporation”; it is a controlled process. The rate of transpiration is affected by environmental factors, and the opening and closing of stomata regulate water loss. In the OCR specification, you need to know the role of guard cells in opening and closing stomata.

蒸腾作用不仅仅是“蒸发”，它是一个受控过程。蒸腾速率受环境因素影响，气孔的开闭可调节水分损失。在 OCR 考纲中，你需要了解保卫细胞在气孔开闭中的作用。

6. The Transpiration Stream – Cohesion-Tension Theory | 蒸腾流——内聚力-张力理论

The cohesion-tension theory explains how water moves up long distances in the xylem against gravity. Water molecules are cohesive: they form hydrogen bonds with each other. As water evaporates from the leaf mesophyll cells during transpiration, it creates a tension (negative pressure) at the top of the xylem. This tension pulls the continuous column of water up from the roots, because the water column is held together by cohesion.

内聚力-张力理论解释了水如何在木质部中长距离逆重力向上移动。水分子具有内聚力：它们之间形成氢键。当蒸腾作用使水分从叶肉细胞蒸发时，在木质部顶端形成张力（负压）。这种张力将连续的水柱从根向上拉动，因为水柱通过内聚力保持在一起。

Adhesion of water molecules to the xylem walls also helps to counteract gravity. The whole column remains unbroken because of the high tensile strength of water. No metabolic energy is used to lift the water — it is a passive physical process driven by the transpiration pull.

水分子对木质部壁的附着力也有助于抵消重力。由于水的高抗张强度，整个水柱保持不断。提升水分不消耗代谢能量——这是一个由蒸腾拉力驱动的被动物理过程。

7. Factors Affecting Transpiration Rate | 影响蒸腾速率的因素

Four main environmental factors influence the rate of transpiration. You must be able to explain their effects and interpret data from potometer experiments.

Light intensity: In bright light, stomata open wider to allow more CO₂ in for photosynthesis, so transpiration rate increases.

光照强度：强光下气孔张开更大以让更多 CO₂ 进入进行光合作用，因此蒸腾速率升高。
Temperature: Higher temperatures increase the kinetic energy of water molecules and increase the water-holding capacity of the air, so the rate of evaporation from mesophyll cells increases and transpiration rises.

温度：温度升高增加水分子动能，并增加空气容纳水蒸气的能力，因此叶肉细胞蒸发速率增加，蒸腾速率上升。
Humidity: High humidity reduces the water potential gradient between the leaf and the atmosphere, slowing down transpiration.

湿度：高湿度降低了叶片与大气之间的水势梯度，减慢蒸腾速率。
Air movement (wind): Moving air removes water vapour from around the leaf surface, maintaining a steep concentration gradient, so transpiration increases. In still air, vapour builds up, reducing the gradient.

空气流动（风）：流动的空气带走叶面周围的水蒸气，保持陡峭的浓度梯度，因此蒸腾速率增加。在静止空气中，水蒸气积累，减小梯度。

A potometer can measure water uptake by a cut shoot, which gives an indirect measure of transpiration rate. You need to control variables when designing investigations.

植物蒸腾计可以测量剪下的枝条吸水量，间接给出蒸腾速率。在设计实验时，你需要控制变量。

8. Potometer Investigations and Calculations | 蒸腾计实验与计算

A bubble potometer consists of a capillary tube with a scale, connected to a plant shoot and a reservoir of water. As the shoot transpires, water is pulled up the capillary tube, and an air bubble introduced into the tube moves along the scale. The rate of bubble movement is proportional to the rate of water uptake, which reflects transpiration rate (assumption: water uptake ≈ transpiration loss).

气泡蒸腾计由一根带刻度的毛细管、连接植物枝条和一个储水器组成。当枝条蒸腾时，水被吸入毛细管，引入管中的气泡沿刻度移动。气泡移动速率与吸水速率成正比，后者反映蒸腾速率（假设：吸水≈蒸腾失水）。

To calculate the rate, divide the distance moved by the bubble by the time taken. Typical units are mm min⁻¹ or cm³ min⁻¹ if you know the cross-sectional area. You must be able to describe precautions: cut the shoot under water to prevent air entering xylem, allow the plant to acclimatise, and use a single variable when testing factors.

计算速率时，用气泡移动的距离除以所用时间。如果知道横截面积，典型单位是 mm min⁻¹ 或 cm³ min⁻¹。你必须能描述注意事项：在水下剪切枝条以防空气进入木质部，让植株适应环境，测试单一变量时控制其他因素。

9. Translocation – Moving Sugars in the Phloem | 易位——韧皮部中糖的运输

Translocation is the movement of sucrose and amino acids from sources to sinks through phloem sieve tubes. Sources are plant regions that produce more organic nutrients than they use, such as mature leaves. Sinks are regions that store or use nutrients, e.g., developing roots, fruits, and young leaves. The direction of translocation can change depending on the plant’s needs.

易位是指蔗糖和氨基酸通过韧皮部筛管从“源”移动到“库”的过程。“源”是产生多于自身消耗的有机养分的植物区域，如成熟叶片。“库”是储存或使用养分的区域，例如果实、发育中的根和新叶。易位方向可根据植物的需要而改变。

The pressure-flow (mass flow) hypothesis is widely accepted. Sucrose is actively loaded from companion cells into sieve tubes at the source, lowering water potential. Water enters by osmosis from xylem, increasing hydrostatic pressure. At the sink, sucrose is actively unloaded or used, causing water to leave by osmosis and reducing pressure. Thus a pressure gradient drives mass flow from source to sink.

压力流（集流）假说被广泛接受。在“源”，蔗糖被主动从伴胞装载至筛管，降低水势。水从木质部通过渗透进入，增加静水压力。在“库”，蔗糖被主动卸载或消耗，水通过渗透离开，压力降低。因此压力梯度驱动从“源”到“库”的集流。

10. Evidence for Translocation – Ringing Experiments and Aphids | 易位的证据——环剥实验与蚜虫

Early evidence for phloem transporting organic substances came from ringing experiments. Removing a ring of bark (which contains phloem) from a woody stem causes swelling above the ring because sugars cannot pass to the roots. The tissues below eventually die, while the shoot remains alive. This shows that phloem transports sugars made in leaves downwards.

韧皮部运输有机物的早期证据来自环剥实验。从木本茎上剥除一圈树皮（含韧皮部）后，环的上方会肿胀，因为糖类无法输送到根部。下方的组织最终死亡，而枝条仍存活。这表明韧皮部向下运输叶片制造的糖类。

Radioactive tracers (e.g., ¹⁴C-labelled CO₂) can be supplied to a leaf; the radioactive carbon is incorporated into sucrose and can later be detected in phloem sap at different locations, confirming translocation. Aphid stylets can be used to sample phloem sap: an aphid is inserted into the stem, and its stylet is cut, allowing sap to be collected and analysed.

放射性示踪剂（如 ¹⁴C 标记的 CO₂）可提供给一片叶；放射性碳被掺入蔗糖，之后可在不同部位的韧皮部汁液中检测到，从而证实了易位。蚜虫口针可用来采集韧皮部汁液：将蚜虫插入茎中，切断其口针，即可收集和分析汁液。

11. Comparing Xylem and Phloem – A Summary Table | 木质部与韧皮部比较——总结表

Being able to compare xylem and phloem tissues quickly is a common exam requirement. Here is a summary table to help you memorise the key differences.

能够迅速比较木质部和韧皮部组织是常见的考试要求。下面的总结表可帮助你记忆关键区别。

Feature	Xylem	Phloem
Substance transported	Water and mineral ions	Sucrose and amino acids
Direction of flow	Up (roots → shoots)	Up and down (source → sink)
Cells	Dead, hollow tubes	Living sieve tubes, companion cells
End walls	Absent (completely broken down)	Sieve plates with pores
Lignin	Present (thickened walls)	Absent
Cytoplasm	None	Present (minimal in sieve tube)
Mechanism	Passive (transpiration pull)	Active loading → mass flow

Feature comparison:

特征比较：

Transported substance: Xylem — water, ions. Phloem — sucrose, amino acids. Direction: Xylem up; Phloem source to sink. Cell type: Xylem dead; Phloem living. End walls: Xylem absent; Phloem sieve plates. Lignin: Xylem present; Phloem absent. Cytoplasm: Xylem none; Phloem minimal. Mechanism: Xylem passive; Phloem active involved.

运输物质：木质部——水、离子。韧皮部——蔗糖、氨基酸。方向：木质部向上；韧皮部从源到库。细胞类型：木质部死细胞；韧皮部活细胞。端壁：木质部无；韧皮部有筛板。木质素：木质部有；韧皮部无。细胞质：木质部无；韧皮部极少。机制：木质部被动；韧皮部涉及主动过程。

12. Exam Tips and Common Pitfalls | 应考技巧与常见误区

In IGCSE OCR Biology, questions on plant transport often require precise use of terminology. Avoid confusing ‘transpiration’ (water loss) with ‘translocation’ (sugar movement). Do not state that xylem vessels are ‘alive’; they are dead at maturity. When describing potometer experiments, you must note that the bubble moves because of water uptake, not directly due to transpiration.

在 IGCSE OCR 生物学中，有关植物运输的题目常要求精确使用术语。避免混淆“蒸腾作用”（水分损失）和“易位”（糖的移动）。不要声称木质部导管是“活的”；它们在成熟时是死亡的。在描述蒸腾计实验时，你必须指出气泡移动是由于吸水，而非直接由蒸腾引起。

For explanations of the cohesion-tension theory, emphasise that it is a passive process — the energy comes from the sun driving evaporation. For translocation, be clear that active transport is involved in loading at the source. Always refer to water potential, not simply ‘concentration’, when discussing osmosis. Finally, practice labelled diagrams of cross-sections of root, stem and leaf, as these are frequently tested.

在解释内聚力-张力理论时，强调它是一个被动过程——能量来自太阳驱动蒸发。对于易位，要明确在“源”的装载涉及主动运输。讨论渗透作用时，始终使用“水势”而不是简单的“浓度”。最后，练习根、茎、叶横切面的标注图，这些经常被考查。

Published by TutorHao | IGCSE Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Plant Transport in IGCSE OCR Biology – Key Concepts Explained | IGCSE OCR 生物：植物运输 考点精讲