Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输

📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输

Every living cell is surrounded by a partially permeable cell membrane, and the movement of substances across this membrane is fundamental to life. In IGCSE Science, you need to understand three key processes: diffusion, osmosis and active transport. These processes explain how cells obtain oxygen and nutrients, remove waste products, and maintain the correct internal conditions.

每一个活细胞都被一层部分通透的细胞膜所包围，物质穿过细胞膜的运动是生命的基础。在 IGCSE 科学课程中，你需要理解三个关键过程：扩散、渗透与主动运输。这些过程解释了细胞如何获取氧气和营养物质、清除代谢废物，并维持正确的内部环境。

1. The Cell Membrane: A Partially Permeable Barrier | 细胞膜：部分通透的屏障

The cell membrane is described as partially permeable, meaning it allows some substances to pass through but not others. Small molecules such as water, oxygen and carbon dioxide can pass through easily, while larger molecules such as sugars and proteins cannot pass through freely.

细胞膜被描述为部分通透，这意味着它允许某些物质通过，而阻止另一些物质通过。水、氧气和二氧化碳等小分子可以轻易穿过，而糖和蛋白质等较大的分子则不能自由通过。

This selective behaviour is the key to understanding how diffusion, osmosis and active transport work. Each process moves substances in a specific direction and requires different conditions:

这种选择性行为是理解扩散、渗透和主动运输如何运作的关键。每个过程都以特定的方向移动物质，并且需要不同的条件：

Diffusion: passive movement from high to low concentration.

扩散：从高浓度到低浓度的被动运动。
Osmosis: passive movement of water across a partially permeable membrane.

渗透：水穿过部分通透膜的被动运动。
Active transport: energy-requiring movement against the concentration gradient.

主动运输：逆浓度梯度、需要能量的运动。

2. Diffusion | 扩散

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient. This is a passive process, meaning it does not require energy. Diffusion occurs because particles have kinetic energy and are in constant, random motion.

扩散是粒子从高浓度区域到低浓度区域、沿浓度梯度向下的净运动。这是一个被动过程，意味着它不需要能量。扩散的发生是因为粒子具有动能，并且处于不断的随机运动中。

In the lungs, oxygen diffuses from the alveoli (where oxygen concentration is high) into the blood (where oxygen concentration is lower). At the same time, carbon dioxide diffuses in the opposite direction. In plant leaves, carbon dioxide diffuses into the leaf through stomata, while oxygen diffuses out during photosynthesis.

在肺中，氧气从肺泡（氧气浓度高）扩散进入血液（氧气浓度较低）。同时，二氧化碳沿相反方向扩散。在植物叶片中，二氧化碳通过气孔扩散进入叶片，而在光合作用中氧气则扩散出去。

net movement: high concentration → low concentration

净运动：高浓度 → 低浓度

3. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素

The rate of diffusion is affected by four main factors. Understanding these allows scientists to predict and control diffusion in biological systems, and they are commonly tested in IGCSE exams.

扩散速率受四个主要因素影响。理解这些因素使科学家能够预测和控制生物系统中的扩散现象，这也是 IGCSE 考试中的常见考点。

Concentration gradient: The greater the difference in concentration, the faster the rate of diffusion.

浓度梯度：浓度差越大，扩散速率越快。
Temperature: Higher temperature increases particle kinetic energy, so particles move faster and diffusion happens more quickly.

温度：较高的温度增加粒子的动能，因此粒子运动更快，扩散发生得更迅速。
Surface area: A larger surface area allows more particles to cross the membrane at the same time, increasing the rate.

表面积：更大的表面积允许更多粒子同时穿过膜，从而增加速率。
Distance: A shorter diffusion distance means particles reach their destination faster, increasing the rate.

距离：较短的扩散距离意味着粒子更快到达目的地，从而增加速率。

For example, the alveoli in the lungs have a very large surface area, a thin wall (short diffusion distance), and are well ventilated to maintain a steep concentration gradient. These features maximise the rate of gas exchange.

例如，肺中的肺泡具有非常大的表面积、很薄的壁（扩散距离短）并且通气良好以维持陡峭的浓度梯度。这些特征使气体交换速率最大化。

4. Surface Area to Volume Ratio | 表面积与体积比

As organisms increase in size, their volume increases faster than their surface area. This means the surface area to volume ratio (SA:V) decreases as an organism grows. A single-celled organism such as an amoeba has a large SA:V ratio, so diffusion alone is sufficient to meet its needs.

随着生物体增大，其体积的增长速度快于表面积。这意味着随着生物体生长，其表面积与体积比（SA:V）会减小。像变形虫这样的单细胞生物具有较大的 SA:V 比，因此仅靠扩散就足以满足其需求。

For example, a cube with side length 1 cm has a surface area of 6 cm² and a volume of 1 cm³, giving a ratio of 6:1. If the side length doubles to 2 cm, the surface area becomes 24 cm² and the volume becomes 8 cm³, giving a ratio of only 3:1.

例如，边长为 1 cm 的立方体表面积为 6 cm²，体积为 1 cm³，比值为 6:1。如果边长加倍至 2 cm，表面积变为 24 cm²，体积变为 8 cm³，比值仅为 3:1。

1 cm cube: SA:V = 6:1 → 2 cm cube: SA:V = 3:1

1 cm 立方体：SA:V = 6:1 → 2 cm 立方体：SA:V = 3:1

This is why multicellular organisms need specialised exchange surfaces such as lungs, gills and root hairs. These structures increase the available surface area and reduce the diffusion distance, compensating for the low SA:V ratio of the whole organism.

这就是为什么多细胞生物需要肺、鳃和根毛等特化的交换表面。这些结构增加了可用表面积并减小了扩散距离，从而弥补了整个生物体 SA:V 比低的不足。

5. Osmosis: The Diffusion of Water | 渗透：水的扩散

Osmosis is a special case of diffusion. It is defined as the net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane. Be careful: in osmosis, we always talk about the movement of water, not solutes.

渗透是扩散的一种特殊情况。它被定义为水分子通过部分通透膜，从水势较高的区域向水势较低区域的净运动。请注意：在渗透中，我们始终讨论的是水的运动，而不是溶质的运动。

Water moves from where there is a higher concentration of water molecules (more dilute solution) to where there is a lower concentration of water molecules (more concentrated solution). The solute molecules are too large to pass through the membrane, so they remain on one side.

水从水分子浓度较高（溶液较稀）的地方移动到水分子浓度较低（溶液较浓）的地方。溶质分子太大而无法穿过膜，因此它们留在原侧。

dilute solution → concentrated solution (water movement)

稀溶液 → 浓溶液（水的运动方向）

6. Osmosis in Plant Cells | 植物细胞中的渗透作用

Plant cells have a rigid cell wall made of cellulose, which affects how they respond to osmosis. When a plant cell is placed in a solution with higher water potential (more dilute), water enters the cell by osmosis. The vacuole swells, and the cytoplasm presses against the cell wall. The cell becomes turgid and firm. This is normal and desirable for plant support.

植物细胞具有由纤维素构成的刚性细胞壁，这影响了它们对渗透作用的响应。当植物细胞被置于水势较高（更稀）的溶液中时，水通过渗透进入细胞。液泡膨胀，细胞质紧压细胞壁。细胞变得充盈（turgid）而坚固。这对植物的支撑是正常且有利的。

When a plant cell is placed in a solution with lower water potential (more concentrated), water leaves the cell by osmosis. The vacuole and cytoplasm shrink, and the cell membrane pulls away from the cell wall. This is called plasmolysis, and the cell is described as flaccid. If too much water is lost, the plant wilts.

当植物细胞被置于水势较低（更浓）的溶液中时，水通过渗透离开细胞。液泡和细胞质收缩，细胞膜从细胞壁上脱离。这被称为质壁分离（plasmolysis），此时细胞被描述为松弛（flaccid）。如果失去过多水分，植物就会枯萎。

Condition 条件	Water movement 水的运动	Cell state 细胞状态
Dilute solution 稀溶液	Into cell 进入细胞	Turgid 充盈
Concentrated solution 浓溶液	Out of cell 离开细胞	Flaccid / plasmolysed 松弛/质壁分离

7. Osmosis in Animal Cells | 动物细胞中的渗透作用

Animal cells do not have a cell wall. As a result, they are much more sensitive to changes in the surrounding solution. If an animal cell is placed in a more dilute (hypotonic) solution, water enters the cell by osmosis and it may swell and burst. This is known as lysis.

动物细胞没有细胞壁。因此，它们对周围溶液的变化更加敏感。如果将动物细胞置于更稀（低渗）的溶液中，水通过渗透进入细胞，细胞可能膨胀并破裂。这被称为裂解（lysis）。

If an animal cell is placed in a more concentrated (hypertonic) solution, water leaves the cell and it shrinks and becomes shrivelled. This is known as crenation. If the solution has the same concentration as the cell contents (isotonic), water enters and leaves at equal rates, and the cell remains unchanged.

如果将动物细胞置于更浓（高渗）的溶液中，水离开细胞，细胞收缩并变得皱缩。这被称为皱缩（crenation）。如果溶液的浓度与细胞内容物相同（等渗），水以相等的速率进出，细胞保持原状。

This is why hospital patients are given saline (salt) drips that are isotonic with their blood plasma. Pure water injected directly into the bloodstream would cause red blood cells to swell and burst.

这就是为什么医院给病人输注与血浆等渗的生理盐水。如果直接将纯水注入血液，会导致红细胞膨胀并破裂。

8. Active Transport | 主动运输

Active transport is the movement of particles from a region of lower concentration to a region of higher concentration, against the concentration gradient. Unlike diffusion and osmosis, active transport requires energy in the form of ATP, which is released from glucose during respiration. It also requires carrier proteins in the cell membrane.

主动运输是粒子从低浓度区域向高浓度区域、逆浓度梯度的运动。与扩散和渗透不同，主动运输需要以 ATP 形式提供的能量，这些能量在细胞呼吸过程中从葡萄糖中释放。它还需要细胞膜中的载体蛋白。

One important example is in the root hair cells of plants. The concentration of mineral ions in the soil is usually lower than inside the root hair cell. To absorb these ions, the plant must use active transport to move them against the concentration gradient. This requires energy from respiration.

一个重要的例子是植物根毛细胞。土壤中矿物离子的浓度通常低于根毛细胞内部的浓度。为了吸收这些离子，植物必须使用主动运输将它们逆浓度梯度移动。这需要呼吸作用提供的能量。

Another example is in the ileum of the small intestine, where glucose is absorbed into the blood. After a meal, glucose concentration in the intestine is high, and it moves into the blood by diffusion. However, as glucose is rapidly removed by the blood, its concentration in the intestine falls below that of the blood. Active transport then continues to pump glucose into the blood, ensuring maximum absorption.

另一个例子是小肠回肠中葡萄糖被吸收进入血液。进食后，肠道中葡萄糖浓度很高，它通过扩散进入血液。然而，随着葡萄糖被血液迅速带走，肠道中的浓度降到低于血液。此时主动运输继续将葡萄糖泵入血液，确保最大程度的吸收。

active transport: low concentration → high concentration (requires ATP)

主动运输：低浓度 → 高浓度（需要 ATP）

9. Comparing the Three Processes | 三种过程对比

The table below summarises the key differences between diffusion, osmosis and active transport. You should be able to reproduce this comparison in an exam.

下表总结了扩散、渗透和主动运输之间的关键区别。你应该能够在考试中复现这一比较。

Feature 特征	Diffusion 扩散	Osmosis 渗透	Active transport 主动运输
Substance moved 运输的物质	Any particles 任意粒子	Water only 仅水	Ions / molecules 离子/分子
Direction 方向	High → low 高 → 低	High water potential → low water potential 高水势 → 低水势	Low → high 低 → 高
Energy required 是否需要能量	No 否	No 否	Yes (ATP) 是（ATP）
Membrane needed 是否需要膜	No 不需要	Yes 需要	Yes (carrier proteins) 需要（载体蛋白）

10. Exam Tips and Common Mistakes | 考试技巧与常见错误

Many students lose marks on this topic because of careless language. Here are the most common mistakes and how to avoid them.

许多学生在这一主题上因为措辞不严谨而丢分。以下是最常见的错误及如何避免它们。

Do not say ‘water moves from low to high solute concentration’: This describes the movement of water correctly in terms of solute, but examiners prefer the precise phrase ‘from higher water potential to lower water potential’ or ‘from a dilute solution to a concentrated solution’.

不要说‘水从低溶质浓度移动到高溶质浓度’：虽然这在溶质意义上描述了水的运动，但考官更倾向于‘从较高水势到较低水势’或‘从稀溶液到浓溶液’这样精确的表述。
Remember that diffusion does not require a membrane: Diffusion can happen in gases and liquids without any membrane. Osmosis requires a partially permeable membrane.

记住扩散不需要膜：扩散可以发生在没有膜的气体和液体中。渗透则需要部分通透膜。
Always include ‘net movement’ in definitions: Particles move in all directions randomly, so the overall movement is the net movement. This word is often required for full marks.

在定义中始终包含‘净运动’：粒子随机向各个方向运动，因此总体运动是净运动。这个词语通常是获得满分所必需的。
Link active transport to energy from respiration: If a question asks why active transport can stop when a plant is in low oxygen conditions, the answer is that respiration produces too little ATP.

将主动运输与呼吸作用产生的能量联系起来：如果题目问为什么植物在缺氧条件下主动运输会停止，答案是呼吸作用产生的 ATP 太少。
Be careful with plant cell diagrams: When drawing plasmolysed cells, ensure the cell wall is drawn as intact and the cell membrane is clearly pulled away from it.

画植物细胞图时要小心：画质壁分离的细胞时，确保细胞壁画得完整，并且细胞膜明显从细胞壁上脱离。

Mastering these three transport processes will help you answer a wide range of IGCSE Biology and Science questions. Always define each process precisely, state the direction of movement, and mention whether energy is required. With practice, you will be able to apply these concepts confidently to unfamiliar scenarios.

掌握这三种运输过程将帮助你回答广泛的 IGCSE 生物和科学问题。始终精确定义每个过程，说明运动方向，并提及是否需要能量。通过练习，你将能够自信地将这些概念应用于不熟悉的场景。

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Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输