📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
In this revision article, we explore three fundamental ways that substances move across cell membranes: diffusion, osmosis, and active transport. These processes are essential for life, allowing cells to exchange gases, absorb nutrients, and remove waste products. Together they form a core topic in the Edexcel IGCSE Science syllabus, and mastering them is key to understanding how living organisms maintain internal balance.
在本复习文章中，我们将探讨物质穿过细胞膜的三种基本方式：扩散、渗透和主动运输。这些过程对生命至关重要，能让细胞交换气体、吸收养分并排出废物。它们共同构成 Edexcel IGCSE 科学大纲中的核心主题，掌握它们对于理解生物体如何维持内部平衡至关重要。
1. Why Cells Need Transport Mechanisms | 为什么细胞需要运输机制
All living cells are surrounded by a cell membrane, which controls what enters and leaves the cell. The membrane is selectively permeable, meaning it allows some molecules to pass through but not others. For example, oxygen and carbon dioxide can easily diffuse through the lipid bilayer, while larger molecules like glucose or charged ions may require special proteins or energy.
所有活细胞都被细胞膜包围，细胞膜控制着出入细胞的物质。细胞膜具有选择透过性，即允许某些分子通过，而阻止其他分子通过。例如，氧气和二氧化碳可以轻松穿过脂双层，而较大的分子如葡萄糖或带电离子则可能需要特殊蛋白质或能量。
Substances move across membranes through three main routes: passive diffusion, osmosis (a special form of diffusion for water), and active transport, which requires energy. Each process has a specific role, and its direction relative to concentration gradients determines whether it is passive or active.
物质通过三种主要途径穿过膜：被动扩散、渗透（水的一种特殊扩散形式）以及需要能量的主动运输。每种过程都有特定作用，其相对于浓度梯度的方向决定了它是被动还是主动。
2. Diffusion: The Movement of Particles | 扩散：粒子的运动
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. This movement is caused by the random kinetic energy of particles, which makes them spread out until they are evenly distributed. Diffusion does not require energy from the cell, so it is a passive process.
扩散是粒子从高浓度区域向低浓度区域（沿浓度梯度）的净运动。这种运动是由粒子的随机动能引起的，使它们不断散布直到均匀分布。扩散不需要细胞提供能量，因此是被动过程。
Common examples in biology include the exchange of oxygen and carbon dioxide in the lungs and the movement of glucose molecules into erythrocytes (red blood cells). In chemistry, it explains why a drop of ink eventually colours the whole beaker of water.
生物学中的常见例子包括肺部氧气和二氧化碳的交换，以及葡萄糖分子进入红细胞的过程。在化学中，它解释了为什么一滴墨水最终会给整杯水染色。
For diffusion to occur efficiently, there must be a concentration gradient, and particles must be able to move freely. The rate of diffusion depends on several factors, which we will examine in the next section.
要使扩散高效发生，必须存在浓度梯度，且粒子能够自由移动。扩散速率取决于多个因素，我们将在下一节探讨。
3. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
The rate of diffusion can be increased or decreased by changing conditions. Four main factors are frequently tested in exams: concentration gradient, temperature, surface area, and distance.
扩散速率可以因条件改变而升高或降低。考试中常考四个主要因素：浓度梯度、温度、表面积和距离。
- Concentration gradient: The steeper the gradient (greater difference in concentration), the faster the net movement of particles.
- Temperature: Higher temperatures give particles more kinetic energy, so they move faster and diffusion speeds up.
- Surface area: A larger membrane surface area allows more particles to cross per unit time.
- Distance: Shorter distances allow particles to reach equilibrium quicker, so thinner membranes increase diffusion rate.
- 浓度梯度：梯度越陡（浓度差越大），粒子的净运动越快。
- 温度：温度越高，粒子动能越大，运动更快，扩散加速。
- 表面积：膜表面积越大，单位时间内穿过膜的粒子越多。
- 距离：距离越短，粒子越快达到平衡，因此更薄的膜可提高扩散速率。
In the human body, alveoli in the lungs are adapted for rapid diffusion by having a huge total surface area, very thin walls (one cell thick), and a steep oxygen gradient maintained by ventilation and blood circulation.
在人体内，肺部的肺泡通过巨大的总表面积、极薄的壁（单层细胞）以及由通气和血液循环维持的陡峭氧气梯度，适应了快速扩散的需求。
4. Osmosis: A Special Case of Diffusion | 渗透：扩散的特殊情况
Osmosis is the net movement of water molecules from a solution with a higher water potential (more dilute) to a solution with a lower water potential (more concentrated) through a selectively permeable membrane. The membrane must allow water molecules to pass but restrict the movement of solutes such as sugar or salt.
渗透是水分子通过选择透过性膜，从水势较高（较稀）的溶液向水势较低（较浓）的溶液进行的净运动。该膜必须允许水分子通过，但限制溶质（如糖或盐）的移动。
Water potential is a measure of the tendency of water to leave a solution. Pure water has the highest water potential (usually set at zero on an arbitrary scale), and adding solutes lowers the water potential, making it more negative. Water always moves from a region of higher water potential to lower water potential.
水势衡量的是水离开溶液的倾向。纯水的水势最高（通常在任意标度上设为零），加入溶质会降低水势，使其变得更负。水总是从高水势区域向低水势区域移动。
You may also see the term ‘partially permeable membrane’ used interchangeably with ‘selectively permeable membrane’. It is important to note that osmosis is passive: it does not require metabolic energy, because water molecules have kinetic energy and naturally collide with the membrane pores.
你也可能看到“半透膜”与“选择透过性膜”互换使用。需要注意，渗透是被动过程：它不需要代谢能量，因为水分子具有动能，会自然撞击膜孔。
5. Osmosis in Animal Cells | 动物细胞中的渗透
Animal cells do not have a cell wall, so they are very sensitive to changes in the water potential of their surroundings. If an animal cell is placed in a solution with a higher water potential than its cytoplasm (a hypotonic solution), water enters by osmosis. The cell swells and may burst – a process called lysis.
动物细胞没有细胞壁，因此对周围环境水势变化非常敏感。如果将动物细胞置于水势高于其细胞质的溶液（低渗溶液）中，水会通过渗透进入。细胞膨胀并可能破裂——这一过程称为溶胞。
Conversely, if an animal cell is placed in a solution with a lower water potential than its cytoplasm (a hypertonic solution), water leaves the cell. The cell shrinks and becomes shrivelled – a process called crenation. In an isotonic solution (same water potential), there is no net movement of water, and the cell remains normal.
相反，如果将动物细胞置于水势低于其细胞质的溶液（高渗溶液）中，水会离开细胞。细胞缩小并起皱——这一过程称为皱缩。在等渗溶液（水势相同）中，水没有净运动，细胞保持正常。
| Solution type | Water potential vs. cell | Effect on animal cell |
| Hypotonic | Higher (more dilute) | Swells and may burst (lysis) |
| Hypertonic | Lower (more concentrated) | Shrinks (crenation) |
| Isotonic | Equal | No net change |
| 溶液类型 | 与细胞的水势比较 | 对动物细胞的影响 |
| 低渗溶液 | 较高（较稀） | 膨胀并可能破裂（溶胞） |
| 高渗溶液 | 较低（较浓） | 缩小（皱缩） |
| 等渗溶液 | 相等 | 无明显变化 |
6. Osmosis in Plant Cells | 植物细胞中的渗透
Plant cells are surrounded by a rigid cell wall made of cellulose. This wall is fully permeable, but it gives the cell mechanical strength. When a plant cell is placed in a hypotonic solution, it absorbs water and becomes turgid. The cell expands and presses against the cell wall, creating turgor pressure. This turgidity supports non-woody parts of plants, such as leaves and stems.
植物细胞被由纤维素组成的刚性细胞壁包围。该细胞壁是完全透性的，但它赋予细胞机械强度。当植物细胞置于低渗溶液中时，它会吸水并变得肿胀（膨压充盈）。细胞膨胀并紧压细胞壁，产生膨胀压。这种充盈状态支撑着植物的非木质部分，如叶片和茎。
If a plant cell is placed in a hypertonic solution, water moves out of the vacuole and cytoplasm, causing the cell to lose volume. The cytoplasm pulls away from the cell wall, a condition called plasmolysis. The plant wilts because turgor pressure is lost.
如果将植物细胞置于高渗溶液中，水从液泡和细胞质中移出，导致细胞体积减小。细胞质与细胞壁分离，这种现象称为质壁分离。植物因失去膨胀压而萎蔫。
In an isotonic solution, a plant cell becomes flaccid – the cell membrane is not pressed against the cell wall, but the plant cell does not shrink or burst because the cell wall prevents rupture. In summary, the cell wall protects plant cells from bursting in hypotonic environments, while animal cells lack this protection.
在等渗溶液中，植物细胞变得松弛（质壁分离前的状态）——细胞膜未紧贴细胞壁，但植物细胞不会缩小或破裂，因为细胞壁阻止了破裂。总之，细胞壁保护植物细胞在低渗环境中不破裂，而动物细胞缺乏这种保护。
7. Active Transport: Using Energy to Move Molecules | 主动运输：利用能量移动分子
Active transport is the movement of particles against a concentration gradient, from a region of lower concentration to a region of higher concentration. This process requires energy, which is supplied by the breakdown of ATP. ATP is produced during respiration.
主动运输是粒子逆浓度梯度、从低浓度区域向高浓度区域的运动。这个过程需要能量，能量由 ATP 分解提供。ATP 在呼吸作用中产生。
The energy is used to change the shape of carrier proteins in the cell membrane. These proteins act as pumps, binding specific molecules or ions on one side of the membrane and releasing them on the other side after an energy-dependent conformational change.
能量被用于改变细胞膜中载体蛋白的形状。这些蛋白质充当泵，在膜的一侧结合特定分子或离子，并在依赖能量的构象变化后将其释放到另一侧。
Active transport is vital for many physiological processes. For example, root hair cells in plants absorb mineral ions such as nitrate and magnesium from the soil. The concentration of these ions is often lower in the soil than inside the root, so active transport is necessary to move them against the gradient.
主动运输对许多生理过程至关重要。例如，植物根毛细胞从土壤中吸收硝酸盐和镁等矿质离子。这些离子在土壤中的浓度通常低于根内部，因此需要主动运输逆梯度移动它们。
In animals, the kidney tubules actively reabsorb glucose from the filtrate back into the blood, and the small intestine uses active transport to absorb glucose and amino acids from digested food even when intestinal concentrations are low.
在动物中，肾小管主动将葡萄糖从滤液中重吸收回血液，小肠利用主动运输吸收消化食物中的葡萄糖和氨基酸，即使肠道浓度较低时也能进行。
8. Comparing Passive and Active Transport | 被动运输与主动运输的比较
The three processes can be systematically compared using several criteria: direction of movement relative to concentration gradient, requirement for energy, and use of carrier proteins.
我们可以通过几个标准系统比较这三种过程：相对于浓度梯度的移动方向、是否需要能量以及是否使用载体蛋白。
| Feature | Diffusion | Osmosis | Active transport |
| Movement | Down gradient | Down water potential gradient | Against gradient |
| Energy (ATP) | Not required | Not required | Required |
| Membrane proteins | Not involved | Not involved (pores only) | Carrier proteins used |
| Substance | Any diffusible molecule | Water only | Ions and specific molecules |
| 特征 | 扩散 | 渗透 | 主动运输 |
| 运动方向 | 顺浓度梯度 | 顺水势梯度 | 逆浓度梯度 |
| 能量（ATP） | 不需要 | 不需要 | 需要 |
| 膜蛋白 | 不参与 | 不参与（仅孔道） | 使用载体蛋白 |
| 运送物质 | 任意可扩散的分子 | 仅水 | 离子和特定分子 |
One common exam question asks you to identify which process is occurring based on the presence or absence of a concentration gradient. Remember that diffusion and osmosis are spontaneous and do not require ATP, whereas active transport always requires ATP and often involves specific membrane pumps.
一个常见考题是基于浓度梯度的存在与否来识别发生的是哪种过程。记住，扩散和渗透是自发的，不需要 ATP；而主动运输总是需要 ATP，并且通常涉及特定的膜泵。
9. Investigating the Processes | 探究这些过程
In the laboratory, you may be asked to investigate diffusion using a non-living model such as a Visking tube. For example, you can fill the tube with starch solution and place it in a beaker of iodine solution. Iodine diffuses into the tube and turns the starch blue-black, while starch cannot diffuse out because its particles are too large.
在实验室中，你可能需要使用非生物模型（如 Visking 管）来探究扩散。例如，将淀粉溶液装入管中，放入装有碘液的烧杯中。碘扩散进管内并将淀粉变成蓝黑色，而淀粉颗粒太大无法扩散出来。
For osmosis, a common investigation uses potato cylinders. Cut equal-sized potato cylinders, weigh them, and place them in different solutions (e.g., pure water and concentrated sucrose). After a set time, reweigh them. The cylinders in water gain mass, while those in concentrated sucrose lose mass. This demonstrates water movement across the partially permeable membrane of the potato cells.
对于渗透实验，常用马铃薯条进行探究。切取等大小马铃薯条，称重，然后放入不同溶液（如纯水和浓蔗糖溶液）中。经过设定时间后，再次称重。水中的薯条质量增加，而浓蔗糖中的薯条质量减少。这证明了水通过马铃薯细胞半透膜的移动。
Active transport can be investigated using yeast cells suspended in a glucose solution. Yeast cells take up glucose via facilitated diffusion when the external concentration is high, but when the concentration is low, uptake continues only if oxygen (for respiration) is present to supply ATP. Adding a respiratory poison stops active transport, confirming its energy dependence.
可以使用悬浮在葡萄糖溶液中的酵母细胞来探究主动运输。当外部葡萄糖浓度高时，酵母细胞通过易化扩散吸收葡萄糖；但当浓度低时，只有在有氧气（用于呼吸作用产生 ATP）的情况下，吸收才会继续。加入呼吸抑制剂会阻止主动运输，从而证实其对能量的依赖。
10. Real-World Applications and Exam Tips | 实际应用与考试技巧
Understanding these transport mechanisms helps explain many everyday phenomena. For instance, salting meat removes water from bacteria by osmosis, preventing them from multiplying. Similarly, soaking raisins in water makes them swell because water enters the cells by osmosis. Fertilisers should not be applied in excessive amounts to crops because they can make the soil solution hypertonic, causing plant roots to lose water and wilt.
理解这些运输机制有助于解释许多日常现象。例如，腌制肉类通过渗透作用使细菌失水，从而抑制其繁殖。同样，将葡萄干泡在水中会使其膨胀，因为水通过渗透进入细胞。给农作物施肥时不能过量，因为过量会使土壤溶液变为高渗，导致植物根部失水而萎蔫。
In exams, be careful with terminology. ‘Turgid’ describes a plant cell that is swollen due to water entry, while ‘flaccid’ means soft and not swollen. ‘Plasmolysis’ specifically refers to the separation of the cell membrane from the cell wall. For animal cells, use ‘lysis’ for bursting and ‘crenation’ for shrinking.
在考试中，术语要小心。“Turgid”（膨胀）描述植物细胞因吸水而肿胀，“Flaccid”（松弛）指柔软不胀。“质壁分离”特指细胞膜与细胞壁分离。对于动物细胞，用“溶胞”表示膨胀破裂，“皱缩”表示缩小。
Another tip: when drawing graphs of osmosis experiments, always label the axes with ‘Mass change (%)’ and ‘Concentration of solution’. The line should cross the x-axis at the point where the solution is isotonic with the cell contents – no mass change occurs there.
另一个技巧：在绘制渗透实验图表时，始终标出坐标轴“质量变化（%）”和“溶液浓度”。曲线应在与细胞内容物等渗的浓度点处穿过 x 轴——该点质量无变化。
Finally, practise explaining processes in a logical order: state the concentration gradient, describe the movement of particles, mention whether energy is needed, and give a biological example. This structured approach will earn full marks on six-mark extended questions.
最后，练习按逻辑顺序解释过程：先说明浓度梯度，描述粒子运动，提及是否需要能量，并给出生物学例子。这种结构化方法可在 6 分拓展题中获得满分。
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