📚 Cell Transport: Diffusion, Osmosis and Active Transport | 细胞运输：扩散、渗透与主动运输
Every living cell is surrounded by a cell membrane that controls the movement of substances in and out. Understanding how molecules travel across membranes is a fundamental part of IGCSE Science (Biology). In this revision guide, we will break down diffusion, osmosis and active transport step by step, compare them clearly, and link each process to everyday biological examples.
每个活细胞都被一层控制物质进出细胞膜所包围。理解分子如何穿过膜是IGCSE科学（生物）的基础部分。在本复习指南中，我们将逐步拆解扩散、渗透和主动运输，清晰比较它们，并将每个过程与日常生物实例联系起来。
1. The Cell Membrane: A Selective Barrier | 细胞膜：选择性的屏障
All cells are surrounded by a partially permeable membrane. This means that some substances can pass through it easily, while others cannot. Small molecules like oxygen, carbon dioxide and water pass through freely; larger molecules like glucose and ions often need help.
所有细胞都被一层部分通透的膜包围。这意味着某些物质可以容易通过，而其他物质不能。像氧气、二氧化碳和水这样的小分子可以自由通过；像葡萄糖和离子这样较大的分子通常需要帮助。
The cell membrane is made of a phospholipid bilayer with embedded proteins. These proteins act as channels or carriers, allowing specific molecules to cross. This property is essential for maintaining homeostasis inside the cell.
细胞膜由磷脂双分子层和嵌入其中的蛋白质组成。这些蛋白质充当通道或载体，允许特定分子通过。这一特性对于维持细胞内的稳态至关重要。
2. Diffusion: Moving Down the Concentration Gradient | 扩散：沿浓度梯度运动
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random movement. It is a passive process, meaning it does not require energy from the cell.
扩散是粒子由于其随机运动从高浓度区域到低浓度区域的净运动。这是一个被动过程，意味着不需要细胞提供能量。
In biology, diffusion is how oxygen enters cells and carbon dioxide leaves them. For example, in the lungs, oxygen diffuses from the air sacs (alveoli) into the bloodstream because oxygen concentration is higher in the alveoli than in the blood.
在生物学中，扩散是氧气进入细胞、二氧化碳离开细胞的方式。例如，在肺部，氧气从肺泡扩散到血液中，因为肺泡中的氧气浓度高于血液中的氧气浓度。
- Simple diffusion: molecules pass directly through the membrane.
- 简单扩散：分子直接穿过膜。
- Facilitated diffusion: molecules move through protein channels or carriers.
- 易化扩散：分子通过蛋白质通道或载体移动。
Diffusion also occurs in the small intestine: digested food molecules like glucose diffuse into the blood. In plants, carbon dioxide diffuses into leaves through stomata for photosynthesis.
扩散也发生在小肠：消化后的食物分子如葡萄糖扩散到血液中。在植物中，二氧化碳通过气孔扩散进入叶片用于光合作用。
3. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
Several factors can speed up or slow down diffusion. You should be able to explain each one in an exam.
有几个因素可以加快或减慢扩散。你应该能够在考试中解释每一个因素。
| Factor | Effect on rate |
| Concentration gradient | Higher gradient → faster diffusion |
| Temperature | Higher temperature → more kinetic energy → faster |
| Surface area | Larger surface area → faster diffusion |
| Thickness of membrane | Thinner membrane → faster diffusion |
In the body, alveoli have a large surface area and very thin walls to maximise oxygen diffusion. Similarly, root hair cells in plants have long extensions to increase surface area for water and mineral uptake, though water absorption is mainly by osmosis.
在体内，肺泡具有巨大的表面积和极薄的壁，以最大化氧气扩散。同样，植物中的根毛细胞具有长延伸以增加水分和矿物质吸收的表面积，尽管水分吸收主要通过渗透。
4. Osmosis: Water Movement Through a Membrane | 渗透：通过膜的水分运动
Osmosis is a special type of diffusion that refers only to the movement of water. It is defined as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) across a partially permeable membrane.
渗透是一种特殊类型的扩散，仅指水分子的运动。它被定义为水分子通过部分通透膜从较高水势（稀释溶液）区域到较低水势（浓缩溶液）区域的净运动。
Note that water potential is a measure of the tendency of water to move. Pure water has the highest water potential. Adding solutes lowers water potential.
注意水势是衡量水移动趋势的指标。纯水具有最高的水势。添加溶质会降低水势。
Water moves from high water potential → low water potential
水从高水势 → 低水势流动
For example, if a plant cell is placed in pure water, water enters the cell because the cell sap has a lower water potential than pure water. If a red blood cell is placed in pure water, water enters and the cell bursts (haemolysis).
例如，如果植物细胞放在纯水中，水进入细胞，因为细胞液的水势低于纯水。如果红细胞放在纯水中，水进入并且细胞破裂（溶血）。
5. Isotonic, Hypotonic and Hypertonic Solutions | 等渗、低渗和高渗溶液
These terms describe the relative concentrations of two solutions.
这些术语描述两种溶液的相对浓度。
- Isotonic: equal concentrations; no net water movement.
- 等渗：浓度相等；没有净水分运动。
- Hypotonic: lower solute concentration (higher water potential) than the cell.
- 低渗：比细胞更低溶质浓度（较高水势）。
- Hypertonic: higher solute concentration (lower water potential) than the cell.
- 高渗：比细胞更高溶质浓度（较低水势）。
Animal cells require an isotonic environment. If placed in a hypertonic solution, they lose water and shrink (crenation). If placed in a hypotonic solution, they gain water and burst (lysis). Plant cells, however, have a cell wall that prevents bursting. In a hypotonic solution, plant cells become turgid; in a hypertonic solution, they become plasmolysed as the cytoplasm pulls away from the wall.
动物细胞需要等渗环境。如果放在高渗溶液中，它们会失水并萎缩（皱缩）。如果放在低渗溶液中，它们会吸水并破裂（溶解）。然而，植物细胞有细胞壁防止破裂。在低渗溶液中，植物细胞变得膨胀（硬挺）；在高渗溶液中，它们发生质壁分离，因为细胞质从细胞壁拉开。
6. Experimental Investigation: Osmosis in Potato Strips | 实验探究：土豆条中的渗透
A classic IGCSE investigation involves placing potato strips into different concentrations of sucrose solution. You can measure the percentage change in mass to determine the direction of water movement.
一个经典的IGCSE实验是将土豆条放入不同浓度的蔗糖溶液中。你可以测量质量变化百分比来确定水运动的方向。
Method: Cut equal-size potato strips, dry them, weigh them, and place each in a different solution for about 30 minutes. Then remove, dry and reweigh.
方法：切下等大小土豆条，擦干，称重，然后将每条放入不同溶液中约30分钟。然后取出，擦干并重新称重。
- In pure water (hypotonic), mass increases because water enters the potato cells.
- 在纯水（低渗）中，质量增加，因为水进入土豆细胞。
- In concentrated sucrose (hypertonic), mass decreases because water leaves the potato cells.
- 在浓蔗糖（高渗）中，质量减少，因为水离开土豆细胞。
If you plot a graph of mass change against concentration, the point where there is no mass change gives the concentration of the cell sap (isotonic point).
如果绘制质量变化对浓度的图表，没有质量变化的点给出细胞液的浓度（等渗点）。
7. Active Transport: Moving Against the Gradient | 主动运输：逆梯度移动
Active transport is the movement of particles against a concentration gradient (from low to high concentration) using energy from respiration. It requires carrier proteins in the membrane and ATP as energy.
主动运输是粒子逆浓度梯度（从低浓度到高浓度）的移动，利用呼吸作用产生的能量。它需要膜中的载体蛋白和ATP作为能量。
Examples of active transport in living organisms include:
生物体中主动运输的例子包括：
- Root hair cells absorbing mineral ions (e.g., nitrate) from the soil, even when the ion concentration in the soil is lower than in the cell.
- 根毛细胞从土壤中吸收矿质离子（如硝酸盐），即使土壤中的离子浓度低于细胞内的浓度。
- Glucose being reabsorbed in the kidney tubules from the filtrate back into the blood.
- 葡萄糖在肾小管中从滤液重吸收回血液。
- Iodine uptake by thyroid gland cells.
- 甲状腺细胞对碘的吸收。
Energy is supplied by cellular respiration. If respiration stops, active transport stops because no ATP is produced. In plants, active transport in root hairs can be reduced by lack of oxygen or low temperature, which slows respiration.
能量由细胞呼吸提供。如果呼吸停止，主动运输也停止，因为没有ATP产生。在植物中，根毛中的主动运输会因缺氧或低温而减少，因为呼吸减慢。
8. Comparing Diffusion, Osmosis and Active Transport | 对比扩散、渗透和主动运输
It is important to compare these three processes clearly. The table below summarises the key similarities and differences.
清楚比较这三个过程非常重要。下表总结了关键相似点和差异。
| Feature | Diffusion | Osmosis | Active transport |
| Type of molecule | Any particles (gases, liquids, solutes) | Water only | Ions, sugars, amino acids |
| Direction of movement | High → low concentration | High water potential → low water potential | Low → high concentration |
| Energy required? | No (passive) | No (passive) | Yes (ATP) |
| Membrane proteins? | Not needed (simple diffusion) | Not needed (water passes through lipid or aquaporins) | Carrier proteins essential |
Another way to remember: diffusion and osmosis are passive; active transport is active. Osmosis is just water diffusion across a partially permeable membrane. Active transport always requires energy and carrier proteins.
另一种记忆方式：扩散和渗透是被动的；主动运输是主动的。渗透只是水通过部分通透膜的扩散。主动运输总是需要能量和载体蛋白。
9. Surface Area to Volume Ratio and Exchange | 表面积与体积之比及交换
Small organisms have a large surface area to volume ratio, so they can rely on simple diffusion for gas exchange. Large organisms, like humans, have a small surface area to volume ratio, so they need specialised exchange surfaces and transport systems.
小生物具有较大的表面积与体积比，因此它们可以依靠简单的扩散进行气体交换。大生物，如人类，表面积与体积比小，因此需要专门的交换表面和运输系统。
Exchange surfaces have adaptations:
交换表面具有适应性特征：
- Large surface area (e.g., alveoli, villi).
- 大表面积（如肺泡、绒毛）。
- Thin walls (short diffusion distance).
- 薄壁（短扩散距离）。
- Rich blood supply (maintains concentration gradient).
- 丰富血液供应（维持浓度梯度）。
- Ventilation or constant flow (renews external medium).
- 通气或持续流动（更新外部介质）。
Calculating surface area to volume ratio: a cube of side 1 cm has a surface area of 6 cm² and a volume of 1 cm³, so the ratio is 6:1. If the side is 2 cm, surface area is 24 cm², volume is 8 cm³, ratio is 3:1. As size increases, the ratio decreases.
计算表面积与体积比：边长为1 cm的立方体表面积为6 cm²，体积为1 cm³，因此比率为6:1。如果边长为2 cm，表面积为24 cm²，体积为8 cm³，比率为3:1。随着尺寸增加，比率减小。
10. Common Misconceptions in Exams | 考试中常见误解
Students often confuse osmosis with diffusion, or forget that active transport goes against the gradient. Here are some traps to avoid.
学生经常混淆渗透与扩散，或者忘记主动运输是逆梯度的。以下是一些要避免的陷阱。
- Misconception: Osmosis can move solutes. – No, osmosis only moves water.
- 误解：渗透可以移动溶质。——不，渗透只移动水。
- Misconception: Diffusion requires energy. – No, diffusion is passive.
- 误解：扩散需要能量。——不，扩散是被动的。
- Misconception: Active transport moves down the gradient. – No, it moves from low to high concentration.
- 误解：主动运输顺梯度移动。——不，它从低浓度向高浓度移动。
- Misconception: Plant cells cannot burst in hypotonic solutions. – They do not burst because of the cell wall, but they become turgid.
- 误解：植物细胞在低渗溶液中不会破裂。——它们不会破裂因为有细胞壁，但它们会膨胀。
In exam questions, always state the direction of water movement (e.g., “water moves into the cell because the external solution has a higher water potential”).
在考试题目中，始终说明水运动的方向（例如“水进入细胞，因为外部溶液具有较高的水势”）。
11. Exam-Style Practice Question | 考试风格练习题
Try this short question to test your understanding.
尝试这个简短问题来测试你的理解。
Question: A student placed a red blood cell in a concentrated salt solution. Explain what happens to the cell and why.
问题：一名学生将红细胞放入浓盐溶液中。解释细胞会发生什么以及为什么。
Answer: The cell loses water and shrinks. This is because the salt solution is hypertonic (has a lower water potential) compared to the cell contents. Water moves out of the cell by osmosis through the cell membrane.
答案：细胞失去水并收缩。这是因为盐溶液与细胞内容物相比是高渗的（具有较低的水势）。水通过渗透经细胞膜离开细胞。
You can also be asked to draw a graph of results from an osmosis experiment, identify the isotonic point, or suggest why the rate of active transport stops when a poison inhibits respiration.
你也可能会被要求绘制渗透实验结果的图表，确定等渗点，或解释为什么当毒物抑制呼吸时主动运输速率停止。
12. Key Terms Checklist | 关键术语清单
Before your exam, make sure you can define and use these terms correctly.
考试前，确保你能够正确定义和使用这些术语。
- Diffusion / 扩散
- Osmosis / 渗透
- Active transport / 主动运输
- Partially permeable membrane / 部分通透膜
- Concentration gradient / 浓度梯度
- Water potential / 水势
- Isotonic / 等渗
- Hypotonic / 低渗
- Hypertonic / 高渗
- Turgid / 膨胀（硬挺）
- Plasmolysis / 质壁分离
- Carrier protein / 载体蛋白
Understanding these processes helps you explain everything from kidney function to how plants drink. Keep practising past paper questions, and always underline the key terms in your answers.
理解这些过程有助于你解释从肾功能到植物喝水的一切。不断练习历年真题，在答案中始终标出关键术语。
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