📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
In IGCSE Edexcel Science, understanding how substances move into and out of cells is essential. This article explains the three key processes: diffusion, osmosis and active transport, with clear examples and exam-focused points.
在 Edexcel IGCSE 科学中，理解物质如何进出细胞至关重要。本文解释三个关键过程：扩散、渗透和主动运输，并配有清晰的示例和考试重点。
1. 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, until equilibrium is reached. It is a passive process, meaning it does not require energy.
扩散是粒子从高浓度区域到低浓度区域沿浓度梯度的净运动，直到达到平衡。这是一个被动过程，意味着不需要能量。
Diffusion happens in gases and in solutions. For example, oxygen diffuses from the alveoli in the lungs into the blood, where oxygen concentration is lower. Carbon dioxide diffuses in the opposite direction.
扩散发生在气体和溶液中。例如，氧气从肺部肺泡扩散到血液中，因为血液中氧气浓度较低。二氧化碳则向相反方向扩散。
Rate of diffusion ∝ surface area × concentration difference / membrane thickness
Factors affecting diffusion rate include temperature, surface area, concentration gradient and diffusion distance.
影响扩散速率的因素包括温度、表面积、浓度梯度和扩散距离。
2. Osmosis | 渗透
Osmosis is the net movement of water molecules from a solution with a higher water potential to a solution with a lower water potential, through a partially permeable membrane. It is also a passive process.
渗透是水分子通过部分通透膜从水势较高的溶液移动到水势较低的溶液的净运动。这也是一个被动过程。
In biology, osmosis is crucial for plant cells. When a plant cell is placed in pure water, water enters the cell, causing it to swell and become turgid. The cell wall prevents it from bursting.
在生物学中，渗透对植物细胞至关重要。当植物细胞放在纯水中时，水进入细胞，使其膨胀并变得充盈。细胞壁防止细胞破裂。
If a plant cell is placed in a concentrated solution, water leaves the cell, causing the cytoplasm to shrink and the cell to become plasmolysed.
如果植物细胞放在浓溶液中，水会离开细胞，导致细胞质收缩，细胞发生质壁分离。
3. 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. This process requires energy from respiration, which is carried out by mitochondria.
主动运输是粒子从低浓度区域到高浓度区域、逆着浓度梯度的运动。这个过程需要呼吸作用提供的能量，由线粒体完成。
Carrier proteins in the cell membrane pick up the particles and change shape to carry them across the membrane. Exam questions often ask you to compare active transport with diffusion.
细胞膜中的载体蛋白拾取粒子并改变形状，将它们携带穿过膜。考试题常常要求你比较主动运输与扩散。
An important example is in the root hair cells of plants, where mineral ions are absorbed from very dilute soil solutions into the root cells, even though their concentration is higher inside the cell.
一个重要的例子是植物根毛细胞，即使细胞内离子浓度更高，仍然从非常稀释的土壤溶液中吸收矿物质离子。
4. Comparison Table | 比较表格
| Feature / 特点 | Diffusion 扩散 | Osmosis 渗透 | Active Transport 主动运输 |
| Energy required 是否需要能量 | No 否 | No 否 | Yes 是 |
| Concentration direction 浓度方向 | High to low 高到低 | High water potential to low water potential 高水势到低水势 | Low to high 低到高 |
| Membrane needed 是否需要膜 | No (not essential) 否（不必须） | Yes, partially permeable 是，部分通透 | Yes, with carrier proteins 是，需要载体蛋白 |
5. Familiar Examples | 熟悉例子
Diffusion example: gas exchange in the lungs and leaves. Oxygen and carbon dioxide diffuse through cell membranes freely.
扩散例子：肺部和叶片中的气体交换。氧气和二氧化碳自由穿过细胞膜扩散。
Osmosis example: absorption of water by plant roots. Water enters root hairs by osmosis from the soil.
渗透例子：植物根部吸收水分。水通过渗透从土壤进入根毛。
Active transport example: uptake of glucose by kidney tubule cells and uptake of mineral ions by root hair cells.
主动运输例子：肾小管细胞吸收葡萄糖，以及根毛细胞吸收矿物质离子。
6. Factors Affecting Diffusion Rate | 影响扩散速率的因素
For gases and solutes, a steeper concentration gradient increases the rate of diffusion. A larger surface area also increases the rate, because more particles can cross the membrane at the same time.
对于气体和溶质，更陡的浓度梯度会增加扩散速率。更大的表面积也会增加速率，因为更多粒子可以同时穿过膜。
Higher temperature provides particles with more kinetic energy, so they move faster. A thinner membrane or shorter diffusion distance also speeds up diffusion.
较高的温度给粒子提供更多动能，使它们移动更快。更薄的膜或更短的扩散距离也会加速扩散。
Rate = (surface area × concentration difference) ÷ membrane thickness
In exams, be careful to state that diffusion is passive and moves down the concentration gradient.
在考试中，注意说明扩散是被动的，并且沿浓度梯度向下移动。
7. Water Potential and Osmosis Direction | 水势与渗透方向
Water potential is a measure of the tendency of water to move. Pure water has the highest water potential (often taken as zero). Dissolved solutes lower the water potential, making it more negative.
水势是衡量水移动趋势的指标。纯水具有最高的水势（通常取为零）。溶解的溶质降低水势，使其更负。
Osmosis always moves water from a higher water potential (less negative) to a lower water potential (more negative). For example, in a potato strip placed in salt solution, water leaves the potato cells and enters the salt solution.
渗透总是使水从水势较高（不那么负）的区域移动到水势较低（更负）的区域。例如，将土豆条放在盐溶液中，水会离开土豆细胞进入盐溶液。
Understanding water potential helps you predict whether cells gain or lose water in different solutions.
理解水势有助于你预测细胞在不同溶液中的吸水或失水情况。
8. Exam Advice: Describing Experiments | 考试建议：描述实验
A common IGCSE practical question involves dialysis tubing or Visking tubing to model a partially permeable membrane. Students fill the tubing with sugar solution and place it in a beaker of distilled water.
一个常见的 IGCSE 实验题使用透析管或 Visking 管来模拟部分通透膜。学生将糖溶液装入管中，并放在装有蒸馏水的烧杯中。
After one hour, the tubing becomes firm because water enters by osmosis. To test for the movement of sugar, test the outside water for glucose. If glucose was small enough to diffuse, the test would be positive.
一小时后，管子变硬，因为水通过渗透进入。为了测试糖是否移动，检测外部水中的葡萄糖。如果葡萄糖小到可以扩散，测试结果将呈阳性。
Always state the control variables, such as temperature, volume and surface area, to gain full marks.
始终说明控制变量，例如温度、体积和表面积，以获得满分。
9. Cell Membrane Structure | 细胞膜结构
The cell membrane is made of a phospholipid bilayer with embedded proteins. Small molecules like oxygen and carbon dioxide can pass through easily. Water also passes through aquaporins, which are protein channels.
细胞膜由磷脂双分子层和嵌入的蛋白质组成。像氧气和二氧化碳这样的小分子可以轻松穿过。水也通过水通道蛋白（一种蛋白质通道）穿过。
Carrier proteins are essential for active transport. They bind to a specific molecule on one side of the membrane, change shape, and release it on the other side. This uses energy from ATP.
载体蛋白对主动运输至关重要。它们与膜一侧的特定分子结合，改变形状，并在另一侧释放。这一过程使用 ATP 中的能量。
Protein channels allow water and ions to pass through by facilitated diffusion, but this is not required in detail for IGCSE.
蛋白质通道允许水和离子通过易化扩散，但 IGCSE 不要求详细掌握。
10. Everyday Applications | 日常应用
Diffusion is used in the absorption of digested food molecules in the small intestine. The villi provide a large surface area for rapid diffusion.
扩散用于小肠中消化食物的吸收。绒毛提供了巨大的表面积，促进快速扩散。
Osmosis is used in food preservation. Adding salt or sugar to food reduces water activity and prevents microbial growth because water leaves the microorganisms by osmosis.
渗透用于食品保存。向食物中加入盐或糖会降低水分活性，并防止微生物生长，因为水通过渗透离开微生物。
Active transport is used in kidney tubules to reabsorb glucose and other useful substances from the filtrate back into the blood, against a concentration gradient.
主动运输用于肾小管，将滤液中的葡萄糖和其他有用物质逆浓度梯度重新吸收回血液。
11. Key Phrases for Exams | 考试关键短语
- For diffusion: “net movement from high to low concentration down a concentration gradient.”
- 对于扩散：”沿浓度梯度从高浓度到低浓度的净运动。”
- For osmosis: “net movement of water across a partially permeable membrane.”
- 对于渗透：”水分子通过部分通透膜的净运动。”
- For active transport: “movement against a concentration gradient using energy from respiration.”
- 对于主动运输：”逆浓度梯度移动，使用呼吸作用产生的能量。”
Always mention “net movement” rather than just “movement,” because particles move in both directions.
始终提及”净运动”而不是仅仅说”运动”，因为粒子是双向移动的。
12. Quick Summary | 快速总结
Diffusion is passive and moves particles down a concentration gradient. Osmosis is a special type of diffusion involving water through a partially permeable membrane. Active transport is the only one of the three that requires energy and moves substances against the gradient.
扩散是被动的，使粒子沿浓度梯度向下移动。渗透是涉及水通过部分通透膜的一种特殊扩散。主动运输是三者中唯一需要能量并逆梯度移动物质的过程。
For IGCSE, remember examples for each process, the direction of movement, and which factors increase the rate. Good luck with your revision!
对于 IGCSE，记住每个过程的例子、移动方向以及哪些因素会增加速率。祝你复习顺利！
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