📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
In IGCSE Biology, understanding how substances move across cell membranes is fundamental. These processes — diffusion, osmosis and active transport — govern water and solute movement, nutrient uptake and waste removal in all living organisms.
在 IGCSE 生物考试中，理解物质如何穿过细胞膜是基础。扩散、渗透和主动运输这些过程，控制着所有生物体内水分与溶质的移动、营养物质的吸收以及废物的排出。
1. Diffusion: The Passive Spreading of Particles | 扩散：粒子的被动散布
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. This occurs due to the random motion of particles and requires no energy input.
扩散是粒子从浓度较高区域沿浓度梯度移动到浓度较低区域的净移动。这是由于粒子的随机运动引起的，不需要能量输入。
For example, when a drop of ink is placed in water, the pigment molecules spread out until the water becomes uniformly coloured. In biology, oxygen diffuses from the alveoli into the blood because the concentration of oxygen is higher in the alveoli than in the blood.
例如，将一滴墨水放入水中，色素分子会不断扩散，直到水颜色均匀。在生物学中，氧气从肺泡扩散进入血液，因为肺泡内氧气浓度高于血液中的氧气浓度。
2. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
Three main factors influence how quickly diffusion occurs: concentration gradient, temperature and the surface area of the membrane.
影响扩散速度的三个主要因素是：浓度梯度、温度和膜的表面积。
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A steeper concentration gradient leads to a faster rate of diffusion.
浓度梯度越大，扩散速率越快。
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Higher temperatures provide particles with more kinetic energy, so they move faster and diffusion speeds up.
温度越高，粒子获得更多动能，运动更快，扩散也加快。
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A larger surface area allows more particles to cross the membrane at the same time.
膜表面积越大，同一时间能穿过膜的粒子越多。
3. Osmosis: The Special Case of Water | 渗透：水的特殊扩散
Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. It is a type of diffusion specifically for water, and it does not require energy.
渗透是水分子通过部分通透性膜，从水势较高区域向水势较低区域的净移动。它是专门针对水的一种扩散类型，不需要能量。
The partially permeable membrane allows water molecules to pass through, but prevents larger solute molecules, such as sucrose, from passing. Therefore, water moves to dilute the more concentrated solution.
部分通透性膜允许水分子通过，但阻止较大的溶质分子（如蔗糖）通过。因此，水会向更浓的溶液方向移动，以稀释它。
4. Investigating Osmosis with Potato or Visking Tubing | 用马铃薯或 Visking 管探究渗透
A common practical is to place cylinders of potato into different concentrations of sucrose solution. If the solution is more dilute than the potato sap, water enters the potato cells by osmosis and the potato increases in mass and firmness.
一个常见实验是将马铃薯条放入不同浓度的蔗糖溶液中。如果外界溶液比马铃薯细胞液更稀，水会通过渗透进入马铃薯细胞，导致马铃薯质量增加、质地变硬。
If the external solution is more concentrated, water leaves the potato cells, and the potato loses mass and becomes flaccid. Visking tubing, which is a partially permeable membrane, can also be used to demonstrate osmosis because it allows water but not large molecules like starch to pass through.
如果外界溶液更浓，水将从马铃薯细胞中流失，马铃薯质量减少并变得软缩。Visking 管也是一种部分通透性膜，可用于演示渗透，因为它允许水通过，但不允许淀粉等大分子通过。
5. Effects of Osmosis on Cells: Animal vs Plant | 渗透对细胞的影响：动物细胞与植物细胞
Animal cells have no cell wall, so they are more affected by changes in external osmotic pressure. In a hypotonic solution (more dilute than the cytoplasm), an animal cell gains water, swells and may burst. In a hypertonic solution, it loses water and shrinks.
动物细胞没有细胞壁，因此受外界渗透压变化的影响更大。在低渗溶液（比细胞质稀）中，动物细胞吸水膨胀，甚至可能破裂。在高渗溶液中，动物细胞失水并皱缩。
Plant cells are protected by a rigid cell wall. In a hypotonic solution, they become turgid, but the cell wall prevents bursting. In a hypertonic solution, the protoplast shrinks away from the cell wall, a process called plasmolysis.
植物细胞有坚硬的细胞壁保护。在低渗溶液中，它们变得充盈（紧张），但细胞壁阻止其破裂。在高渗溶液中，原生质体收缩脱离细胞壁，这一过程称为质壁分离。
6. Active Transport: Moving Against the Gradient | 主动运输：逆梯度移动
Active transport is the movement of substances across a cell membrane from a region of lower concentration to a region of higher concentration — against the concentration gradient. This process requires energy in the form of ATP and involves specific carrier proteins in the membrane.
主动运输是物质从浓度较低区域穿过细胞膜向浓度较高区域移动的过程，即逆浓度梯度。这个过程需要以 ATP 形式提供的能量，并涉及膜上特定的载体蛋白。
For example, in the small intestine, glucose is absorbed into the blood even when the blood concentration of glucose is higher than that in the intestine. Active transport allows this to happen.
例如，在小肠中，即使血液中葡萄糖浓度高于肠腔中的浓度，葡萄糖仍能被吸收进入血液。主动运输使这一点得以实现。
7. Carrier Proteins and Energy | 载体蛋白与能量
In active transport, specific carrier proteins bind to the molecule or ion. These proteins then change shape, using ATP, to move the substance through the membrane and release it on the other side.
在主动运输中，特定的载体蛋白与分子或离子结合。然后这些蛋白利用 ATP 改变形状，将物质穿过膜并在另一侧释放。
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Energy required | No | No | Yes (ATP) |
| Direction of movement | Down gradient | Down water potential gradient | Against gradient |
| Substances | Any particles | Water only | Ions & molecules |
| Membrane proteins | Not needed | Not needed (membrane must be partially permeable) | Carrier proteins required |
8. Examples of Active Transport in Living Organisms | 主动运输在生物体中的实例
In addition to glucose absorption in the gut, root hair cells in plants use active transport to take up mineral ions from the soil. The soil solution is very dilute, and the concentration of ions inside the root hair cell is much higher. Active transport enables the plant to absorb these essential ions.
除了肠道吸收葡萄糖外，植物根毛细胞也利用主动运输从土壤中吸收矿物质离子。土壤溶液非常稀，而根毛细胞内的离子浓度远高于外界。主动运输使植物能够吸收这些必需的离子。
In the kidneys, active transport reabsorbs glucose and essential salts from the filtrate back into the blood, preventing their loss in urine.
在肾脏中，主动运输从滤液中重新吸收葡萄糖和必需盐类进入血液，防止其随尿液流失。
9. Surface Area to Volume Ratio and Exchange | 表面积与体积之比及物质交换
Diffusion, osmosis and active transport all depend on the surface area available for exchange. Small organisms have a large surface area relative to their volume, so diffusion alone is sufficient to supply their needs.
扩散、渗透和主动运输都依赖于可用于交换的表面积。微小生物的表面积与体积之比较大，因此仅靠扩散就足以满足其需求。
Large multicellular organisms have a small surface area to volume ratio, so they require specialised exchange surfaces, such as alveoli in the lungs and villi in the small intestine, as well as transport systems to distribute substances.
大型多细胞生物的表面积与体积之比较小，因此需要特化的交换表面，如肺中的肺泡和小肠中的绒毛，以及运输系统来分配物质。
10. Exam Tips and Common Mistakes | 考试技巧与常见错误
Many students confuse osmosis with diffusion. Remember that osmosis always involves water and a partially permeable membrane. Another common error is writing ‘active transport requires no energy’ — it does require energy, whereas diffusion and osmosis do not.
许多学生混淆渗透和扩散。记住：渗透总是涉及水和部分通透性膜。另一个常见错误是写着“主动运输不需要能量”——实际上它需要能量，而扩散和渗透不需要。
When comparing solutions, use the terms ‘hypotonic’ (less concentrated), ‘isotonic’ (same concentration) and ‘hypertonic’ (more concentrated). Always state the direction of movement and mention whether energy is used.
比较溶液时，使用术语“低渗”（浓度较低）、“等渗”（浓度相同）和“高渗”（浓度较高）。务必说明移动方向，并指出是否消耗能量。
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