📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
In IGCSE Science, understanding how substances move across cell membranes is essential. Three key processes — diffusion, osmosis and active transport — explain how cells take in nutrients, remove waste and maintain internal balance. This revision guide breaks them down with clear definitions, practical examples and exam-style comparisons.
在 IGCSE 科学中，理解物质如何穿过细胞膜至关重要。扩散、渗透和主动运输是三大关键过程，它们解释了细胞如何摄取营养、排出废物并维持内部平衡。本复习指南通过清晰定义、实际例子和考试式对比，帮助你彻底掌握这些内容。
1. Diffusion: Net Movement Down a Concentration Gradient | 扩散：沿浓度梯度的净移动
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 passive — it requires no energy from the cell. It happens because particles are in constant random motion. Diffusion continues until equilibrium is reached, but even then particles still move, just with no net movement.
扩散是粒子从高浓度区域向低浓度区域（沿浓度梯度）的净移动。这种移动是被动的，不需要细胞提供能量。由于粒子始终处于无规则运动中，扩散会持续到达到平衡为止；即使达到平衡，粒子仍在运动，只是没有净移动。
- Oxygen diffuses into cells from blood where its concentration is higher.
- Carbon dioxide diffuses out of cells into blood where its concentration is lower.
- 氧气从浓度较高的血液中扩散进入细胞。
- 二氧化碳从浓度较高的细胞中扩散进入血液。
Factors that increase diffusion rate: larger surface area, steeper concentration gradient, higher temperature, and shorter distance (as in thin alveoli walls).
增加扩散速率的因素包括：更大的表面积、更陡的浓度梯度、更高的温度，以及更短的距离（如薄薄的肺泡壁）。
2. Osmosis: Water’s Special Form of Diffusion | 渗透：水的特殊扩散形式
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. It is also passive. The membrane allows water to pass through but not larger solute particles. Water moves to dilute the more concentrated solution.
渗透是水分子穿过部分通透膜，从水势较高区域向水势较低区域的净移动。它同样是被动过程。这种膜允许水通过，但阻止较大的溶质粒子通过。水会向更浓的溶液方向移动，以稀释之。
Water potential ∝ concentration of free water molecules
水势 ∝ 自由水分子浓度
- In animal cells, osmosis can cause cells to swell and burst (haemolysis) if placed in pure water.
- In plant cells, turgor pressure provides support; lack of water causes plasmolysis.
- 在动物细胞中，放入纯水中时渗透会使细胞膨胀甚至破裂（溶血）。
- 在植物细胞中，膨压提供支撑；缺水则导致质壁分离。
3. Active Transport: Moving Against the Gradient | 主动运输：逆梯度移动
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 in the form of ATP, produced during respiration. It also requires carrier proteins in the cell membrane that act as pumps.
主动运输是粒子从低浓度区域向高浓度区域（逆浓度梯度）的移动。该过程需要呼吸作用产生的 ATP 形式的能量，还需要细胞膜中充当泵的载体蛋白。
| Process | Direction vs gradient | Energy needed? | Example |
| Diffusion | Down | No | O₂ from alveoli to blood |
| Osmosis | Down (water) | No | Water into root hair cells |
| Active transport | Against | Yes (ATP) | Mineral ions into root hair cells |
| 过程 | 相对梯度方向 | 需要能量？ | 例子 |
| 扩散 | 顺梯度 | 否 | 氧气从肺泡进入血液 |
| 渗透 | 顺梯度（水） | 否 | 水进入根毛细胞 |
| 主动运输 | 逆梯度 | 是（ATP） | 矿物离子进入根毛细胞 |
4. Partially Permeable Membranes | 部分通透膜
Cell membranes are described as partially permeable because they allow some molecules to pass through freely, while others are blocked. Small molecules like water, oxygen and carbon dioxide pass easily through the phospholipid bilayer. Larger molecules like glucose and ions require specific transport proteins.
细胞膜被称为部分通透膜，因为它允许某些分子自由通过，而阻止另一些。水、氧气和二氧化碳等小分子能轻松穿过磷脂双分子层。葡萄糖和离子等较大分子则需要特定的运输蛋白。
The membrane’s structure — fluid mosaic model — includes phospholipids, cholesterol, proteins and carbohydrates. This structure is vital for selective transport and cell communication.
膜的流体镶嵌模型包括磷脂、胆固醇、蛋白质和碳水化合物。这种结构对选择性运输和细胞通讯至关重要。
5. Diffusion in Gases: The Alveoli and Leaves | 气体扩散：肺泡与叶片
In the lungs, oxygen diffuses from the alveoli into blood capillaries, and carbon dioxide diffuses in the opposite direction. The alveoli are adapted for rapid diffusion: they are numerous (huge surface area), have very thin walls (one cell thick), are surrounded by dense capillaries, and are moist.
在肺部，氧气从肺泡扩散进入毛细血管，而二氧化碳沿相反方向扩散。肺泡适应快速扩散的特点：数量多（表面积巨大）、壁极薄（仅一层细胞）、周围密布毛细血管，且保持湿润。
In plants, gases diffuse through stomata. Carbon dioxide enters leaves for photosynthesis, and oxygen exits. The internal air spaces and thin mesophyll cells reduce diffusion distance.
在植物中，气体通过气孔扩散。二氧化碳进入叶片用于光合作用，氧气则逸出。内部气隙和薄的海绵组织细胞缩短了扩散距离。
6. Osmosis in Plant Cells: Turgor and Plasmolysis | 植物细胞中的渗透：膨压与质壁分离
When a plant cell is placed in a dilute solution, water enters the vacuole by osmosis. The cell becomes turgid, and the cytoplasm pushes against the cell wall. The cell wall prevents bursting. Turgid cells support herbaceous plants. When placed in a concentrated solution, water leaves the cell. The cytoplasm shrinks and the cell becomes flaccid; eventually plasmolysis occurs when the membrane pulls away from the wall.
将植物细胞置于稀溶液中时，水分通过渗透进入液泡，细胞变得充盈（膨胀），细胞质挤压细胞壁。细胞壁防止细胞破裂。充盈的细胞支撑着草本植物。当将细胞置于浓溶液中时，水分渗出，细胞质收缩，细胞变得松弛；最终细胞膜与细胞壁分离，发生质壁分离。
7. Active Transport in Humans and Plants | 人类与植物中的主动运输
In the human small intestine, glucose is absorbed by active transport when its concentration in the intestinal lumen is lower than in the blood. This ensures efficient absorption of all available glucose. In kidneys, glucose is reabsorbed from the filtrate into the blood by active transport, despite being at a low concentration in the filtrate.
在人的小肠中，当肠腔中葡萄糖浓度低于血液时，葡萄糖通过主动运输被吸收，以确保有效吸收所有可利用的葡萄糖。在肾脏中，尽管滤液中的葡萄糖浓度很低，仍通过主动运输从滤液中重吸收进入血液。
In plants, root hair cells absorb mineral ions such as nitrate and magnesium from the soil using active transport. The concentration of these ions is usually lower in the soil than inside the root, so energy must be used to pump them in. This is why roots need oxygen for respiration — to produce ATP.
在植物中，根毛细胞利用主动运输从土壤中吸收硝酸盐和镁等矿物离子。土壤中这些离子的浓度通常低于根内部，因此必须消耗能量将其泵入。这就是为什么根部需要氧气进行呼吸作用 — 以产生 ATP。
8. Comparing the Three Processes | 三大过程对比
Examiners often ask you to compare diffusion, osmosis and active transport. Use these key points:
考官常要求你对比扩散、渗透与主动运输。请记住这些要点：
- All three are involved in moving substances across membranes.
- Diffusion and osmosis are passive; active transport requires ATP.
- Diffusion moves any particles down their concentration gradient; osmosis specifically involves water moving across a partially permeable membrane; active transport moves particles against the gradient.
- 三者都与跨膜物质移动有关。
- 扩散和渗透是被动的；主动运输需要 ATP。
- 扩散使任意粒子顺浓度梯度移动；渗透特指水穿过部分通透膜；主动运输使粒子逆梯度移动。
Remember: Passive = no energy, down gradient; Active = energy used, against gradient
记住：被动 = 无能量，顺梯度；主动 = 耗能量，逆梯度
9. Experimental Techniques | 实验技术
For diffusion, you can observe the movement of potassium permanganate crystals in water — a purple colour spreads slowly. For osmosis, use Visking tubing (partially permeable) filled with sugar solution, placed in a beaker of water. The tubing increases in mass and volume because water enters. A potato strip experiment shows how osmosis changes mass in different salt concentrations.
对于扩散，可以观察高锰酸钾晶体在水中的运动 — 紫色缓慢扩散。对于渗透，使用装有糖溶液的玻璃纸袋（部分通透）放入盛水的烧杯中，纸袋的质量和体积会增加，因为水进入。马铃薯条实验可以显示不同盐浓度下渗透如何改变质量。
In active transport experiments, a sample of yeast cells is incubated with a glucose solution. ATP is required, so adding a respiratory inhibitor reduces glucose uptake, demonstrating energy dependence.
在主动运输实验中，将酵母细胞样品与葡萄糖溶液一起培养。由于需要 ATP，加入呼吸抑制剂会减少葡萄糖吸收，从而证明其对能量的依赖。
10. Common Exam Mistakes | 常见考试误区
Many students confuse “high concentration” with “high water potential”. In osmosis, water moves from high water potential (more water molecules) to low water potential (fewer water molecules). Also, diffusion does not stop at equilibrium — particles continue moving, but the net movement is zero.
许多学生将“高浓度”与“高水势”混为一谈。在渗透中，水从高水势（水分子多）流向低水势（水分子少）。另外，扩散在平衡时并未停止 — 粒子仍在运动，只是净移动为零。
Do not say “water flows along a concentration gradient” — it flows along a water potential gradient. And never state that active transport happens “with the gradient” — it always moves against it.
不要说“水沿浓度梯度流动” — 它沿水势梯度流动。也绝不能说主动运输是“顺梯度”进行的 — 它总是逆梯度移动。
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