📚 Diffusion and Osmosis | 扩散与渗透作用
Diffusion and osmosis are fundamental processes that explain how substances move across cell membranes. Understanding these concepts is essential for Edexcel IGCSE Biology, as they appear in topics ranging from transport in cells to practical investigations.
扩散和渗透是解释物质如何穿过细胞膜的基本过程。理解这些概念对 Edexcel IGCSE 生物学至关重要，它们出现在从细胞内部运输到实验调查的各个主题中。
1. What is Diffusion? | 什么是扩散？
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 does not require energy input from the cell.
扩散是粒子从高浓度区域到低浓度区域沿浓度梯度的净移动。这是由于粒子的随机运动引起的，不需要细胞提供能量。
- Net movement stops when particles are evenly distributed, although random motion continues.
- Diffusion is passive – no ATP or metabolic energy is used.
- 当粒子均匀分布时净移动停止，但随机运动仍在继续。
- 扩散是被动过程——不使用 ATP 或代谢能量。
Diffusion rate ∝ concentration gradient × surface area ÷ distance
扩散速率 ∝ 浓度梯度 × 表面积 ÷ 距离
2. Factors Affecting Diffusion Rate | 影响扩散速率的因素
Three main factors control how quickly diffusion occurs: the concentration gradient, the temperature, and the surface area to volume ratio. A steeper gradient, higher temperature, or larger surface area all increase the rate of diffusion.
三个主要因素控制扩散发生的速度：浓度梯度、温度和表面积与体积比。梯度越陡、温度越高或表面积越大，都会增加扩散速率。
| Factor | Effect on diffusion | Reason |
| Concentration gradient | Increase → faster | More particles move down the gradient per unit time |
| Temperature | Increase → faster | Particles gain kinetic energy and move more rapidly |
| Surface area | Increase → faster | More area available for particles to cross |
| 因素 | 对扩散的影响 | 原因 |
| 浓度梯度 | 增加 → 更快 | 单位时间内更多粒子沿梯度移动 |
| 温度 | 增加 → 更快 | 粒子获得动能并移动得更快 |
| 表面积 | 增加 → 更快 | 有更多面积可供粒子穿过 |
3. Diffusion in Living Organisms | 生物体中的扩散
Diffusion is vital for gas exchange in lungs, leaves, and gills. Oxygen diffuses into the blood because its concentration is higher in the alveoli than in the blood. Carbon dioxide diffuses in the opposite direction.
扩散对肺、叶片和鳃中的气体交换至关重要。由于肺泡中的氧气浓度高于血液，氧气扩散进入血液。二氧化碳则向相反方向扩散。
- Alveoli have thin walls (one cell thick) and a large surface area to maximise diffusion.
- In plant leaves, carbon dioxide diffuses into the mesophyll cells through stomata.
- 肺泡壁薄（仅一层细胞厚）且表面积大，以最大化扩散效率。
- 在植物叶片中，二氧化碳通过气孔扩散进入叶肉细胞。
4. What is Osmosis? | 什么是渗透？
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane. It is a special case of diffusion involving water only.
渗透是水分子从水势较高的区域通过部分通透性膜向水势较低区域的净移动。它是仅涉及水的扩散的特殊情况。
Water potential (Ψ) – a measure of the tendency of water to move
水势（Ψ）——衡量水移动趋势的指标
Water always moves from high water potential (dilute solution) to low water potential (concentrated solution). The membrane allows water molecules to pass, but not solute particles.
水总是从高水势（稀溶液）向低水势（浓溶液）移动。膜允许水分子通过，但不允许溶质粒子通过。
5. Animal Cells in Different Solutions | 动物细胞在不同溶液中的表现
Animal cells do not have a cell wall. When placed in a hypotonic solution (more dilute), water enters the cell, causing it to swell and eventually burst; this is called lysis. In a hypertonic solution (more concentrated), water leaves the cell, causing it to shrink; this is called crenation.
动物细胞没有细胞壁。当置于低渗溶液（更稀释）中时，水进入细胞，使其膨胀并最终破裂，这称为裂解。在高渗溶液（更浓）中，水离开细胞，使其收缩，这称为皱缩。
- In an isotonic solution, water movements in and out are balanced, so the cell stays unchanged.
- Red blood cells are often used to demonstrate osmosis because they are easy to observe.
- 在等渗溶液中，水的进出平衡，因此细胞保持不变。
- 红细胞常用于演示渗透作用，因为它们容易观察。
6. Plant Cells in Different Solutions | 植物细胞在不同溶液中的表现
Plant cells have a strong cell wall. In a hypotonic solution, water enters and the cell becomes turgid – the cytoplasm pushes against the cell wall. In a hypertonic solution, water leaves, the cytoplasm shrinks away from the cell wall, and the cell becomes plasmolysed.
植物细胞有坚固的细胞壁。在低渗溶液中，水进入，细胞变得膨胀——细胞质挤压细胞壁。在高渗溶液中，水离开，细胞质从细胞壁缩回，细胞发生质壁分离。
Turgor pressure supports herbaceous plants and young shoots
膨胀压支撑草本植物和幼嫩枝条
| Solution type | Water movement | Appearance |
| Hypotonic | Into cell | Turgid, firm |
| Hypertonic | Out of cell | Plasmolysed, flaccid |
| 溶液类型 | 水的移动 | 外观 |
| 低渗 | 进入细胞 | 膨胀、坚硬 |
| 高渗 | 离开细胞 | 质壁分离、松软 |
7. Active Transport | 主动运输
Active transport is the movement of particles from a lower concentration to a higher concentration, against the concentration gradient. This process requires energy in the form of ATP and uses carrier proteins embedded in the membrane.
主动运输是粒子从低浓度向高浓度（逆浓度梯度）移动的过程。该过程需要以 ATP 形式提供能量，并利用膜内嵌的载体蛋白。
- Examples include mineral ion uptake by root hairs and glucose absorption in the ileum.
- Without active transport, cells could not absorb essential substances when their internal concentration is already high.
- 示例包括根毛吸收矿物离子和小肠回肠吸收葡萄糖。
- 如果没有主动运输，细胞在内部浓度已经很高时无法吸收必需物质。
8. Investigating Osmosis – Potato Experiment | 探究渗透作用——马铃薯实验
In Edexcel IGCSE, you may need to investigate osmosis using potato cylinders. Place equal-sized cylinders in different sucrose solutions and measure their change in mass after a set period.
在 Edexcel IGCSE 中，你可能需要使用马铃薯条探究渗透作用。将等大小的马铃薯条放入不同浓度的蔗糖溶液中，一段时间后测量其质量变化。
- Use a cork borer to cut identical potatoes.
- Dry them carefully with paper towels before weighing.
- Control variables: temperature, time, potato type, and surface area.
- 使用打孔器切出相同的马铃薯条。
- 称重前用纸巾小心擦干。
- 控制变量：温度、时间、马铃薯品种和表面积。
The concentration where no change in mass occurs is the isotonic concentration – the potato cell sap has the same water potential as the external solution.
质量不发生变化的浓度即为等渗浓度——马铃薯细胞液的水势与外部溶液相同。
9. Calculating Percentage Change in Mass | 计算质量变化百分比
To compare results fairly, always calculate the percentage change in mass, not just the raw change. This accounts for initial size differences between potato cylinders.
为公平比较结果，务必计算质量变化百分比，而不仅仅是原始变化量。这样可以消除马铃薯条初始大小差异的影响。
Percentage change = (final mass − initial mass) ÷ initial mass × 100%
质量变化百分比 =（最终质量 − 初始质量）÷ 初始质量 × 100%
- A positive value indicates a net gain of water (osmosis into the potato).
- A negative value indicates a net loss of water (osmosis out of the potato).
- 正值表示水的净增加（渗透进入马铃薯）。
- 负值表示水的净损失（渗透离开马铃薯）。
10. Osmosis and Food Preservation | 渗透作用与食品保存
Salting and sugaring foods preserve them by drawing water out of microorganisms through osmosis. Without water, bacteria and fungi cannot grow and reproduce.
盐渍和糖渍食品通过渗透作用将微生物体内的水分吸出，从而保存食物。没有水，细菌和真菌无法生长繁殖。
- Jam contains high sugar concentration, so water leaves any microbes.
- Salt on meat creates a hypertonic environment for bacteria.
- 果酱含高浓度糖分，因此微生物中的水会流失。
- 肉上的盐为细菌创造了高渗环境。
11. Common Exam Mistakes and Tips | 常见考试错误与技巧
Many students confuse osmosis with diffusion or forget to mention ‘partially permeable membrane’ in their definition. Always use precise scientific language.
许多学生混淆渗透与扩散，或在定义中忘记提及“部分通透性膜”。务必使用精确的科学语言。
- State ‘net movement’ not just ‘movement’ – particles move both ways.
- For osmosis, always say ‘water molecules’ not ‘particles’.
- Use ‘higher concentration’ for diffusion, but ‘higher water potential’ for osmosis.
- 要写“净移动”而不是“移动”——粒子双向运动。
- 对于渗透，总是说“水分子”而不是“粒子”。
- 扩散用“较高浓度”，渗透用“较高水势”。
In multiple-choice questions, look for the key phrase ‘against the concentration gradient’ – this indicates active transport, not diffusion.
在选择题中，寻找关键词“逆浓度梯度”——这表示主动运输，而非扩散。
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