📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
In Edexcel IGCSE Science (Biology), understanding how substances move into and out of cells is essential. This article explains the three key processes — diffusion, osmosis and active transport — with their definitions, examples, practical investigations and exam-style comparisons.
在 Edexcel IGCSE 科学（生物）中，理解物质如何进出细胞至关重要。本文讲解扩散、渗透和主动运输这三个关键过程，包括定义、实例、实验探究以及考试风格的对比。
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, until equilibrium is reached. It is a passive process, meaning it does not require energy.
扩散是粒子从浓度较高区域向浓度较低区域（沿浓度梯度）的净移动，直到达到平衡。这是一个被动过程，即不需要能量。
In gases and liquids, particles move randomly; diffusion results from their kinetic energy.
In cells, oxygen and carbon dioxide diffuse through the cell membrane.
The rate of diffusion increases with a greater concentration gradient, higher temperature, or larger surface area.
在气体和液体中，粒子随机运动；扩散源于它们的动能。
在细胞中，氧气和二氧化碳通过细胞膜扩散。
浓度梯度越大、温度越高或表面积越大，扩散速率越快。
Rate of diffusion ∝ (surface area × concentration gradient) ÷ membrane thickness
扩散速率 ∝（表面积 × 浓度梯度）÷ 膜厚度
2. Factors Affecting Diffusion | 影响扩散的因素
Several factors affect how quickly particles diffuse across a membrane. In exam questions, you may be asked to explain these using the kinetic particle model.
多个因素影响粒子穿过膜的扩散快慢。在考试题目中，你可能会被要求用粒子运动模型解释这些因素。
| Factor / 因素 | Effect / 影响 |
| Concentration gradient / 浓度梯度 | Steeper gradient → faster diffusion / 梯度越大，扩散越快 |
| Temperature / 温度 | Higher temperature → more kinetic energy → faster diffusion / 温度越高，动能越大，扩散越快 |
| Surface area / 表面积 | Larger surface area → more space for particles → faster diffusion / 表面积越大，粒子通过的空间越多，扩散越快 |
| Distance / 距离 | Shorter distance → faster diffusion / 距离越短，扩散越快 |
3. What is Osmosis? | 什么是渗透？
Osmosis is 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. It is also a passive process.
渗透是水分子通过部分通透性膜，从水势较高区域（稀溶液）向水势较低区域（浓溶液）的净移动。这也是一个被动过程。
Water moves through the membrane, but larger solute particles cannot pass through.
The 'water potential' describes the ability of water to move; pure water has the highest water potential (0).
Osmosis is a special type of diffusion involving only water molecules.
水分子穿过膜，而较大的溶质粒子不能通过。
“水势”描述水移动的能力；纯水具有最高水势（0）。
渗透是一种只涉及水分子的特殊扩散。
Water potential: pure water (0) > dilute solution > concentrated solution (more negative)
水势：纯水（0）＞ 稀溶液 ＞ 浓溶液（更负）
4. Osmosis in Animal Cells | 动物细胞中的渗透
Animal cells do not have a cell wall, so they are strongly affected by changes in the surrounding solution.
动物细胞没有细胞壁，因此受周围溶液变化影响很大。
Isotonic solution: water enters and leaves at the same rate; the cell stays normal.
Hypotonic solution (more dilute): water enters the cell, which swells and may burst (lysis).
Hypertonic solution (more concentrated): water leaves the cell, which shrinks (crenation).
等渗溶液：水进入和离开的速率相等；细胞保持正常。
低渗溶液（更稀）：水进入细胞，细胞膨胀并可能破裂（溶胞）。
高渗溶液（更浓）：水离开细胞，细胞皱缩（质壁分离状）。
5. Osmosis in Plant Cells | 植物细胞中的渗透
Plant cells have a rigid cell wall, so they respond differently to osmotic changes.
植物细胞具有坚硬的细胞壁，因此对渗透变化的反应不同。
In a hypotonic solution, water enters; the vacuole expands and pushes against the cell wall, making the cell turgid. Turgidity supports the plant.
In a hypertonic solution, water leaves; the vacuole shrinks and the cytoplasm pulls away from the cell wall, causing plasmolysis.
In an isotonic solution, the cell is flaccid (limp).
在低渗溶液中，水进入；液泡膨胀并挤压细胞壁，使细胞膨胀。膨胀支持植物挺立。
在高渗溶液中，水离开；液泡缩小，细胞质与细胞壁分离，导致质壁分离。
在等渗溶液中，细胞为松弛（柔软）状态。
6. Practical Investigation: Osmosis in Potatoes | 实验探究：马铃薯中的渗透
A common Edexcel practical involves placing potato cylinders into different sucrose solutions and measuring mass changes.
一个常见的 Edexcel 实验是将马铃薯条放入不同蔗糖溶液中，测量质量变化。
Cut identical potato cylinders and weigh them.
Place each cylinder in a different concentration of sucrose solution (e.g., 0%, 5%, 10%, 20%).
Leave for a set time (e.g., 30 minutes), then remove, dry and reweigh.
Calculate percentage change in mass: (final mass − initial mass) ÷ initial mass × 100.
切下相同的马铃薯条并称重。
将每条分别放入不同浓度的蔗糖溶液（如 0%、5%、10%、20%）。
静置一定时间（如30分钟），取出、擦干并重新称重。
计算质量变化百分比：（最终质量 − 初始质量）÷ 初始质量 × 100。
In the 0% solution, the potato gains mass because water enters by osmosis. In very concentrated solutions, the potato loses mass. The concentration where there is no mass change is the isotonic point of the potato tissue.
在 0% 溶液中，马铃薯因渗透吸水而质量增加。在很浓的溶液中，马铃薯失水而质量减少。质量不变时的浓度即为马铃薯组织的等渗点。
7. What is Active Transport? | 什么是主动运输？
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 released by respiration, and it uses carrier proteins in the membrane.
主动运输是粒子从浓度较低区域向浓度较高区域（逆浓度梯度）的移动。该过程需要呼吸作用释放的能量，并利用膜中的载体蛋白。
Energy is stored in ATP (adenosine triphosphate), produced during respiration.
Carrier proteins change shape to pump particles across the membrane.
Unlike diffusion and osmosis, active transport can move substances even when the concentration is already high inside the cell.
能量以 ATP（三磷酸腺苷）形式储存，由呼吸作用产生。
载体蛋白改变形状，将粒子泵过膜。
与扩散和渗透不同，主动运输即使在细胞内浓度已很高时也能移动物质。
8. Examples of Active Transport in Living Organisms | 生物体内主动运输的实例
There are two key examples you must learn for IGCSE exams.
IGCSE 考试中你需要掌握两个关键实例。
Root hair cells in plants: absorb mineral ions (e.g., nitrate, magnesium) from very dilute soil solutions against a concentration gradient.
Kidney tubules in animals: reabsorb glucose from the filtrate back into the blood, even when blood glucose is high.
植物根毛细胞：逆浓度梯度从极稀的土壤溶液中吸收矿质离子（如硝酸盐、镁离子）。
动物肾小管：将滤液中的葡萄糖重新吸收回血液，即使血糖已经很高。
Active transport: low concentration → high concentration (uphill, requires energy)
主动运输：低浓度 → 高浓度（逆梯度，需要能量）
9. Comparing Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输对比
Exam questions often ask you to compare these processes in a table. Make sure you remember the key differences.
考试题目常要求你在表格中比较这些过程。请务必记住关键区别。
| Feature / 特征 | Diffusion / 扩散 | Osmosis / 渗透 | Active transport / 主动运输 |
| Substance / 物质 | Gas / dissolved particles / 气体/溶质粒子 | Water only / 仅水 | Ions / glucose / 离子/葡萄糖 |
| Direction / 方向 | Down gradient / 顺梯度 | Down water potential gradient / 顺水势梯度 | Against gradient / 逆梯度 |
| Energy needed / 是否需要能量 | No / 否 | No / 否 | Yes (ATP) / 是（ATP） |
| Membrane protein needed / 是否需要膜蛋白 | Usually no (simple) / 通常不需（简单扩散） | No (through membrane) / 不需（直接穿过膜） | Yes (carrier proteins) / 需要（载体蛋白） |
10. Exam Tips and Common Mistakes | 考试技巧与常见错误
Many students lose marks by confusing osmosis with diffusion or by forgetting to mention 'partially permeable membrane' in an osmosis definition.
很多学生因混淆渗透与扩散，或在渗透定义中忘记提到“部分通透性膜”而失分。
Always use the term 'net movement' in definitions of diffusion and osmosis.
For osmosis, state 'water molecules' not 'particles' — the membrane is not permeable to solute.
For active transport, mention 'against the concentration gradient' and 'energy from respiration'.
In potato experiments, calculate percentage change in mass, not just mass change, so results are comparable.
在扩散和渗透的定义中始终使用“净移动”一词。
对于渗透，要说“水分子”而不是“粒子”——膜对溶质不通透。
对于主动运输，要提到“逆浓度梯度”和“呼吸作用释放的能量”。
在马铃薯实验中，计算质量变化百分比而不只是质量变化，以保证结果可比。
11. Quick Revision Questions | 快速复习题
Test yourself with these short questions before your exam.
考试前用这些短问题自测。
Q1. Define diffusion in terms of concentration gradient.
Q1. 用浓度梯度定义扩散。
Q2. Explain why a red blood cell bursts when placed in distilled water.
Q2. 解释为什么红细胞放入蒸馏水中会破裂。
Q3. Give two differences between osmosis and active transport.
Q3. 给出渗透和主动运输的两个区别。
Q4. A plant loses water from its roots when the surrounding soil is more concentrated than the root cells. Why?
Q4. 当周围土壤比根细胞更浓时，植物根部失水。为什么？
Q5. Name the process by which root hair cells absorb nitrate ions from dilute soil.
Q5. 根毛细胞从稀土壤中吸收硝酸根离子的过程叫什么？
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