📚 Edexcel IGCSE Biology: Diffusion, Osmosis and Active Transport | 爱德思 IGCSE 生物：扩散、渗透与主动运输
Movement of substances across cell membranes is essential for life. In IGCSE Biology, three main processes – diffusion, osmosis, and active transport – govern how molecules enter and leave cells. This article explains each process, their differences, and how they are tested in the Edexcel exam.
物质跨细胞膜的运动对生命至关重要。在 IGCSE 生物中，扩散、渗透和主动运输三个主要过程控制着分子进出细胞。本文详细讲解每种过程的原理、区别以及爱德思考试中的常见考点。
1. Diffusion | 扩散
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until the particles are evenly distributed. It is a passive process – it does not require energy.
扩散是粒子从高浓度区域沿浓度梯度向低浓度区域的净运动，直到粒子均匀分布。这是一个被动过程，不需要能量。
Key features 关键特征
Net movement occurs from high to low concentration.
净运动方向是从高浓度到低浓度。
It is driven by the random motion of particles.
由粒子的随机运动驱动。
It does not require metabolic energy (ATP).
不需要代谢能量（ATP）。
It continues until concentrations become equal.
持续进行直到浓度相等。
Factors affecting diffusion rate 影响扩散速率的因素
| Factor 因素 | Effect on rate 对速率的影响 |
|---|---|
| Concentration gradient | The steeper the gradient, the faster the diffusion. |
| 浓度梯度 | 梯度越大，扩散越快。 |
| Temperature | Higher temperature increases kinetic energy, so particles move faster. |
| 温度 | 温度升高，动能增大，粒子运动更快。 |
| Surface area | Larger surface area allows more particles to cross in the same time. |
| 表面积 | 表面积越大，相同时间内通过的粒子越多。 |
| Distance | Shorter distance means faster diffusion (e.g., thin walls of alveoli). |
| 距离 | 距离越短，扩散越快（如肺泡壁薄）。 |
Examples in living organisms 生物中的实例
Oxygen diffuses from alveoli into blood and carbon dioxide diffuses from blood into alveoli.
氧气从肺泡扩散入血液，二氧化碳从血液扩散入肺泡。
Glucose and amino acids diffuse from the small intestine into the blood (after digestion).
葡萄糖和氨基酸在小肠消化后扩散进入血液。
2. Osmosis | 渗透
Osmosis is a special type of diffusion that involves water molecules. It 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) through a partially permeable membrane.
渗透是涉及水分子的一种特殊扩散。它是水分子通过部分通透膜，从水势高（稀溶液）的区域向水势低（浓溶液）的区域的净运动。
Water potential 水势
Water potential is a measure of the tendency of water to move. Pure water has the highest water potential (0). Adding solutes lowers water potential, making it more negative.
水势是衡量水分子运动趋势的指标。纯水水势最高（为0）。加入溶质会降低水势，使其更负。
Effect on animal cells 对动物细胞的影响
In a hypotonic solution (higher water potential than the cell), water enters the cell. The cell may burst (lysis).
在低渗溶液中（水势高于细胞），水进入细胞，细胞可能破裂（溶胞）。
In a hypertonic solution (lower water potential), water leaves the cell. The cell shrinks (crenation).
在高渗溶液中（水势低于细胞），水离开细胞，细胞收缩（皱缩）。
In an isotonic solution, no net movement occurs.
在等渗溶液中，无净水运动。
Effect on plant cells 对植物细胞的影响
In a hypotonic solution, water enters the plant cell. The vacuole swells, pushing the cytoplasm against the cell wall, making the cell turgid. Turgidity provides support.
在低渗溶液中，水进入植物细胞。液泡膨胀，细胞质紧贴细胞壁，细胞变得膨压充盈（坚挺）。膨压支撑植物。
In a hypertonic solution, water leaves the plant cell. The vacuole shrinks, the cytoplasm pulls away from the cell wall – plasmolysis occurs.
在高渗溶液中，水离开植物细胞。液泡缩小，细胞质与细胞壁分离，出现质壁分离。
Because the cell wall is rigid and fully permeable, plant cells do not burst; they become turgid.
由于细胞壁坚硬且全透，植物细胞不会破裂，只会变得膨压充盈。
3. Active Transport | 主动运输
Active transport is the movement of particles against a concentration gradient, i.e., from a region of lower concentration to a region of higher concentration. This process requires energy (ATP) released by respiration, and involves carrier proteins in the cell membrane.
主动运输是粒子逆浓度梯度运动，即从低浓度区域向高浓度区域移动。这个过程需要呼吸作用释放的能量（ATP），并涉及细胞膜上的载体蛋白。
Mechanism 机制
Carrier protein binds with the particle (e.g., a mineral ion).
载体蛋白与粒子（如矿质离子）结合。
ATP is hydrolysed to provide energy for the protein to change shape.
ATP 被水解，为蛋白质改变形状提供能量。
The particle is transported across the membrane and released.
粒子被转运过膜并释放。
Examples 实例
Root hair cells absorb mineral ions (e.g., nitrate, magnesium) from the soil, where concentrations are lower than inside the root.
根毛细胞从土壤中吸收矿质离子（如硝酸盐、镁），土壤中的浓度低于根内。
The small intestine absorbs glucose into the blood after digestion, even when blood glucose is high.
小肠在消化后吸收葡萄糖进入血液，即使血糖浓度较高。
Kidney tubules reabsorb glucose from the filtrate back into the blood.
肾小管将滤液中的葡萄糖重新吸收回血液。
4. Comparison of Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输的比较
Understanding the differences is critical for exam questions. Use the table below for quick revision.
理解三者之间的区别对考试非常关键。使用下面的表格快速复习。
| Feature 特征 | Diffusion 扩散 | Osmosis 渗透 | Active transport 主动运输 |
|---|---|---|---|
| Substance moved 物质 | Any particles (gas, dissolved solid) | Water molecules only | Ions or molecules (e.g., glucose, amino acids) |
| 越过膜的物质 | 任何粒子（气体、溶质） | 只有水分子 | 离子或分子（如葡萄糖、氨基酸） |
| Direction 方向 | High → low concentration | High water potential → low water potential | Low → high concentration |
| 方向 | 高浓度 → 低浓度 | 高水势 → 低水势 | 低浓度 → 高浓度 |
| Energy required 是否需要能量 | No 否 | No (passive) 否（被动） | Yes (ATP) 是（ATP） |
| Membrane protein needed 需要膜蛋白 | No (simple diffusion) or channel (facilitated) | No, but aquaporins help | Carrier protein required |
| 膜蛋白需求 | 不需要（简单扩散）或通道蛋白（易化扩散） | 不需要，但水通道可协助 | 需要载体蛋白 |
| Example 实例 | O₂ entering blood in alveoli | Water entering root hair cells | Nitrate uptake by root cells |
| 实例 | 肺泡中 O₂ 进入血液 | 水分进入根毛细胞 | 根细胞吸收硝酸盐 |
5. Experimental Investigations | 实验探究
In Edexcel IGCSE, you may be asked to describe or evaluate experiments related to osmosis or diffusion.
在爱德思 IGCSE 中，您可能会被要求描述或评价与渗透或扩散相关的实验。
Visking tubing experiment 透析袋实验
Visking tubing is a partially permeable membrane. When filled with a starch and glucose solution and placed in distilled water, glucose diffuses out, but starch cannot. This demonstrates selective permeability.
透析袋是一种部分通透膜。当袋内装有淀粉和葡萄糖混合液并放在蒸馏水中时，葡萄糖能扩散出来，而淀粉不能。这证明了选择性通透性。
Potato strips experiment 马铃薯条实验
Cut potato strips of equal size, weigh them, and place in different sucrose concentrations (e.g., pure water, 0.5 mol/dm³, 1 mol/dm³).
切取等大的马铃薯条，称重后放入不同浓度的蔗糖溶液（如纯水、0.5 mol/dm³、1 mol/dm³）中。
After a set time, reweigh. In pure water, the strip gains mass (turgid). In concentrated solution, it loses mass (flaccid, plasmolysis).
一定时间后再次称重。在纯水中，薯条质量增加（膨压充盈）。在浓溶液中，质量减少（软弱，质壁分离）。
The change in mass can be used to estimate the water potential of the potato tissue (isotonic point).
质量变化可用于估算马铃薯组织的水势（等渗点）。
When answering experimental questions, always state the effect of variables: independent variable (e.g., concentration), dependent variable (mass change), and controlled variables (temperature, surface area, time).
回答实验问题时，务必说明变量：自变量（如浓度）、因变量（质量变化）和控制变量（温度、表面积、时间）。
6. Exam Tips and Common Mistakes | 考试技巧与常见错误
Let’s highlight some key points that Edexcel examiners look for.
我们重点强调爱德思考官看重的几个关键点。
Use the word 'net movement' instead of just 'movement' – it shows you know particles move randomly.
使用“净运动”而不是“运动”——表明你了解粒子随机运动。
For osmosis, always mention 'partially permeable membrane' and 'water potential gradient'.
对于渗透，务必提到“部分通透膜”和“水势梯度”。
Do not confuse 'turgid' with 'flaccid'. Turgid means swollen and stiff; flaccid means soft and limp.
不要混淆“膨压充盈”和“软弱”。Turgid 指膨胀而硬挺；Flaccid 指柔软、萎软。
When explaining active transport, say 'requires energy released from respiration' – not just 'energy'.
解释主动运输时，要说“需要呼吸作用释放的能量”，而不仅仅是“能量”。
For 'semi-permeable' vs 'partially permeable' – both are accepted, but use consistently.
“半透膜”与“部分通透膜”均可接受，但请保持一致。
In graphs of mass change vs concentration, the point where the line crosses zero change is the isotonic concentration.
在质量变化-浓度图中，曲线穿过零变化的点即为等渗浓度。
7. Real-World Applications | 实际应用
Understanding these processes helps explain many biological phenomena and technologies.
理解这些过程有助于解释许多生物现象和科技应用。
Food preservation: high sugar or salt solutions cause water loss from microorganisms via osmosis, preventing their growth.
食品保藏：高糖或高盐溶液使微生物通过渗透失水，抑制其生长。
Fertiliser application: if too much fertiliser is applied, the soil solution becomes hypertonic, causing plant roots to lose water – the plant wilts.
施肥过量：若施肥过多，土壤溶液变为高渗，植物根部失水，植物萎蔫。
Kidney dialysis: uses diffusion across a partially permeable membrane to remove waste urea from blood.
肾透析：利用部分通透膜上的扩散作用清除血液中的尿素废物。
Kidney tubule reabsorption: active transport allows glucose to be fully reabsorbed even when blood glucose is high.
肾小管重吸收：主动运输使葡萄糖即使在高血糖时也能完全被重吸收。
8. Quick Revision Checklist | 快速复习清单
Use this checklist before the exam to ensure you know all necessary points.
考前使用此清单，确保掌握所有要点。
Define diffusion and list factors that affect its rate.
定义扩散并列出影响其速率的因素。
Explain osmosis in terms of water potential.
用水势解释渗透。
Describe what happens to animal and plant cells in different solutions.
描述动物和植物细胞在不同溶液中的变化。
Define active transport and give two examples.
定义主动运输并给出两个例子。
Compare all three processes in a table.
用表格比较三种过程。
Design an experiment to determine the concentration of a cell's contents.
设计实验测定细胞内容物的浓度。
Keep this guide handy for revision, and always practise past paper questions to become familiar with the wording expected by Edexcel.
请将本文用作复习参考，并多练习真题，熟悉爱德思考官期望的表达方式。
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