📚 Topic 403: Diffusion, Osmosis and Active Transport | 主题403：扩散、渗透与主动转运
This revision guide explores three essential processes that control the movement of substances in and out of cells: diffusion, osmosis and active transport. These concepts are central to the Edexcel IGCSE Science (Biology) syllabus, and understanding them clearly will help you answer both multiple-choice and longer-answer questions.
本篇复习指南将探讨控制物质进出细胞的三个基本过程：扩散、渗透与主动转运。这些概念是 Edexcel IGCSE 科学（生物）大纲的核心内容，清晰理解它们将有助于你回答选择题和长答题。
1. Diffusion: The Passive Spread of Particles | 1. 扩散：粒子的被动扩散
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down their concentration gradient, until they are evenly distributed. It is a passive process, meaning it does not require energy.
扩散是指粒子从高浓度区域沿浓度梯度净移动至低浓度区域，直至均匀分布。这是一个被动过程，意味着它不需要能量。
- Key point: Diffusion happens in gases and liquids because particles are free to move. It does not happen in solids.
- 关键点：扩散发生在气体和液体中，因为粒子可以自由移动；它在固体中不会发生。
- Example: Oxygen diffuses from the alveoli in the lungs into the blood because oxygen concentration is higher in the alveoli than in the deoxygenated blood.
- 例子：氧气从肺部的肺泡扩散进入血液，因为肺泡中的氧气浓度高于去氧血液中的浓度。
Rate of diffusion ∝ surface area × concentration difference ÷ diffusion distance
This relationship reminds you which factors affect how quickly diffusion occurs. Exam questions often ask you to explain how the rate can be increased.
这一关系提醒你哪些因素会影响扩散速度。考试题经常要求你解释如何加快扩散速率。
2. Factors That Affect Diffusion | 2. 影响扩散的因素
Three main factors change the rate of diffusion: temperature, concentration gradient and surface area. A fourth factor, distance, is also important, especially in single-celled organisms versus multicellular organisms.
影响扩散速率的主要因素有三个：温度、浓度梯度和表面积。第四个因素——距离——也很重要，尤其是在单细胞生物与多细胞生物的对比中。
1. Temperature: Increasing temperature gives particles more kinetic energy, so they move faster and diffusion speeds up.
1. 温度：温度升高使粒子获得更多动能，因此粒子运动更快，扩散加速。
2. Concentration gradient: The steeper the gradient (larger difference in concentration), the faster the rate of diffusion.
2. 浓度梯度：梯度越陡（浓度差越大），扩散速率越快。
3. Surface area: A larger surface area allows more particles to cross a membrane at the same time, increasing the rate. This is why villi in the small intestine are adapted for absorption.
3. 表面积：更大的表面积允许更多粒子同时穿过膜，从而提高速率。这就是小肠绒毛适应吸收功能的原因。
4. Distance: Shorter distances allow faster diffusion. Single-celled organisms have a short diffusion distance, which is why they do not need a circulatory system.
4. 距离：距离越短，扩散越快。单细胞生物的扩散距离很短，因此它们不需要循环系统。
3. Osmosis: Water Movement Across a Membrane | 3. 渗透：水分子穿过膜的运动
Osmosis is a special type of diffusion involving water. It is defined as the net movement of water molecules from a region of higher water potential (or higher water concentration) to a region of lower water potential (or lower water concentration) across a partially permeable membrane.
渗透是一种涉及水分子的特殊扩散。它被定义为水分子穿过部分透性膜，从水势高（或水浓度高）的区域净移动至水势低（或水浓度低）的区域。
A partially permeable membrane allows water molecules to pass through, but not larger solute molecules such as sugar. This is a key detail that distinguishes osmosis from simple diffusion.
部分透性膜允许水分子通过，但不允许较大的溶质分子（如糖）通过。这是区分渗透与简单扩散的关键细节。
Osmosis: water moves from high water potential → low water potential
In practical terms, water moves towards the solution with the higher solute concentration. For example, if a potato chip is placed in concentrated salt solution, it loses water and becomes flaccid.
在实际中，水会向溶质浓度更高的溶液方向移动。例如，将薯条放入浓盐溶液中，它会失水并变得软缩。
4. Tonicity: Hypotonic, Isotonic and Hypertonic | 4. 张力：低渗、等渗与高渗
When comparing two solutions, examiners expect you to use the terms hypotonic, isotonic and hypertonic correctly. These terms relate to the solute concentration of one solution relative to another.
比较两种溶液时，考官期望你正确使用低渗、等渗和高渗三个术语。这些术语涉及一种溶液相对于另一种溶液的溶质浓度。
| Term | Solute concentration compared to cell | Effect on animal cell | Effect on plant cell |
|---|---|---|---|
| Hypotonic | Lower solute (more water) | Cell swells and may burst (lysis) | Cell becomes turgid (firm) |
| Isotonic | Same solute concentration | No net water movement | No net water movement |
| Hypertonic | Higher solute (less water) | Cell shrinks (crenation) | Cell becomes plasmolysed (flaccid) |
Plant cells are protected by their cell wall, which prevents bursting. When placed in a hypotonic solution, plant cells become turgid and this provides support for the plant. When placed in a hypertonic solution, the cytoplasm pulls away from the cell wall, a process called plasmolysis.
植物细胞由细胞壁保护，不会胀破。将植物细胞放入低渗溶液中时，细胞会变得膨胀坚挺（turgid），为植物提供支撑。放入高渗溶液时，细胞质会与细胞壁分离，这一过程称为质壁分离。
5. Active Transport: Moving Against the Gradient | 5. 主动转运：逆浓度梯度移动
Active transport is the movement of substances from a lower concentration to a higher concentration (against the concentration gradient), using energy released by respiration. It always requires a membrane protein called a carrier protein.
主动转运是指物质从低浓度向高浓度（逆浓度梯度）运动，并使用呼吸作用释放的能量。它总是需要一种叫做载体蛋白的膜蛋白。
- Energy source: The energy comes from ATP, produced during aerobic respiration in the mitochondria.
- 能量来源：能量来自ATP，由线粒体中的有氧呼吸产生。
- Example: Root hair cells absorb mineral ions like nitrate from the soil, even though the concentration of ions in the soil is lower than inside the cell.
- 例子：根毛细胞从土壤中吸收硝酸根等矿质离子，即使土壤中离子浓度低于细胞内部。
- Another example: The small intestine absorbs glucose against a concentration gradient after a meal, when blood glucose may already be high.
- 另一个例子：餐后血糖可能已经很高，小肠仍能逆浓度梯度吸收葡萄糖。
If respiration stops, active transport stops because no ATP is produced. This is why cells that carry out active transport, such as kidney cells and root hair cells, contain many mitochondria.
如果呼吸停止，主动转运就会停止，因为没有ATP产生。这就是为什么进行主动转运的细胞（如肾细胞和根毛细胞）含有大量线粒体。
6. Comparing Diffusion, Osmosis and Active Transport | 6. 扩散、渗透与主动转运的比较
Exam questions often ask you to complete a comparison table. Make sure you can distinguish these processes in three ways: the direction of movement, whether energy is required, and whether a membrane is needed.
考试题常要求你完成比较表。请确保你能从三个维度区分这些过程：移动方向、是否需要能量、以及是否需要膜。
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| Type of molecule | Any small molecule or ion (gas or liquid) | Water only | Ions, sugars, amino acids |
| Direction | High concentration → low concentration | High water potential → low water potential | Low concentration → high concentration |
| Energy required | No | No | Yes (ATP) |
| Membrane needed | Not necessarily (occurs in open space) | Partially permeable membrane required | Carrier proteins in a membrane required |
Remember that osmosis is just diffusion of water, but because water moves through a partially permeable membrane, osmotic effects have special consequences for cells.
记住，渗透只是水分子的扩散，但因为水要通过部分透性膜，所以渗透效应对细胞有特殊的影响。
7. Investigating Osmosis and Diffusion | 7. 探究渗透与扩散的实验
You should be familiar with the classic potato and visking tubing experiments. These are common in paper questions because they allow examiners to test your practical skills.
你应该熟悉经典的马铃薯和玻璃纸（visking tubing）实验。这些实验在试卷中很常见，因为考官可以借此测试你的实验技能。
Potato experiment: Cylinders of potato are placed in different concentrations of salt or sugar solution. The percentage change in mass is calculated. In pure water, the mass increases because water enters the potato cells by osmosis. In concentrated solution, the mass decreases because water leaves the cells.
马铃薯实验：将马铃薯条放入不同浓度的盐或糖溶液中，计算质量变化百分比。在纯水中，由于渗透作用水进入马铃薯细胞，质量增加；在浓溶液中，水离开细胞，质量减少。
Visking tubing experiment: Visking tubing acts as a partially permeable membrane. A solution of starch and glucose is placed inside the tube, and the tube is sealed and placed in a beaker of water. After some time, the water in the beaker tests positive for glucose but negative for starch. This proves that glucose molecules are small enough to diffuse out, while starch is too large.
玻璃纸实验：玻璃纸可作为部分透性膜。将淀粉和葡萄糖的混合液放入玻璃纸袋内，密封后置于装有水的烧杯中。一段时间后，烧杯中的水检测出葡萄糖，但检测不到淀粉。这证明葡萄糖分子小到可以扩散出来，而淀粉太大不能通过。
When plotting results, most exam questions ask you to estimate the concentration of the solution that is isotonic to the potato cell, where no net mass change occurs.
画图时，大多数考试题会要求你估算与马铃薯细胞等渗的溶液浓度，即质量没有净变化时的浓度。
8. Applications in Living Organisms | 8. 这些过程在生物体中的应用
Diffusion, osmosis and active transport are not just abstract concepts; they are essential for survival.
扩散、渗透和主动转运不仅是抽象概念，它们对生存至关重要。
- Gas exchange in the lungs: Oxygen diffuses into the blood and carbon dioxide diffuses out, maintaining a steep concentration gradient for rapid gas exchange.
- 肺部气体交换：氧气扩散进入血液，二氧化碳扩散出来，维持陡峭的浓度梯度以实现快速气体交换。
- Water absorption in plant roots: Water enters root hair cells by osmosis because there is a higher water potential in the soil than in the root hair cell.
- 植物根部吸水：由于土壤水势高于根毛细胞，水通过渗透进入根毛细胞。
- Mineral uptake by roots: Mineral ions are absorbed by active transport, which requires energy from respiration.
- 根部吸收矿物质：矿质离子通过主动转运吸收，需要呼吸作用提供能量。
- Absorption in the small intestine: Glucose and amino acids are absorbed by active transport into the bloodstream. Villi provide a large surface area to maximise absorption.
- 小肠吸收：葡萄糖和氨基酸通过主动转运进入血液。小肠绒毛提供巨大的表面积，从而最大化吸收效率。
In the kidneys, active transport enables the selective reabsorption of useful solutes from the filtrate back into the blood against their concentration gradient.
在肾脏中，主动转运使有用的溶质从滤液中逆浓度梯度重新吸收回血液。
9. Common Misconceptions and Exam Traps | 9. 常见误解和考试陷阱
Many students lose marks by using the word “movement” without saying “net movement”. Particles never stop moving; diffusion describes the overall direction of movement.
许多学生因为没有写出”净移动”而失分。粒子从不会停止运动；扩散描述的是运动的总体方向。
Trap 1: Confusing osmosis with diffusion of any substance. Osmosis is specifically the movement of water across a partially permeable membrane.
陷阱1：将渗透与任何物质的扩散混为一谈。渗透特指水穿过部分透性膜的运动。
Trap 2: Saying active transport requires ‘energy’ but not specifying the source. Always say ‘energy released by respiration’ or ‘ATP’.
陷阱2：只说主动转运需要”能量”，却不说明来源。务必说”呼吸作用释放的能量”或”ATP”。
Trap 3: When describing water potential, remember that adding solute lowers water potential. Pure water has the highest water potential.
陷阱3：描述水势时，记住加入溶质会降低水势。纯水具有最高的水势。
Trap 4: In the potato experiment, do not forget to dry the potato cylinders with a paper towel before measuring final mass, to avoid excess water affecting results.
陷阱4：在马铃薯实验中，最后称量前不要忘记用纸巾吸干薯条表面水分，以免多余水分影响结果。
10. Quick Revision Checklist | 10. 快速复习检查表
Use this checklist to make sure you have mastered every core idea before the exam.
使用此检查表，确保你在考试前掌握每一个核心理念。
- Can you define diffusion using the terms ‘net movement’ and ‘concentration gradient’?
- 你能用”净移动”和”浓度梯度”来定义扩散吗？
- Can you list three factors that increase the rate of diffusion?
- 你能列出三个加快扩散速率的因素吗？
- Can you define osmosis using ‘water molecules’ and ‘partially permeable membrane’?
- 你能用”水分子”和”部分透性膜”来定义渗透吗？
- Can you explain why plant cells become turgid in pure water but animal cells burst?
- 你能解释为什么植物细胞在纯水中变得坚挺而动物细胞会胀破吗？
- Can you give two examples of active transport in living organisms?
- 你能举出两个生物体中主动转运的例子吗？
- Can you describe the potato osmosis experiment, including control variables?
- 你能描述马铃薯渗透实验及控制变量吗？
If you can answer all of these confidently, you are ready for Topic 403 questions.
如果你能自信地回答所有这些问题，你就已经准备好应对403主题的题目了。
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