📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
In IGCSE Biology, understanding how molecules move across cell membranes is essential. This revision guide covers three key processes: diffusion, osmosis, and active transport, with their definitions, examples, and common exam traps.
在 IGCSE 生物中，理解分子如何穿过细胞膜是至关重要的。本复习指南涵盖三个关键过程：扩散、渗透和主动运输，包括它们的定义、实例和常见考试陷阱。
1. The Cell Membrane as a Barrier | 细胞膜作为屏障
The cell membrane controls what enters and leaves the cell. It is selectively permeable, meaning it allows some substances to pass through but not others. Small molecules like oxygen, water, carbon dioxide, and glucose can cross easily, while larger molecules such as proteins and starch cannot.
细胞膜控制着进出细胞的物质。它具有选择透过性，意思是允许某些物质通过而不允许其他物质通过。像氧气、水、二氧化碳和葡萄糖这样的小分子容易穿过，而蛋白质和淀粉等较大的分子则不能。
This property is fundamental to diffusion, osmosis, and active transport. The membrane also contains carrier proteins and channel proteins, which are especially important for active transport and facilitated diffusion.
这一特性是扩散、渗透和主动运输的基础。细胞膜还含有载体蛋白和通道蛋白，它们对主动运输和易化扩散尤为重要。
2. Diffusion: Definition and Examples | 扩散：定义与实例
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. This is a passive process – it does not require energy from respiration.
扩散是粒子从浓度较高的区域向浓度较低的区域沿浓度梯度的净运动，直到粒子均匀分布。这是一个被动过程——不需要来自呼吸作用的能量。
Examples in biology include oxygen moving from alveoli into the blood, carbon dioxide moving from the blood into alveoli, and glucose molecules diffusing into the bloodstream from the small intestine.
生物学中的实例包括氧气从肺泡进入血液，二氧化碳从血液进入肺泡，以及葡萄糖分子从小肠进入血液。
3. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
Several factors influence how fast diffusion occurs:
有几个因素影响扩散发生的速度：
Concentration gradient – a steeper gradient increases the rate.
浓度梯度——梯度越陡，速率越快。
Temperature – higher temperature gives particles more kinetic energy, so they move faster.
温度——温度越高，粒子获得更多动能，所以运动更快。
Surface area – a larger surface area allows more particles to cross in the same time.
表面积——更大的表面积允许更多粒子在同一时间内穿过。
Distance – a shorter distance means faster diffusion (important in thin alveoli walls).
距离——距离越短，扩散越快（在薄的肺泡壁中很重要）。
Size of molecules – smaller molecules diffuse faster than larger ones.
分子大小——较小的分子比较大的分子扩散更快。
Rate of diffusion ∝ (surface area × concentration gradient) / distance
You should remember this relationship and be able to apply it to adaptations of exchange surfaces, such as alveoli, villi, and gills.
你应该记住这一关系，并能够将其应用于交换表面的适应特征，如肺泡、绒毛和鳃。
4. Osmosis: A Special Case of Diffusion | 渗透：扩散的特殊情况
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) through a partially permeable membrane. It is also a passive process.
渗透是水分子从水势较高（稀溶液）的区域通过部分透性膜向水势较低（浓溶液）的区域进行的净运动。它也是一个被动过程。
Water potential is a measure of the tendency of water to move. Pure water has the highest water potential (zero), and adding solutes lowers the water potential (makes it more negative).
水势是衡量水运动趋势的指标。纯水具有最高的水势（零），加入溶质会降低水势（使其更负）。
5. Osmosis in Animal Cells | 动物细胞中的渗透
Animal cells do not have a rigid cell wall, so osmosis can cause them to swell or shrink dramatically. When placed in pure water, water enters the cell, causing it to swell and eventually burst. This is called lysis.
动物细胞没有坚硬的细胞壁，因此渗透会使它们剧烈膨胀或收缩。当放入纯水中时，水进入细胞，导致细胞膨胀并最终破裂。这称为裂解。
When placed in a concentrated solution, water leaves the cell, causing it to shrink and shrivel. This is called crenation. Red blood cells are often used to demonstrate these effects.
当放入浓溶液时，水离开细胞，导致细胞收缩并皱缩。这称为皱缩。红细胞常被用来演示这些效应。
6. Osmosis in Plant Cells | 植物细胞中的渗透
Plant cells have a cell wall that prevents bursting. In a dilute solution, water enters the cell, the vacuole expands, and the cell becomes turgid. Turgidity provides support for herbaceous plants.
植物细胞有细胞壁，防止胀破。在稀溶液中，水进入细胞，液泡膨胀，细胞变得膨胀。膨胀为草本植物提供支撑。
In a concentrated solution, water leaves the cell, the vacuole shrinks, and the cytoplasm pulls away from the cell wall. This is called plasmolysis. If the cell is placed back into water, it can recover.
在浓溶液中，水离开细胞，液泡收缩，细胞质与细胞壁分离。这称为质壁分离。如果将细胞放回水中，它可以恢复。
7. Active Transport: Moving Against the Gradient | 主动运输：逆梯度运动
Active transport is the movement of particles from a region of lower concentration to a region of higher concentration (against the concentration gradient). This requires energy from cellular respiration and involves carrier proteins in the membrane.
主动运输是粒子从浓度较低的区域向浓度较高的区域（逆浓度梯度）的运动。这需要细胞呼吸提供的能量，并涉及膜中的载体蛋白。
A classic example is the absorption of mineral ions by root hairs. The soil solution has a lower concentration of minerals than the root hair cells, so active transport is needed to take them up. Another example is the uptake of glucose in the small intestine when there is a low glucose concentration in the gut.
一个经典例子是根毛对矿质离子的吸收。土壤溶液中的矿物质浓度低于根毛细胞，因此需要主动运输来吸收它们。另一个例子是小肠在肠腔内葡萄糖浓度较低时对葡萄糖的吸收。
8. Comparing Diffusion, Osmosis and Active Transport | 比较扩散、渗透与主动运输
Use this table to summarise the key differences:
使用此表总结关键区别：
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Substance moved | Any small molecule | Water only | Ions and larger molecules |
| Direction | Down concentration gradient | Down water potential gradient | Against concentration gradient |
| Energy required? | No | No | Yes (from respiration) |
| Membrane proteins involved? | Not usually | No (water passes through pores) | Carrier proteins |
Exam tip: Do not say oxygen diffuses from high to low concentration in osmosis. Osmosis always refers to water moving through a partially permeable membrane.
考试提示：不要声称氧气在渗透中从高浓度向低浓度扩散。渗透总是指水通过部分透性膜的运动。
9. Investigating Osmosis: Potato Experiment | 渗透探究：马铃薯实验
To investigate osmosis, you can place cylinders of potato in different concentrations of sucrose solution and measure the change in mass or length. In pure water, the potato gains mass; in a very concentrated solution, it loses mass. The point where there is no change in mass represents the concentration of the potato cell sap.
为了探究渗透，你可以将马铃薯条放入不同浓度的蔗糖溶液中，并测量质量或长度的变化。在纯水中，马铃薯质量增加；在非常浓的溶液中，质量减少。质量不变的点代表马铃薯细胞液的浓度。
This experiment is common in IGCSE practical exams. Always repeat measurements, calculate mean percentage change, and control the temperature and time.
这个实验在 IGCSE 实验考试中很常见。务必重复测量，计算平均百分比变化，并控制温度和时间。
10. Common Misconceptions and Exam Tips | 常见误解与考试技巧
Many students lose marks due to careless language. Here are some key points:
许多学生因粗心的语言而丢分。以下是一些关键点：
Diffusion is the net movement of particles, not the random movement itself.
扩散是粒子的净运动，而不是随机运动本身。
Osmosis is only about water, not sugar or ions.
渗透只涉及水，不涉及糖或离子。
Active transport requires energy from respiration, not directly from the sun or from diffusion.
主动运输需要来自呼吸作用的能量，而不是直接来自太阳或扩散。
“Down the concentration gradient” means from high to low concentration. “Against” means from low to high.
“顺浓度梯度”意味着从高到低。“逆”意味着从低到高。
When writing answers, mention: direction (with/against gradient), energy requirement (passive/active), and any membrane proteins involved. This earns full marks in “explain” questions.
写答案时，要提及：方向（顺/逆梯度）、能量需求（被动/主动）以及是否涉及膜蛋白。这在“解释”类问题中能得满分。
Published by TutorHao | Biology Revision Series | aleveler.com
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