📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
Every living cell is a busy factory. It must take in raw materials such as oxygen, glucose and mineral ions, and release waste products such as carbon dioxide. Understanding how substances cross cell membranes is essential for Edexcel IGCSE Science, as questions on cell transport appear frequently in both Biology Paper 1 and Paper 2, and form the foundation of topics from digestion to respiration.
每个活细胞都是一座繁忙的工厂。细胞必须吸收氧气、葡萄糖和矿物质离子等原料，同时释放二氧化碳等废物。理解物质如何穿越细胞膜是 Edexcel IGCSE 科学的核心要求，细胞运输相关题目在生物 Paper 1 和 Paper 2 中经常出现，也是消化、呼吸等众多主题的基础。
1. Diffusion: The Foundation of Passive Transport | 扩散：被动运输的基础
Diffusion is defined as the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of the random movement of particles. The key word here is “net”, meaning the overall movement. Even when particles are evenly spread, individual particles continue to move randomly, but there is no net movement in any one direction; this state is called dynamic equilibrium.
扩散的定义是：粒子由于自身的随机运动，从浓度较高的区域向浓度较低的区域净移动，即沿浓度梯度向下移动。这里的关键词是”净”（net），指的是整体上的移动方向。即使粒子已经均匀分布，单个粒子仍在随机运动，但任何方向上都不再有净移动，这种状态称为动态平衡。
Diffusion is a passive process, which means it does not require any energy input from the cell. The driving force comes purely from the kinetic energy of the particles themselves. Small, non-polar molecules such as oxygen and carbon dioxide diffuse easily through the lipid bilayer of the cell membrane, while larger molecules such as glucose may require special transport proteins.
扩散是一个被动过程，即不需要细胞消耗任何能量。驱动力完全来自粒子自身的动能。氧气和二氧化碳等小型非极性分子可以轻松穿过细胞膜的脂双层，而葡萄糖等较大分子则需要特殊的转运蛋白协助。
2. Factors That Control the Rate of Diffusion | 控制扩散速率的因素
Four main factors affect how quickly diffusion occurs. You must be able to explain all four in the exam, and you should be familiar with Fick’s law, which summarises them in one simple expression.
有四个主要因素影响扩散速度。你需要在考试中解释这四个因素，并且应熟悉菲克定律（Fick’s law），它用一个简单表达式概括了这些因素。
The first factor is the concentration gradient: the steeper the difference in concentration between two regions, the faster the rate of diffusion. The second factor is temperature: higher temperatures give particles more kinetic energy, so they move faster and collide more often, increasing the rate. The third factor is surface area: a larger surface area allows more particles to cross a membrane at the same time. The fourth factor is the diffusion distance: the shorter the distance, the faster particles can cross, which is why exchange surfaces are always extremely thin.
第一个因素是浓度梯度：两个区域之间的浓度差越大，扩散速率越快。第二个因素是温度：温度越高，粒子动能越大，运动越快、碰撞越频繁，扩散速率随之提高。第三个因素是表面积：表面积越大，同一时间能穿过膜的粒子就越多。第四个因素是扩散距离：距离越短，粒子穿越越快，因此交换表面总是极其薄。
Rate of diffusion ∝ (Surface area × Concentration difference) ÷ Diffusion distance
This relationship is Fick’s law. In exam answers, always state which factor is changed and then link it logically to the rate. For example: “The alveoli have a large surface area, which increases the rate of oxygen diffusion into the blood.”
这一关系就是菲克定律。在答题时，务必说明哪个因素发生了变化，并逻辑清晰地将其与速率联系起来。例如：”肺泡具有巨大的表面积，因此增加了氧气扩散进入血液的速率。”
3. Diffusion in Action: Gas Exchange in Living Organisms | 扩散的实际应用：生物体内的气体交换
Diffusion is the mechanism behind gas exchange in many organisms. In the human lungs, oxygen diffuses from the air inside the alveoli, where its concentration is high, into the blood, where its concentration is low. Meanwhile, carbon dioxide diffuses in the opposite direction. The alveoli are adapted for rapid diffusion because they have very thin walls (one cell thick), a large total surface area, and a dense network of capillaries that maintains a steep concentration gradient by constantly carrying blood away.
扩散是许多生物体进行气体交换的机制。在人体肺部，氧气从浓度高的肺泡空气扩散进入浓度低的血液；与此同时，二氧化碳向相反方向扩散。肺泡对快速扩散有特殊适应：壁极薄（仅一个细胞厚）、总表面积巨大，并且有密集的毛细血管网，通过不断带走血液来维持陡峭的浓度梯度。
In fish, gills use a clever counter-current system: blood flows through the gill lamellae in the opposite direction to water flowing over the gills. This maintains a concentration gradient of oxygen along the entire length of the lamellae, maximising oxygen uptake. In plants, carbon dioxide diffuses into leaves through stomata and then through air spaces to reach photosynthesising cells; oxygen produced in photosynthesis diffuses out the same way.
鱼类的鳃采用精巧的逆流系统：血液在鳃片上流动的方向与水流过鳃的方向相反。这能在鳃片全长维持氧气浓度梯度，使摄氧量最大化。在植物中，二氧化碳通过气孔扩散进入叶片，再经过细胞间隙到达进行光合作用的细胞；光合作用产生的氧气则沿相同路径扩散出去。
4. Osmosis: A Special Case of Water Movement | 渗透作用：水分运动的特殊形式
Osmosis is defined as the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. Water potential is a measure of the tendency of water to move; pure water has the highest water potential, and adding solutes lowers it. A dilute solution therefore has a higher water potential than a concentrated solution.
渗透作用的定义是：水分子通过部分通透性膜（partial permeable membrane），从水势较高的区域净移动到水势较低的区域。水势衡量的是水分移动的趋势；纯水的水势最高，加入溶质会降低水势。因此，稀溶液的水势高于浓溶液。
Notice that osmosis is simply the diffusion of water, but with two crucial restrictions. First, only water molecules move through the partially permeable membrane; solute particles are too large to pass through the pores. Second, osmosis always involves a membrane, whereas diffusion does not require one. In animals, cell membranes are partially permeable, allowing water to pass but blocking many dissolved substances.
请注意，渗透作用本质上是水的扩散，但有两个关键限制条件。第一，只有水分子能穿过部分通透性膜；溶质粒子太大，无法通过膜孔。第二，渗透作用一定涉及膜，而扩散则不需要膜。动物细胞膜是部分通透性的，允许水通过，但阻挡许多溶解物质。
5. Water Potential and the Fate of Cells | 水势与细胞的命运
When a plant cell is placed in pure water or a very dilute solution, water enters the cell by osmosis. The cell swells and pushes against its rigid cell wall; it becomes turgid. Turgidity provides support for herbaceous plants. In contrast, if a plant cell is placed in a concentrated solution, water leaves the cell, the cytoplasm shrinks and pulls away from the cell wall; this is called plasmolysis.
当植物细胞置于纯水或极稀溶液中时，水分通过渗透作用进入细胞。细胞膨胀并推挤刚性细胞壁，变得膨胀（turgid）。膨胀为草本植物提供支撑。相反，若将植物细胞放入浓溶液，水分离开细胞，细胞质收缩并与细胞壁分离，这一现象称为质壁分离（plasmolysis）。
Animal cells behave differently because they have no cell wall. An animal cell placed in pure water will swell and eventually burst, a process called lysis in general and haemolysis in red blood cells. An animal cell placed in a concentrated solution will lose water and shrink, becoming crenated. To avoid these fates, animal cells must be bathed in an isotonic solution, where the water potential inside and outside the cell is equal and there is no net movement of water.
动物细胞的行为不同，因为它们没有细胞壁。动物细胞放入纯水中会膨胀并最终破裂，这一过程在红细胞中称为溶血（haemolysis）。动物细胞放入浓溶液会失水收缩，变得皱缩（crenated）。为避免这些结局，动物细胞必须处于等渗溶液中，使细胞内外水势相等，水分没有净移动。
| Solution type | Plant cell | Animal cell |
| Hypotonic (dilute) | Turgid, firm | Swells and bursts (lysis) |
| Isotonic | No net change | No net change |
| Hypertonic (concentrated) | Plasmolysed, flaccid | Shrinks (crenated) |
6. Active Transport: Energy-Driven Movement | 主动运输：耗能驱动的移动
Active transport is the movement of substances from a region of lower concentration to a region of higher concentration, against the concentration gradient, using energy released from respiration. Unlike diffusion and osmosis, active transport requires a cell to spend energy, so it is an active process. This energy is released from glucose during respiration and stored as ATP, which powers carrier proteins in the cell membrane.
主动运输是指物质逆浓度梯度移动，即从浓度低的区域向浓度高的区域移动，并消耗呼吸作用释放的能量。与扩散和渗透不同，主动运输需要细胞消耗能量，因此是一个主动过程。能量来自葡萄糖在呼吸作用中释放并储存于 ATP，为细胞膜上的载体蛋白供能。
Carrier proteins are specific: each one transports only one type of molecule or ion. When the molecule binds to the carrier protein, ATP is used to change the shape of the protein, which then pumps the molecule across the membrane to the other side. A classic Edexcel example is the root hair cell, which absorbs mineral ions such as nitrate and magnesium from the soil, even though the concentration of these ions inside the root is already much higher than in the dilute soil solution.
载体蛋白具有特异性：每种载体只运输一种特定类型的分子或离子。当分子与载体蛋白结合时，ATP 被用于改变蛋白质形状，将其泵送穿过膜到达另一侧。Edexcel 教材中的经典例子是根毛细胞：即使根内离子浓度已远高于稀薄的土壤溶液，根毛细胞仍能吸收硝酸盐和镁离子等矿质离子。
Another key example is the small intestine. Glucose is absorbed into the blood from the lumen of the ileum partly by diffusion, but when the concentration of glucose in the blood rises, diffusion alone cannot move enough glucose. Active transport then pumps glucose out of the epithelial cells into the blood, ensuring efficient absorption. In the kidneys, active transport is used to reabsorb useful substances such as glucose and amino acids back into the blood from the filtrate.
另一个关键例子是小肠。葡萄糖从回肠管腔进入血液部分依靠扩散；但当血液中葡萄糖浓度升高时，仅靠扩散不足以吸收足够多的葡萄糖。此时主动运输将葡萄糖从上皮细胞泵入血液，确保高效吸收。在肾脏中，主动运输用于将葡萄糖和氨基酸等有用物质从滤液中重吸收回血液。
7. Side-by-Side Comparison Table | 三种运输方式对比表
The following table summarises the key differences between diffusion, osmosis and active transport. You should memorise this table because comparison questions appear in almost every exam series.
下表总结了扩散、渗透和主动运输之间的关键区别。你应该牢记此表，因为比较类题目几乎在每次考试中都会出现。
| Feature | Diffusion | Osmosis | Active transport |
| Substance moved | Any particles (gases, dissolved molecules) | Water only | Ions and molecules |
| Direction of movement | High to low concentration | High to low water potential | Low to high concentration |
| Energy required | No | No | Yes (ATP from respiration) |
| Membrane required | No | Yes, partially permeable | Yes, with carrier proteins |
| Example | O₂ into blood from alveoli | Water into root hairs | Nitrate ions into root hair cells |
8. Core Practical: Investigating Osmosis with Potato Tissue | 核心实验：用马铃薯组织探究渗透作用
The Edexcel core practical for osmosis uses potato cylinders to measure how the concentration of the surrounding solution affects water movement. The method is straightforward but must be described with precision in the exam. First, use a cork borer to cut several identical cylinders of potato, then blot them dry with paper towel and measure their initial mass using a balance.
Edexcel 核心实验利用马铃薯条来测量周围溶液浓度如何影响水分移动。方法简单，但考试中描述时必须精确。首先，用打孔器切出若干根相同的马铃薯条，用吸水纸吸干表面水分，并用天平测量初始质量。
Next, place one potato cylinder into each of a set of sucrose solutions with known concentrations, for example 0.0, 0.2, 0.4, 0.6 and 0.8 mol/dm³. Leave them for about 30 to 40 minutes so that osmosis can occur. Remove each cylinder, blot it dry and measure its final mass. Calculate the percentage change in mass for each solution.
接下来，将每根马铃薯条分别放入一组已知浓度的蔗糖溶液中，例如 0.0、0.2、0.4、0.6 和 0.8 mol/dm³。放置约 30 至 40 分钟，让渗透作用充分发生。取出每条马铃薯，吸干水分并测量最终质量，然后计算每个溶液中的质量变化百分比。
Percentage change in mass = (Final mass − Initial mass) ÷ Initial mass × 100%
In dilute solutions, the potato gains mass because water enters by osmosis; in concentrated solutions, it loses mass because water leaves. Plotting percentage change against concentration produces a graph with a straight-line region; the point where the line crosses the x-axis indicates the concentration that is isotonic to the potato cells, where no net water movement occurs.
在稀溶液中，马铃薯质量增加，因为水分通过渗透进入；在浓溶液中，质量减少，因为水分离开。将质量变化百分比对浓度作图，可得到具有直线段的曲线；直线与 x 轴的交点即为与马铃薯细胞等渗的浓度，在该浓度下没有净水分移动。
Control variables in this experiment include temperature, the time the cylinders are left in the solution, the size and variety of the potato cylinders, and the volume of solution used. By keeping these constant, you ensure that any change in mass is due only to the concentration of the sucrose solution.
本实验的对照变量包括温度、马铃薯条在溶液中放置的时间、马铃薯条的大小和品种，以及所用溶液的体积。通过保持这些变量不变，可以确保质量变化完全由蔗糖溶液浓度引起。
9. Real-World Applications in Medicine and Agriculture | 在医学和农业中的实际应用
Osmosis and diffusion have enormous practical importance. In medicine, kidney dialysis uses diffusion across a partially permeable membrane to remove waste substances such as urea from the blood of patients with kidney failure. The dialysis fluid has the same concentration of useful solutes as normal blood, so only
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