📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
In IGCSE Edexcel Science (Double Award), understanding how substances move into and out of cells is essential. This article explains the three main transport processes — diffusion, osmosis and active transport — with clear definitions, examples and comparisons for exam success.
在 Edexcel IGCSE 科学（双奖）中，理解物质如何进出细胞至关重要。本文将解释三种主要的运输过程——扩散、渗透和主动运输，并提供清晰的定义、示例和对比，帮助你考试成功。
1. Why Cells Need Transport | 为什么细胞需要运输
Cells need to take in oxygen, glucose and other nutrients while removing waste products like carbon dioxide. The cell membrane controls what enters and leaves, using passive or active processes depending on the concentration gradient and energy availability.
细胞需要摄入氧气、葡萄糖和其他营养物质，同时排出二氧化碳等废物。细胞膜控制物质的进出，根据浓度梯度和能量可用性，采用被动或主动的运输过程。
- Passive transport: no energy required, includes diffusion and osmosis.
- 主动运输：需要能量（ATP），用于逆浓度梯度运输。
2. Diffusion: Definition and Key Ideas | 扩散：定义与核心概念
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient, until equilibrium is reached. It is a passive process that does not require energy.
扩散是粒子从高浓度区域向低浓度区域（沿浓度梯度方向）进行净移动，直到达到平衡的过程。这是一个不需要能量的被动过程。
The key word is ‘net’ movement. Even at equilibrium, particles continue to move, but there is no net movement in any direction.
关键词是“净”移动。即使在平衡时，粒子仍在运动，但没有任意方向的净移动。
3. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
The rate of diffusion is influenced by several factors. In IGCSE investigations, you should be able to explain each one.
扩散速率受多种因素影响。在 IGCSE 实验中，你需要能够解释每一种因素。
| Factor | Effect |
| Concentration gradient | Steeper gradient → faster diffusion |
| Temperature | Higher temperature → more kinetic energy → faster diffusion |
| Surface area | Larger surface area → faster diffusion |
| Thickness of membrane | Thinner membrane → faster diffusion |
For small unicellular organisms, diffusion alone is enough to supply oxygen and remove waste because the distances are short. Larger organisms need specialised exchange surfaces and transport systems.
对于单细胞生物，仅靠扩散就足以供应氧气和排出废物，因为距离很短。大型生物需要专门的交换表面和运输系统。
4. Examples of Diffusion in Biological Systems | 生物系统中的扩散实例
Oxygen diffuses from the alveoli in the lungs into the blood because the oxygen concentration is higher in the alveoli and lower in the blood. Carbon dioxide diffuses in the opposite direction.
氧气从肺部的肺泡扩散进入血液，因为肺泡中氧气浓度较高，血液中较低。二氧化碳则向相反方向扩散。
In plants, carbon dioxide diffuses into leaves through stomata for photosynthesis, and oxygen diffuses out as a waste product. In the small intestine, digested food molecules diffuse into the blood.
在植物中，二氧化碳通过气孔扩散进入叶片用于光合作用，氧气作为废物扩散出去。在小肠中，消化后的食物分子扩散进入血液。
5. Osmosis: A Special Case of Diffusion | 渗透：扩散的特例
Osmosis is 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.
渗透是水分子通过部分通透性膜，从水势较高的区域向水势较低的区域进行净移动。水势是衡量水移动趋势的指标。
It is important to describe osmosis in terms of ‘water potential’ rather than ‘concentration’ to gain full marks in exams.
在考试中，用“水势”而不是“浓度”来描述渗透非常重要，这样才能得满分。
6. Water Potential and Concentration Gradients | 水势与浓度梯度
Pure water has the highest water potential (zero). Adding solutes lowers water potential, making it more negative. Therefore, a dilute solution has a higher water potential than a concentrated solution.
纯水具有最高的水势（为零）。加入溶质会降低水势，使其变为负值。因此，稀溶液的水势高于浓溶液。
Water moves from high water potential (-2) → low water potential (-7)
This gradient determines the direction of osmosis. Water always moves towards the region with lower water potential (more negative).
这个梯度决定了渗透的方向。水总是向水势较低（更负）的区域移动。
7. Osmosis in Animal Cells | 渗透在动物细胞中的作用
Animal cells have no cell wall. If placed in pure water (high water potential), water enters the cell by osmosis, causing it to swell and potentially burst. This is called lysis.
动物细胞没有细胞壁。如果放在纯水中（高水势），水会通过渗透进入细胞，导致细胞膨胀甚至破裂。这称为溶胞。
If placed in a concentrated solution (low water potential), water leaves the cell, causing it to shrivel. This is called crenation. To prevent these problems, animals control solute concentration in their body fluids.
如果放在浓溶液中（低水势），水会离开细胞，导致细胞皱缩。这称为皱缩。为了防止这些问题，动物会控制体液中的溶质浓度。
8. Osmosis in Plant Cells | 渗透在植物细胞中的作用
Plant cells have a cell wall that resists expansion. When water enters a plant cell by osmosis, the vacuole swells and pushes the cytoplasm against the cell wall. The cell becomes turgid, providing support.
植物细胞有抵抗膨胀的细胞壁。当水通过渗透进入植物细胞时，液泡膨胀并将细胞质推向细胞壁。细胞变得紧胀（turgid），提供支撑。
When water leaves, the cell becomes flaccid. If too much water is lost, the cell membrane pulls away from the cell wall, a condition called plasmolysis. This happens when plants are watered with concentrated salt water.
当水分流失时，细胞变得松弛（flaccid）。如果失水过多，细胞膜会与细胞壁分离，这种状态称为质壁分离。当植物用浓盐水浇灌时会发生这种情况。
9. Active Transport: Moving Against the Gradient | 主动运输：逆梯度移动
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 respiration, in the form of ATP, and involves carrier proteins in the membrane.
主动运输是粒子从低浓度区域向高浓度区域（逆浓度梯度）的移动。这需要呼吸作用产生的能量（ATP），并且涉及膜上的载体蛋白。
In the small intestine, glucose can be absorbed into the blood even when blood glucose is higher than in the gut. This is active transport. In plants, root hair cells use active transport to absorb mineral ions from the dilute soil solution.
在小肠中，即使血糖浓度高于肠道中，葡萄糖仍能被吸收进入血液。这就是主动运输。在植物中，根毛细胞利用主动运输从稀薄的土壤溶液中吸收矿物质离子。
10. Comparison and Exam Focus | 对比与考试重点
In the exam, you may be asked to compare diffusion, osmosis and active transport. The following summary is a reliable revision tool.
考试中，你可能会被要求对比扩散、渗透和主动运输。下面的总结是可靠的复习工具。
| Process | Energy required? | Direction relative to gradient | Membrane involvement |
| Diffusion | No | Down gradient | Not required (simple diffusion) |
| Osmosis | No | Down water potential | Required (partially permeable) |
| Active transport | Yes (ATP) | Against gradient | Required (carrier proteins) |
When answering exam questions, always use the correct scientific terms: ‘net movement’, ‘partially permeable membrane’, ‘water potential’, ‘ATP’, ‘carrier protein’. These key phrases help you earn marks.
回答考试问题时，始终使用正确的科学术语：“净移动”、“部分通透性膜”、“水势”、“ATP”、“载体蛋白”。这些关键词能帮助你得分。
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