📚 Cell Transport: Diffusion, Osmosis and Active Transport | 细胞运输：扩散、渗透与主动运输
In IGCSE Edexcel Science, understanding how substances move into and out of cells is one of the most frequently assessed topics. Three processes — diffusion, osmosis and active transport — govern the movement of molecules across cell membranes. Mastering their definitions, key factors and real-world examples will help you to answer both multiple-choice questions and extended-response questions with confidence.
在 IGCSE Edexcel 科学课程中，理解物质如何进出细胞是最常考的考点之一。扩散、渗透与主动运输这三个过程控制着分子穿越细胞膜的运动。掌握它们的定义、关键影响因素和实际应用，将帮助你有信心地应对选择题和拓展回答题。
1. The Cell Membrane | 细胞膜：选择性透过屏障
Every substance entering or leaving a cell must cross the cell membrane. This membrane is described as selectively permeable: small molecules such as water, oxygen and carbon dioxide pass through freely, while larger molecules such as proteins and starch cannot cross without assistance. This selective property is the foundation of all three transport processes you need to know for the exam.
所有进入或离开细胞的物质都必须穿过细胞膜。细胞膜被描述为选择性透过膜：水、氧气、二氧化碳等小分子可自由通过，而蛋白质、淀粉等较大分子则无法轻易穿过。这种选择性是三大运输过程的基础。
- Phospholipid bilayer — the basic framework of the membrane. 磷脂双分子层——膜的基本结构框架。
- Carrier and channel proteins — embedded in the membrane and involved in transport. 载体蛋白与通道蛋白——镶嵌在膜中，参与物质运输。
- Selective permeability — the membrane allows some molecules through but not others. 选择性透过——膜允许某些分子通过，但阻止另一些分子通过。
The exact behaviour of these membrane components determines whether a molecule moves by diffusion, osmosis or active transport. Exam questions often describe a membrane experiment and ask you to explain the results using these ideas.
膜组分的具体行为决定了分子是以扩散、渗透还是主动运输的方式运动。试题常描述一个膜实验，并要求你运用这些概念解释实验结果。
2. Diffusion: Definition and Key Factors | 扩散：定义与关键因素
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient. It is a passive process, so it requires no energy from the cell. Diffusion occurs in liquids and gases; particles move randomly, but the net direction is always from high to low concentration until equilibrium is reached.
扩散是颗粒从高浓度区域沿浓度梯度向低浓度区域发生的净移动。这是一个被动过程，不需要细胞提供能量。扩散发生在液体和气体中；颗粒随机运动，但净方向总是从高浓度到低浓度，直到达到平衡。
Net movement: high concentration → low concentration (down the gradient)
净移动：高浓度 → 低浓度（沿梯度向下）
Several factors affect the rate of diffusion. A steeper concentration gradient, a higher temperature, a larger surface area and a shorter diffusion distance all increase the rate. You should be able to explain each factor in terms of particle movement and collisions.
多个因素影响扩散速率。浓度梯度越陡、温度越高、表面积越大、扩散距离越短，都会加快扩散速率。你需要能够从颗粒运动和碰撞的角度解释每个因素。
| Factor 因素 | Effect on rate of diffusion 对扩散速率的影响 |
| Concentration gradient 浓度梯度 | Steeper gradient → faster diffusion 梯度越陡，扩散越快 |
| Temperature 温度 | Higher temperature → more kinetic energy → faster 温度越高，动能越大，扩散越快 |
| Surface area 表面积 | Larger surface area → more particles crossing per second 表面积越大，每秒穿过的颗粒越多 |
| Distance 距离 | Shorter distance → faster diffusion 距离越短，扩散越快 |
3. Diffusion in Living Organisms | 生物体中的扩散实例
In the lungs, oxygen diffuses from the alveoli, where its concentration is high, into the blood, where its concentration is lower. Carbon dioxide diffuses in the opposite direction, from the blood into the alveoli. In fish gills and plant leaves, gas exchange also depends entirely on diffusion along concentration gradients. These examples are frequently used in exam questions about adaptation.
在肺中，氧气从浓度高的肺泡扩散进入浓度较低的血液；二氧化碳则沿相反方向从血液扩散进入肺泡。在鱼鳃和植物叶片中，气体交换也完全依赖沿浓度梯度的扩散。这些例子常出现在考查“适应”的试题中。
- Alveoli 肺泡：large surface area, thin walls and moist surfaces make gas exchange efficient. 表面积大、壁薄、表面湿润，使气体交换高效进行。
- Stomata 气孔：allow carbon dioxide to diffuse into leaves during photosynthesis and oxygen to diffuse out. 让二氧化碳在
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