📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
Cells are constantly exchanging substances with their surroundings. Three key processes make this possible: diffusion, osmosis and active transport. Understanding their differences is essential for IGCSE Science.
细胞时刻都在与周围环境进行物质交换。扩散、渗透和主动运输是实现这种交换的三种关键方式。理解它们之间的区别是IGCSE科学学习的重要内容。
1. Diffusion | 扩散
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until equilibrium is reached. This is a passive process, so it does not require energy.
扩散是指粒子从浓度较高的区域向浓度较低的区域净移动的过程，即沿浓度梯度方向进行，直到达到平衡。这是一个被动过程，因此不需要能量。
- Small molecules such as oxygen and carbon dioxide diffuse easily through cell membranes.
- 溶解氧和二氧化碳等小分子能容易地穿过细胞膜进行扩散。
- In the lungs, oxygen diffuses from the alveoli into the blood because oxygen concentration is higher in the alveoli.
- 在肺部，氧气从肺泡扩散进入血液，因为肺泡内氧气浓度更高。
- Carbon dioxide diffuses in the opposite direction to be breathed out.
- 二氧化碳则沿相反方向扩散，最终被呼出体外。
2. Factors Affecting Diffusion | 影响扩散的因素
Several factors affect the rate of diffusion: concentration gradient, temperature, surface area and diffusion distance. A steeper concentration gradient, higher temperature, larger surface area and shorter distance all increase the rate of diffusion.
影响扩散速率的因素包括：浓度梯度、温度、表面积和扩散距离。浓度梯度越大、温度越高、表面积越大、距离越短，扩散速率就越快。
Rate of diffusion ∝ (concentration gradient × surface area × temperature) / distance
扩散速率 ∝（浓度梯度 × 表面积 × 温度）÷ 扩散距离
- Higher temperature gives particles more kinetic energy, so they move faster and collide more often.
- 温度升高使粒子获得更多动能，运动更快，碰撞更频繁。
- Smaller organisms have a large surface area to volume ratio, which makes diffusion more efficient.
- 体型较小的生物具有较大的相对表面积，因此扩散效率更高。
3. Osmosis | 渗透作用
Osmosis is a special type of diffusion involving water molecules. It is the net movement of water from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
渗透是一种涉及水分子运动的特殊扩散。它是指水分子通过半透膜，从水势较高（即溶液较稀）的区域向水势较低（即溶液较浓）的区域净移动的过程。
- The partially permeable membrane allows water molecules to pass through but not larger solute molecules.
- 半透膜允许水分子通过，但较大的溶质分子不能通过。
- In animal cells, if placed in pure water, water enters the cell and the cell may burst because it has no cell wall.
- 动物细胞若放入纯水中，水会进入细胞，最终可能导致细胞破裂，因为动物细胞没有细胞壁。
- In plant cells, the cell wall prevents bursting; instead the cell becomes turgid.
- 植物细胞因有细胞壁而不会破裂，而是会变得充盈（膨胀）。
4. Water Potential | 水势
Water potential is a measure of the tendency of water to move from one place to another. Pure water has the highest water potential, usually taken as zero. Adding solutes lowers water potential, making it more negative.
水势是衡量水从一处向另一处移动趋势的指标。纯水的水势最高，通常规定为零。加入溶质会降低水势，使其变得更负。
- A dilute solution has a higher (less negative) water potential than a concentrated solution.
- 稀溶液的水势高于（即负值更小）浓溶液的水势。
- Water moves by osmosis from higher water potential to lower water potential.
- 水通过渗透作用从水势高处向水势低处移动。
Pure water = 0 kPa; adding solute → negative values
纯水水势 = 0 kPa；加入溶质 → 负值
5. Active Transport | 主动运输
Active transport is the movement of particles from a region of lower concentration to a region of higher concentration, against the concentration gradient. This process requires energy carried by ATP molecules and uses carrier proteins in the cell membrane.
主动运输是指粒子从浓度较低的区域向浓度较高的区域移动，即逆浓度梯度运输的过程。这一过程需要ATP分子提供能量，并依赖细胞膜上的载体蛋白。
- Energy is released by respiration and used to change the shape of carrier proteins.
- 呼吸作用释放能量，用于改变载体蛋白的形状。
- In plant roots, mineral ions are absorbed from the soil even when soil concentration is lower than inside the root.
- 植物根细胞能吸收土壤中的矿质离子，即使土壤中离子浓度低于根细胞内部。
- In the small intestine, glucose is absorbed into the blood against its concentration gradient after digestion.
- 在小肠中，消化后的葡萄糖可以逆浓度梯度被吸收进入血液。
6. Comparing the Three Processes | 三种过程的比较
Diffusion and osmosis are passive processes, while active transport requires energy. Diffusion moves any small particles, osmosis moves only water, and active transport moves substances against the gradient.
扩散和渗透是被动过程，而主动运输需要消耗能量。扩散可移动任何小粒子，渗透只涉及水分子的移动，主动运输则可以逆浓度梯度运输物质。
| Process | Energy | Direction | Membrane needed? |
| Diffusion | None | High → low | No |
| Osmosis | None | High water potential → low water potential | Yes (partially permeable) |
| Active transport | ATP | Low → high | Yes (carrier proteins) |
7. Practical Applications | 实际应用
Osmosis is used to preserve food. High concentrations of sugar or salt draw water out of microorganisms, preventing them from growing. Active transport is used by kidney cells to reabsorb glucose from the filtrate.
渗透作用可用于食品保存。高浓度的糖或盐会使微生物细胞失水，从而抑制其生长。肾脏细胞则利用主动运输从原尿中重吸收葡萄糖。
- Salting fish and making jams rely on osmosis to remove water.
- 腌鱼和制作果酱就是利用渗透作用来脱水。
- Plants use active transport to take up nitrate ions from the soil.
- 植物通过主动运输从土壤中吸收硝酸根离子。
8. Experimental Design | 实验设计
To investigate osmosis, you can place potato cylinders in different concentrations of sugar or salt solution and measure their change in mass. If the solution is more concentrated, water leaves the potato and mass decreases.
研究渗透作用时，可以将马铃薯条分别放入不同浓度的糖水或盐水中，测量其质量变化。若外界溶液浓度更高，马铃薯条会失水，质量下降。
- Equal-sized potato cylinders are used to keep surface area and volume the same.
- 使用等大的马铃薯条可保证表面积和体积相同。
- Pat dry the potato before weighing to remove surface water.
- 称重前用纸巾吸干表面水分，避免误差。
- Repeat the experiment several times and calculate a mean.
- 重复实验多次并计算平均值，提高结果可靠性。
9. Common Exam Pitfalls | 常见考试易错点
Many students confuse 'high concentration of water' with 'high water concentration'. In osmosis, always refer to water potential, not solute concentration, to avoid mistakes.
许多同学容易混淆“水的浓度高”和“水势高”。在渗透作用中，应使用“水势”来描述，而不是直接说“溶质浓度”，这样可以避免出错。
- Do not say 'water diffuses' — say 'water molecules move by osmosis'.
- 不要说“水扩散”，应该说“水分子通过渗透作用移动”。
- Active transport always requires energy, so if the question mentions respiration, link the two.
- 主动运输一定需要能量，若题目提到呼吸作用，应将其与能量供应联系起来。
- In animals, osmosis can cause cells to shrink (crenation) or swell and burst (lysis).
- 在动物细胞中，渗透可导致细胞皱缩（质壁分离）或膨胀破裂（溶血）。
10. Summary | 总结
Diffusion and osmosis are passive; active transport requires energy. Diffusion moves any small particles down a concentration gradient; osmosis moves water through a partially permeable membrane; active transport moves substances against the gradient using protein carriers.
扩散和渗透是被动过程；主动运输需要能量。扩散使小粒子顺浓度梯度移动；渗透使水分子通过半透膜移动；主动运输则借助载体蛋白逆浓度梯度运输物质。
Make sure you can define each process precisely, give biological examples, and interpret experimental data involving mass changes or concentration differences. These skills are frequently tested in the Edexcel IGCSE Science examination.
请务必准确掌握每个过程的定义，能够列举生物实例，并会分析涉及质量变化或浓度差异的实验数据。这些能力在爱德思IGCSE科学考试中经常被考查。
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