📚 Movement of Substances: Diffusion, Osmosis and Active Transport | 物质运输：扩散、渗透与主动运输
Cells must exchange substances with their surroundings to stay alive. In the Edexcel IGCSE Science syllabus, movement of substances into and out of cells is a core biology idea that links to nutrition, respiration, tissues, organs and homeostasis. This article explains diffusion, osmosis and active transport, compares them clearly, and gives exam style tips so you can use the correct scientific language.
细胞必须与周围环境进行物质交换才能维持生命。在 Edexcel IGCSE 科学课程中，物质进出细胞是生物学的核心概念，它与营养、呼吸、组织、器官和稳态都有密切联系。本文将讲解扩散、渗透和主动运输，并对三者进行比较，同时提供考试答题技巧，帮助你使用准确的科学语言。
1. The Particle Model and Concentration Gradient | 粒子模型与浓度梯度
All matter is made of many tiny particles. In gases and liquids, particles move around randomly and constantly. This random movement allows substances to spread out from a region where they are more concentrated to a region where they are less concentrated.
所有物质都由大量微小粒子构成。在气体和液体中，粒子不停地做无规则运动。这种随机运动使物质能够从浓度较高的区域向浓度较低的区域扩散。
The difference in concentration between two areas is called the concentration gradient. The steeper the concentration gradient, the faster the net movement of particles. In an exam, always use the term "concentration gradient" when explaining the direction of diffusion or osmosis.
两个区域之间的浓度差别称为浓度梯度。浓度梯度越陡，粒子的净移动速度就越快。在考试中，解释扩散或渗透方向时一定要使用"浓度梯度"这一术语。
2. Diffusion | 扩散
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Because particles move down a concentration gradient, diffusion is a passive process and does not need energy from respiration.
扩散是粒子从高浓度区域向低浓度区域的净移动。由于粒子沿浓度梯度向下移动，扩散是一个被动过程，不需要呼吸作用提供能量。
Examples of diffusion in living organisms include:
Oxygen diffuses from the alveoli in the lungs into the blood, because oxygen concentration is higher in the alveoli than in the blood.
氧气从肺部肺泡扩散进入血液，因为肺泡中氧气浓度比血液中的高。
Carbon dioxide diffuses from the blood into the alveoli so that it can be exhaled.
二氧化碳从血液中扩散进入肺泡，然后通过呼气排出体外。
Oxygen diffuses from respiring cells into mitochondria where it is used in aerobic respiration.
氧气从正在呼吸的细胞中扩散进入线粒体，用于有氧呼吸。
3. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
You need to be able to explain how different factors increase or decrease the rate of diffusion.
你需要能够解释不同因素如何加快或减慢扩散速率。
Surface area: a larger surface area allows more particles to cross the membrane at 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膜。
Concentration gradient: a steeper concentration gradient increases the rate of diffusion.
浓度梯度：浓度梯度越陡，扩散速率越快。
Thickness of membrane: a thinner membrane means particles have a shorter distance to travel, so diffusion is faster.
膜厚度：膜越薄，粒子需要穿过的距离越短，扩散越快。
Temperature: higher temperature gives particles more kinetic energy, so they move faster.
温度：温度越高，粒子动能越大，运动越快。
4. Osmosis | 渗透
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.
渗透是指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的过程。
A partially permeable membrane allows small molecules, such as water, to pass through, but it prevents larger solute molecules, such as sugar, from passing through. In biology, this membrane is usually a cell membrane.
部分通透性膜允许小分子如水通过，但阻止较大的溶质分子如糖通过。在生物中，这个膜通常指细胞膜。
If a Visking tubing bag containing sugar solution is placed in pure water, water molecules move into the bag because the water potential inside the bag is lower than the water potential outside. The bag becomes larger and heavier.
如果将装有糖溶液的玻璃纸袋放入纯水中，由于袋内水势低于袋外水势，水分子会进入袋内，袋子会变大变重。
5. Water Potential and Solution Concentration | 水势与溶液浓度
Water potential is a measure of the tendency of water molecules to move from one place to another. Pure water has the highest water potential. When solutes are dissolved in water, the water potential is lowered.
水势衡量的是水分子从一个地方移动到另一个地方的倾向。纯水具有最高的水势。当溶质溶解在水中时，水势会降低。
The more concentrated a solution is, the lower its water potential. Therefore a concentrated sugar solution has a lower water potential than a dilute sugar solution. Water will move towards the more concentrated solution if a partially permeable membrane separates them.
溶液浓度越高，水势越低。因此浓糖溶液的水势低于稀糖溶液。如果两种溶液被部分通透性膜隔开，水会向更浓的溶液方向移动。
6. 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 from respiration and uses carrier proteins in the cell 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移动的过程。这个过程需要呼吸作用提供能量，并需要细胞膜上的载体蛋白参与。
Examples of active transport include:
Root hair cells absorbing mineral ions from the very dilute solution in the soil.
根毛细胞从土壤中浓度很低的溶液中吸收矿物质离子。
Cells lining the small intestine absorbing glucose from the lumen when glucose concentration in the blood is higher.
小肠内壁细胞在血液中葡萄糖浓度较高时，仍能从肠腔中吸收葡萄糖。
Kidney tubule cells reabsorbing useful glucose back into the blood after filtration.
肾小管细胞在过滤后重新吸收有用的葡萄糖回血液中。
7. Comparing Diffusion, Osmosis and Active Transport | 比较扩散、渗透和主动运输
The table below summarises the key differences. You should be able to copy this table from memory.
下表总结了三个过程的主要区别。你应该能够凭记忆写出这个表格。
| Feature | Diffusion | Osmosis | Active transport |
| Direction | High to low concentration | High to low water potential | Low to high concentration |
| Energy needed | No | No | Yes, from respiration |
| Membrane needed | Not always | Partially permeable membrane | Cell membrane with carrier proteins |
| Example | Oxygen into cells | Water into root hair cells | Mineral ions into root hair cells |
8. Osmosis in Plant Cells: Turgor and Plasmolysis | 植物细胞中的渗透：膨压与质壁分离
When a plant cell is placed in a solution with a higher water potential than the cell sap, water enters by osmosis. The vacuole expands and the cytoplasm pushes against the cell wall. The cell becomes turgid. Turgid cells keep stems and leaves firm.
当植物细胞放在水势高于细胞液的溶液中时，水会通过渗透进入细胞。液泡膨胀，细胞质压向细胞壁，细胞变得紧张膨大，这就是膨压。膨压能让茎和叶保持挺立。
If a plant cell is placed in a concentrated solution with a lower water potential, water leaves the cell by osmosis. The vacuole shrinks and the cytoplasm pulls away from the cell wall. This is called plasmolysis.
如果植物细胞放在水势较低的浓溶液中，水会通过渗透离开细胞。液泡缩小，细胞质从细胞壁上脱离，这个过程称为质壁分离。
The cell wall is strong and does not burst easily, unlike animal cells, because it provides support and prevents excessive expansion.
植物细胞壁强韧，不像动物细胞那样容易胀破，因为它能提供支持并防止细胞过度膨胀。
9. Osmosis in Animal Cells | 动物细胞中的渗透
Animal cells do not have a cell wall. If an animal cell is placed in pure water, water enters rapidly by osmosis, the cell membrane stretches, and the cell may burst.
动物细胞没有细胞壁。如果将动物细胞放入纯水中，水会迅速通过渗透进入细胞，细胞膜被拉伸，最终细胞可能破裂。
For example, red blood cells placed in pure water swell and burst. This is called haemolysis.
例如，红血细胞放入纯水中会膨胀并破裂，这种现象称为溶血。
If animal cells are placed in a concentrated solution, they lose water and shrink. In red blood cells, this shrinking is called crenation.
如果动物细胞放入浓溶液中，细胞会失水并缩小。在红血细胞中，这种皱缩现象称为皱缩。
10. Adaptations of Exchange Surfaces | 交换表面的适应特征
Diffusion and active transport are both important in exchange surfaces. Exchange surfaces are adapted to move substances quickly.
扩散和主动运输在物质交换表面都很重要。交换表面具有加快物质移动的适应特征。
Alveoli have a large surface area, thin walls, and a rich blood supply. Breathing and blood flow maintain a steep concentration gradient for oxygen and carbon dioxide.
肺泡表面积大，壁薄，血液供应丰富。呼吸和血流使氧气和二氧化碳保持陡峭的浓度梯度。
Root hair cells have a large surface area and contain many mitochondria. Mitochondria provide energy for active transport of mineral ions.
根毛细胞具有巨大的表面积，且含有大量线粒体。线粒体为矿物质的主动运输提供能量。
Villi in the small intestine increase surface area for absorbing digested food. They have thin walls and dense capillary networks to quickly remove absorbed glucose and amino acids.
小肠绒毛增加吸收营养物质的表面积。绒毛壁薄，毛细血管密集，能迅速运走吸收的葡萄糖和氨基酸。
Fish gills contain many thin filaments with a large surface area. Water flows over them in one direction, while blood flows in the opposite direction, maintaining a steep concentration gradient.
鱼鳃有许多细丝，表面积大。水流和血流方向相反，形成逆流交换系统，能维持陡峭的浓度梯度。
11. Core Practical: Investigating Osmosis | 核心实验：探究渗透
A common Edexcel IGCSE practical involves using Visking tubing to investigate osmosis.
Edexcel IGCSE 常见实验中，会使用玻璃纸袋来探究渗透作用。
Procedure: tie one end of a Visking tubing bag, add a sugar solution, tie the other end, weigh the bag, then place it in pure water. After a set time, remove it, dry the outside, and weigh it again.
步骤：将玻璃纸袋一端扎紧，加入糖溶液，扎紧另一端，称量袋子质量，然后放入纯水中。一段时间后取出，擦干
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