📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
Every living cell must exchange materials with its environment. Oxygen, carbon dioxide, water, glucose and mineral ions all need to pass through the cell membrane. Understanding the three key transport processes – diffusion, osmosis and active transport – is essential for IGCSE Biology.
每一个活细胞都必须与其环境交换物质。氧气、二氧化碳、水、葡萄糖和矿物质离子都需要穿过细胞膜。理解扩散、渗透和主动运输这三种关键运输过程，是IGCSE生物学的核心要求。
1. Cell Membranes and Transport | 细胞膜与物质运输
The cell membrane is partially permeable. It allows small molecules such as water, oxygen and carbon dioxide to pass through easily, but it restricts the movement of larger molecules and charged particles such as ions and proteins.
细胞膜是部分渗透性的。它允许水、氧气和二氧化碳等小分子自由通过，但会限制较大的分子以及离子和蛋白质等带电粒子的移动。
Molecules are always moving. In liquids and gases, particles move randomly and constantly. This movement provides the physical basis for diffusion and osmosis.
分子总在不停运动。在液体和气体中，粒子随机且持续地运动。这种运动为扩散和渗透提供了物理基础。
2. Diffusion: Definition and Process | 扩散：定义与过程
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until the particles are evenly spread.
扩散是粒子从浓度较高的区域净移动到浓度较低的区域，即沿浓度梯度方向进行，直到粒子均匀分布为止。
Diffusion is a passive process. It does not require energy from respiration. The particles move because of their own kinetic energy.
扩散是一个被动过程。它不需要呼吸作用提供能量。粒子依靠自身的动能移动。
For example, when a drop of ink is placed in water, the ink particles spread out through the water until the water becomes uniformly coloured.
例如，将一滴墨水滴入水中，墨水粒子会扩散到整杯水中，直到水均匀变色。
3. Examples of Diffusion in Living Organisms | 生物体内扩散的实例
In the lungs, oxygen diffuses from the air sacs (alveoli) into the blood because the oxygen concentration is higher in the alveoli than in the deoxygenated blood. At the same time, carbon dioxide diffuses from the blood into the alveoli.
在肺部，氧气从肺泡扩散进入血液，因为肺泡中的氧气浓度高于缺氧血液中的氧气浓度。同时，二氧化碳从血液扩散进入肺泡。
In plants, carbon dioxide diffuses into leaves through stomata for photosynthesis. Oxygen produced during photosynthesis diffuses out of the leaf into the atmosphere.
在植物中，二氧化碳通过气孔扩散进入叶片，用于光合作用。光合作用产生的氧气则从叶片扩散到大气中。
In the small intestine, digested food molecules such as glucose and amino acids diffuse across the intestinal wall into the blood capillaries.
在小肠中，消化后的食物分子如葡萄糖和氨基酸，扩散穿过肠壁进入毛细血管。
4. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
The rate of diffusion is affected by four main factors:
扩散速率主要受四个因素影响：
- Concentration gradient: The steeper the gradient, the faster the rate of diffusion.
- Temperature: Higher temperature increases particle kinetic energy, so particles move faster and diffusion is quicker.
- Surface area: A larger surface area allows more particles to cross at the same time.
- Distance: A shorter diffusion distance means particles reach the lower concentration region more quickly.
- 浓度梯度：梯度越陡，扩散速率越快。
- 温度：温度升高，粒子动能增加，运动加快，扩散更快。
- 表面积：表面积越大，同一时间内能穿过膜的粒子越多。
- 距离：扩散距离越短，粒子到达浓度较低区域越快。
These factors explain why alveoli are thin-walled and highly folded, and why active muscles require a rapid blood supply.
这些因素解释了为什么肺泡壁薄且高度折叠，以及为什么剧烈运动的肌肉需要快速的血液供应。
5. Osmosis: A Special Case of Diffusion | 渗透：扩散的特殊情况
Osmosis is the net movement of water molecules from a region of higher water concentration to a region of lower water concentration, across a partially permeable membrane.
渗透是水分子通过部分渗透性膜，从水浓度较高的区域净移动到水浓度较低的区域。
Osmosis is a passive process. It does not require metabolic energy. Water moves through tiny pores or phospholipid bilayer of the membrane.
渗透是一个被动过程。它不需要代谢能量。水通过细胞膜上的微小孔道或磷脂双分子层移动。
It is important to describe water movement in terms of 'water concentration', not just 'solute concentration'. A solution with more solute has a lower water concentration.
描述水分运动时必须使用“水浓度”这一概念，而不只是“溶质浓度”。溶质较多的溶液，其水浓度较低。
6. Water Potential and Osmotic Terms | 水势与渗透术语
Scientists use the term 'water potential' to describe the tendency of water to move. Pure water has the highest water potential. Adding solutes lowers water potential.
科学家使用“水势”一词来描述水运动的趋势。纯水具有最高的水势。加入溶质会降低水势。
Water always moves from a region of higher water potential to a region of lower water potential. This movement is called osmosis when it occurs across a partially permeable membrane.
水总是从水势较高的区域流向水势较低的区域。当当这种移动发生时通过部分渗透性膜就称为渗透。
Key terms to remember:
需要记住的关键术语：
- Isotonic: Two solutions have the same water potential.
- Hypotonic: A solution has a higher water potential (more dilute) than another.
- Hypertonic: A solution has a lower water potential (more concentrated) than another.
- 等渗：两种溶液具有相同的水势。
- 低渗：一种溶液比另一种溶液具有更高的水势（更稀）。
- 高渗：一种溶液比另一种溶液具有更低的水势（更浓）。
7. Animal Cells in Different Solutions | 动物细胞在不同溶液中的变化
Animal cells do not have a cell wall. They are therefore strongly affected by changes in the water concentration of their surroundings.
动物细胞没有细胞壁，因此受周围水浓度变化的影响很大。
In a solution with a higher water potential than the cell (hypotonic), water enters the cell by osmosis. The cell swells and may burst. This is called lysis.
在比细胞水势更高的溶液（低渗溶液）中，水通过渗透进入细胞。细胞膨胀，可能破裂。这称为溶胞。
In a solution with a lower water potential than the cell (hypertonic), water leaves the cell by osmosis. The cell shrinks and becomes wrinkled. This is called crenation.
在比细胞水势更低的溶液（高渗溶液）中，水通过渗透离开细胞。细胞萎缩并出现皱缩。这称为质壁分离中的细胞皱缩。
In an isotonic solution, there is no net movement of water and the cell stays the same size.
在等渗溶液中，水没有净移动，细胞保持原大小。
8. Plant Cells in Different Solutions | 植物细胞在不同溶液中的变化
Plant cells have a rigid cell wall, so they do not burst when placed in a hypotonic solution. Instead, the cell becomes turgid. Turgidity supports non-woody plants.
植物细胞具有坚硬的细胞壁，因此在低渗溶液中不会破裂。相反，细胞会变得膨胀（turgescent）。膨胀性支撑着非木质植物。
In a hypertonic solution, water leaves the plant cell. The vacuole shrinks and the cell membrane pulls away from the cell wall. This is called plasmolysis.
在高渗溶液中，水离开植物细胞。液泡缩小，细胞膜从细胞壁上脱离。这称为质壁分离。
A normal plant cell is slightly turgid because its vacuole contains solutes, giving a lower water potential than pure water in the soil.
正常植物细胞略微膨胀，因为液泡中含有溶质，其水势低于土壤中的纯水。
9. 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 a concentration gradient. This process requires energy from respiration.
主动运输是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。这个过程需要呼吸作用提供能量。
Active transport uses carrier proteins in the cell membrane. The energy, in the form of ATP, changes the shape of the carrier protein to move the particle across the membrane.
主动运输利用细胞膜中的载体蛋白。能量以ATP形式推动载体蛋白改变形状，将粒子运过膜。
Examples include the absorption of glucose and mineral ions by root hair cells in plants, and the reabsorption of glucose in the kidney tubules of humans.
例如，植物根毛细胞吸收葡萄糖和矿质离子，以及人体肾小管对葡萄糖的重吸收。
10. Comparing Diffusion, Osmosis and Active Transport | 三种运输方式的比较
The table below summarises the key differences between the three processes.
下表总结了三种过程的主要区别。
| Feature | Diffusion | Osmosis | Active Transport |
| Energy required? | No | No | Yes (ATP) |
| Direction of movement | Down concentration gradient | Down water potential gradient | Against concentration gradient |
| Molecule moved | Any particles (gas, solutes) | Water only | Ions, glucose, amino acids |
| Membrane needed? | Not always | Yes, partially permeable | Yes, with carrier proteins |
| Example | Oxygen into blood | Water into root hair | Mineral ions into root hair |
In the table, note that diffusion does not always require a membrane, but in biological systems it often occurs across one.
注意：扩散并不总是需要膜，但在生物体系中通常跨膜发生。
11. Practical: Visking Tubing Experiment | 实验：Visking管（透析袋）
Visking tubing is a partially permeable membrane. It can be used to investigate osmosis and diffusion.
Visking管是一种部分渗透性膜，可用于研究渗透和扩散。
Fill the tubing with starch solution and place it in a beaker of iodine solution. Iodine slowly diffuses into the tubing because iodine molecules are small. The starch inside turns blue-black. Starch molecules are too large to leave the tubing.
将淀粉溶液装入管中，放入装有碘液的烧杯。碘分子较小因而能缓慢扩散进入管中。管内的淀粉遇碘变蓝黑色。而淀粉分子太大不能逸出管外。
For osmosis, fill the tubing with sugar solution and place it in pure water. Water enters the tubing and the tubing swells or gains mass. This proves that water moves from high water potential to low water potential.
为观察渗透，将糖溶液装入管中并放入纯水中。水进入管中，管会膨胀或质量增加。这证明水从高水势流向低水势。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
When answering questions on transport, always use the words 'net movement' and 'concentration gradient'. Avoid saying particles 'run out of energy' because diffusion relies on kinetic energy, not respiration energy.
回答运输类题目时，务必使用“净移动”和“浓度梯度”等词语。避免说粒子“耗尽能量”，因为扩散依赖的是动能，而非呼吸能量。
Remember that osmosis only refers to water movement. Do not write that 'sucrose moves' during osmosis. Solutes do not move in osmosis.
请记住渗透仅指水分子的运动。不要写“蔗糖移动”。在渗透过程中溶质不移动。
For active transport, always mention 'against the concentration gradient' and 'requires energy'. Both are needed for full marks.
对于主动运输，必须提到“逆浓度梯度”和“需要能量”。这两点必不可少才能得满分。
Practise drawing simple arrows on diagrams to show the direction of movement and label the type of transport used.
练习在图上用箭头标出移动方向，并注明所采用的运输类型。
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