📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动转运
Cells are the basic units of life. To survive, every cell must exchange substances with its surroundings. This exchange occurs through three main processes: diffusion, osmosis and active transport. Understanding these processes is essential for IGCSE Science, as they explain how nutrients enter cells, how waste leaves, and how water balance is maintained.
细胞是生命的基本单位。为了生存，每个细胞都必须与周围环境交换物质。这种交换主要通过三种方式进行：扩散、渗透和主动转运。理解这些过程对于IGCSE科学至关重要，因为它们解释了营养物质如何进入细胞、废物如何排出，以及水分平衡如何维持。
1. What is Diffusion? | 什么是扩散？
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. This movement is passive, meaning it does not require energy. Diffusion occurs because particles are constantly moving randomly, and over time they spread out evenly.
扩散是粒子从高浓度区域向低浓度区域（沿浓度梯度）的净移动。这个过程是被动的，不需要能量。扩散的发生是因为粒子不断进行随机运动，随着时间推移，它们会均匀分布。
- Diffusion can happen in gases and liquids, but not in solids because particles in solids are fixed.
- 扩散可以发生在气体和液体中，但不能发生在固体中，因为固体中的粒子位置固定。
- The steeper the concentration gradient, the faster the rate of diffusion.
- 浓度梯度越大，扩散速率越快。
- Diffusion continues until particles are evenly distributed, reaching dynamic equilibrium.
- 扩散持续进行直到粒子均匀分布，达到动态平衡。
2. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
Several factors increase or decrease how quickly diffusion occurs. The temperature, concentration gradient, surface area and distance all play important roles. In the human body, these factors are optimised to allow rapid exchange of gases in the lungs and nutrients in the intestines.
有几个因素会加快或减慢扩散速度。温度、浓度梯度、表面积和距离都起着重要作用。在人体内，这些因素经过优化，使得肺部的气体交换和肠道中的营养吸收能够快速进行。
| Factor | 因素 | Effect on rate | 对速率的影响 |
|---|---|
| Temperature | 温度 | gher temperature increases kinetic energy, so particles move faster → diffusion speeds up. |
| 温度 | 温度 | 温度升高，粒子动能增大，运动加快 → 扩散加速。 |
| Concentration gradient | 浓度梯度 | A steeper gradient gives a greater driving force, so diffusion is faster. |
| 浓度梯度 | 梯度越陡，驱动力越大，扩散越快。 |
| Surface area | 表面积 | A larger surface area allows more particles to cross at once, increasing the rate. |
| 表面积 | 表面积越大，单位时间内更多粒子能够通过，速率增加。 |
| Distance | 距离 | A shorter distance means particles take less time to reach the low-concentration region. |
| 距离 | 距离越短，粒子到达低浓度区域所需的时间越少。 |
3. Examples of Diffusion in Living Organisms | 生物体内扩散的实例
Diffusion is vital for gas exchange. In the lungs, oxygen (O₂) moves from the alveoli (high concentration) into the blood (low concentration), while carbon dioxide (CO₂) moves in the opposite direction. Similarly, in plant leaves, CO₂ diffuses into cells through stomata for photosynthesis, and O₂ diffuses out as a waste product.
扩散对于气体交换至关重要。在肺中，氧气（O₂）从肺泡（高浓度）进入血液（低浓度），而二氧化碳（CO₂）则向相反方向移动。同样，在植物叶片中，CO₂通过气孔扩散进入细胞用于光合作用，O₂则作为废物扩散出去。
- In fish, oxygen diffuses from water into gill capillaries across the gill filaments.
- 在鱼类中，氧气从水扩散到鳃毛细血管中，穿过鳃丝。
- In the small intestine, digested food molecules such as glucose and amino acids diffuse into the blood.
- 在小肠中，诸如葡萄糖和氨基酸等消化后的食物分子扩散进入血液。
- In the kidneys, nitrogenous waste urea diffuses from blood into the nephron for excretion.
- 在肾脏中，含氮废物尿素从血液扩散进入肾单位以排出。
4. What is Osmosis? | 什么是渗透？
Osmosis is a special type of diffusion involving water. It is the net movement of water molecules from a region of higher water potential (dilute solution, more water molecules) to a region of lower water potential (concentrated solution, fewer water molecules), through a partially permeable membrane. This process is also passive and requires no energy.
渗透是扩散的一种特殊类型，涉及水分子的移动。它是水分子通过部分通透性膜从水势较高区域（稀溶液，水分子多）净移动至水势较低区域（浓溶液，水分子少）的过程。该过程同样是被动的，不需要能量。
Water moves from high water potential → low water potential
水从高水势 → 低水势移动
The partially permeable membrane allows water molecules to pass through, but not larger solute molecules such as sucrose or salt ions. Water potential is measured in kilopascals (kPa). Pure water has the highest water potential, which is zero. Adding solutes lowers water potential, making it more negative.
部分通透性膜允许水分子通过，但不允许较大的溶质分子（如蔗糖或盐离子）通过。水势以千帕（kPa）为单位测量。纯水具有最高的水势，即为零。加入溶质会降低水势，使其变得更负。
5. Osmosis in Plant Cells | 植物细胞中的渗透
Plant cells have a rigid cell wall. When placed in a dilute solution (higher water potential outside), water enters the cell by osmosis. The vacuole swells and pushes the cytoplasm against the cell wall, making the cell turgid. Turgidity provides support for non-woody plants.
植物细胞有坚硬的细胞壁。当置于稀溶液（外部水势更高）中时，水通过渗透进入细胞。液泡膨胀并将细胞质推向细胞壁，使细胞变得膨胀（质壁分离状态相反）。膨胀为草本植物提供支撑。
- Turgid cells are firm and help keep stems and leaves upright.
- 膨胀的细胞非常坚硬，帮助茎和叶保持直立。
- If a plant is placed in a concentrated solution, water leaves the cell, and the cytoplasm shrinks away from the cell wall. This is called plasmolysis.
- 如果植物置于浓溶液中，水分会离开细胞，细胞质收缩并脱离细胞壁，这称为质壁分离。
- Plasmolysis makes the plant wilt (become flaccid).
- 质壁分离使植物萎蔫（变得柔软）。
6. Osmosis in Animal Cells | 动物细胞中的渗透
Animal cells do not have a cell wall. Therefore, they are more vulnerable to changes in water concentration. If an animal cell is placed in a dilute solution, water enters and the cell swells. Eventually, it may burst, a process called haemolysis (in red blood cells).
动物细胞没有细胞壁，因此更容易受到水浓度变化的影响。如果动物细胞置于稀溶液中，水进入细胞并使其膨胀。最终，细胞可能破裂，这一过程在红细胞中称为溶血。
In a concentrated solution, water leaves the cell, causing it to shrink and become crenated. This is why the body must carefully regulate blood plasma concentration to keep cells in an isotonic environment, where there is no net water movement.
在浓溶液中，水离开细胞，导致细胞萎缩并变成皱缩状。这就是为什么身体必须仔细调节血浆浓度，使细胞处于等渗环境中，此时没有净水移动。
| Solution type | 溶液类型 | Effect on animal cell | 对动物细胞的影响 | Effect on plant cell | 对植物细胞的影响 |
|---|---|---|
| Hypotonic (dilute) | 低渗（稀） | Cell swells and may burst (haemolysis) | Cell becomes turgid, but does not burst |
| 低渗（稀） | 细胞膨胀并可能破裂（溶血） | 细胞变为膨胀，但不会破裂 |
| Isotonic (equal) | 等渗（相等） | No net movement | No net movement |
| 等渗（相等） | 无净移动 | 无净移动 |
| Hypertonic (concentrated) | 高渗（浓） | Cell shrinks (crenation) | Plasmolysis; cell becomes flaccid |
| 高渗（浓） | 细胞萎缩（皱缩） | 质壁分离；细胞变柔软 |
7. What is Active Transport? | 什么是主动转运？
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, which is released during cellular respiration. Active transport uses carrier proteins embedded in the cell membrane to move substances.
主动转运是粒子从低浓度区域向高浓度区域的移动，即逆浓度梯度。这个过程需要能量，能量来自细胞呼吸。主动转运使用细胞膜中的载体蛋白来移动物质。
Active transport: low concentration → high concentration (requires energy)
主动转运：低浓度 → 高浓度（需要能量）
For example, in the roots of plants, mineral ions such as nitrate (NO₃⁻) are present in higher concentrations inside the root cells than in the soil. Active transport allows these ions to be absorbed against the concentration gradient. In the human gut, glucose is absorbed into the blood by active transport when its concentration in the intestine is lower than that in the blood.
例如，在植物根部，硝酸根离子（NO₃⁻）在根细胞内的浓度高于土壤中的浓度。主动转运允许这些离子逆浓度梯度被吸收。在人体肠道中，当肠腔内葡萄糖浓度低于血液时，葡萄糖通过主动转运被吸收进入血液。
8. Comparison of Diffusion, Osmosis and Active Transport | 扩散、渗透与主动转运的比较
All three processes are essential for life, but they differ in direction, energy requirement and specific molecules involved. The table below summarises the key differences.
这三个过程对生命都至关重要，但它们在方向、能量需求和所涉及的具体分子上有所不同。下表总结了主要区别。
| Feature | 特征 | Diffusion | 扩散 | Osmosis | 渗透 | Active transport | 主动转运 |
|---|---|---|---|
| Substance moved | 移动的物质 | Any particles (gases, solutes) | 任何粒子（气体、溶质） | Water molecules only | 仅水分子 | Ions, sugars, amino acids | 离子、糖类、氨基酸 |
| Direction | 方向 | Down concentration gradient (high → low) | Down water potential gradient (high → low) | Against concentration gradient (low → high) |
| 方向 | 沿浓度梯度（高 → 低） | 沿水势梯度（高 → 低） | 逆浓度梯度（低 → 高） |
| Energy (ATP) required? | 需要能量（ATP）吗？ | No | 不需要 | No | 不需要 | Yes | 需要 |
| Membrane proteins used? | 使用膜蛋白吗？ | Usually no (simple diffusion) | Often through aquaporins (channel proteins) | Carrier proteins | 载体蛋白 |
9. Investigating Osmosis in Potatoes | 探究马铃薯中的渗透作用
A common IGCSE practical is to place potato strips into different concentrations of sucrose solution. By measuring the change in mass before and after, you can determine whether water entered or left the cells.
一个常见的IGCSE实验是将马铃薯条放入不同浓度的蔗糖溶液中。通过测量实验前后的质量变化，你可以判断水是进入还是离开了细胞。
- If the external solution is more dilute than the cell sap, the potato strip gains mass because water enters by osmosis.
- 如果外部溶液比细胞液更稀，马铃薯条质量增加，因为水通过渗透进入。
- If the external solution is more concentrated, the potato strip loses mass and becomes floppy.
- 如果外部溶液更浓，马铃薯条质量减少并变得柔软。
- The point where no mass change occurs gives the concentration of the cell sap (isotonic point).
- 质量不变的点对应细胞液的浓度（等渗点）。
Another experiment uses Visking tubing, which is partially permeable, to demonstrate diffusion of small molecules like glucose and iodine, while large molecules such as starch remain inside. This models the gut lining.
另一个实验使用Visking透析袋（部分通透性膜）来演示小分子（如葡萄糖和碘）的扩散，而淀粉等大分子留在袋内。这模拟了肠道内壁。
10. Why These Processes Matter in Homeostasis | 这些过程在体内稳态中的重要性
Homeostasis is the maintenance of a stable internal environment. Diffusion, osmosis and active transport work together to regulate water balance, pH, ion levels and nutrient uptake. For example, in the kidneys, glomerular filtrate is adjusted by selective reabsorption, where glucose and ions are actively transported back into the blood, while water is reabsorbed by osmosis in the collecting duct.
稳态是维持稳定的内部环境。扩散、渗透和主动转运共同调节水平衡、pH、离子水平以及营养物质的吸收。例如，在肾脏中，肾小球滤液通过选择性重吸收进行调整，其中葡萄糖和离子通过主动转运回到血液，而水在集合管中通过渗透被重吸收。
In plants, the opening and closing of stomata depend on changes in turgor pressure driven by osmosis in guard cells. Without these transport processes, cells would not be able to take up nutrients, remove waste, or maintain their shape. Therefore, understanding them helps you explain many biological phenomena, from plant wilting to gas exchange in humans.
在植物中，气孔的开放和关闭取决于由保卫细胞渗透作用驱动的膨压变化。没有这些转运过程，细胞将无法摄取营养、排出废物或维持其形态。因此，理解它们有助于你解释许多生物现象，从植物萎蔫到人体气体交换。
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