📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
Every living cell must exchange substances with its surroundings. This movement of molecules across membranes can happen passively or actively, depending on the concentration gradient and the energy available. In this revision article, we explore the three key transport processes in IGCSE Biology: diffusion, osmosis and active transport.
每一个活细胞都必须与周围环境交换物质。分子跨膜移动的方式可以是被动的，也可以是主动的，这取决于浓度梯度和可用能量。在本复习文章中，我们探讨IGCSE生物学的三个关键运输过程：扩散、渗透和主动运输。
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, until they are evenly spread out. It is a passive process, meaning it does not require energy from respiration.
扩散是粒子从高浓度区域向低浓度区域、顺浓度梯度进行的净移动，直到它们均匀分布。这是一个被动过程，意味着它不需要呼吸作用提供的能量。
Particles move randomly, but the overall (net) direction is from high to low concentration.
粒子随机运动，但整体（净）方向是从高浓度到低浓度。
Diffusion stops when the concentrations are equal, though particles still move back and forth.
当浓度相等时扩散停止，尽管粒子仍在来回运动。
Examples in living organisms: oxygen diffusing from alveoli into blood, and carbon dioxide diffusing from blood into alveoli.
生物体内的例子：氧气从肺泡扩散进入血液，二氧化碳从血液扩散进入肺泡。
2. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
Several physical factors determine how quickly a substance diffuses. Understanding these helps explain why certain adaptations exist in organisms.
有几个物理因素决定物质扩散的快慢。理解这些因素有助于解释生物体为何具有某些适应性结构。
Concentration gradient: the steeper the gradient, the faster the diffusion rate.
浓度梯度：梯度越大，扩散速率越快。
Temperature: higher temperature gives particles more kinetic energy, so they move faster.
温度：温度越高，粒子动能越大，移动越快。
Surface area: a larger surface area allows more particles to cross per unit time.
表面积：表面积越大，单位时间内穿过的粒子越多。
Distance: a shorter distance (e.g. thin alveolar walls) speeds up diffusion.
距离：距离越短（例如肺泡壁薄）可加快扩散。
3. What is Osmosis? | 什么是渗透？
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane. It is a special kind of diffusion that involves only water.
渗透是水分子从水势较高区域通过部分通透膜向水势较低区域进行的净移动。这是一种只涉及水的特殊扩散。
water moves from high water potential → low water potential
水从高水势 → 低水势
The membrane allows water molecules to pass through, but not larger solute particles such as sugar or salt ions.
这种膜允许水分子通过，但不允许较大的溶质颗粒（如糖或盐离子）通过。
4. Key Terms: Tonicity | 关键术语：张力
Osmosis is described using three terms that compare the solute concentration of two solutions.
渗透可以用三个术语来描述，用于比较两种溶液的溶质浓度。
Isotonic: two solutions have the same solute concentration; there is no net water movement.
等渗：两种溶液的溶质浓度相同；没有水的净移动。
Hypotonic: the solution has a lower solute concentration than the other; water enters the cell.
低渗：溶液比另一边的溶质浓度低；水进入细胞。
Hypertonic: the solution has a higher solute concentration than the other; water leaves the cell.
高渗：溶液比另一边的溶质浓度高；水离开细胞。
5. Osmosis in Animal Cells | 动物细胞中的渗透
Animal cells have no cell wall, so they are highly sensitive to changes in external water concentration.
动物细胞没有细胞壁，因此对外界水分浓度变化非常敏感。
In a hypotonic solution, water enters the cell causing it to swell and potentially burst (lysis).
在低渗溶液中，水进入细胞使其膨胀，甚至可能破裂（裂解）。
In a hypertonic solution, water leaves the cell causing it to shrink (crenation).
在高渗溶液中，水离开细胞使其皱缩（皱缩现象）。
Normal animal cells are bathed in isotonic tissue fluid to prevent damage.
正常动物细胞浸泡在等渗的组织液中，以防止损伤。
6. Osmosis in Plant Cells | 植物细胞中的渗透
Plant cells have a rigid cell wall, which provides support and prevents bursting. Instead, plant cells become turgid or flaccid.
植物细胞有坚硬的细胞壁，提供支撑并防止破裂。相反，植物细胞会变得膨胀（紧涨）或萎蔫（松软）。
In a hypotonic solution, water enters the vacuole; the cell becomes turgid and presses against the cell wall, giving support.
在低渗溶液中，水进入液泡；细胞变得膨胀，紧压细胞壁，提供支撑。
In a hypertonic solution, water leaves the cell; the cell becomes flaccid and plasmolysis may occur when the cytoplasm pulls away from the wall.
在高渗溶液中，水离开细胞；细胞变得松软，当细胞质与细胞壁分离时可能发生质壁分离。
7. What is Active Transport? | 什么是主动运输？
Active transport is the movement of particles from a region of lower concentration to a region of higher concentration (against the concentration gradient). It requires energy from respiration, in the form of ATP.
主动运输是粒子从低浓度区域向高浓度区域（逆浓度梯度）的移动。它需要呼吸作用产生的能量，以ATP形式提供。
Active transport: low concentration → high concentration (uses ATP)
主动运输：低浓度 → 高浓度（使用ATP）
This process also involves specific carrier proteins in the membrane that bind to the particle and change shape to move it across.
这个过程还涉及膜中的特定载体蛋白，它们与粒子结合并通过改变形状将其运过膜。
8. Examples of Active Transport in Organisms | 生物体中主动运输的实例
Active transport is essential for absorbing essential substances that may be present in very low concentrations outside cells.
主动运输对于吸收细胞内低浓度的必需物质至关重要。
In the human kidney, glucose and ions are reabsorbed from the filtrate back into the blood against a concentration gradient.
在人体肾脏中，葡萄糖和离子逆浓度梯度从滤液中重吸收回血液。
In the small intestine, glucose is absorbed into the blood even when blood glucose is higher than the intestine contents.
在小肠中，即使血糖浓度高于肠内容物，葡萄糖仍能被吸收进入血液。
In plant root hair cells, mineral ions such as nitrate and magnesium are absorbed from the very dilute soil solution.
在植物根毛细胞中，硝酸根和镁等矿质离子从非常稀薄的土壤溶液中吸收。
9. Comparison of the Three Processes | 三种过程的比较
The table below summarises the key similarities and differences between diffusion, osmosis and active transport.
下表总结了扩散、渗透和主动运输之间的主要异同。
| Feature | Diffusion | Osmosis | Active Transport |
| Direction of movement | High → low | High water potential → low water potential | Low → high |
| Membrane needed? | No (but can cross membranes) | Yes, partially permeable | Yes, carrier proteins |
| Energy required? | No | No | Yes (ATP) |
| Substances moved | Any particles (gas, solutes) | Water only | Ions, sugars, amino acids |
10. Surface Area to Volume Ratio and Transport | 表面积与体积比和运输
The efficiency of diffusion depends on the surface area available compared to the volume of the cell. Small cells have a large surface area to volume ratio, which allows rapid exchange of substances.
扩散效率取决于可利用的表面积与细胞体积之比。小细胞具有较大的表面积与体积比，从而能够快速交换物质。
As organisms grow, their volume increases faster than their surface area, so the ratio decreases. This is why large organisms need specialised exchange surfaces, such as lungs, gills and roots, and transport systems such as blood.
随着生物体生长，其体积增长快于表面积，导致比例下降。这就是为什么大型生物需要专门的交换表面（如肺、鳃和根），以及血液等运输系统。
11. Experimental Demonstrations | 实验演示
You need to know the classic experiments used to investigate osmosis and diffusion.
你需要了解用于研究渗透和扩散的经典实验。
Visking tubing (dialysis tubing): a bag filled with starch or sugar solution is placed in water; only small molecules pass through, demonstrating selective permeability.
维斯科管（透析管）：将装有淀粉或糖溶液的袋子放入水中；只有小分子能通过，证明选择透过性。
Potato strips: placing cylinders of potato in different sucrose solutions causes them to gain or lose mass, showing osmosis.
土豆条：将土豆圆柱体放入不同蔗糖溶液中，会使其增重或减重，从而显示渗透作用。
Ink drop in water: a drop of ink spreads out, demonstrating simple diffusion.
水滴入水中的墨水扩散：一滴墨水散开，演示简单扩散。
12. Common Mistakes and Exam Tips | 常见错误与考试提示
In exams, students often confuse osmosis with diffusion or forget to mention the partially permeable membrane. Always define the direction using the correct terminology.
在考试中，学生常把渗透与扩散混淆，或忘记提及部分通透膜。始终使用正确的术语来描述方向。
In an osmosis question, say "water moves from a high water potential to a low water potential" not "from a low solute concentration" (though this is equivalent, using 'water potential' is safer).
在渗透问题中，要说“水从高水势移到低水势”，而不要说“从低溶质浓度”（尽管两者等价，但使用“水势”更稳妥）。
Remember that respiration provides the energy for active transport, not diffusion or osmosis.
记住：呼吸作用为主动运输提供能量，而不是为扩散或渗透。
For plant cells, use 'turgid' and 'flaccid' correctly; 'plasmolysis' only when the cell is in a hypertonic solution.
对于植物细胞，正确使用“紧涨”和“松软”；只有在细胞处于高渗溶液中时才使用“质壁分离”。
State that diffusion and osmosis are passive processes; they do not require extra energy from respiration.
说明扩散和渗透是被动过程；它们不需要额外来自呼吸作用的能量。
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