📚 Diffusion, Osmosis and Active Transport | 扩散、渗透与主动运输
In IGCSE Edexcel Science, understanding how substances move into and out of cells is fundamental. Diffusion, osmosis and active transport are three key processes that allow cells to take in nutrients, remove waste and maintain internal balance. Each process plays a distinct role depending on energy requirements and concentration gradients. Mastering these concepts is essential for explaining how organisms survive and interact with their environment.
在IGCSE爱德思科学课程中，理解物质如何进出细胞是基础知识。扩散、渗透和主动运输是细胞摄取营养、排出废物并维持内部平衡的三种关键过程。每种过程根据能量需求和浓度梯度发挥着不同的作用。掌握这些概念对于解释生物如何生存以及与环境互动至关重要。
1. Introduction to Movement of Substances | 物质运输概述
All living cells require a constant supply of substances such as oxygen, glucose and mineral ions, and must remove waste products like carbon dioxide and urea. These movements happen across the cell membrane, which is selectively permeable — it allows some molecules to pass through but not others. The three main transport processes are diffusion, osmosis and active transport, each driven by different forces.
所有活细胞都需要持续供应氧气、葡萄糖和矿物质离子等物质，并必须清除二氧化碳和尿素等废物。这些物质穿过细胞膜进行运输，细胞膜具有选择透过性——允许某些分子通过，但阻止其他分子。三种主要的运输方式是扩散、渗透和主动运输，它们分别由不同的驱动力推动。
2. Definition and Principles of Diffusion | 扩散的定义与原理
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process, meaning it does not require energy from respiration. Particles move randomly due to their kinetic energy, and over time they become evenly distributed. This continues until equilibrium is reached, although individual particles still move in both directions.
扩散是指粒子从浓度较高的区域沿浓度梯度向浓度较低的区域净移动。它是一种被动过程，不需要呼吸作用提供的能量。粒子因自身动能而随机运动，随着时间推移会达到均匀分布。直至平衡时，单个粒子仍在两个方向运动，但净移动为零。
Examples in the human body include oxygen diffusing from the alveoli into the blood and carbon dioxide diffusing out of the blood into the lungs. In plants, carbon dioxide diffuses into leaves through stomata for photosynthesis. Diffusion also occurs in non‑living systems, such as a drop of ink spreading in water.
人体中的例子包括氧气从肺泡扩散到血液中，二氧化碳从血液扩散到肺中呼出。在植物中，二氧化碳通过气孔扩散到叶片中进行光合作用。扩散也发生在非生物系统中，例如墨滴在水中扩散。
3. Factors Affecting the Rate of Diffusion | 影响扩散速率的因素
The rate of diffusion depends on several factors. A larger concentration gradient speeds up diffusion because there is a steeper difference in particle numbers. Higher temperatures increase kinetic energy, making particles move faster. A greater surface area of the membrane allows more particles to cross at once. Finally, a shorter diffusion distance reduces the time needed for particles to travel. These relationships are summarised by Fick's law.
扩散速率取决于几个因素。浓度梯度越大，扩散越快，因为粒子数量的差异更陡峭。温度越高，动能越大，粒子运动更快。膜的表面积越大，同时穿过的粒子就越多。最后，扩散距离越短，粒子移动所需时间越少。这些关系可以用菲克定律来概括。
Rate of diffusion ∝ (surface area × concentration difference) / thickness of membrane
扩散速率 ∝ (表面积 × 浓度差) / 膜的厚度
4. Osmosis: A Special Case of Diffusion | 渗透：扩散的特例
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. Water potential (Ψ, psi) is a measure of the tendency of water to move; pure water has the highest water potential (0 kPa), and adding solutes lowers it (more negative). Osmosis is passive and only involves water.
渗透是水分子从较高水势的区域穿过部分通透膜向较低水势的区域净移动。水势（Ψ）是衡量水分移动趋势的指标；纯水的水势最高（0 kPa），加入溶质会降低水势（变得更负）。渗透是被动过程，仅涉及水的移动。
A partially permeable membrane has tiny pores that allow small water molecules to pass but block larger solute molecules. Osmosis continues until the water potential on both sides is equal, or until hydrostatic pressure prevents further water entry. The direction of water movement is determined entirely by the relative water potentials.
部分通透膜具有微小孔隙，允许小水分子通过，但阻挡较大的溶质分子。渗透持续进行，直到两侧水势相等，或静水压力阻止水分进一步进入。水分子运动的方向完全取决于相对水势的高低。
5. Water Potential and Osmosis in Plant and Animal Cells | 水势及渗透在动植物细胞中的作用
In animal cells, placing them in a hypotonic solution (higher water potential, lower solute concentration) causes water to enter by osmosis, leading to swelling and possible bursting (lysis). In an isotonic solution, there is no net water movement and the cell remains normal. In a hypertonic solution (lower water potential, higher solute concentration), water leaves the cell, causing it to shrink (crenation).
在动物细胞中，将其置于低渗溶液（较高水势、较低溶质浓度）中会导致水通过渗透进入，细胞膨胀并可能破裂（细胞溶解）。在等渗溶液中，无水净移动，细胞保持正常。在高渗溶液（较低水势、较高溶质浓度）中，水分从细胞流失，导致细胞皱缩（细胞皱缩）。
Plant cells behave differently because they have a strong cellulose cell wall. In a hypotonic solution, water enters the cell, the vacuole swells and the cytoplasm pushes against the cell wall, making the cell turgid — this supports the plant. In a hypertonic solution, water leaves, the vacuole shrinks, and the cell membrane pulls away from the cell wall; this is called plasmolysis, and the cell becomes flaccid.
植物细胞的表现不同，因为它们有坚韧的纤维素细胞壁。在低渗溶液中，水分进入细胞，液泡膨胀，细胞质推挤细胞壁，使细胞膨胀（硬挺）——这为植物提供支持。在高渗溶液中，水分流失，液泡缩小，细胞膜从细胞壁拉开；这称为质壁分离，细胞变得萎软。
6. Active Transport: Moving Against the Gradient | 主动运输：逆浓度梯度转运
Active transport is the movement of particles from a region of lower concentration to a region of higher concentration, against the concentration gradient. Because this goes against the natural direction of diffusion, energy is required — in the form of ATP from respiration. Carrier proteins in the cell membrane bind to the specific solute and change shape to pump it across, using the energy released from ATP breakdown.
主动运输是将粒子从较低浓度区域移向较高浓度区域，即逆浓度梯度转运。由于这种移动与扩散的自然方向相反，需要能量——来自呼吸作用产生的ATP。细胞膜上的载体蛋白与特定溶质结合，利用ATP分解释放的能量改变形状，将溶质泵过膜去。
Key examples include the uptake of mineral ions (e.g. nitrate, potassium) by root hair cells from the soil, where ion concentrations are often lower than inside the root. In the human small intestine, glucose is actively transported from the gut lumen into the blood, even when glucose concentration inside the villi is already higher. Active transport allows cells to absorb nutrients efficiently even from dilute environments.
典型例子包括根毛细胞从土壤中吸收矿物质离子（如硝酸根、钾离子），尽管土壤中的离子浓度常常低于根细胞内。在人体小肠中，葡萄糖被主动运输从肠腔转运到血液中，即使绒毛内的葡萄糖浓度已经较高。主动运输使细胞即使在稀溶液中也能高效吸收营养。
7. Comparison of Diffusion, Osmosis and Active Transport | 扩散、渗透和主动运输的比较
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Substance moved | Solute particles (gas or liquid) | Water only | Solutes (ions, glucose, amino acids) |
| Concentration gradient | Down (high → low) | Down (high water potential → low water potential) | Against (low → high) |
| Energy required? | No (passive) | No (passive) | Yes (ATP, active) |
| Membrane requirement | Not essential (can occur anywhere) | Partially permeable membrane required | Cell membrane with carrier proteins |
| Key examples | O₂ and CO₂ exchange in lungs; perfume in air | Water uptake by roots; red blood cell in plasma | Ion uptake by roots; glucose absorption in ileum |
Understanding these differences is crucial for predicting how cells will respond to their environment. While diffusion and osmosis balance concentrations without energy input, active transport enables cells to stockpile useful substances even when the external supply is scarce.
理解这些差异对于预测细胞如何对环境作出反应至关重要。扩散和渗透能在不消耗能量的情况下平衡浓度，而主动运输使细胞即使在外部供给稀少时也能富集有用物质。
8. Importance in Living Organisms | 在生物体中的重要性
These three transport mechanisms are vital across all kingdoms of life. Diffusion supplies oxygen for respiration and removes carbon dioxide, while osmosis regulates water balance in tissues and cells, preventing dehydration or over‑hydration. Active transport is essential for nutrient absorption in the gut and for maintaining ion gradients in nerve cells. Without them, metabolism would cease.
这三种运输机制对生物界的所有生命都至关重要。扩散为呼吸作用提供氧气并清除二氧化碳，渗透调节组织和细胞的水分平衡，防止脱水或水分过多。主动运输对于肠道中的营养吸收和维持神经细胞中的离子梯度必不可少。若没有它们，新陈代谢将停止。
In plants, osmosis creates turgor pressure that keeps stems and leaves upright. Mineral ions taken up by active transport are used to synthesise proteins, chlorophyll and nucleic acids. In the nitrogen cycle, root hair cells actively absorb nitrate ions, which are then assimilated into amino acids, showing how transport processes underpin entire ecosystems.
在植物中，渗透产生膨胀压，使茎和叶保持挺立。通过主动运输吸收的矿物质离子被用于合成蛋白质、叶绿素和核酸。在氮循环中，根毛细胞主动吸收硝酸根离子，随后将其同化为氨基酸，这表明运输过程支撑着整个生态系统。
9. Practical Investigations | 实验探究
Simple laboratory experiments can demonstrate these processes. To show diffusion, place a potassium permanganate crystal in a beaker of still water; the purple colour spreads evenly over time. Repeating the experiment at two different temperatures shows the effect of temperature on the rate of diffusion.
简单的实验室实验可以演示这些过程。要展示扩散，可将高锰酸钾晶体放入盛有静止水的烧杯中；紫色会随着时间均匀扩散。在两个不同温度下重复实验，可显示温度对扩散速率的影响。
Osmosis can be investigated using potato cylinders placed in different concentrations of sucrose solution. After a set time, the change in length or mass of the cylinders indicates whether water entered (turgid, mass increase) or left (flaccid, mass decrease). A graph of percentage change in mass against concentration reveals the water potential of potato cells where the line crosses zero (no net water movement).
渗透作用可使用土豆条在不同浓度的蔗糖溶液中进行探究。经过一定时间后，土豆条长度或质量的变化显示出水分是进入（涨大增重）还是流失（缩小减轻）。绘制质量变化百分比对浓度的曲线图，当曲线与零刻度相交时，可得出土豆细胞的水势（无水净移动）。
Active transport is more challenging to demonstrate directly, but using a respiration inhibitor like cyanide (or safer alternatives like sodium azide in a controlled classroom setting) can stop active uptake of ions by yeast cells, while diffusion continues. Comparing dye or ion movement in energy‑depleted versus normal cells highlights the role of ATP.
主动运输较难直接演示，但可以使用呼吸抑制剂（如氰化物，或在受控课堂环境中使用更安全的替代品如叠氮化钠）来停止酵母细胞对离子的主动吸收，而扩散继续进行。比较能量耗尽细胞与正常细胞的染料或离子移动，可突出ATP的作用。
10. Exam Tips and Common Misconceptions | 考试技巧与常见误区
Students often confuse diffusion and osmosis: remember that osmosis is only water and requires a partially permeable membrane. Do not say "concentration of water" in osmosis—use "water potential". Also, active transport requires carrier proteins and energy, not just the presence of a membrane. Always state that it moves substances against the concentration gradient.
学生们经常混淆扩散和渗透：记住渗透只涉及水分，且需要部分通透膜。在渗透中不要使用"水的浓度"，而应使用"水势"。此外，主动运输需要载体蛋白和能量，而不仅仅依赖膜的存在。务必说明它逆浓度梯度转运物质。
In practical questions, graphs showing mass change in osmosis often have a point where the line crosses the x‑axis. That point corresponds to the water potential of the tissue. Be precise with descriptions: 'the potato cylinder increased in mass because water entered by osmosis from a region of higher water potential to a region of lower water potential inside the cells.'
在实验题中，显示渗透质量变化的曲线图常有一条线与x轴相交。该交点对应于组织的水势。描述要精确： "土豆条质量增加是因为水通过渗透从较高水势的区域进入细胞内较低水势的区域。"
11. Linking Transport Processes to Other Topics | 将运输过程与其他主题的联系
These concepts underpin many areas of IGCSE Science. In human biology, the structure of the alveolus (large surface area, thin walls) maximises diffusion of gases. In plant biology, the opening and closing of stomata depend on osmosis‑caused changes in guard cell turgor. Active transport explains how nitrates are taken up even when concentrations in the soil are low, linking to the nitrogen cycle and protein synthesis.
这些概念支撑着IGCSE科学的许多领域。在人体生物学中，肺泡的结构（大面积、薄壁）最大化了气体的扩散。在植物生物学中，气孔的张开和闭合取决于保卫细胞渗透引起的膨胀变化。主动运输解释了即使土壤浓度低时也能吸收硝酸盐，从而与氮循环和蛋白质合成相关联。
12. Summary and Key Points | 总结与要点
Diffusion, osmosis and active transport are the three main ways substances cross cell membranes. Diffusion and osmosis are passive, moving down concentration or water potential gradients. Active transport uses energy from ATP to pump substances against the gradient. Their roles are distinct: diffusion handles dissolved gases and small solutes; osmosis manages water balance; active transport enables accumulation of essential ions and nutrients. A solid understanding of these processes is the key to explaining many physiological and experimental observations in IGCSE Science.
扩散、渗透和主动运输是物质穿过细胞膜的三种主要方式。扩散和渗透是被动过程，沿浓度梯度或水势梯度移动。主动运输利用ATP提供的能量逆梯度泵送物质。它们的作用各不相同：扩散处理溶解气体和小分子溶质；渗透管理水分平衡；主动运输使必须离子和营养的富集成为可能。扎实地理解这些过程是解释IGCSE科学中许多生理和实验观察的关键。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图