📚 Diffusion, Osmosis & Active Transport | 扩散、渗透与主动运输
Cells are the basic units of life, and they must exchange materials such as oxygen, carbon dioxide, water, and nutrients with their surroundings. This exchange occurs across the cell membrane, which is selectively permeable — meaning it allows some substances to pass through while preventing others. Understanding how substances move across membranes is essential for IGCSE Science, as it explains many processes from gas exchange in the lungs to water uptake in plant roots.
细胞是生命的基本单位，必须与周围环境交换氧气、二氧化碳、水和营养物质等物质。这种交换发生在细胞膜上，细胞膜具有选择性通透性——即允许某些物质通过，而阻止其他物质通过。理解物质如何穿过膜是IGCSE科学的核心内容，它解释了从肺部气体交换到植物根部吸水等许多过程。
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 is a passive process, meaning it does not require energy from respiration. Diffusion occurs because particles are in constant random motion; over time, they spread out evenly to fill the available space.
扩散是粒子从高浓度区域向低浓度区域、沿浓度梯度进行的净移动。这是一个被动过程，意味着不需要呼吸作用提供能量。扩散之所以发生，是因为粒子处于持续的无规则运动中；随着时间的推移，它们会均匀地分散到可用的空间中。
Net movement means that more particles move from high to low concentration than in the opposite direction. | 净移动是指从高浓度向低浓度移动的粒子数多于相反方向移动的粒子数。
Diffusion continues until the particles are evenly distributed, but they still move around randomly. | 扩散持续到粒子均匀分布为止，但粒子仍然在随机运动。
Simple diffusion occurs directly through the phospholipid bilayer of the cell membrane for small non-polar molecules. | 简单扩散通过细胞膜的磷脂双分子层直接发生，适用于小的非极性分子。
2. Factors Affecting Diffusion | 影响扩散的因素
The rate of diffusion is influenced by several key factors: concentration gradient, temperature, surface area, and the distance over which diffusion takes place. A steeper concentration gradient leads to a faster rate, as does a higher temperature because particles have more kinetic energy. A larger surface area provides more space for particles to cross, while a shorter distance reduces the time needed for diffusion.
扩散速率受几个关键因素影响：浓度梯度、温度、表面积以及扩散发生所经过的距离。浓度梯度越陡，速率越快；温度越高，粒子动能越大，速率也越快。更大的表面积提供了更多粒子穿过的空间，而更短的距离缩短了扩散所需的时间。
| Factor | Effect on Diffusion Rate | Explanation |
| Concentration gradient ↑ | Rate ↑ | More particles move down the gradient per unit time. |
| Temperature ↑ | Rate ↑ | Particles have more kinetic energy and move faster. |
| Surface area ↑ | Rate ↑ | More membrane area available for crossing. |
| Membrane thickness ↑ | Rate ↓ | Greater distance requires longer time. |
This is why the alveoli in the lungs are adapted for diffusion: they provide a huge surface area and very thin walls (one cell thick). | 这就是为什么肺部的肺泡适应于扩散：它们提供了巨大的表面积和极薄的壁（单层细胞厚度）。
3. Diffusion in Living Organisms | 扩散在生物体中的作用
Diffusion is vital for gas exchange and the movement of small molecules. In the lungs, oxygen diffuses from the air in the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli. In plants, carbon dioxide diffuses into leaves through stomata, and oxygen diffuses out during photosynthesis.
扩散对气体交换和小分子运动至关重要。在肺部，氧气从肺泡内的空气扩散进入血液，而二氧化碳从血液扩散进入肺泡。在植物中，二氧化碳通过气孔扩散进入叶片，光合作用产生的氧气则扩散出去。
In fish, oxygen diffuses from water into the blood across the gill lamellae; the countercurrent flow maintains a steep concentration gradient. | 在鱼类中，氧气通过鳃小片从水扩散进入血液；逆流系统维持了陡峭的浓度梯度。
In humans, glucose is small enough to diffuse across capillary walls, but larger proteins are not. | 在人体中，葡萄糖小到可以穿过毛细血管壁，而较大的蛋白质则不能。
In unicellular organisms like Amoeba, diffusion alone is sufficient for gas exchange because of their large surface area to volume ratio. | 在像变形虫这样的单细胞生物中，由于其巨大的表面积与体积比，仅靠扩散就足以进行气体交换。
4. What is Osmosis? | 什么是渗透？
Osmosis is a special type of diffusion involving water molecules. It is the net movement of water molecules from a region of higher water potential (higher concentration of water) to a region of lower water potential, across a partially permeable membrane. Water potential is a measure of the tendency of water to move; pure water has the highest water potential (usually taken as zero).
渗透是一种涉及水分子扩散的特殊类型。它是水分子从水势较高（水浓度较高）的区域穿过部分通透性膜，移动到水势较低区域的净移动。水势是衡量水运动趋势的指标；纯水具有最高的水势（通常视为零）。
Water moves from high water potential → low water potential
水分子从高水势 → 低水势移动
Important: Osmosis only refers to water movement, and it requires a partially permeable membrane. The membrane lets water molecules pass through but not larger solute molecules. | 重要：渗透仅指水分子的运动，并且需要部分通透性膜。膜允许水分子通过，但不允许较大的溶质分子通过。
5. Water Potential and Solute Concentration | 水势与溶质浓度
The water potential of a solution is lowered when solute is added. This is because solute particles 'dilute' the water and reduce the proportion of free water molecules. Therefore, a concentrated solution has a lower water potential (more negative) than a dilute solution.
当加入溶质时，溶液的水势会降低。这是因为溶质颗粒“稀释”了水，减少了自由水分子的比例。因此，浓溶液的水势比稀溶液的水势更低（更负）。
Pure water has a water potential of 0 kPa. | 纯水的水势为0 kPa。
A concentrated sugar solution has a negative water potential, e.g. −100 kPa. | 浓糖溶液的水势为负值，例如−100 kPa。
Water moves from a region with a less negative water potential to one with a more negative water potential. | 水从水势较不负的区域移动到水势更负的区域。
In plant cells, the cell wall is fully permeable, but the cell membrane is partially permeable. This combination is crucial for osmosis. | 在植物细胞中，细胞壁是全通透性的，但细胞膜是部分通透性的。这种组合对渗透至关重要。
6. Osmosis in Plant Cells | 渗透在植物细胞中的作用
When a plant cell is placed in a dilute solution (high water potential), water enters the cell by osmosis. The vacuole and cytoplasm swell, pressing the cell membrane against the cell wall. The cell becomes turgid. Plant cells depend on turgor pressure for support in non-woody parts.
当植物细胞置于稀溶液（高水势）中时，水通过渗透进入细胞。液泡和细胞质膨胀，将细胞膜压向细胞壁。细胞变得坚挺（膨胀）。植物非木质部分的支持依赖于膨压。
When a plant cell is placed in a concentrated solution (low water potential), water leaves the cell by osmosis. The cell membrane pulls away from the cell wall, causing plasmolysis. This makes the plant wilt.
当植物细胞置于浓溶液（低水势）中时，水通过渗透离开细胞。细胞膜与细胞壁分离，导致质壁分离。这会使植物萎蔫。
| External solution | Movement of water | Plant cell appearance |
| Dilute (hypotonic) | Into the cell | Turgid (firm) |
| Concentrated (hypertonic) | Out of the cell | Plasmolysed (flaccid) |
| Isotonic (same concentration) | No net movement | Normal |
7. Osmosis in Animal Cells | 渗透在动物细胞中的作用
Animal cells do not have a cell wall, so they cannot withstand large changes in water volume. If placed in a dilute solution, water enters the cell and it may swell and burst (haemolysis in red blood cells). If placed in a concentrated solution, water leaves the cell and it shrivels (crenation).
动物细胞没有细胞壁，因此不能承受水体积的大幅变化。如果置于稀溶液中，水进入细胞，细胞可能膨胀并破裂（红细胞中称为溶血）。如果置于浓溶液中，水离开细胞，细胞萎缩（皱缩）。
Red blood cells in pure water will burst quickly. | 红细胞在纯水中会迅速破裂。
Cells in the body are bathed in tissue fluid with a similar water potential to the cell contents, preventing osmosis-related damage. | 体内的细胞浸泡在与细胞内容物水势相似的组织液中，防止渗透造成的损伤。
Salt or sugar can be used to preserve food because they create a hypertonic environment that causes bacteria to lose water and die. | 盐或糖可用于保存食物，因为它们创造高渗环境，使细菌失水死亡。
8. 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, which is released from glucose during respiration. It also requires carrier proteins in the cell membrane, which act as ‘pumps’ to move specific molecules.
主动运输是粒子从低浓度区域向高浓度区域、逆浓度梯度的运动。这个过程需要能量，能量来自呼吸作用中葡萄糖的释放。它还需要细胞膜中的载体蛋白，载体蛋白扮演“泵”的角色，移动特定的分子。
Active transport: low concentration → high concentration (requires energy)
主动运输：低浓度 → 高浓度（需要能量）
Carrier proteins bind to a specific molecule, change shape, and release it on the other side of the membrane. | 载体蛋白与特定分子结合，改变形状，并将其释放到膜的另一侧。
Energy is used in the form of ATP, produced during aerobic respiration. | 能量以ATP的形式使用，ATP在有氧呼吸中产生。
If respiration is inhibited (e.g. by lack of oxygen), active transport stops. | 如果呼吸作用受到抑制（例如缺氧），主动运输就会停止。
9. Active Transport in Roots and Intestines | 主动运输在根和肠道中的作用
Plants absorb mineral ions from the soil, even when the ion concentration in the soil is much lower than in the root hair cells. This requires active transport across the root hair cell membrane. The root hair cells also have many mitochondria to provide the necessary ATP.
植物从土壤中吸收矿质离子，即使土壤中的离子浓度远低于根毛细胞内的浓度。这需要通过根毛细胞膜进行主动运输。根毛细胞还含有大量线粒体以提供必要的ATP。
In the human small intestine, glucose is absorbed into the blood by active transport when the concentration in the gut is lower than in the blood. This ensures that all glucose can be absorbed, not just when it is present at high concentrations.
在人体小肠中，当肠腔中的葡萄糖浓度低于血液时，葡萄糖通过主动运输被吸收进入血液。这确保所有葡萄糖都能被吸收，而不仅仅是在高浓度时。
Both root hair cells and intestinal villus cells have a large surface area and high numbers of mitochondria. | 根毛细胞和小肠绒毛细胞都具有较大的表面积和大量线粒体。
Ions such as nitrate and magnesium are actively transported into roots for protein and chlorophyll production. | 硝酸盐和镁离子等被主动运输进入根部，用于蛋白质和叶绿素的合成。
Glucose that is not absorbed by diffusion can still be taken in by active transport. | 通过扩散未被吸收的葡萄糖仍然可以通过主动运输被摄入。
10. Comparison of the Three Transport Processes | 三种运输方式的比较
Diffusion, osmosis, and active transport are all ways substances move across cell membranes, but they differ in direction, energy requirement, and the types of substances involved. The table below summarises the key differences.
扩散、渗透和主动运输都是物质穿过细胞膜的方式，但它们在方向、能量需求以及涉及的物质类型上有所不同。下表总结了主要差异。
| Feature | Diffusion | Osmosis | Active Transport |
| Direction of movement | High → low concentration | High water potential → low water potential | Low → high concentration |
| Energy required? | No | No | Yes (ATP) |
| Membrane involved? | Not always needed (can occur in air) | Partially permeable membrane required | Carrier proteins required |
| Substances moved | Small molecules (O₂, CO₂, glucose) | Water only | Ions, glucose, amino acids |
| Example in biology | Oxygen into red blood cells | Water into root hair cells | Nitrate ions into root hair cells |
Remember: osmosis is just the diffusion of water across a selectively permeable membrane, while active transport is the only one of the three that works against the concentration gradient and requires energy.
记住：渗透只是水分子穿过选择性通透膜的扩散，而主动运输是三者中唯一逆浓度梯度工作且需要能量的过程。
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