📚 Cell Transport: Diffusion, Osmosis & Active Transport | 细胞运输：扩散、渗透与主动运输
Cells are the basic units of life, and they constantly exchange materials with their surroundings. This exchange relies on three main transport mechanisms: diffusion, osmosis, and active transport. Each process plays a critical role in maintaining the internal environment of the cell and supporting life functions.
细胞是生命的基本单位，它们不断与周围环境交换物质。这种交换依赖三种主要的运输机制：扩散、渗透和主动运输。每一种过程都在维持细胞内部环境和支持生命功能方面发挥着关键作用。
1. Introduction to Cell Transport | 细胞运输导言
Cell transport refers to the movement of substances across the cell membrane. The cell membrane is selectively permeable, meaning it allows some molecules to pass through while blocking others. This selective movement is essential for obtaining nutrients, removing waste, and maintaining homeostasis.
细胞运输是指物质穿过细胞膜的运动。细胞膜具有选择透过性，即它允许某些分子通过而阻止其他分子通过。这种选择性运动对于获取营养物质、清除废物和维持稳态至关重要。
There are two broad categories of transport: passive transport, which requires no energy, and active transport, which requires energy in the form of ATP. Diffusion and osmosis are examples of passive transport, while active transport uses carrier proteins to pump substances against their concentration gradient.
运输分为两大类：不需要能量的被动运输和需要以ATP形式提供能量的主动运输。扩散和渗透是被动运输的例子，而主动运输则利用载体蛋白将物质逆浓度梯度泵入或泵出细胞。
2. Diffusion | 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 random and occurs due to the kinetic energy of particles. It continues until equilibrium is reached, where the particles are evenly distributed.
扩散是粒子从高浓度区域到低浓度区域（沿浓度梯度）的净移动。这种移动是随机的，由粒子的动能驱动。它一直持续到达到平衡状态，即粒子均匀分布为止。
Diffusion can occur in gases and liquids. For example, oxygen diffuses into red blood cells from the alveoli of the lungs, while carbon dioxide diffuses out. In organisms, diffusion across cell membranes is vital for gas exchange and the uptake of small molecules such as glucose and amino acids.
扩散可以发生在气体和液体中。例如，氧气从肺部肺泡扩散进入红细胞，而二氧化碳则扩散出去。在生物体内，通过细胞膜的扩散对于气体交换以及葡萄糖、氨基酸等小分子的吸收至关重要。
Rate of diffusion ∝ (Surface area × Concentration difference) / Membrane thickness
The above equation shows that the rate of diffusion increases with a larger surface area, a greater concentration difference, and a thinner membrane. This is why alveoli and villi have specialised structures to maximise diffusion efficiency.
上述方程表明，扩散速率随着表面积增大、浓度差增大以及膜厚度减小而增加。这就是为什么肺泡和小肠绒毛具有特化结构以最大化扩散效率的原因。
3. Factors Affecting Diffusion | 影响扩散的因素
Several factors influence the rate and extent of diffusion. Understanding these factors is essential for explaining biological phenomena and for experimental design.
有多种因素影响扩散的速率和程度。理解这些因素对解释生物学现象和实验设计至关重要。
- Temperature: Higher temperatures increase the kinetic energy of particles, leading to faster diffusion.
- 温度：较高的温度增加了粒子的动能，导致扩散更快。
- Concentration gradient: A steeper gradient means a higher rate of diffusion until equilibrium is reached.
- 浓度梯度：梯度越陡，扩散速率越高，直到达到平衡。
- Surface area: A larger surface area provides more space for particles to cross the membrane.
- 表面积：更大的表面积提供了更多让粒子穿过膜的空间。
- Distance: Shorter distances allow faster diffusion because particles have less distance to travel.
- 距离：距离越短，粒子需要移动的路程越少，扩散越快。
- Size of particles: Smaller particles diffuse more quickly than larger ones at the same temperature.
- 粒子大小：在相同温度下，较小的粒子比较大的粒子扩散得更快。
4. Osmosis | 渗透作用
Osmosis is a special type of diffusion involving water molecules. It is defined as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a selectively permeable membrane.
渗透作用是涉及水分子的一种特殊扩散。它定义为水分子通过选择透过性膜，从水势较高（稀溶液）的区域向水势较低（浓溶液）区域的净移动。
Water potential is denoted by the Greek letter ψ (psi). Pure water has the highest water potential, set as zero. Adding solutes lowers the water potential, making it negative. Therefore, water always moves towards the more negative water potential.
水势用希腊字母ψ（psi）表示。纯水的水势最高，设定为零。加入溶质会降低水势，使其变为负值。因此，水总是向着水势更负的方向移动。
In osmosis, the selectively permeable membrane allows water molecules to pass through but prevents the passage of larger solute particles. The process continues until the water potential on both sides is equal, or until the pressure opposes further movement.
在渗透作用中，选择透过性膜允许水分子通过，但阻止较大的溶质粒子通过。该过程持续到两侧水势相等，或直到压力阻止进一步移动为止。
5. Osmosis in Plant Cells | 植物细胞中的渗透作用
Plant cells have a rigid cell wall made of cellulose, which affects how they respond to different external solutions. The cell wall is fully permeable, but the cell membrane controls osmosis.
植物细胞具有由纤维素组成的刚性细胞壁，这影响了它们对不同外部溶液的反应。细胞壁是完全透性的，但细胞膜控制渗透作用。
- In a dilute solution (higher water potential outside): Water enters the cell, causing the vacuole to swell. The cytoplasm presses against the cell wall, and the cell becomes turgid. Turgidity provides support to the plant.
- 在稀溶液中（外界水势较高）：水进入细胞，导致液泡膨胀。细胞质压向细胞壁，细胞变得膨胀。膨胀为植物提供支撑。
- In a concentrated solution (lower water potential outside): Water leaves the cell, the vacuole shrinks, and the cytoplasm pulls away from the cell wall, a process called plasmolysis. The cell becomes flaccid and may wilt.
- 在浓溶液中（外界水势较低）：水离开细胞，液泡缩小，细胞质与细胞壁分离，这一过程称为质壁分离。细胞变得松弛，可能枯萎。
- In an isotonic solution (same water potential): No net movement of water occurs; the cell remains unchanged.
- 在等渗溶液中（水势相同）：没有水的净移动；细胞保持不变。
6. Osmosis in Animal Cells | 动物细胞中的渗透作用
Animal cells do not have a cell wall, so they are more sensitive to changes in the external water potential. If too much water enters, they may burst; if too much water leaves, they may shrink.
动物细胞没有细胞壁，因此对外界水势变化更为敏感。如果进入的水过多，它们可能会胀破；如果失去的水过多，它们可能会萎缩。
In a dilute solution (hypotonic), animal cells swell and can undergo lysis (bursting) because the cell membrane cannot withstand the increased internal pressure. In a concentrated solution (hypertonic), animal cells lose water and become crenated (shriveled). Only in an isotonic solution do animal cells remain normal.
在稀溶液（低渗）中，动物细胞会膨胀并可能发生裂解（胀破），因为细胞膜无法承受增大的内部压力。在浓溶液（高渗）中，动物细胞失水并变得皱缩（萎缩）。只有在等渗溶液中，动物细胞才能保持正常。
This is why medical saline solutions are isotonic with blood plasma (0.9% NaCl), so that red blood cells are not damaged during intravenous drips.
这就是为什么医用生理盐水与血浆等渗（0.9% NaCl），这样红细胞在静脉滴注时不会被破坏。
7. 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 provided by ATP. ATP is produced during cellular respiration.
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。这个过程需要能量，由ATP提供。ATP是在细胞呼吸过程中产生的。
Active transport involves specific carrier proteins embedded in the cell membrane. When a particle binds to a carrier protein, the protein uses ATP to change shape, releasing the particle on the opposite side of the membrane. These carrier proteins are often called pumps.
主动运输涉及嵌入细胞膜中的特异性载体蛋白。当粒子与载体蛋白结合时，载体蛋白利用ATP改变形状，在膜的另一侧释放粒子。这些载体蛋白通常被称为泵。
A classic example is the absorption of glucose by the villi in the small intestine. Glucose is moved from the gut lumen (low concentration) into the epithelial cells (high concentration) via active transport. Another example is the uptake of mineral ions by root hair cells in plants, where the ion concentration in the soil is often lower than in the root cells.
一个典型例子是小肠绒毛吸收葡萄糖。葡萄糖通过主动运输从肠腔（低浓度）进入上皮细胞（高浓度）。另一个例子是植物根毛细胞吸收矿质离子，因为土壤中的离子浓度通常低于根细胞内的浓度。
8. Comparing Passive and Active Transport | 被动运输与主动运输的比较
It is useful to compare the three transport processes across several key features. The following table summarises the differences between diffusion, osmosis, and active transport.
比较这三种运输过程在几个关键特征上的异同很有帮助。下表总结了扩散、渗透和主动运输之间的区别。
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Direction relative to gradient | Down gradient | Down water potential gradient | Against gradient |
| Energy required? | No | No | Yes (ATP) |
| Membrane protein needed? | No (simple diffusion) or channel/carrier (facilitated) | No, but uses aquaporins in some cases | Carrier proteins (pumps) |
| Substances transported | Gases, small non-polar molecules | Water only | Ions, sugars, amino acids |
| Example | O₂ entering cells | Water absorption in root hairs | Glucose uptake in ileum |
9. Effects on Cells and Tissues | 对细胞和组织的影响
The balance of water movement across membranes is critical for the health of tissues and organs. In animals, the extracellular fluid is regulated by the kidneys to maintain isotonic conditions. If cells lose or gain too much water, their functions are impaired.
跨膜水移动的平衡对组织和器官的健康至关重要。在动物中，细胞外液由肾脏调节以维持等渗条件。如果细胞失去或获得过多的水，其功能就会受损。
In plants, turgor pressure due to osmosis in cells provides mechanical support. When water is scarce, plants lose turgor and wilt. Plasmolysis in plant cells can be observed under a microscope when the cell membrane detaches from the cell wall. This is a useful technique for determining the concentration of cell sap.
在植物中，细胞渗透作用产生的膨胀压提供机械支撑。当水分不足时，植物失去膨胀并枯萎。在显微镜下可以观察到，当细胞膜与细胞壁分离时，植物细胞发生质壁分离。这是测定细胞液浓度的一种有用技术。
Facilitated diffusion is another passive transport type, where specific carrier or channel proteins help larger or charged molecules cross the membrane without energy input. This is important for glucose uptake in red blood cells and ion transport in neurons.
易化扩散是另一种被动运输类型，其中特异性载体蛋白或通道蛋白帮助较大或带电荷的分子无需能量即可穿过膜。这对于红细胞吸收葡萄糖和神经元中的离子运输非常重要。
10. Investigating Osmosis (Practical) | 探究渗透作用（实验）
A common experiment to investigate osmosis uses potato cylinders or visking tubing. For potato cylinders, different concentrations of sucrose solution are used, and the change in mass or length is measured.
一个常见的探究渗透作用的实验使用马铃薯条或纤维素透析袋（Visking管）。对于马铃薯条，使用不同浓度的蔗糖溶液，并测量质量或长度的变化。
Procedure: Cut equal-sized potato cylinders, blot them dry, record initial masses, and place each in a different concentration of sucrose solution (e.g., 0%, 5%, 10%, 20%). After 30 minutes, remove, blot, and record final masses. Calculate the percentage change in mass.
实验步骤：切取等长的马铃薯条，擦干，记录初始质量，然后分别放入不同浓度的蔗糖溶液中（如0%、5%、10%、20%）。30分钟后取出，擦干，记录最终质量。计算质量变化百分比。
Percentage change in mass = (Final mass – Initial mass) / Initial mass × 100%
If the solution is more dilute than the cell sap, the potato gains mass due to water entering by osmosis. If the solution is more concentrated, the potato loses mass. The concentration at which there is no mass change is the approximate concentration of the cell sap.
如果溶液比细胞液稀释，马铃薯会因渗透作用进入的水而质量增加。如果溶液更浓，马铃薯质量减少。质量不发生变化的浓度即为细胞液的近似浓度。
When using visking tubing, a prepared solution is placed inside the tubing, which is then sealed and suspended in a beaker of distilled water or a solution. The mass change over time indicates the direction and rate of osmosis.
使用透析袋时，将配好的溶液放入袋中，封口后悬浮在盛有蒸馏水或溶液的烧杯中。随时间的质量变化可以指示渗透作用的方向和速率。
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