📚 Cell Membranes and Transport | 细胞膜与物质运输
Cell membranes are fundamental to all living cells because they control the movement of substances into and out of the cell. In the Cambridge A-Level Biology syllabus, you need to explain membrane structure using the fluid mosaic model and link that structure to transport processes such as diffusion, osmosis, active transport, endocytosis and exocytosis. This article covers the key ideas, common exam applications and practical investigations to help you build accurate, exam-ready explanations.
细胞膜对所有活细胞都至关重要，因为它控制着物质进出细胞。在剑桥 A-Level 生物课程中，你需要用流动镶嵌模型解释膜的结构，并把结构与扩散、渗透、主动运输、胞吞和胞吐等运输过程联系起来。本文涵盖关键概念、常见考试应用和实验探究，帮助你建立准确、适合应考的解释。
1. The Fluid Mosaic Model | 流动镶嵌模型
The fluid mosaic model describes the cell membrane as a dynamic structure made of a phospholipid bilayer with proteins, cholesterol, glycolipids and glycoproteins embedded or attached. The term ‘fluid’ means that phospholipids and many proteins can move laterally within the layer, while ‘mosaic’ refers to the patchwork of different proteins scattered throughout the bilayer.
流动镶嵌模型将细胞膜描述为由磷脂双分子层以及嵌入或附着其中的蛋白质、胆固醇、糖脂和糖蛋白组成的动态结构。“流动”是指磷脂和许多蛋白质可以在膜层内横向移动，“镶嵌”则是指不同蛋白质在双分子层中分散分布，形成拼图状结构。
Evidence for this model includes freeze-fracture electron micrographs, which show proteins embedded within the bilayer, and cell fusion experiments in which fluorescently labelled membrane proteins mix after two cells fuse. Both observations support the idea that membrane components are not fixed in place.
支持该模型的证据包括冷冻断裂电子显微照片，它们显示蛋白质嵌入双分子层内部；还有细胞融合实验，在两种细胞融合后，荧光标记的膜蛋白会相互混合。这些观察结果都支持膜成分并非固定不动的观点。
2. Phospholipid Bilayer Structure | 磷脂双分子层结构
Phospholipids are amphipathic molecules: they have a hydrophilic phosphate head that faces the aqueous environments inside and outside the cell, and two hydrophobic fatty acid tails that point inward, away from water. This arrangement forms a stable bilayer that acts as a selective barrier.
磷脂是双亲性分子：它们有一个亲水的磷酸头，朝向细胞内外两侧的水环境；还有两条疏水的脂肪酸尾，朝内避开水分。这种排列方式形成了稳定的双分子层，起到选择性屏障的作用。
The hydrophobic core blocks the free passage of ions and most polar molecules, while small non-polar molecules such as O₂ and CO₂ can diffuse directly through the bilayer. The phospholipid bilayer also gives the membrane flexibility and the ability to self-seal.
疏水核心阻止离子和大多数极性分子自由通过，而氧气和二氧化碳等小型非极性分子可以直接穿过双分子层扩散。磷脂双分子层还赋予膜柔韧性以及自我封闭的能力。
3. Membrane Proteins | 膜蛋白
Membrane proteins are classified as integral proteins, which span the whole bilayer, or peripheral proteins, which are bound to the inner or outer surface. Integral proteins include channel proteins and carrier proteins, both of which are involved in transport.
膜蛋白分为贯穿整个双分子层的整合蛋白，以及结合在膜内表面或外表面的外周蛋白。整合蛋白包括通道蛋白和载体蛋白，二者都参与物质运输。
| Type 类型 | Function 功能 |
|---|---|
| Channel protein 通道蛋白 | Forms a hydrophilic pore for facilitated diffusion 形成亲水孔道，用于易化扩散 |
| Carrier protein 载体蛋白 | Changes shape to transport molecules in facilitated diffusion or active transport 改变构象，在易化扩散或主动运输中转运分子 |
| Receptor protein 受体蛋白 | Binds signalling molecules and triggers a cellular response 结合信号分子并引发细胞反应 |
| Enzyme 酶 | Catalyses reactions at the membrane surface 在膜表面催化反应 |
| Adhesion protein 黏附蛋白 | Helps cells attach to each other or to the extracellular matrix 帮助细胞彼此黏附或黏附于细胞外基质 |
Exam questions often ask you to link the three-dimensional shape of a carrier protein to its specificity. Because the binding site is complementary to a specific solute, carrier proteins can transport only certain molecules or ions.
考试题目经常要求你把载体蛋白的三维形状与其特异性联系起来。由于结合位点与特定溶质互补，载体蛋白只能运输特定的分子或离子。
4. Cholesterol and Membrane Fluidity | 胆固醇与膜流动性
Cholesterol is a lipid found in animal cell membranes. It fits between phospholipid molecules, where it regulates membrane fluidity and mechanical stability. At high temperatures, cholesterol reduces excessive fluidity by restricting phospholipid movement; at low temperatures, it prevents phospholipids from packing too closely together, so the membrane remains fluid.
胆固醇是动物细胞膜中存在的一种脂质。它嵌在磷脂分子之间，调节膜的流动性和机械稳定性。在高温下，胆固醇通过限制磷脂运动来降低过度流动性；在低温下，它防止磷脂过于紧密地排列，因此膜仍能保持流动性。
Cholesterol also makes the membrane less permeable to very small water-soluble ions and helps maintain the shape of animal cells. Plant cell membranes contain related sterols, but A-Level answers should state that cholesterol is characteristic of animal membranes.
胆固醇还降低了膜对极小的水溶性离子的通透性，并有助于维持动物细胞的形状。植物细胞膜含有相关的甾醇，但在 A-Level 答案中应说明胆固醇是动物细胞膜的特征成分。
5. Glycolipids and Glycoproteins | 糖脂与糖蛋白
Glycolipids are membrane lipids with short carbohydrate chains attached, while glycoproteins are membrane proteins with carbohydrate chains attached. Together they form a carbohydrate-rich outer layer called the glycocalyx.
糖脂是带有短糖链的膜脂，糖蛋白是带有糖链的膜蛋白。它们共同在细胞膜外侧形成富含糖类的覆盖层，称为糖萼。
These molecules are important in cell recognition, cell adhesion and immune responses. For example, the ABO blood group antigens on red blood cells are glycoproteins and glycolipids, and receptor glycoproteins allow hormones or drugs to bind specifically to target cells.
这些分子在细胞识别、细胞黏附和免疫反应中起重要作用。例如，红细胞表面的 ABO 血型抗原就是糖蛋白和糖脂，而受体糖蛋白则使激素或药物能够特异性地结合靶细胞。
6. Passive Transport: Diffusion and Facilitated Diffusion | 被动运输：扩散与易化扩散
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient. It is a passive process, so it does not require metabolic energy in the form of ATP.
扩散是粒子从高浓度区域向低浓度区域的净移动，即沿浓度梯度进行。这是一种被动过程，因此不需要以 ATP 形式提供代谢能量。
Small non-polar molecules such as O₂ and CO₂ diffuse directly through the phospholipid bilayer. Larger or charged particles, such as glucose and ions, require facilitated diffusion through channel proteins or carrier proteins. Facilitated diffusion is still passive, but it depends on specific membrane proteins and shows saturation at high solute concentrations.
氧气和二氧化碳等小型非极性分子可直接通过磷脂双分子层扩散。葡萄糖和离子等较大或带电荷的粒子则需要通过通道蛋白或载体蛋白进行易化扩散。易化扩散仍然是被动的，但它依赖特定的膜蛋白，并且在高溶质浓度下会出现饱和现象。
Rate of diffusion ∝ (surface area × concentration gradient) ÷ diffusion distance
扩散速率 ∝ (表面积 × 浓度梯度) ÷ 扩散距离
This relationship helps explain why exchange surfaces such as alveoli and root hairs have large surface areas, thin membranes and steep concentration gradients maintained by ventilation or active transport.
这一关系有助于解释为什么肺泡和根毛等交换表面具有较大的表面积、较薄的膜以及通过通气或主动运输维持的陡峭浓度梯度。
7. Osmosis and Water Potential | 渗透作用与水势
Osmosis is the diffusion of water molecules through a selectively permeable membrane from a solution with higher water potential to a solution with lower water potential. Water potential is the pressure exerted by water molecules and is affected by solute potential and pressure potential.
渗透作用是水分子通过选择性透过膜，从水势较高的溶液向水势较低的溶液扩散的过程。水势是水分子产生的压力，受溶质势和压力势的影响。
Ψ = Ψₛ + Ψₚ
Where Ψ is water potential, Ψₛ is solute potential and Ψₚ is pressure potential. Adding solute lowers Ψₛ, making the water potential more negative because the solute reduces the free movement of water molecules.
式中 Ψ 是水势，Ψₛ 是溶质势，Ψₚ 是压力势。加入溶质会降低 Ψₛ，使水势变得更负，因为溶质减少了水分子的自由运动。
In an animal cell placed in a hypotonic solution, water enters by osmosis and the cell may burst, whereas in a hypertonic solution it shrinks. A plant cell in a hypotonic solution becomes turgid because the cell wall exerts pressure potential and prevents bursting.
动物细胞置于低渗溶液时，水通过渗透作用进入细胞，细胞可能胀破；而在高渗溶液中细胞会皱缩。植物细胞在低渗溶液中会变得硬挺，因为细胞壁产生压力势，阻止细胞破裂。
8. Active Transport | 主动运输
Active transport is the movement of molecules or ions against their concentration gradient, from a region of lower concentration to a region of higher concentration. This process requires energy in the form of ATP and uses specific carrier proteins.
主动运输是分子或离子逆浓度梯度移动的过程，即从低浓度区域向高浓度区域运输。该过程需要以 ATP 形式提供能量，并使用特定的载体蛋白。
A key example is the sodium-potassium pump in animal cells, which moves Na⁺ out of the cell and K⁺ into the cell against their gradients. In plant root hair cells, active transport moves nitrate ions from the soil into the cell, even when the concentration inside the cell is already higher.
一个典型例子是动物细胞中的钠钾泵，它逆浓度梯度将 Na⁺ 泵出细胞，并将 K⁺ 泵入细胞。在植物根毛细胞中，主动运输将硝酸根离子从土壤运入细胞，即使细胞内的浓度已经更高。
- Active transport depends on carrier protein shape change powered by ATP hydrolysis.
- 主动运输依赖载体蛋白在 ATP 水解驱动下发生构象变化。
- Inhibitors of respiration, such as cyanide, reduce ATP production and therefore inhibit active transport.
- 呼吸作用抑制剂（如氰化物）会减少 ATP 生成，从而抑制主动运输。
9. Bulk Transport: Endocytosis and Exocytosis | 大分子运输：胞吞与胞吐
Large molecules or particles are transported across the membrane by bulk transport mechanisms. Endocytosis is the uptake of material into the cell by infolding of the membrane to form a vesicle, while exocytosis is the release of material from the cell when a vesicle fuses with the membrane.
大分子或颗粒通过大分子运输机制跨膜转运。胞吞是指细胞膜向内凹陷形成囊泡，将物质摄入细胞；胞吐则是指囊泡与细胞膜融合，将物质释放到细胞外。
Phagocytosis is a form of endocytosis used by white blood cells to engulf bacteria, while pinocytosis takes in extracellular fluid. Exocytosis is used to secrete digestive enzymes, hormones and neurotransmitters or to export cell wall materials.
吞噬是白细胞吞噬细菌时使用的一种胞吞形式，胞饮则摄入细胞外液。胞吐用于分泌消化酶、激素和神经递质，或输出细胞壁物质。
Both endocytosis and exocytosis require ATP because the membrane must move and fuse, so they are active processes. They also demonstrate the fluidity of the membrane described by the fluid mosaic model.
胞吞和胞吐都需要 ATP，因为膜必须发生移动和融合，所以它们是主动过程。这些过程也体现了流动镶嵌模型所描述的膜流动性。
10. Factors Affecting Membrane Transport | 影响膜运输的因素
Several factors affect the rate of movement across cell membranes. Temperature increases the kinetic energy of molecules, so diffusion and facilitated diffusion generally speed up as temperature rises, until membrane proteins denature or the membrane becomes too leaky.
多种因素会影响物质跨膜运输的速率。温度升高会增加分子的动能，因此扩散和易化扩散通常会加快，直到膜蛋白变性或膜结构破坏而导致通透性过度增加。
| Factor 因素 | Effect 影响 |
|---|---|
| Concentration gradient 浓度梯度 | Steeper gradient increases diffusion rate 梯度越陡，扩散速率越快 |
| Surface area 表面积 | Greater area increases transport 表面积越大，运输越快 |
| Diffusion distance 扩散距离 | Thicker membrane slows diffusion 膜越厚，扩散越慢 |
| Temperature 温度 | Kinetic energy increases, but high heat disrupts proteins 动能增加，但高温会破坏蛋白质 |
| pH and solvents pH 和溶剂 | Extreme pH or organic solvents denature membrane proteins and dissolve lipids 极端的 pH 或有机溶剂会使膜蛋白变性并溶解脂质 |
For active transport, any factor that reduces ATP production, such as low oxygen or respiratory inhibitors, will lower the transport rate. For osmosis, the difference in water potential between the two sides is the main driving force.
对于主动运输，任何减少 ATP 产生的因素，如低氧或呼吸抑制剂，都会降低运输速率。对于渗透作用，膜两侧的水势差是主要驱动力。
11. Investigating Membrane Permeability | 膜通透性探究
A common Cambridge practical uses beetroot cells to investigate membrane permeability. Beetroot vacuoles contain a red pigment called betalain, which leaks out when the cell membrane and tonoplast are damaged. The intensity of pigment released can be measured using a colorimeter.
剑桥考试中常见的实验使用甜菜根细胞来探究膜的通透性。甜菜根液泡中含有一种红色色素，称为甜菜红素。当细胞膜和液泡膜受损时，色素会泄漏出来。释放出的色素强度可以用比色计测量。
In a typical investigation, identical beetroot discs are placed in water baths at different temperatures for a set time. Higher temperatures increase membrane permeability because proteins denature and phospholipids gain kinetic energy; the surrounding solution becomes darker red, giving a higher absorbance reading.
在典型探究中，将相同的甜菜根圆片放入不同温度的水浴中处理相同时间。较高温度会增加膜通透性，因为蛋白质变性且磷脂动能增加；周围溶液颜色变得更深红，因此吸光度读数更高。
- Control variables include beetroot age, disc size, washing method, incubation time and solvent volume.
- 控制变量包括甜菜根的年龄、圆片大小、洗涤方法、孵育时间和溶剂体积。
- Organic solvents such as ethanol also increase pigment release because they dissolve membrane lipids.
- 乙醇等有机溶剂也会增加色素释放，因为它们会溶解膜脂。
12. Exam Tips for Membrane Questions | 膜相关考题应试技巧
When answering membrane transport questions, always use precise terminology: say ‘net movement down a concentration gradient’ for diffusion, and state that facilitated diffusion uses channel or carrier proteins but no ATP. For active transport, mention ‘against the concentration gradient’ and ‘using ATP’.
回答膜运输问题时，始终使用准确术语：描述扩散时要说“沿浓度梯度净移动”，并说明易化扩散使用通道蛋白或载体蛋白，但不消耗 ATP。对于主动运输，要提到“逆浓度梯度”和“使用 ATP”。
Avoid common errors such as saying that water moves from low to high concentration during osmosis. Instead, refer to water potential: water moves from a region of higher water potential to a region of lower water potential. Also avoid saying that the membrane is static or that all proteins move freely.
避免常见错误，例如说渗透作用中水从低浓度向高浓度移动。应改为提及水势：水从水势较高区域移向水势较低区域。还要避免说膜是静止的，或说所有蛋白质都能自由移动。
For practical questions, comment on reliability and validity: repeat readings, use a colorimeter rather than subjective colour judgement, and control temperature accurately. Linking the observable colour change to membrane damage caused by protein denaturation and lipid dissolution will earn higher marks.
对于实验题，要评价可靠性和有效性：重复读数、使用比色计而不是主观颜色判断，并准确控制温度。把观察到的颜色变化与蛋白质变性和脂质溶解导致的膜损伤联系起来，能获得更高分数。
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