📚 Cell Membranes: Key Points for OCR Biology | 细胞膜：OCR 生物考点精讲
Cell membranes, also known as plasma membranes, form the boundary of every living cell. They are fundamental to life, controlling what enters and leaves the cell, enabling cell communication, and maintaining internal conditions. In OCR A-Level Biology, a deep understanding of the structure and function of cell membranes is essential, as it links to topics such as transport, signalling, and cell division. This article provides an in-depth, exam-focused review of the key membrane concepts you need to master.
细胞膜，又称质膜，构成了每个活细胞的边界。它们对生命至关重要，控制物质进出细胞，实现细胞通讯，并维持内部环境。在 OCR A-Level 生物课程中，深刻理解细胞膜的结构和功能是必要的，因为它与物质运输、信号转导和细胞分裂等主题紧密相连。本文深入且紧扣考点地梳理了你需要掌握的关键膜概念。
1. The Fluid Mosaic Model | 流动镶嵌模型
The currently accepted model of membrane structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972. In this model, the membrane is a dynamic, two-dimensional fluid in which phospholipids and proteins can move laterally within the layer. The term ‘mosaic’ refers to the patchwork of proteins floating in or on the phospholipid bilayer, like tiles in a mosaic.
目前公认的膜结构模型是流动镶嵌模型，由 Singer 和 Nicolson 于 1972 年提出。在该模型中，膜是一种动态的二维流体，磷脂和蛋白质可以在层内横向移动。’镶嵌’一词指的是蛋白质像马赛克瓷砖一样漂浮在磷脂双分子层中或附着于其表面，形成拼图样的分布。
Key evidence for the fluid mosaic model includes freeze-fracture electron microscopy, which splits the membrane along the hydrophobic interior, revealing embedded proteins. Fluorescence recovery after photobleaching (FRAP) also demonstrates that membrane components are mobile, as fluorescently labelled proteins can diffuse back into a bleached area.
支持流动镶嵌模型的关键证据包括冷冻断裂电子显微镜技术，该技术沿疏水内部分裂膜，暴露出嵌入的蛋白质。光漂白荧光恢复技术（FRAP）也证明膜组分是流动的，因为荧光标记的蛋白质可以扩散回被漂白的区域。
2. Phospholipid Bilayer Structure | 磷脂双分子层结构
Phospholipids are the main structural components of the membrane. Each phospholipid molecule consists of a hydrophilic (water-loving) phosphate-containing ‘head’ and two hydrophobic (water-fearing) fatty acid ‘tails’. In an aqueous environment, phospholipids spontaneously form a bilayer: the hydrophilic heads orient outwards towards the water on both sides of the membrane, while the hydrophobic tails shield themselves in the interior.
磷脂是膜的主要结构组分。每个磷脂分子由一个亲水（喜水）的含磷酸基’头部’和两条疏水（惧水）的脂肪酸’尾部’组成。在水性环境中，磷脂自发形成双分子层：亲水头部朝外，面向膜两侧的水环境，而疏水尾部则藏在内部，相互屏蔽。
This arrangement creates a stable barrier that is selectively permeable. Small, non-polar molecules (such as O₂ and CO₂) can dissolve in the hydrophobic core and diffuse across rapidly, whereas large, polar molecules and ions cannot easily pass through without help.
这种排列形成了一层具有选择透过性的稳定屏障。小的非极性分子（如 O₂ 和 CO₂）能溶于疏水核心并快速扩散通过，而大的极性分子和离子则无法轻易单独穿过。
The fatty acid tails can be saturated or unsaturated. Unsaturated fatty acids contain double bonds that introduce kinks, preventing tight packing of the phospholipids. This increases membrane fluidity. Organisms living in cold environments often incorporate more unsaturated fatty acids into their membranes to maintain fluidity.
脂肪酸尾部可以是饱和的或不饱和的。不饱和脂肪酸含有双键，会引入扭结，阻止磷脂紧密排列，从而增加膜流动性。生活在寒冷环境中的生物常在其膜中掺入更多不饱和脂肪酸，以维持流动性。
3. Membrane Proteins: Intrinsic and Extrinsic | 膜蛋白：内在蛋白与外在蛋白
Proteins embedded in or associated with the membrane perform a wide range of essential functions. They are classified as intrinsic (integral) or extrinsic (peripheral). Intrinsic proteins span the entire bilayer or are firmly embedded within it, held by hydrophobic interactions with the fatty acid tails. Many intrinsic proteins are transmembrane, crossing the membrane once or multiple times.
嵌入或结合在膜上的蛋白质行使着多种关键功能。它们分为内在蛋白（整合蛋白）和外在蛋白（外周蛋白）。内在蛋白跨越整个双分子层或深嵌其中，通过与脂肪酸尾部的疏水相互作用固定。许多内在蛋白是跨膜蛋白，可一次或多次穿膜。
Extrinsic proteins are located on the inner or outer surface of the membrane. They are often bound to intrinsic proteins or to the phospholipid heads via hydrogen bonds and ionic interactions. These can be easily removed by changes in pH or salt concentration without disrupting the bilayer.
外在蛋白位于膜的内表面或外表面。它们通常通过氢键和离子相互作用结合到内在蛋白或磷脂头部上。改变 pH 或盐浓度可轻易将其移除，而不破坏双分子层。
Channel proteins and carrier proteins are two crucial types of intrinsic proteins involved in transport. Channel proteins form hydrophilic pores for passive movement of ions or water, while carrier proteins change shape to shuttle specific molecules across the membrane. Other intrinsic proteins act as receptors or enzymes.
通道蛋白和载体蛋白是参与运输的两种关键内在蛋白。通道蛋白形成亲水孔道，供离子或水被动通过，而载体蛋白通过改变形状来携运特定分子过膜。其他内在蛋白则充当受体或酶。
4. Cholesterol and Membrane Fluidity | 胆固醇与膜流动性
Cholesterol is a lipid molecule found in the membranes of animal cells. It is positioned between phospholipid molecules, with its hydrophilic hydroxyl group near the phospholipid heads and its small hydrophobic region interacting with the tails. Cholesterol modulates membrane fluidity, making it more stable against temperature fluctuations.
胆固醇是动物细胞膜中的一种脂类分子。它位于磷脂分子之间，其亲水的羟基靠近磷脂头部，而小的疏水区域与尾部相互作用。胆固醇调节膜的流动性，使其对温度波动更加稳定。
At moderate to high temperatures, cholesterol restrains phospholipid movement, reducing fluidity and preventing excessive leakage. At low temperatures, it disrupts tight packing of phospholipid tails, preventing the membrane from becoming too rigid. This buffering role is vital for cells exposed to varying temperatures.
在中高温下，胆固醇限制磷脂运动，降低流动性并防止过度渗漏。在低温下，它打乱了磷脂尾部的紧密堆积，防止膜变得过于僵硬。这种缓冲作用对暴露于温度变化的细胞至关重要。
5. Glycoproteins and Glycolipids: The Glycocalyx | 糖蛋白与糖脂：糖萼
Glycoproteins and glycolipids are molecules with carbohydrate chains attached to membrane proteins or phospholipids. These carbohydrate groups project exclusively to the exterior surface of the plasma membrane, forming a sugary coating called the glycocalyx.
糖蛋白和糖脂是膜蛋白或磷脂上连接有碳水化合物链的分子。这些碳水化合物基团只伸向细胞膜的外表面，形成一层糖衣，称为糖萼。
The glycocalyx serves several important functions. It protects the cell surface from mechanical and chemical damage, facilitates cell-cell recognition by acting as specific markers (e.g., ABO blood group antigens are glycoproteins/glycolipids), and enables the binding of signalling molecules like hormones. The carbohydrate chains also contribute to the cell’s overall negative charge, which can influence interactions between cells.
糖萼具有多种重要功能。它保护细胞表面免受机械和化学损伤，通过作为特异性标记物促进细胞识别（例如 ABO 血型抗原就是糖蛋白/糖脂），并能使激素等信号分子结合。碳水化合物链还使细胞整体带负电，这会影响细胞间的相互作用。
6. Selective Permeability and Transport Across Membranes | 选择透过性与跨膜运输
Based on its structure, the plasma membrane exhibits selective permeability. Only certain substances can cross the membrane unaided. Non-polar, small molecules like O₂, CO₂, and steroid hormones readily diffuse through the phospholipid bilayer. Very small polar molecules such as water and urea can pass through slowly, while large polar molecules (glucose) and ions (Na⁺, K⁺, Cl⁻) require transport proteins.
基于其结构，细胞膜表现出选择透过性。只有某些物质能独立穿过膜。非极性小分子如 O₂、CO₂ 和类固醇激素能轻易扩散通过磷脂双分子层。很小的极性分子如水和尿素可缓慢通过，而较大的极性分子（葡萄糖）和离子（Na⁺、K⁺、Cl⁻）则需要转运蛋白。
Transport processes are broadly classified as passive, which do not require metabolic energy (ATP), or active, which directly or indirectly use energy. Different transport mechanisms enable cells to maintain an internal environment distinct from the outside.
运输过程大致分为不消耗代谢能（ATP）的被动运输，以及直接或间接利用能量的主动运输。不同的运输机制使细胞能维持与外界截然不同的内部环境。
7. Passive Transport: Diffusion and Facilitated Diffusion | 被动运输：扩散与易化扩散
Simple diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process driven by the kinetic energy of the molecules themselves. For cells, small, non-polar molecules cross the membrane this way directly through the phospholipid bilayer.
简单扩散是分子从高浓度区域向低浓度区域的净移动，沿浓度梯度进行。这是一个被动过程，由分子自身的动能驱动。对细胞而言，小的非极性分子就是以这种方式直接穿过磷脂双分子层的。
Facilitated diffusion also occurs down a concentration gradient but requires the assistance of membrane transport proteins. It is still passive and does not use ATP. Channel proteins provide aqueous pathways for specific ions (e.g., voltage-gated Na⁺ channels) or water (aquaporins). Carrier proteins bind specific solutes, undergo a conformational change, and release the molecule on the other side, as in the case of glucose transporters.
易化扩散也沿浓度梯度进行，但需要膜转运蛋白的协助。它仍是被动的，不消耗 ATP。通道蛋白为特定离子（如电压门控 Na⁺ 通道）或水（水通道蛋白）提供水性通路。载体蛋白结合特定溶质，经历构象变化，将分子释放到另一侧，例如葡萄糖转运蛋白。
8. Osmosis and Water Potential | 渗透作用与水势
Osmosis is the net movement of water molecules through a partially permeable membrane from a region of higher water potential to a region of lower water potential. Water potential (ψ) is a measure of the tendency of water to move, measured in pressure units (kPa). Pure water at atmospheric pressure has a water potential of zero. Adding solutes lowers the water potential (making it more negative).
渗透作用是水分子通过部分透性膜，从水势较高的区域向水势较低的区域的净移动。水势（ψ）是衡量水移动趋势的指标，以压力单位（kPa）表示。大气压下纯水的水势为零。加入溶质会降低水势（使其更负）。
The water potential equation is crucial for OCR exam answers:
ψ = ψₛ + ψₚ
where ψₛ is the solute potential (always negative or zero) and ψₚ is the pressure potential (positive in turgid plant cells, zero in open systems). In animal cells, osmosis can cause lysis or crenation, while in plant cells it results in turgid or plasmolysed states due to the cell wall.
水势方程对 OCR 考试答题至关重要：ψ = ψₛ + ψₚ，其中 ψₛ 为溶质势（始终为负或零），ψₚ 为压力势（在饱满植物细胞中为正，开放体系中为零）。在动物细胞中，渗透作用可导致溶血或皱缩；而在植物细胞中，由于细胞壁的存在，它会引发饱满或质壁分离状态。
9. Active Transport and Bulk Transport | 主动运输与大分子运输
Active transport moves substances against their concentration gradient, from lower to higher concentration, using energy in the form of ATP. This process is mediated by specific carrier proteins, often called pumps. The sodium–potassium pump (Na⁺/K⁺-ATPase) is a classic example: it exports three Na⁺ ions out of the cell and imports two K⁺ ions, directly hydrolysing ATP in the process.
主动运输利用 ATP 形式的能量使物质逆浓度梯度移动，从低浓度到高浓度。这一过程由特定的载体蛋白（常称为泵）介导。钠钾泵（Na⁺/K⁺-ATP 酶）是一个经典例子：它将三个 Na⁺ 离子泵出细胞，同时泵入两个 K⁺ 离子，并在此过程中直接水解 ATP。
Co-transport is a form of secondary active transport where the movement of one substance down its gradient drives the uphill movement of another. For instance, in the small intestine, the Na⁺ gradient established by the Na⁺/K⁺ pump is used to absorb glucose against its gradient via a symporter protein.
协同转运是一种次级主动运输，一种物质顺浓度梯度的运动驱动另一种物质逆梯度向上运动。例如在小肠中，由 Na⁺/K⁺ 泵建立的 Na⁺ 梯度被用来通过同向转运蛋白逆浓度梯度吸收葡萄糖。
Bulk transport mechanisms, endocytosis and exocytosis, allow large particles or volumes of fluid to cross the membrane. These processes involve the formation and fusion of vesicles, requiring ATP. Phagocytosis is the uptake of solid particles, while pinocytosis takes in liquid droplets. Receptor-mediated endocytosis is highly specific, involving clathrin-coated pits.
大分子运输机制——胞吞与胞吐，使大的颗粒或大量液体能够跨膜。这些过程涉及囊泡的形成和融合，需要 ATP。吞噬作用是摄取固体颗粒，而胞饮作用则摄取液滴。受体介导的胞吞高度特异，涉及网格蛋白包被的小凹。
10. Cell Signalling and Membrane Receptors | 细胞信号与膜受体
Cell membranes are vital for communication. Receptor proteins on the cell surface bind specific signalling molecules (ligands), such as hormones or neurotransmitters, and trigger a cascade of events inside the cell. This often involves second messengers like cyclic AMP (cAMP) and a phosphorylation cascade.
细胞膜对于通讯至关重要。细胞表面受体蛋白结合特定的信号分子（配体），如激素或神经递质，并在细胞内引发级联事件。这常涉及第二信使如环腺苷酸（cAMP）和磷酸化级联反应。
For example, glucagon binds to its receptor on liver cell membranes, activating a G-protein that stimulates adenylyl cyclase to produce cAMP. This in turn activates protein kinase A, which phosphorylates enzymes to promote glycogen breakdown and glucose release. The entire pathway depends on the membrane’s selective receptor and compartmentalisation.
例如，胰高血糖素与肝细胞膜上的受体结合，激活 G 蛋白，刺激腺苷酸环化酶产生 cAMP。cAMP 进而激活蛋白激酶 A，该酶磷酸化相关酶，促进糖原分解和葡萄糖释放。整个通路依赖于膜的选择性受体和分区功能。
11. Factors Affecting Membrane Permeability | 影响膜通透性的因素
Several factors can disrupt membrane structure and increase its permeability. High temperature provides kinetic energy to phospholipids, causing them to move more vigorously. Initially, this increases permeability slightly as the bilayer becomes more fluid, but beyond a critical temperature, phospholipid–protein interactions weaken and proteins denature, leaving large gaps. This causes a sharp, irreversible increase in permeability.
多种因素能破坏膜结构，增加其通透性。高温为磷脂提供动能，使其运动加剧。起初，随着双分子层流动性增强，通透性略微增加；但超过某一临界温度后，磷脂与蛋白质的相互作用减弱，蛋白质变性，留下大缺口，导致通透性急剧且不可逆地上升。
Organic solvents such as ethanol dissolve lipids in the membrane, destroying the bilayer and increasing permeability. As the concentration of ethanol increases, more lipids are extracted, and permeability rises. This effect is used in practical investigations to model membrane damage.
有机溶剂如乙醇能溶解膜中的脂类，破坏双分子层，从而增加通透性。随着乙醇浓度升高，更多的脂类被提取，通透性随之上升。该效应被用于探究实验以模拟膜损伤。
12. Investigating Membrane Permeability: Beetroot Practical | 探究膜通透性：甜菜根实验
Beetroot (Beta vulgaris) provides an accessible model for investigating membrane permeability. The vacuole of beetroot cells contains a red pigment, betalain, which cannot normally pass through intact tonoplast and plasma membranes. When membranes are damaged, the pigment leaks out, and the intensity of colour in the surrounding solution can be measured quantitatively using a colorimeter.
甜菜（Beta vulgaris）为探究膜通透性提供了一个易于操作的模型。甜菜细胞的液泡中含有红色色素——甜菜红素，该色素通常不能穿过完整的液泡膜和细胞膜。当膜受损时，色素会泄漏出来，周围溶液的颜色深浅可用比色计进行定量测量。
A typical OCR practical involves cutting uniform beetroot discs, washing away surface pigment, and incubating them in water baths at different temperatures (e.g., 0, 20, 40, 60, 80 °C) for a standardised time. After incubation, the absorbance or percentage transmission of the bathing solution is recorded. A higher absorbance indicates more pigment leakage, thus greater membrane damage.
典型的 OCR 实验包括切取大小均一的甜菜圆片，洗去表面色素，然后将它们在不同温度（如 0, 20, 40, 60, 80 °C）的水浴中温育相同时间。温育后，记录浸泡液的吸光度或透光率百分比。吸光度越高，表明色素泄漏越多，膜损伤越严重。
Similarly, the effect of ethanol concentration can be tested by incubating discs in different ethanol solutions. A key control is using distilled water to establish baseline leakage. Variables must be carefully controlled: beetroot disc size, incubation time, volume of water, and wavelength of the colorimeter. This practical reinforces concepts of variables, data presentation, and drawing conclusions about membrane structure.
类似地，可通过将甜菜圆片孵育在不同乙醇浓度中来测试乙醇的影响。关键对照是使用蒸馏水确定基线泄漏水平。必须仔细控制的变量包括：甜菜圆片大小、温育时间、水体积以及比色计的波长。该实验强化了变量控制、数据呈现和根据膜结构得出结论等概念。
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