📚 Structure of Cell Membranes | 细胞膜的结构
All cells are surrounded by a partially permeable plasma membrane that controls exchange between the cell and its environment. Membrane structure also underpins the function of organelles such as mitochondria, chloroplasts and the endoplasmic reticulum. Cambridge A-Level Biology requires understanding of the fluid mosaic model, the component molecules of membranes, and how membrane structure relates to functions such as transport and cell signalling.
所有细胞都被一层部分通透的质膜包围，这层膜控制着细胞与外界环境之间的物质交换。膜结构也为线粒体、叶绿体和内质网等细胞器的功能提供了基础。剑桥 A-Level 生物学要求理解流动镶嵌模型、膜的组成分子，以及膜结构如何与物质运输和细胞信号传导等功能相关联。
1. The Phospholipid Bilayer | 磷脂双分子层
The core of every biological membrane is a bilayer of phospholipid molecules. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails. In an aqueous environment, phospholipids spontaneously arrange themselves into a bilayer so that the hydrophilic heads face the aqueous cytoplasm and the external environment, while the hydrophobic tails form a non-aqueous core.
所有生物膜的核心都是磷脂双分子层。每个磷脂分子都有一个亲水的磷酸头部和两条疏水的脂肪酸尾部。在水环境中，磷脂自发排列成双分子层，亲水头部朝向含水的细胞质和外界环境，而疏水尾部则形成非水性的核心。
This arrangement does not require energy because it allows the hydrophobic tails to avoid water while the hydrophilic heads interact with water. The bilayer forms a stable barrier that separates two aqueous compartments and is also able to reseal if torn.
这种排列不需要能量，因为疏水尾部避开水，而亲水头部与水相互作用。双分子层形成一道稳定的屏障，将两个水相区域分隔开，并且如果被撕裂还能够自行重新封闭。
2. Amphipathic Nature of Phospholipids | 磷脂的两亲性
Phospholipids are described as amphipathic because they contain both hydrophilic and hydrophobic regions within the same molecule. The phosphate head is polar and attracted to water, while the fatty acid tails are non-polar and repelled by water.
磷脂被称为两亲性分子，因为它们在同一个分子中同时含有亲水区和疏水区。磷酸头部是极性的，被水吸引；脂肪酸尾部是非极性的，被水排斥。
This dual property explains why phospholipids form bilayers, micelles or liposomes in water. It also explains the self-sealing behaviour of membranes and their selective permeability, since the hydrophobic interior acts as a barrier to most water-soluble substances.
这种双重性质解释了为什么磷脂在水中会形成双分子层、微团或脂质体。它也解释了膜的自我封闭行为以及选择透过性，因为疏水内部对大多数水溶性物质构成屏障。
3. The Fluid Mosaic Model | 流动镶嵌模型
The fluid mosaic model was proposed by Singer and Nicolson in 1972. It describes membranes as a fluid phospholipid bilayer in which proteins are embedded or attached. The term ‘fluid’ refers to the lateral movement of lipids and many proteins, while ‘mosaic’ refers to the patchwork of different proteins scattered within the bilayer.
流动镶嵌模型由 Singer 和 Nicolson 于 1972 年提出。它将膜描述为流动的磷脂双分子层，蛋白质镶嵌或附着于其中。”流动” 指脂质和许多蛋白质可以侧向移动，”镶嵌” 指不同蛋白质在双分子层中形成的拼图状分布。
According to this model, membrane components are not fixed in place. Phospholipids can move sideways within their own leaflet, and many proteins drift laterally. Some proteins are anchored to the cytoskeleton, which restricts their movement and helps maintain cell shape.
根据这一模型，膜的组成成分并不是固定不动的。磷脂可以在其所在的单层内侧向移动，许多蛋白质也能横向漂移。一些蛋白质锚定在细胞骨架上，这限制了它们的运动并有助于维持细胞形状。
The fluid mosaic model replaced earlier ‘sandwich’ models that placed proteins only on the outside of a continuous lipid layer. Those models could not explain membrane fluidity or the movement of proteins observed experimentally.
流动镶嵌模型取代了早期将蛋白质仅置于连续脂质层外侧的 “三明治” 模型。那些模型无法解释实验观察到的膜流动性和蛋白质运动。
4. Membrane Proteins | 膜蛋白
Membrane proteins are classified as integral or peripheral. Integral proteins are embedded within the phospholipid bilayer. Some span the entire bilayer and are called transmembrane proteins; channel proteins and carrier proteins are examples. Peripheral proteins are bound to the membrane surface, often attached to integral proteins or to phospholipid heads by non-covalent interactions.
膜蛋白分为内在蛋白和外周蛋白。内在蛋白嵌入磷脂双分子层内部。有些贯穿整个双分子层，称为跨膜蛋白；通道蛋白和载体蛋白就是例子。外周蛋白结合在膜表面，通常通过非共价相互作用附着于内在蛋白或磷脂头部。
The regions of transmembrane proteins that pass through the hydrophobic core are mainly composed of non-polar amino acids. Their hydrophobic side chains interact with the fatty acid tails of phospholipids, which stabilises the protein within the membrane.
跨膜蛋白穿过疏水核心的区域主要由非极性氨基酸组成。它们的疏水侧链与磷脂的脂肪酸尾部相互作用，从而使蛋白质在膜内保持稳定。
5. Functions of Membrane Proteins | 膜蛋白的功能
Transport proteins allow specific ions and molecules to cross the bilayer. Channel proteins form hydrophilic pores, while carrier proteins change shape to move substances across. Receptor proteins bind hormones and other signalling molecules, triggering intracellular responses. Enzymes in membranes catalyse reactions such as those in electron transport chains. Cell adhesion proteins link cells together, and recognition glycoproteins allow the immune system to distinguish self from non-self cells.
转运蛋白允许特定的离子和分子穿过双分子层。通道蛋白形成亲水孔道，载体蛋白则通过改变形状来运输物质。受体蛋白结合激素和其他信号分子，触发细胞内反应。膜上的酶催化电子传递链等反应。细胞粘附蛋白将细胞连接在一起，识别糖蛋白使免疫系统能够区分自身和非自身细胞。
A single plasma membrane may contain hundreds of different proteins. This variety gives each cell type unique permeability, recognition and signalling properties, allowing cells to specialise for different functions in tissues and organs.
一个质膜可能含有数百种不同的蛋白质。这种多样性赋予每种细胞类型独特的通透性、识别和信号传导特性，使细胞能够在组织和器官中特化出不同的功能。
6. Cholesterol in Animal Membranes | 胆固醇在动物膜中的作用
Cholesterol is a lipid found in animal cell membranes. It inserts between phospholipid molecules, with its hydrophilic hydroxyl group positioned near the phosphate heads and its hydrophobic ring structure lying among the fatty acid tails. Cholesterol acts as a fluidity buffer: at low temperatures it prevents phospholipid tails from packing too closely, and at high temperatures it restricts excessive movement.
胆固醇是存在于动物细胞膜中的一种脂质。它插入磷脂分子之间，其亲水的羟基靠近磷酸头部，疏水的环状结构位于脂肪酸尾部之间。胆固醇充当流动性的缓冲剂：低温时防止磷脂尾部排列过于紧密，高温时限制过度运动。
Cholesterol also increases the mechanical stability of the membrane and reduces its permeability to small water-soluble molecules. Plant cell membranes contain related sterols, while prokaryotic membranes usually lack cholesterol. The presence of cholesterol is therefore an important factor in the fluidity and integrity of animal membranes.
胆固醇还增加膜的机械稳定性，并降低膜对小水溶性分子的通透性。植物细胞膜含有相关的固醇类物质，而原核生物膜通常不含胆固醇。因此，胆固醇的存在是影响动物膜流动性和完整性的重要因素。
7. Glycolipids and Glycoproteins | 糖脂与糖蛋白
Glycolipids and glycoproteins are molecules with short carbohydrate chains attached to lipids or proteins. They are found mainly on the outer surface of the plasma membrane, where their carbohydrate groups extend into the extracellular space and form a layer called the glycocalyx.
糖脂和糖蛋白是带有短糖链的脂质或蛋白质分子。它们主要位于质膜的外表面，其糖基伸向细胞外空间，形成称为糖萼的一层结构。
These carbohydrate chains act as recognition sites, protect the cell surface from mechanical and chemical damage, and enable cell-cell adhesion. Blood group antigens are examples of membrane glycolipids and glycoproteins that differ between individuals and are recognised by the immune system.
这些糖链充当识别位点，保护细胞表面免受机械和化学损伤，并介导细胞间粘附。血型抗原就是膜糖脂和糖蛋白的例子，它们在个体之间存在差异并被免疫系统识别。
8. Membrane Asymmetry | 膜的不对称性
The two leaflets of a biological membrane have different lipid and protein compositions. Glycolipids and glycoproteins are usually restricted to the outer leaflet, while certain phospholipids such as phosphatidylserine are more common in the inner leaflet facing the cytoplasm.
生物膜的两个单层具有不同的脂质和蛋白质组成。糖脂和糖蛋白通常只分布在外层，而某些磷脂（如磷脂酰丝氨酸）更多分布在内层，即面向细胞质的一侧。
This asymmetry is functionally important. For example, the appearance of phosphatidylserine on the outer surface can signal a cell for apoptosis. Membrane asymmetry is maintained by enzymes that move specific lipids between leaflets only when required, rather than by random diffusion.
这种不对称性在功能上很重要。例如，磷脂酰丝氨酸出现在外表面可以标记细胞进行凋亡。膜的不对称性由酶来维持，这些酶仅在需要时将特定脂质在两个单层之间移动，而不是通过随机扩散。
9. Factors Affecting Membrane Fluidity | 影响膜流动性的因素
Membrane fluidity depends on temperature, fatty acid composition and cholesterol content. Unsaturated fatty acids contain one or more double bonds, which introduce kinks in the hydrocarbon chains. These kinks prevent tight packing and therefore increase fluidity. Saturated fatty acids are straight, pack closely together and decrease fluidity.
膜流动性取决于温度、脂肪酸组成和胆固醇含量。不饱和脂肪酸含有一个或多个双键，使烃链产生弯曲。这些弯曲阻止了紧密堆积，因此增加流动性。饱和脂肪酸呈直链，排列紧密，降低流动性。
Shorter fatty acid tails also increase fluidity because they form fewer intermolecular interactions. As temperature rises, membranes become more fluid, and excessive fluidity can make them leaky. Organisms can adjust the saturation and length of fatty acids in their membranes to maintain appropriate fluidity across environmental temperature changes.
较短的脂肪酸尾部也增加流动性，因为它们形成的分子间相互作用较少。随着温度升高，膜变得更流动，过度流动会使膜发生渗漏。生物体可以调节膜中脂肪酸的饱和度和长度，以在环境温度变化时保持适当的流动性。
10. Selective Permeability | 选择透过性
The phospholipid bilayer is selectively permeable. Small non-polar molecules such as O₂ and CO₂ diffuse freely through the hydrophobic core. Water and urea cross slowly, while ions such as Na⁺, K⁺ and Cl⁻ and large polar molecules such as glucose cannot cross the bilayer without the help of transport proteins.
磷脂双分子层具有选择透过性。小的非极性分子（如 O₂ 和 CO₂）可以自由穿过疏水核心。水和尿素穿过较慢，而离子（如 Na⁺、K⁺ 和 Cl⁻）以及葡萄糖等大的极性分子不能在没有转运蛋白帮助的情况下穿过双分子层。
This selective permeability allows cells to maintain internal conditions that differ from the external environment. For example, the plasma membrane keeps many solutes inside the cell while excluding harmful substances, and it enables the formation of ion gradients that are essential for nerve impulse transmission and ATP synthesis.
这种选择透过性使细胞能够维持与外界环境不同的内部条件。例如，质膜将许多溶质保留在细胞内部，同时阻挡有害物质，并且能够形成离子梯度，这对神经冲动的传递和 ATP 的合成至关重要。
11. Membrane Receptors and Cell Signalling | 膜受体与细胞信号
Receptor proteins in the membrane allow cells to detect and respond to extracellular signals. When a signalling molecule such as a hormone binds to its specific receptor, the receptor changes shape. This conformational change activates intracellular signalling pathways, often involving G proteins, enzymes or second messengers such as cyclic AMP.
膜中的受体蛋白使细胞能够检测并响应细胞外信号。当激素等信号分子与其特异性受体结合时，受体发生构象变化。这种构象变化激活细胞内信号通路，通常涉及 G 蛋白、酶或环磷酸腺苷等第二信使。
Glycoproteins and glycolipids often act as recognition sites in this process, enabling the specific binding of signalling molecules. Membrane receptors therefore link extracellular events to cytoplasmic responses, allowing cells to respond to their environment and coordinate activities within tissues.
糖蛋白和糖脂在此过程中通常充当识别位点，使信号分子能够特异性结合。因此，膜受体将细胞外事件与细胞质反应联系起来，使细胞能够对环境作出响应并协调组织内的活动。
12. Evidence for the Fluid Mosaic Model | 流动镶嵌模型的证据
Freeze-fracture electron microscopy splits membranes along the hydrophobic core and reveals embedded proteins as particles on the exposed faces. Fluorescence recovery after photobleaching (FRAP) shows that labelled proteins and lipids can diffuse laterally within the membrane after a region has been bleached.
冷冻断裂电子显微镜沿疏水核心将膜劈开，显示嵌入的蛋白质在暴露面上呈颗粒状。荧光漂白恢复实验（FRAP）表明，在某个区域被漂白后，标记的蛋白质和脂质可以在膜内横向扩散。
These observations support the idea that membranes are fluid structures with proteins embedded in a lipid bilayer. The model also explains how membranes can fuse, change shape during endocytosis and exocytosis, and allow the movement of protein complexes within the plane of the membrane.
这些观察支持膜是由嵌在脂质双分子层中的蛋白质组成的流动结构这一观点。该模型还解释了膜如何融合、在胞吞和胞吐过程中改变形状，以及蛋白质复合物如何在膜平面内移动。
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