📚 Structure of Membranes | 膜的结构
Cell membranes are fundamental components of all living cells, acting as dynamic barriers that maintain internal conditions and mediate interactions with the external environment. The currently accepted model of membrane structure is the fluid mosaic model, which describes the membrane as a fluid bilayer of phospholipids with embedded proteins, cholesterol, and carbohydrates. Understanding the structure of membranes is essential for explaining key cellular processes such as transport, signaling, and recognition.
细胞膜是所有活细胞的基本组成成分，扮演着动态屏障的角色，维持细胞内部条件并介导与外部环境的相互作用。目前公认的膜结构模型是流动镶嵌模型，该模型将膜描述为由磷脂组成的流动双层结构，其中嵌有蛋白质、胆固醇和碳水化合物。理解膜的结构对于解释运输、信号传导和识别等关键细胞过程至关重要。
1. Introduction to Cell Membranes | 细胞膜简介
All prokaryotic and eukaryotic cells are bounded by a plasma membrane, a thin structure typically 7–8 nm thick. This membrane is not a simple envelope but a highly organized molecular assembly that defines the cell’s boundary.
所有原核和真核细胞都由质膜包裹，质膜是一种通常厚度为 7–8 nm 的薄层结构。这层膜并非简单的包膜，而是一个高度组织化的分子组合，界定了细胞的边界。
Membranes serve multiple functions: they act as a selectively permeable barrier, compartmentalize cellular activities, and provide sites for the attachment of enzymes and signaling molecules.
膜具有多种功能：它们充当选择性通透屏障，分隔细胞活动，并为酶和信号分子的附着提供位点。
In eukaryotic cells, internal membranes also form organelles such as the nucleus, mitochondria, and endoplasmic reticulum, reflecting the same basic bilayer structure that characterises the plasma membrane.
在真核细胞中，内膜也形成诸如细胞核、线粒体和内质网等细胞器，均体现与质膜相同的双层基本结构。
2. The Fluid Mosaic Model | 流动镶嵌模型
The fluid mosaic model, proposed by Singer and Nicolson in 1972, is the widely accepted description of membrane structure. The term ‘fluid’ refers to the lipid bilayer in which molecules can move laterally, while ‘mosaic’ describes the patchwork of proteins floating in or on the fluid bilayer.
流动镶嵌模型由 Singer 和 Nicolson 于 1972 年提出，是目前被广泛接受的膜结构描述。“流动”一词指脂质双分子层中的分子可以侧向移动，而“镶嵌”则描述了漂浮在流动双层中或其上的蛋白质拼凑体。
Evidence from freeze-fracture electron microscopy and fluorescence labeling supports this model, showing that proteins are not fixed in place but are mobile within the plane of the membrane, constantly shifting to meet the cell’s functional needs.
来自冷冻断裂电子显微镜和荧光标记的证据支持这一模型，表明蛋白质并非固定不动，而是可以在膜的平面内移动，不断调整以满足细胞的功能需求。
3. Phospholipid Bilayer: Structure and Properties | 磷脂双分子层：结构与特性
The fundamental scaffold of all biological membranes is a bilayer of phospholipids. Each phospholipid molecule consists of a hydrophilic (water-loving) head and two hydrophobic (water-hating) fatty acid tails.
所有生物膜的基本骨架是磷脂双分子层。每个磷脂分子由一个亲水性（喜水）头部和两条疏水性（憎水）脂肪酸尾部组成。
The head contains a phosphate group and glycerol, which are polar or charged, allowing interaction with water. The tails are long hydrocarbon chains that are nonpolar and therefore avoid contact with aqueous environments.
头部含有磷酸基团和甘油，它们呈极性或带电，可与水相互作用。尾部是长的烃链，为非极性，因此会避免与水环境接触。
In an aqueous environment, phospholipids spontaneously arrange into a bilayer: the hydrophilic heads face outward toward the water on both sides, while the hydrophobic tails face inward, shielded from water. This self-assembly is driven by hydrophobic interactions and is crucial for membrane stability.
在水溶液环境中，磷脂自发排列成双分子层：亲水头部朝外，面向两侧的水；疏水尾部朝内，避开水分。这种自组装由疏水相互作用驱动，对膜的稳定性至关重要。
Phospholipids can vary in tail length and saturation; unsaturated fatty acids with kinks (due to cis double bonds) increase membrane fluidity because the chains cannot pack as closely together.
磷脂的尾部长度和饱和程度各异；带有扭结（由于顺式双键）的不饱和脂肪酸可增加膜的流动性，因为链不能紧密堆积在一起。
4. Membrane Proteins: Integral and Peripheral | 膜蛋白：整合蛋白与外周蛋白
Proteins are essential components of membranes, accounting for about 50% of the mass of most plasma membranes. There are two main categories: integral (intrinsic) proteins and peripheral (extrinsic) proteins.
蛋白质是膜的重要组分，约占大多数质膜质量的 50%。主要分为两类：整合（内在）蛋白和外周（外在）蛋白。
Integral proteins are firmly embedded within the phospholipid bilayer; many span the entire membrane (transmembrane proteins) and have hydrophobic regions that interact with the lipid tails. Their structure typically includes alpha-helical domains that crisscross the membrane.
整合蛋白牢固地嵌入磷脂双分子层中；许多跨越整个膜（跨膜蛋白），并具有与脂质尾部相互作用的疏水区域。它们的结构通常包括贯穿膜层的 α 螺旋结构域。
Peripheral proteins are attached to the surface of the membrane, often bound to integral proteins or to the polar head groups of phospholipids, and they can be removed without disrupting the bilayer. These proteins often play regulatory or structural roles on the cytoplasmic side.
外周蛋白附着在膜的表面，通常结合于整合蛋白或磷脂的极性头部基团，可以不破坏双层而将其移除。这些蛋白质通常在胞质侧发挥调节或结构作用。
Membrane proteins serve diverse functions: transport (channels, carriers), enzymatic activity, signal transduction, cell-cell recognition, and attachment to the cytoskeleton or extracellular matrix. For instance, channel proteins form hydrophilic pores for ions such as Na⁺, K⁺, and Ca²⁺, while carrier proteins undergo conformational changes to transport specific molecules like glucose.
膜蛋白具有多种功能：运输（通道、载体）、酶活性、信号转导、细胞间识别以及与细胞骨架或细胞外基质的附着。例如，通道蛋白形成亲水性孔道供 Na⁺、K⁺ 和 Ca²⁺ 等离子通过，而载体蛋白经历构象变化以转运特定分子如葡萄糖。
5. Cholesterol in Animal Cell Membranes | 动物细胞膜中的胆固醇
Cholesterol is a lipid molecule found in the membranes of animal cells, positioned between phospholipid molecules within the bilayer. It has a small polar hydroxyl group that interacts with the phospholipid heads, while its rigid steroid ring structure and hydrocarbon tail insert among the fatty acid tails.
胆固醇是一种脂质分子，存在于动物细胞膜中，位于双分子层内磷脂分子之间。它有一个小的极性羟基，与磷脂头部相互作用，而其刚性的甾环结构和烃尾部则插入脂肪酸尾部之间。
Cholesterol modulates membrane fluidity: at high temperatures, it restrains excessive movement of lipids, reducing fluidity; at low temperatures, it prevents tight packing of fatty acid tails, maintaining fluidity and preventing solidification. Thus, cholesterol acts as a fluidity buffer.
胆固醇调节膜的流动性：在高温下，它限制脂质的过度运动，降低流动性；在低温下，它阻止脂肪酸尾部紧密堆积，维持流动性并防止凝固。因此，胆固醇充当流动性缓冲剂。
Plant cell membranes contain related sterols but generally have less cholesterol; bacterial membranes typically lack sterols, using other mechanisms such as branched-chain fatty acids to adjust fluidity.
植物细胞膜含有相关的甾醇，但通常胆固醇较少；细菌膜通常缺乏甾醇，利用支链脂肪酸等其他机制调节流动性。
6. Glycolipids and Glycoproteins: The Glycocalyx | 糖脂与糖蛋白：糖萼
Glycolipids and glycoproteins are molecules with carbohydrate chains attached to lipids or proteins, respectively, on the extracellular surface of the membrane. These carbohydrate chains extend outward, forming a fuzzy layer called the glycocalyx.
糖脂和糖蛋白是分别在膜的外表面附有碳水化合物链的分子，碳水化合物分别连接在脂质或蛋白质上。这些碳水化合物链向外延伸，形成称为糖萼的模糊层。
The glycocalyx is involved in cell protection, lubrication, and recognition. Specific sugar sequences on the glycocalyx act as recognition sites for hormones, antibodies, and other cells; for example, the ABO blood group antigens are glycolipids with different carbohydrate terminal sugars.
糖萼参与细胞保护、润滑和识别。糖萼上特定的糖序列充当激素、抗体和其他细胞的识别位点；例如，ABO 血型抗原就是具有不同碳水化合物末端糖的糖脂。
Glycoproteins also play key roles in the immune system, allowing white blood cells to distinguish self from non-self. The pattern of glycosylation can change in disease states and influences how cells interact with their environment.
糖蛋白也在免疫系统中发挥关键作用，使白细胞能够区分自身与异己。糖基化模式在疾病状态下会发生改变，并影响细胞与环境的相互作用。
7. Fluidity of the Membrane | 膜的流动性
Membrane fluidity is a critical property that allows lateral diffusion of lipids and proteins, enabling membrane
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