Cell Membranes and Transport | 细胞膜与物质运输

📚 Cell Membranes and Transport | 细胞膜与物质运输

Cell membranes are fundamental to life, acting as selective barriers that define the boundaries of cells and organelles. They regulate the movement of substances in and out of the cell, allowing nutrient uptake, waste removal, and the maintenance of internal conditions. Understanding the structure of the plasma membrane and the mechanisms of transport across it is essential for insights into cell physiology, signalling, and homeostasis. This article covers the fluid mosaic model, membrane components, passive and active transport, osmosis, and bulk transport, along with experimental approaches and physiological examples.

细胞膜是生命的基础，作为选择性屏障界定了细胞及细胞器的边界。它们调控物质进出细胞，实现营养摄取、废物排出以及维持内部环境。理解质膜的结构及其跨膜运输机制对于认识细胞生理、信号传导和稳态至关重要。本文涵盖液态镶嵌模型、膜组成、被动与主动运输、渗透作用、批量运输，以及实验方法和生理实例。

1. The Fluid Mosaic Model | 液态镶嵌模型

The currently accepted model of cell membrane structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972. This model describes the membrane as a fluid phospholipid bilayer in which proteins are embedded or associated, forming a mosaic pattern. The fluidity allows lipids and proteins to move laterally within the layer, enabling membrane flexibility and the self-repair of small tears. The model also includes cholesterol molecules in animal cell membranes, which modulate fluidity, and carbohydrate chains attached to proteins or lipids on the extracellular side, playing roles in cell recognition and adhesion.

目前公认的细胞膜结构模型是 Singer 和 Nicolson 于 1972 年提出的液态镶嵌模型。该模型将膜描述为流动的磷脂双分子层，其中镶嵌或结合着蛋白质，形成镶嵌图案。流动性使脂质和蛋白质能在层内横向移动，从而赋予膜柔韧性并能自我修复微小破裂。模型中还包括动物细胞膜中的胆固醇分子，它们调节流动性；以及连接在胞外侧蛋白质或脂质上的糖链，在细胞识别和黏附中发挥作用。

2. Phospholipid Bilayer and Membrane Components | 磷脂双分子层与膜成分

The core of the membrane is the phospholipid bilayer. Each phospholipid molecule has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. This amphipathic nature drives the spontaneous formation of a bilayer in aqueous environments, with heads facing outward and tails buried inside. The hydrophobic interior acts as a barrier to ions and most polar molecules, while allowing non-polar substances to diffuse through. In addition to phospholipids, membranes contain cholesterol (in animal cells), proteins, and glycolipids/glycoproteins, each with specific functions.

膜的核心是磷脂双分子层。每个磷脂分子有一个亲水（喜水）的磷酸头部和两条疏水（畏水）的脂肪酸尾部。这种两亲性促使在水性环境中自发形成双分子层，头部朝外而尾部埋藏在内。疏水内部构成离子和大多数极性分子的屏障，同时允许非极性物质扩散通过。除磷脂外，膜中还含有胆固醇（动物细胞）、蛋白质以及糖脂/糖蛋白，各自具有特定功能。

The combination of these components results in a selectively permeable membrane that is both dynamic and robust. The exact composition varies between membranes of different organelles and cell types, reflecting their specific roles.

这些组分的结合形成了一种具有选择通透性的膜，既动态又稳固。不同细胞器和细胞类型的膜在精确组成上有所差异，这反映出它们各自的特定作用。

3. Membrane Proteins: Intrinsic and Extrinsic | 膜蛋白：内在蛋白与外在蛋白

Integral (intrinsic) proteins are firmly embedded within the phospholipid bilayer. Many are transmembrane proteins that span the entire membrane, exposing regions on both the cytoplasmic and extracellular sides. They often have hydrophobic amino acid side chains interacting with the lipid core and hydrophilic regions protruding into aqueous environments. Examples include channel proteins, carrier proteins, and pumps. Peripheral (extrinsic) proteins are not embedded in the hydrophobic core; they are attached to the surface of the membrane, often by binding to integral proteins or head groups of phospholipids. They can be easily removed by changes in pH or ionic strength and include spectrin in red blood cells and some enzymes.

整合蛋白（内在蛋白）牢固地嵌于磷脂双分子层内。许多为跨膜蛋白，贯穿整个膜，在细胞质侧和胞外侧均暴露出区域。它们常有疏水氨基酸侧链与脂质核心相互作用，亲水区域则伸出至水相环境中。例子包括通道蛋白、载体蛋白和泵。外周蛋白（外在蛋白）并不嵌在疏水核心内；它们附着在膜表面，通常与整合蛋白或磷脂头部结合。改变pH或离子强度可轻易将其移除，红细胞中的血影蛋白以及某些酶即属此类。

4. Functions of Membrane Proteins | 膜蛋白的功能

Membrane proteins perform a wide range of functions essential for cell survival. Transport proteins – channels and carriers – facilitate the movement of specific substances across the membrane. Some act as enzymes, catalysing reactions such as those in the electron transport chain on the inner mitochondrial membrane. Receptor proteins bind extracellular signalling molecules (ligands) and trigger intracellular responses. Cell adhesion molecules (CAMs) enable cells to attach to one another and to the extracellular matrix. Antigens, such as the ABO blood group glycoproteins, serve as identification markers. Finally, some proteins anchor the cytoskeleton, maintaining cell shape and stability.

膜蛋白行使着细胞生存所必需的多种功能。运输蛋白——通道和载体——协助特定物质跨膜移动。有些充当酶，催化线粒体内膜上电子传递链等反应。受体蛋白结合胞外信号分子（配体）并触发胞内应答。细胞黏附分子（CAMs）使细胞彼此以及与细胞外基质相连接。抗原，如ABO血型糖蛋白，充当身份标记。最后，有些蛋白锚定细胞骨架，维持细胞形态和稳定性。

• Transport: channel proteins and carrier proteins / 运输: 通道蛋白和载体蛋白
• Enzymatic activity: ATP synthase / 酶活性: ATP合酶
• Signal transduction: G-protein-coupled receptors / 信号转导: G蛋白偶联受体
• Cell-cell recognition: MHC molecules, blood group antigens / 细胞识别: MHC分子、血型抗原
• Intercellular joining: tight junctions, desmosomes / 细胞间连接: 紧密连接、桥粒
• Attachment: integrins linking to cytoskeleton and ECM / 附着: 整合素连接细胞骨架与细胞外基质

5. Passive Transport: Simple Diffusion | 被动运输：简单扩散

Simple diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, without the expenditure of metabolic energy. This process is driven solely by the kinetic energy of the particles. Small, non-polar molecules such as oxygen (O₂) and carbon dioxide (CO₂) can diffuse directly through the phospholipid bilayer, as can small uncharged polar molecules like urea and water (to some extent). The rate of diffusion is described by Fick’s law, which identifies the main factors governing how fast a substance can cross a membrane.

简单扩散是指分子或离子从高浓度区域向低浓度区域净移动，沿浓度梯度进行，不消耗代谢能量。该过程完全由粒子的动能驱动。小的非极性分子如氧气（O₂）和二氧化碳（CO₂）能直接通过磷脂双分子层扩散，小的不带电极性分子如尿素和水（在一定程度上）也能如此。扩散速率由菲克定律描述，该定律明确了支配物质跨膜速率的主要因素。

Rate of diffusion ∝ (Surface area × Concentration gradient) / Distance of diffusion

扩散速率 ∝ (表面积 × 浓度梯度) / 扩散距离

6. Facilitated Diffusion: Channel and Carrier Proteins | 易化扩散：通道蛋白和载体蛋白

Facilitated diffusion allows the passage of polar or charged molecules, such as glucose, amino acids, and ions, that cannot freely cross the hydrophobic core. This is still passive transport, as the movement occurs down the concentration gradient without ATP consumption. Two types of transport protein mediate facilitated diffusion: channel proteins and carrier proteins. Channel proteins form hydrophilic pores that allow specific ions or water molecules to flow through. Many are gated, opening or closing in response to signals like voltage changes (voltage-gated) or ligand binding (ligand-gated). Carrier proteins, on the other hand, bind the specific solute on one side, undergo a conformational change, and release the solute on the other side. The glucose transporter GLUT1 is a classic example of a carrier protein.

易化扩散使极性或带电分子（如葡萄糖、氨基酸、离子）得以通过疏水核心，这些分子不能自由穿过。这仍然是被动运输，因为运动沿浓度梯度进行，不消耗ATP。两类运输蛋白介导易化扩散：通道蛋白和载体蛋白。通道蛋白形成亲水孔道，允特定离子或水分子流过。许多通道是门控的，响应电压变化（电压门控）或配体结合（配体门控）等信号而开启或关闭。载体蛋白则在膜一侧结合特定溶质，发生构象变化，并在另一侧释放溶质。葡萄糖转运蛋白GLUT1是载体蛋白的经典例子。