Models of Cell Membrane Structure | 细胞膜的结构模型

📚 Models of Cell Membrane Structure | 细胞膜的结构模型

Cell membranes are fundamental to life, controlling what enters and leaves every cell. Understanding how scientists developed structural models of the membrane is a core requirement of the CIE A-Level Biology syllabus, and forms the basis for understanding transport, cell signalling and immunity.

细胞膜是生命的基础，控制着每个细胞与外界之间的物质交换。理解科学家如何逐步构建细胞膜结构模型，是 CIE A-Level 生物考纲的核心要求，也是理解物质运输、细胞信号传递和免疫机制的基础。

1. Why Do We Need Models? | 为什么需要模型？

Models are simplified representations of reality. Because membranes are only about 7-10 nm thick, they cannot be seen clearly even with a light microscope, so scientists used indirect evidence – from chemical analysis, permeability studies and electron microscopy – to propose successive models that were refined as new data emerged.

模型是对现实的简化表达。由于细胞膜厚度仅有约 7-10 nm，即使使用光学显微镜也无法直接看清其结构，因此科学家只能借助化学分析、通透性实验和电子显微镜等间接证据，不断提出新模型，并随着新数据的出现而逐步修正旧模型。

Each new model did not appear from nowhere; it was built upon the strengths of previous models and addressed their weaknesses. In examinations, you are often asked to trace this historical development and to explain why each new model replaced the old one.

每一个新模型都不是凭空出现的；它建立在旧模型优点的基础上，并针对旧模型的缺陷进行修正。在考试中，你常被要求梳理这一历史发展脉络，并解释为何新模型取代了旧模型。

2. Overton’s Lipid Theory | 奥弗顿的脂质学说

In the 1890s, Charles Overton noticed that substances with high lipid solubility entered cells much faster than water-soluble substances of similar size. For example, lipid-soluble dyes and anaesthetics penetrate cells almost instantly, whereas sugars and inorganic ions enter far more slowly.

19 世纪 90 年代，查尔斯·奥弗顿发现脂溶性强的物质进入细胞的速度远快于大小相近但水溶性的物质。例如，脂溶性染料和麻醉剂几乎能瞬间穿透细胞，而糖类和无机离子进入细胞则要缓慢得多。

Overton concluded that the cell surface must contain a layer of lipid, because lipid-soluble molecules could dissolve through this lipid barrier, while water-soluble molecules could not cross it easily. This was the first major clue about the chemical nature of the cell membrane.

奥弗顿由此推断，细胞表面必定含有一层脂质，因为脂溶性分子能够溶解并穿过这层脂质屏障，而水溶性分子则难以通过。这是关于细胞膜化学本质的第一个重要线索。

• Key evidence: lipid-soluble molecules cross rapidly | 关键证据：脂溶性分子穿越迅速

• Conclusion: cell membrane has a lipid component | 结论：细胞膜含有脂质成分

• Limitation: no information about how lipids are arranged | 局限：未提供脂质如何排布的信息

3. Gorter and Grendel’s Bilayer Discovery | 戈特与格伦德尔的磷脂双分子层发现

In 1925, Evert Gorter and François Grendel extracted lipids from red blood cell membranes using acetone. They then spread the extracted lipids as a monolayer on a water surface in a Langmuir trough, and measured the area occupied by the monolayer.

1925 年，戈特和格伦德尔用丙酮从红细胞膜中提取脂质，随后将提取出的脂质在朗缪尔槽的水面上铺展成单分子层，并测量了该单分子层所占的面积。

They found that the area of the lipid monolayer was approximately twice the total surface area of the original red blood cells. Since red blood cells have no nuclei or internal membranes, all the lipids extracted came from the plasma membrane alone, making the calculation reliable.

他们发现该脂质单分子层的面积约为原来红细胞总表面积的两倍。由于红细胞没有细胞核和内部膜结构，提取出的全部脂质均来自质膜本身，使这一计算十分可靠。

Area of lipid monolayer ≈ 2 × Area of cell membrane

脂质单分子层面积 ≈ 2 × 细胞膜面积

This led to the conclusion that the membrane consists of a double layer of phospholipids, with hydrophobic tails facing inward and hydrophilic heads facing outward, in contact with aqueous environments on both sides.

这一结果使科学家得出结论：细胞膜由两层磷脂分子构成，疏水尾部朝向内侧相对排列，亲水头部朝向外侧与水环境接触。

4. The Danielli-Davson Sandwich Model | 丹尼利-戴维森三明治模型

In 1935, James Danielli and Hugh Davson proposed that the lipid bilayer was coated on both sides with a layer of globular proteins, forming a ‘protein-lipid-protein’ sandwich. This model suggested the membrane was relatively rigid and uniform, with protein-lined pores to allow small molecules through.

1935 年，丹尼利和戴维森提出脂质双分子层的两侧各覆盖着一层球状蛋白质，形成“蛋白质-脂质-蛋白质”的三明治结构。该模型认为膜相对坚硬且均匀，并含有由蛋白质衬里的孔道允许小分子通过。

This model was attractive because it explained the low surface tension of membranes (proteins reduce surface tension) and was consistent with early electron micrographs showing dark lines at the edges of membranes. However, it left many functional questions unanswered.

该模型之所以受到青睐，是因为它能解释膜表面张力较低的现象（蛋白质可降低表面张力），并且与早期电镜照片中膜边缘呈现的深色线条相符。然而，它在功能层面留下了许多未解之谜。

5. Robertson’s Unit Membrane Model | 罗伯逊单位膜模型

In the 1950s, J. David Robertson used high-resolution electron microscopy on osmium-stained membranes and observed a characteristic trilaminar pattern: two dark lines with a lighter line between them, giving a total thickness of around 7.5 nm.

20 世纪 50 年代，罗伯逊利用锇酸染色的高分辨率电子显微镜观察膜，发现了特征性的三层结构：两条深色线夹着一条浅色线，总厚度约 7.5 nm。

Robertson interpreted the dark lines as layers of protein and the light line as the lipid tails, naming this the ‘unit membrane model’. He proposed that all biological membranes, regardless of source, share this same basic structure.

罗伯逊将深色线

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