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The birth of the fluid mosaic model: How did Simon and Nicholson subvert the understanding of membrane structure?
In the field of biology, the structure of cell membranes has always been a hot issue studied by scientists. In 1972, Seymour Jonathan Singer and Garth Nicholson made a major discovery, the fluid mosaic model, which overturned people's traditional understanding of cell membranes. The proposal of this model not only explains the composition of the cell membrane, but also lays a solid foundation for further research.
The fluid mosaic model describes that the cell membrane is composed of a bilayer of lipids, which is mainly composed of hydrophilic phospholipid molecules. Within this layer of lipids, various types of proteins are embedded, giving the cell membrane its flexibility and elasticity. The core idea of this model is that the cell membrane is a two-dimensional liquid with embedded proteins randomly distributed on the membrane surface.
Predictions from the fluid mosaic model suggest that the long-distance distribution of any integrin across the plane of the membrane is nearly random.
Chemical composition and experimental evidence
Singer and Nicholson's fluid mosaic model has gained widespread support. The formation of this model relies on a large amount of experimental data, including labeling experiments, X-ray diffraction and calorimetry. These studies demonstrate that the diffusion rate of integral membrane proteins embedded in membranes is affected by the viscosity of the lipid bilayer and emphasize the dynamic nature of molecules in cell membranes.
Before the emergence of the fluid mosaic model, existing models such as the Robertson unit membrane model and the Davson-Danieli three-layer model failed to fully explain the dynamics of the cell membrane. These older models typically viewed the protein as a monolayer adjacent to the lipid layer and did not integrate it into the phospholipid bilayer.
Subsequent development and membrane asymmetry
With the deepening of research, scientists have discovered that the double layer of the cell membrane is not symmetrical, but has obvious asymmetry. This asymmetry allows the two sides of the membrane to contain different proteins and lipids, thereby supporting the spatial segregation of membrane-related biological processes. Cholesterol and cholesterol-interacting proteins can concentrate in lipid rafts, thereby limiting the transmission of cell signals.
In 1984, Mourides and Bloom proposed the "mattress model" to further explore the interaction between lipids and proteins.
Membrane curvature and lipid movement
In fact, the structure of the cell membrane is not always flat. The local curvature of membranes is often affected by asymmetry and non-bilayer lipid organization. The famous BAR domain can bind phosphatidylinositol, assist in vesicle formation, organelle formation and cell division, and plays an important role in the development of membrane curvature.
In the 1970s, scientists first recognized that individual lipid molecules diffuse freely laterally inside each layer of a membrane. The speed of this process is very fast. On average, each lipid molecule can diffuse about 2 microns in about 1 second. These dynamic processes have profound effects on the fluidity and function of cell membranes.
Restrictions, and lipid rafts and protein complexes
However, there are limits to the lateral diffusion of lipids and proteins in membranes, which are mainly caused by the structural effects of the membrane region. Lipid rafts are membrane nanoplatforms composed of specific lipids and proteins and have important biological functions.
Proteins and glycoproteins in the cell membrane do not exist independently, but run in the membrane as diffusion complexes, which have an important functional impact on cell transport and signal transduction.
Future exploration and thinking
The proposal of the fluid mosaic model has undoubtedly deepened our understanding of the structure of cell membranes. However, with the advancement of science and technology, more biophysical phenomena such as protein-lipid interactions still need to be studied in depth. In the future, will we be able to unlock all the mysteries of the cell membrane and further reveal its importance in biology?
