Singer-Nicolson model

L., Matyus, L., Waldmann, T.A., Damjanovich, S. (2003) Dynamic, yet structured the cell membrane three decades after the Singer-Nicolson model. Proc. Natl. Acad. Sci. USA 100, 8053-8058. 

G. Vereb, J. Szollosi, J. Matko, P. Nagy, T. Farkas, L. Vigh, L. Matyus, T.A. Waldmann, S. Damjanovich, Dynamic, Yet Structured The Cell Membrane Three Decades After the Singer-Nicolson Model , Proc. Nad. Acad. Sci. USA, 100,8053 (2003)... 

Figure 1 Singer-Nicolson model of fluid membrane. (From Reference 2.)...

The Singer-Nicolson model of the membrane played a very important role in understanding membrane structure and function. However, many properties of biomembranes are not consistent with this model. In recent years, a growing consensus points at more complex membrane structure, which can be characterized as dynamically structured fluid mosaic. Compared with the original fluid mosaic model, the emphasis has shifted from fluidity to mosaicity. Experimental observations have led to the membrane microdomain concept that describes compartmen-talization/organization of membrane components into stable or transient domains. 

Because the assumption of simple Brownian diffusion breaks down, the diffusion in biomembranes cannot be described by a single diffusion coefficient. For instance, FRAP experiments in the plasma membrane showed that the observed translational diffusion rates depend on the size of the initial photobleached spot, which is also inconsistent with a simple Singer-Nicolson model. 

To reconcile this apparent contradiction the membrane skeleton fence and anchored transmembrane picket model was proposed (54). According to this model, transmembrane proteins anchored to and lined up along the membrane skeleton (fence) effectively act as a row of posts for the fence against the free diffusion of lipids (Fig. 11). This model is consistent with the observation that the hop rate of transmembrane proteins increases after the partial removal of the cytoplasmic domain of transmembrane proteins, but it is not affected by the removal of the major fraction of the extracellular domains of transmembrane proteins or extracellular matrix. Within the compartment borders, membrane molecules undergo simple Brownian diffusion. In a sense, the Singer-Nicolson model is adequate for dimensions of about 10 x lOnm, the special scale of the original cartoon depicted by the authors in 1972. However, beyond such distances simple extensions of the fluid mosaic model fail and a substantial paradigm shift is required from a two-dimensional continuum fluid to the compartmentalized fluid. 

Looking at the Singer-Nicolson model (1) biomembranes mainly consist of lipids and proteins, the latter being either partially (peripheral proteins) or completely (integral proteins) embedded into the lipid matrix. These proteins contribute a major part to the stability of natural membranes (2). The use of these proteins, however, to stabilize an artificial membrane system via incorporation is rather limited and not yet realizable. The stabiliztation of the lipid matrix itself, therefore, appears to be a more convenient method also providing a greater potential use. 

Goni FM. The basic structure and dynamics of ceU membranes an update of the Singer-Nicolson model. Biochim Biophys Acta. 2014 1838(6) 1467-1476. 

Fig. 2.3. Diagrammatic representation of the molecular organisation of the tegument plasma membrane (based on the fluid mosaic model of membrane structure of Singer Nicolson (1972)). The carbohydrate moieties of the membrane glycoproteins and glycolipids are exposed on the external face as the glycocalyx. (After Smyth Halton, 1983.)...

Figure 2 The Singer and Nicolson model for cell membiane...

In 1972, S. J. Singer and G. L. Nicolson proposed the fluid mosaic model for membrane structure, which suggested that membranes are dynamic structures composed of proteins and phospholipids. In this model, the phospholipid bilayer is a fluid matrix, in essence, a two-dimensional solvent for proteins. Both lipids and proteins are capable of rotational and lateral movement. 

Singer, S. J., and Nicolson, G. L., 1972. The fluid mosaic model of the structure of cell membranes. Science 175 720-731. 

The first membrane model to be widely accepted was that proposed by Danielli and Davson in 1935 [528]. On the basis of the observation that proteins could be adsorbed to oil droplets obtained from mackerel eggs and other research, the two scientists at University College in London proposed the sandwich of lipids model (Fig. 7.2), where a bilayer is covered on both sides by a layer of protein. The model underwent revisions over the years, as more was learned from electron microscopic and X-ray diffraction studies. It was eventually replaced in the 1970s by the current model of the membrane, known as the fluid mosaic model, proposed by Singer and Nicolson [529,530]. In the new model (Fig. 7.3), the lipid bilayer was retained, but the proteins were proposed to be globular and to freely float within the lipid bilayer, some spanning the entire bilayer. 

A detailed justification of the surfactant parameter approach is still the subject of theoretical investigations, and we will return to several issues below. We mention that the surfactant parameter approach is consistent with the fluid mosaic model of Singer and Nicolson. It tells us that the self-assembly of amphiphiles is driven by the strong segregation of water and hydrocarbon chains, and that packing effects dominate the self-assembly process. 

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