Biomembranes Singer-Nicolson model

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. 

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. 

The ability of lipid molecules to associate in an aqueous environment into infinite lipid bilayers is the basic mechanism behind the formation of biomembranes and the ability to separate compartments of functional units by such membranes was an important step in the development of organized life. The significance of the ordered but fluid state of lipid molecules, particularly for the interaction with membrane proteins, was revealed by studies of lipid-water systems during the 1960s (see Chapman etal., 1967). This knowledge was summarized (Singer and Nicolson, 1972) in the well-known fluid-mosaic model of the cell membrane. 

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