Singer-Nicholson model

Our present ideas about the nature of biological membranes, which are so fundamental to all biochemical processes, are based on the Singer-Nicholson mosaic model. This model of the membrane is based on a phospholipid bilayer that is, however, asymmetrical. In the outside monolayer, phosphatidylcholine (lecithin) predominates, whereas the inner monolayer on the cytoplasmic side is rich in a mixture of phos-phatidylethanolamine, phosphatidylserine, and phosphatidylinositol. Cholesterol molecules are also inserted into the bilayer, with their 3-hydroxyl group pointed toward the aqueous side. The hydrophobic fatty acid tails and the steran skeleton of cholesterol... 

Two important hypotheses which describe interactions between proteins and hydrophobic systems are the fluid mosaic model for membrane structure proposed by Singer and Nicholson (60) and the amphipathic apolipoprotein structure proposed by Segreste/ al. (56). In Singer s model for membrane structure shown in Fig. 1, it is proposed that membrane proteins float in a sea of lipid. The nonpolar regions of the protein are in contact with the nonpolar lipids present in the membrane, whereas the polar surface is exposed to the aqueous environment. 

Membrane proteins in many cases are randomly distributed through the plane of the membrane. This was one of the corollaries of the fluid mosaic model of Singer and Nicholson and has been experimentally verified using electron microscopy. Electron micrographs show that integral membrane proteins are often randomly distributed in the membrane, with no apparent long-range order. 

Fig. 6.9 Characteristic structures of biological membranes. (A) The fluid mosaic model (S. J. Singer and G. L. Nicholson) where the phospholipid component is predominant. (B) The mitochondrial membrane where the proteins prevail over the phospholipids...

The Singer and Nicholson (13) model for the plasma membrane, which now receives much support, is basically a smectic liquid crystal consisting of one bilayer of phospholipid (Figure 4a). The phospholipid bilayer contains cholesterol at a concentration which depends on cell type. Embedded in the lipid liquid crystal he protein molecules. Some of these protein molecules transverse the entire lipid bilayer and communicate both with the inside and the outside of the cells. Some of these may... 

Figure 4. Structure of the Singer and Nicholson fluid mosaic model for the plasma membrane...

Singer, S.J. and Nicholson, G.L. (1972). The fluid mosaic model of the structure of cell membranes. Science, New York 175,720-731. 

Fig. 1. The lipid-globular protein fluid mosaic model of membrane structure. The solid bodies represent globular proteins, some of which span the phospholipid bilayer represented by the open circles. Adapted from Singer and Nicholson (1972).

The membranes of cells are largely composed on of a phospholipid bilayer and proteins. Most of our current information concerning biological membranes is summarized by the fluid mosaic model proposed by S. J. Singer and G. L. Nicholson in 1972. This is the model depicted in Figure 10.10. 

To describe the behavior of the lipids within the plane of the bilayer, Forsiind [53] has adapted the model originally developed by Singer and Nicholson [54] to describe behavior of lipids and proteins in cellular membranes. Accordingly, the "Domain Mosaic model [53] postulates that the bulk of the stratum comeum lipids segregate into crystalline/ gel domains that are characterized by low permeability. Bordering these domains are highly... 

Subsequent observations such as the known rapid lateral diffusion of lipid and protein in the plane of the membrane and knowledge that proteins are often inserted into and through the lipid matrix have been added to the points made above. These considerations have been allowed for in the proposal by Singer and Nicholson in 1972 of their fluid mosaic model for membrane structure (Figure 6.10). In the diagram an asymmetric lipid bilayer forms the basis of the membrane structure with proteins spanning the membrane or embedded into the hydrophobic core region. 

The basic structure of membranes is usually envisaged as the fluid-mosaic model of Singer and Nicholson. In this, amphiphilic lipids are arranged in a bimolecular layer with the hydrophobic moieties in the centre of the membrane and the hydrophilic head-groups at the two surfaces. Stability is achieved by various ionic or hydrophobic interactions between the lipids and the membrane proteins. The latter can be embedded within the membrane (intrinsic proteins) or attached to the surface (extrinsic proteins). Many membrane proteins are large enough to actually span the membrane. 

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