Biological membrane, fluid-mosaic model

Our knowledge of biological membrane ultrastructure has increased considerably over the years as a result of rapid advances in instrumentation. Although there is still controversy over the most correct biological membrane model, the concept of membrane structure presented by Davson and Danielli of a lipid bilayer is perhaps the one best accepted [12,13]. The most current version of that basic model, illustrated in Fig. 7, is referred to as the fluid mosaic model of membrane structure. This model is consistent with what we have learned about the existence of specific ion channels and receptors within and along surface membranes. 

Finally, Singer and Nicolson produced the fluid mosaic model for membrane structure. This model retained the phospholipid bilayer as the basic structure underlying biological membranes and proposed that the bilayer is fluid. Proteins were considered to be suspended in the fluid bilayer as discrete, individual units. The fluid mosaic model is now accepted as an accurate representation of the fundamental structure of biological membranes. 

The fluid mosaic model of membrane structure pictures biological membranes that are composed of lipid bilayers in which proteins are embedded. Membrane lipids contain polar head groups and nonpolar hydrocarbon tails. The hydrocarbon tails of phospholipids are derived from saturated and unsaturated long-chain fatty acids containing an even number of carbon atoms. The lipids and proteins diffuse rapidly in the lipid bilayer but seldom cross from one side to the other. 

Fatty acids have important roles in membrane structure and there are several ways by which they can potentially influence the functions of membrane proteins (and indeed some intracellular proteins). The fluid mosaic model of membrane structure describes biological membranes as dynamic and responsive structures. It is now also recognized that domains exist in membranes, where lipid-protein and lipid-lipid interactions may be highly... 

Most of the phospholipids in the lipid bilayer contain unsaturated fatty adds. Because of the kinks in the carbon chains at the cis double bonds, the phospholipids do not fit closely together. As a result, the lipid bilayer is not a rigid, fixed structure, but one that is dynamic and fluid-like. This liquid-like bilayer also contains proteins, carbohydrates, and cholesterol molecules. For this reason, the model of biological membranes is referred to as the fluid mosaic model of membranes. 

Singer, S. J., 1976, The fluid mosaic model of membrane structure, in The Structure of Biological Membranes (S. Abrahamsson and I. Pascher, eds.), p. 443, Plenum, New York. 

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...

Wisniewska, A., J. Draus, and W. K. Subczynski. 2006. Is fluid mosaic model of biological membranes fully relevant Studies on lipid organization in model and biological membranes. Cell. Mol. Biol. Lett. 8 147-154. 

The fluid mosaic model conveniently describes how the constituent molecules are ordered, and it correctly describes, in first order, some of the membrane s properties. However, it does not give explicit insight into why the biological membrane has a particular structure, and how this depends on the properties of the constituent molecules and the physicochemical conditions surrounding it. For this reason, only qualitative and no quantitative use can be made of this model as it pertains to permeation properties, for example. It is instructive to review the physicochemical principles that are responsible for typical membrane characteristics. In such a survey, it is necessary to discuss simplified cases of self-assembly first, before the complexity of the biological system may be understood. The focus of this quest for principles will therefore be more on the level of the molecular nature of the membrane, rather than viewing a... 

The structure of biological and model membranes is frequently viewed in the context of the fluid mosaic model [4], Since biological membranes are composed of a mixture of various lipids, proteins, and carbohydrates the supra-structure or lateral organization of the components is not necessarily random. In order to model biological membranes, lipid assemblies of increasing complexity were studied. Extensive investigation of multicomponent monolayers (at the air-water interface) as well as bilayers have been reported. 

Membranes are composed of lipids and proteins in varying combinations particular to each species, cell type, and organelle. The fluid mosaic model describes features common to all biological membranes. The lipid bilayer is the basic structural unit. Fatty acyl chains of phospholipids and the steroid nucleus of sterols are oriented toward the interior of the bilayer their hydrophobic interactions stabilize the bilayer but give it flexibility. 

The fluid-mosaic model for biological membranes as envisioned by Singer and Nicolson. Integral membrane proteins are embedded in the lipid bilayer peripheral proteins are attached more loosely to protruding regions of the integral proteins. The proteins are free to diffuse laterally or to rotate about an axis perpendicular to the plane of the membrane. For further information, see S. J. Singer and G. L. Nicolson, The fluid mosaic model of the structure of cell membranes, Science 175 720, 1972. 

The fluid mosaic model is now known to be correct for the structure of biological membranes, in which the membranes are considered as two-dimensional solutions of oriented lipids and globular proteins. 

The Fluid Mosaic Model of Biological Membrane Structure Figure 113 shows the currently accepted fluid mosaic model of biological membrane structure. This model was presented in detail in a review article by S. J. Singer in 1971. In the article, Singer presented the three models of membrane structure that had been proposed by that time ... 

The major discovery in the field of biologic membranes is undoubtedly the finding that the biomembrane is a liquid-crystal-line lipid bilayer with embedded proteins (1-3). This so-called fluid-mosaic model (1) has been the central paradigm in membrane biology for more than three decades and has been very successful in rationalizing a large body of experimental observations. The model includes two references to the lipid... 

Lipids diffuse freely in fluid model membranes. FRAP measurements show full recovery and diffusion coefficients on the order of magnitude of 10 cm /sec. Free diffusion with a similar rate is often observed for lipids in the biomembrane. However, many cell membrane proteins show lower diffusion rates and incomplete recovery after photobleaching. For membrane proteins, dramatically different behavior in model and biological membranes is a common case. In model membranes, membrane proteins also diffuse freely and their diffusion coefficients are often similar to the diffusion coefficients of lipids. On the contrary, in biomembranes, the diffusion of proteins is 2-3 orders of magnitude slower and the fluorescence recovery is often incomplete. This observation points to limitations of the fluid mosaic model as will be discussed below. 

On the basis of the dynamic properties of proteins in membranes, S. Jonathan Singer and Garth Nicolson proposed the concept of a fluid mosaic model for the overall organization of biological membranes in 1972 (Figure 1230). The essence of their model is that membranes are two-dimensional solutions of oriented lipids and globular proteins. The lipid bilayer has a dual role it is both a solvent for integral membrane proteins and a permeability barrier. Membrane proteins are free to diffuse laterally in the lipid matrix unless restricted by special interactions. 

Figure 9.2 shows a diagram of the fluid mosaic model of a biological membrane. 

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