Fluid mosaic model, biological membrane structure

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... 

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. 

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. 

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. 

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 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 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 ... 

Upon contact with the microbial cells, the diluent molecules dissolve in the cytoplasmic membrane and cause its swelling [19]. This leads to disruption of the transmembrane potential, pH gradient across the cytoplasmic membrane, and it ultimately leads to disruption of the ion-active pumps [20]. Fluid-mosaic model has been widely accepted as the best description of the real-time structure of biological membranes, and it states that the cytoplasmic membrane exists in a fluid state under physiological conditions [21]. 

Most of the properties attributed to living organisms (e.g., movement, growth, reproduction, and metabolism) depend, either directly or indirectly, on membranes. All biological membranes have the same general structure. As previously mentioned (Chapter 2), membranes contain lipid and protein molecules. In the currently accepted concept of membranes, referred to as the fluid mosaic model, membrane is a bimolecular lipid layer (lipid bilayer). The proteins, most of which float within the lipid bilayer, largely determine a membrane s biological functions. Because of the importance of membranes in biochemical processes, the remainder of Chapter 11 is devoted to a discussion of their structure and functions. 

We have seen that hiological memhranes have hoth lipid and protein componen ts. How do these two parts comhine to produce a hiological membrane Currently, the fluid-mosaic model is the most widely accepted description of hiological memhranes. The term mosaic imphes that the two components exist side hy side without forming some other substance of intermediate nature. The basic structure of biological membranes is that of the lipid bilayer, with the proteins embedded in the bUayer structure (Figure 8.18). 

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