Cell membranes fluid-mosaic model

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

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

Fig. 7 Diagrammatic representation of the fluid mosaic model of the cell membrane. The basic structure of the membrane is that of a lipid bilayer in which the lipid portion (long tails) points inward and the polar portion (round head ) points outward. The membrane is penenetrated by transmembrane (or integral) proteins. Attached to the surface of the membrane are peripheral proteins (inner surface) and carbohydrates that bind to lipid and protein molecules (outer surface). (Modified from Ref. 14.)...

Figure 23 Representation of a cell membrane according to the fluid mosaic model (Singer, 1974). In this model, the aqueous phospholipid interfacial microdomain separates the water compartment from the apolar membrane interior. [Redrawn from Burton et al. (1992) with permission from the publisher.]...

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. 

Figure 8.15 is a sketch of one possible relationship between the lipid bilayer and the membrane proteins. Molecules are free to move laterally in these membranes hence the structure pictured in Figure 8.15 is called the fluid mosaic model of a cell membrane. 

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. 

Membranes of plant and animal cells are typically composed of 40-50 % lipids and 50-60% proteins. There are wide variations in the types of lipids and proteins as well as in their ratios. Arrangements of lipids and proteins in membranes are best considered in terms of the fluid-mosaic model, proposed by Singer and Nicolson % According to this model, the matrix of the membrane (a lipid bilayer composed of phospholipids and glycolipids) incorporates proteins, either on the surface or in the interior, and acts as permeability barrier (Fig. 2). Furthermore, other cellular functions such as recognition, fusion, endocytosis, intercellular interaction, transport, and osmosis are all membrane mediated processes. 

Cel Membrane. The fluid mosaic model of the cell membrane is one in which the phospholipids provide the basic order and integrity of the cell through amphiphilic interaction with the aqueous environment. 

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 cell membranes are predominantly a lipid matrix or can be considered a lipid barrier with an average width of a membrane being approximately 75 A. The membrane is described as the fluid mosaic model (Figure 6.2) which consist of (1) a bilayer of phospholipids with hydrocarbons oriented inward (hydrophobic phase), (2) hydrophilic heads oriented outward (hydrophilic phase), and (3) associated intra- and extracellular proteins and transverse the membrane. The ratio of lipid to protein varies from 5 1 for the myelin membrane to 1 5 for the inner structure of the mitochondria. However, 100% of the myelin membrane surface is lipid bilayer, whereas the inner membrane of the mitochondria may have only 40% lipid bilayer surface. In this example the proportion of membrane surface that is lipid will clearly influence distribution of toxicants of varying lipophilicity. 

Perhaps the most important step in the development of our current understanding of biomembranes was the introduction of the fluid mosaic model [28] (Figure 1.4) [40]. This model describes the cell membrane as a fluid two-dimensional lipid bilayer matrix of about 50 A thickness with its associated proteins. It allows for the lateral diffusion of both lipids and proteins in the plane of the membrane [41] but contains little structural detail. This model has been further developed and it has been assumed that the membrane consists of solid domains coexisting with areas of fluid-disordered membrane lipids that may also contain proteins [42]. This concept has... 

The conventional model developed to explain cell membrane characteristics influencing drug permeability is routinely referred to as the fluid-mosaic model (Figures 2.1 and 2.2). In this model the main components, for our purposes, are a phospholipid (e.g., sphingomyelin and phosphatidylcholine) bilayer (8 nm), with polar moieties at both the external and internal surfaces, and with proteins periodically traversing the phospholipid plane perpendicularly. 

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