Fluid-mosaic model phospholipids

In 1972, S. J. Singer and G. L. Nicolson proposed the fluid mosaic model for membrane structure, which suggested that membranes are dynamic structures composed of proteins and phospholipids. In this model, the phospholipid bilayer is a fluid matrix, in essence, a two-dimensional solvent for proteins. Both lipids and proteins are capable of rotational and lateral movement. 

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...

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.]...

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 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. 

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. 

Figure 9.24 The fluid-mosaic model of plasma membrane. Phospholipids with darkened heads are on the cytosol side of the bilayer, and the lipids with unfilled heads are on the outer surface. In intrinsic proteins one or more a-helical segments are in contact with the hydrophobic environment of the bilayer. They usually have hydrophobic amino acid side chains. Carbohydrate is indicated by hexagons. The membrane potential (negative inside) is indicated by AV. (Reproduced by permission from Vance DE, Vance JE. Biochemistry of Lipids and Membranes. Menlo Park Benjamin/Cummings, 1985, p. 26.)...

The basic tenets of the fluid mosaic model of membrane structure shown in Figure III-44 are widely accepted. The phospholipid bilayer is the basic structural feature of the membrane, and proteins or multiprotein complexes are viewed as island embedded in a sea made up by the lipid bilayer. Membranes are fluid in the sense that lipids and proteins can diffuse freely in the plane of the... 

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).

In the popular fluid mosaic model for biomembranes, membrane proteins and other membrane-embedded molecules are in a two-dimensional fluid formed by the phospholipids. Such a fluid state allows free motion of constituents within the membrane bilayer and is extremely important for membrane function. The term "membrane fluidity" is a general concept, which refers to the ease of motion for molecules in the highly anisotropic membrane environment. We give a brief description of physical parameters associated with membrane fluidity, such as rotational and translational diffusion rates, order parameters etc., and review physical methods used for their determination. We also show limitations of the fluid mosaic model and discuss recent developments in membrane science that pertain to fluidity, such as evidence for compartmentalization of the biomembrane by the cell cytoskeleton. 

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 has been very useful in explaining membrane behavior. However, the description of membrane as proteins floating in a phospholipid sea is oversimplified. Describe some components of membrane that are restricted in their lateral motion. 

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. 

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. 

According to the fluid-mosaic model of membrane structure ( ), cell membranes consist of a fluid phospholipid bilayer. Embedded within this bilayer are globular proteins essential to membrane function. A large class of phospholipids present in membranes are phosphoglycerides (1 ). The fatty acid in the number 2 position is often unsaturated ( ). In plants, the unsaturated fatty acid is frequently linolenic acid (1 ) with 3 double bonds (18 3 i 15) ... 

Biological membranes are made of lipid bUayers. The most satisfactory model for the arrangement of phospholipids, proteins, and cholesterol in plant and animal membranes is the fluid-mosaic model proposed in 1972 by S. J. Singer and G. Nicolson. The term mosaic signifies that the various components in the membrane coexist side by side, as discrete units, rather than combining to form new molecules or ions. Fluid signifies that the same sort of fluidity exists in membranes that we have already seen in lipid bilayers. Furthermore, the protein components of membranes float in the bilayer and can move laterally along the plane of the membrane. 

When placed in aqueous solution, phospholipids spontaneously form lipid bilayers. According to the fluid-mosaic model, membrane phospholipids form lipid bilayers with membrane proteins associated with the bilayer as both peripheral and integral proteins. 

Fluid-mosaic model A model of a biological membrane consisting of a phospholipid bilayer, with proteins, carbohydrates, and other Upids embedded in, and on the surface of, the bilayer. 

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