Cell plasma membrane fluid mosaic model

M.S. Bretscher, M.C. Raff, Mammalian Plasma Membrane , Nature, 258,43 (1975) S.J. Singer, G.L. Nicolson, Fluid Mosaic Model of Structiue of Cell Membranes , Science, 175,720 (1972)... 

Membrane fluidity is one of the most important assumptions of the fluid mosaic model of membrane structure. One measure of membrane fluidity, the ability of membrane components to diffuse laterally, can be demonstrated when cells from two different species are fused to form a heterokaryon (Figure 1 ID). (Certain viruses or chemicals are used to promote cell-cell fusion.) The plasma membrane proteins of each cell type can be tracked because they are labeled with different fluorescent markers. Initially, the proteins are confined to their own side of the heterokaryon membrane. As time passes, the two fluorescent markers intermix, indicating that proteins move freely in the lipid bilayer. 

The currently accepted structure of B. is the fluid mosaic model. Lipid molecules and membrane proteins are free to diffuse laterally and to spin within the bilayer in which they are located. However, a flip-flop motion from the inner to the outer surface, or vice versa, is energetically unfavorable, because it would require movement of hydrophilic substituents through the hydrophobic phase. Hence this type of motion is almost never displayed by proteins, and it occurs much less readily than translational motion in the case of lipids. Since there is little movement of material between the inner and outer layers of the bilayer, the two faces of the B. can have different compositions. For membrane proteins, this asymmetry is absolute, and, at least in the plasma membrane, different proportions of lipid classes exist in the two monolayers. Attached carbohydrate residues appear to be located only on the noncytosolic surface. Carbohydrate groups extending from the B. participate in cell recognition, cell adhesion, possibly in intercellular communication, and they also contribute to the distinct immunological character of the cell. 

The plasma membrane (Figure 9.2) encapsulates the cell and physically separates the cytoplasm from the external environment. All substances which enter or leave the cell must pass through the plasma membrane which plays an important role in the selective uptake of nutrients from the extracellular medium and the discharge of waste products of metabolism from the cell. The plasma membrane is the most extensively researched and best understood of all cell membranes and its properties have led to the development of models of membrane structure from the fundamental lipid bilayer composed of amphipathic phospholipids (Section 8.5) to the currently most widely accepted model called the fluid mosaic model. 

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