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| Fluid Mosaic Model |
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| In the year 1972 Singer and Nicolson proposed a model for explaining the membrane structure, taking into account all the known facts. According to this model, cell membrane consists of a highly viscous fluid matrix of a bilayer of phospholipids having globular proteins associated with them. This model came to be known as fluid mosaic model. |
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| The phospholipid molecules in the cell membrane have their polar, hydrophilic heads towards outer surface and the nonpolar, hydrophobic tails towards inner surface. This arrangement forms a water resistant barrier through which only lipid soluble substances can pass through |
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| The proteins in cell membrane are of two kinds. Some of the proteins are found at the periphery, partly projecting out of the lipid layer. These are the extrinsic proteins. These proteins can be easily extracted. Some of the protein molecules are found totally embedded in the phospholipid matrix. Those are the intrinsic or integral proteins, which represent nearly 70% of the membrane proteins. These proteins cannot be extracted. Some of the integral proteins have very large molecules that extend throughout the phospholipid matrix, projecting out on both surfaces. These proteins are called tunnel proteins or transmembrane proteins. They are believed to have channels for the passage of water-soluble substances. The scattered arrangement of protein molecules in the phospholipid matrix gives the appearance of a mosaic pattern. Hence, the name to the model. |
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| fig. 15.2 - Fluid Mosaic Model |
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| A few molecules of oligosaccharides are found attached to the free ends of protein molecules on the outer surface of the membrane. Sometimes they are also found attached to phospholipid layer. |
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| The cell membrane and the membranes of various cell structures exhibit the same structure and as such perform the same functions. However, the amount and arrangement of phospholipids and proteins varies in different cells and organelles. This variation accords a specificity that is unique to each membrane. |
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| The bio-membranes exhibit the following properties |
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 The constituent molecules can move freely in the membrane. |
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 The membranes are constantly renewed during a cell's life span. |
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 The membranes grow with the growth of cell and its organelles. |
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 The membranes can regenerate to some extent in cases of minor disruptions. |
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 The membranes can contract and expand during cell movements. |
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 The membranes allow interactions of cells such as recognition of self and fusion of cell. |
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 The membranes can regulate the flow of materials passing through them. |
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| These properties indicate that biomembranes are fluid and dynamic. |
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