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 The term biomembrane is used to describe the plasma membrane (cell membrane) and the membranes that surround the various cell organelles. |
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 The plasma membrane forms a protective covering around the cell to maintain its size and shape. In addition, it is also responsible for the interactions that occur between the cell and its environment. |
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 Plasma membrane is described as a semipermeable membrane since it allows only particles of a particular size, shape, texture and chemical composition to pass through. |
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 The finer details of chemical composition and nature of plasma membrane is provided by the fluid mosaic model proposed by Singer and Nicholson in 1972. |
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 According to this model, the plasma membrane consists of a fluid phospholipid bilayer with closely held peripheral proteins on its surface and firmly held integral proteins embedded in it. |
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 The phospholipid molecules have their polar heads directed outwards and nonpolar tails directed inwards. |
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 The proteins present in the membrane may function as carriers or receptors or enzymes. |
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 Transport across membranes involves several mechanisms. There are processes like diffusion and osmosis, which do not require energy or activity of the cell. |
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 Diffusion involves movement of molecules from a region of higher concentration, to a region of lower concentration. |
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 Osmosis is a special type of diffusion, which involves the movement of water molecules along the concentration gradient through a semipermeable membrane. |
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 The movement of some substances occurs against a concentration gradient with the expenditure of energy. This process is called active transport. Particularly ions and polar molecules are transported by this mechanism. |
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 There are three types of transport proteins in the cell membrane channel proteins which form an aqueous channel, passive transport carrier proteins which bind to specific solute molecules and active transport carrier proteins which bind to specific molecules but move with the expenditure of energy. |
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 The cell can take in any non-specific fluid droplets by a process called pinocytosis, which involves a folding in of the cell membrane and the pinching off of a pinocytic vesicle. |
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 A cell takes in larger macromolecular solid substances by a process called phagocytosis, which also involves a folding in of the cell membrane. |
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 A cell eliminates macromolecules or its own secretions by a reverse process called exocytosis. |
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