Exocytosis
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Exocytosis is a process by which the contents of a cell vacuole are released to the exterior through fusion of the vacuole membrane with the cell membrane. The cell forms a vesicle around material that needs to be removed or secreted. The vesicle is transported to the cell membrane. The vesicle membrane fuses with the cell membrane and releases the contents. Exocytosis restores the normal amount of plasma membrane. Any molecules dissolved in the fluid contents of these vesicles are discharged into extracellular – this is called secretion. Any integral membrane proteins exposed to the interior surface of the vesicles will now be displayed at the cell surface because the vesicles turn inside out as they fuse with the plasma membrane. Thus exocytosis does not simply replace plasma membrane but ensures that the plasma membrane will display its characteristic cell-surface proteins. Exocytosis is the opposite of endocytosis. So the cell must have a mechanism to restore the normal amount of plasma membrane. Exocytosis is that mechanism.