<P> In cell biology, a vesicle is a small structure within a cell, or extracellular, consisting of fluid enclosed by a lipid bilayer . Vesicles form naturally during the processes of secretion (exocytosis), uptake (endocytosis) and transport of materials within the cytoplasm . Alternatively, they may be prepared artificially, in which case they are called liposomes (not to be confused with lysosomes). If there is only one phospholipid bilayer, they are called unilamellar liposome vesicles; otherwise they are called multilamellar . The membrane enclosing the vesicle is also a lamellar phase, similar to that of the plasma membrane and vesicles can fuse with the plasma membrane to release their contents outside the cell . Vesicles can also fuse with other organelles within the cell . </P> <P> Vesicles perform a variety of functions . Because it is separated from the cytosol, the inside of the vesicle can be made to be different from the cytosolic environment . For this reason, vesicles are a basic tool used by the cell for organizing cellular substances . Vesicles are involved in metabolism, transport, buoyancy control, and temporary storage of food and enzymes . They can also act as chemical reaction chambers . </P>

Where do vesicles originate that are involved in secretion