A vesicle is a membrane-bound sac found in many cells, which is formed when part of a compartment membrane buds off . Vesicles are vital for the taking up, release and transport of molecules within and between cells. Many cells take up molecules via receptor-mediated endocytosis, the process by which binding of a particular protein or other complex causes invagination of the membrane, which eventually breaks off forming a vesicle containing the desired protein or substance . The opposite of this process is exocytosis, whereby a vesicle fuses to a membrane, releasing its contents.
Also see vesicles.