Shown to the right is an animation illustrating how secretion vesicles approach the plasma membrane, fuse with the plasma membrane and dump their soluble contents outside of the cell. This process is called exocytosis and it is mechanism by which cells can secrete molecules like proteins. For example, the epithelial cells in the breast use secretion vesicles to put the major protein of milk (casein) outside of the cell which synthesized it. Note that the membrane of the secretion vesicle becomes the plasma membrane. Thus, cells increase in surface area as they secrete substances. This increase in surface area can be counter balanced by the up take of plasma membrane in the process of endocytosis. Also note that the proteins once exposed on the inner surface of the secretion vesicle become exposed on the outer surface of the plasma membrane.