Exocytosis (ek-soh-sy-TOH-sis; from Greek ἔξω "out" and English cyto- "cell" from Gk. κύτος "receptacle") is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.