Cell secretions - e.g. Hormones, neurotransmitters - are packaged in secretory vesicles at the Golgi apparatus. The secretory vesicles are then transported to the cell surface for release. Vesicles are mainly involved in the transportation of material in/out or within the cell. They are made of at least one layer of the phospholipid-bilayer; which is the major constituent of the cell membrane. The term used for the transportion of solid material into the cell is endocytosis; when this happens the vesicle carrying the material will join to the cell membrane and become part of it, releasing its contents into the cytoplasm. Also pinocytosis is a synonym of endocytosis, which literally means cell-drinking, involves the absorption of aqueous solutes in the same process as endocytosis. Exocytosis involves transporting material out of the cell; the reverse of endocytosis, which happens in much the same way. Vesicles can also transport material within the cell. For example, vesicles transport proteins from the rough endoplasmic reticulum to the golgi apparatus. Some veiscles serve as storage vessels, such as, the synaptic vesicle which can store neurotransmitters; these stores are regulated, or released, via a flow of calcium ions.