Exocytosis is a process of cellular excretion in which substances contained in vesicles are discharged from the cell by fusion of the vesicular membrane with the outer cell membrane. Or, put it in other words, exocytosis is a cellular process in which cells eject waste products or chemical transmitters (such as hormones) from the interior of the cell. Exocytosis is similar in function to endocytosis but working in the opposite direction.
In multicellular organisms there are two types of exocytosis: 1) Ca2+ triggered non-constitutive and 2) non Ca2+ triggered constitutive. Exocytosis in neuronal chemical synapses is Ca2+ triggered and serves interneuronal signalling. Constitutive exocytosis is performed by all cells and serves the release of components of the extracellular matrix, or just delivery of newly-synthesized membrane proteins that are incorporated in the plasma membrane after the fusion of the transport vesicle. Exocytosis is the opposite of endocytosis.
There are five steps to exocytosis: 1) in this first step, the vesicle containing the waste product is transported through the cytoplasm towards the part of the cell from which it will be eliminated; 2) as the vesicle approaches the cell membrane, it is secured and pulled towards the part of the cell from which it will be eliminated; 3) in third step, the vesicle comes in contact with the cell membrane, where it begins to chemical and physically merge with the proteins in the cell membrane; 4) the fourth step involves the chemical preparations for the last step of exocytosis; 5) in the last step, the proteins forming the walls of the vesicle merge with the cell membrane and breach, pushing the vesicle contents (waste products or chemical transmitters) out of the cell. This step is the primary mechanism for the increase in size of the cell's plasma membrane.