Definition, function and procedure:
exocytosis (alternative spelling: exocytosis, from the Greek 'exo' = outside) denotes the process of delivery of substances from the intracellular space (inside the cell) into the intercellular space (outside the cell). Endocytosis is the opposite of exocytosis, during which the cell ingests particles inwards.
Waste and by-products from metabolic processes of the cell can not be stored indefinitely in the cell interior, but require regular removal from the cell. For this purpose, the Golgi apparatus constricts vesicles filled with the waste, the so-called exosomes. The exosome prevents a direct clash between waste material and cytoplasm, since even unnecessary by-products can still interact with cell organelles. As soon as the exosome hits the cell membrane, it fuses with it and empties the contents into the intercellular space.
In addition, exocytosis plays an indispensable role in the cell-driven removal of pollutants and pathogens, as well as in the release of neurotransmitters and hormones. For example, in the case of nerve cell communication, transport vesicles store neurotransmitters (e.g., acetylcholine, adrenaline, dopamine, GABA, histamine, serotonin) and fuse with the presynaptic endings upon electrical stimulation. As a result, the stored neurotransmitters enter the synaptic gap.
A central example of exocytosis is insulin secretion. Production of insulin takes place in the pancreas and its release is activated by an increase in glucose content. Additional factors for insulin delivery are free fatty acids and amino acids. Consequently, calcium ions from the extracellular space enter the beta cells. The discharge finally begins with the fusion of the insulin vesicles with the cell membrane of the beta cells, which may cause the contents of the cell to flow out.