In biology, exocytosis is the process of a biological cell releasing substances into the extracellular fluid (its environment). Exocytosis is the opposite of endocytosis.
Vesicles that contain the substances to be released are transported to the plasma membrane and fuse with it. This accomplishes three tasks:
- The total surface of the plasma membrane increases (by the surface of the fused vesicle). This is important for the regulation of cell size, e.g., during cell growth.
- The substances within the vesicle are released into the exterior. This can be waste products or toxins, but also signalling molecules like hormones or neurotransmitters during synaptic transmission.
- Proteins that are embedded in the vesicle membrane are now part of the plasma membrane. The side of the protein that was facing the inside of the vesicle is now facing the outside of the cell. This mechanism is important for the regulation of transmembrane receptors and transporters.