Exocytosis

Exocytosis is the main way cells export large molecules that cannot cross the plasma membrane on their own. The cargo is packed into a vesicle inside the cell; the vesicle travels to the cell surface, its membrane merges with the plasma membrane, and the contents pour into the space outside. It is the reverse of endocytosis, and like endocytosis it is a form of bulk transport that costs the cell energy.

Cells use it in two broad ways. In constitutive exocytosis, vesicles from the Golgi apparatus fuse with the surface all the time, delivering new membrane proteins and lipids and releasing materials such as extracellular matrix proteins. In regulated exocytosis, filled vesicles wait near the membrane until a signal arrives, often a rise in calcium ions inside the cell. This is how nerve endings release neurotransmitters into the synaptic cleft, how beta cells of the pancreas secrete insulin and how mast cells release histamine. Proteins called SNAREs, on the vesicle and on the target membrane, wind tightly around each other and pull the two membranes close enough to fuse.

Because the vesicle membrane becomes part of the plasma membrane, exocytosis also adds surface area to the cell, which endocytosis balances by drawing membrane back in.