Exocytosis
In exocytosis, a cell exposes cellular products to select organelles or outside the cell. Proteins sequestered within an intracellular vesicle are let loose as the vesicle membrane fuses with a cell’s plasma membrane. The inner surface of the vesicle becomes the outer surface of the membrane.
Membrane fusion is an energetic exercise, requiring the coordinated interaction of adaptor molecules on both the vesicle and plasma membrane. The adaptors are highly selective, only allowing vesicles to fuse with membranes of certain organelles or the cell’s plasma membrane.
Once the appropriate adaptors bind with each other in an elaborate docking maneuver, stored ATP energy is released, forming a fusion pore between the vesicle membrane and plasma membrane.
Vesicle contents are released as the pore widens. Ultimately, the vesicle is either absorbed in the plasma membrane or recycled to the cytoplasm.
Exocytosis can be constitutive or regulated. Constitutive exocytosis occurs all the time; placing proteins, such as receptors, on the outside of a plasma membrane.
Regulated exocytosis is triggered when a cell receives a signal from outside. Calcium ions are typically involved in triggering.
The received signal is a substance, such as a hormone or neurotransmitter, which binds to a specific receptor on the cell surface. This activates the synthesis or release of a 2nd messenger within the cell.
Many secreted cellular products are for the tissue type in which the cell resides. Otherwise, outputs are transmitted to a more distant part of the organism. Cholesterol and hormones are exemplary secreted products.
Most exocytic products are enzymes or other proteins which have undergone rigorous quality control in the endoplasmic reticulum and Golgi complex. At the downstream end of the Golgi network, cellular products are sorted and accumulated in exocytic vesicles.
Exocytosis is a common vehicle for intercellular communication. Immune systems are extensive employers of exocytosis.
A cell harboring a virus displays viral by-products on its surface; a danger sign that attracts immune cells. Upon arrival, an immune cell tells an infected cell to self-destruct, to save its neighbors. Failing that, a good cell gone bad may be engulfed via phagocytosis.
