First, it's a cycle -- Exocytosis balances endocytosis so cell surface area stays the same. Now let's describe exocytosis as being endocytosis in reverse - essentially a vesicle inside cell moved outside of cell where a vesicle fuses with interior plasma membrane and releases contents out of cell
Don't forget that there are 3 types of endocytosis:
- Receptor mediated endocytosis - most selective (it is an endocytotic mechanism in which specific molecules are ingested into the cell)
- Phagocytosis (it is the process by which a cell—often a phagocyte or a protist—engulfs a solid particle to form an internal vesicle known as a phagosome)
- Pinocytosis (bulk phase endocytosis) - least selective (aka: cell drinking, fluid endocytosis, and bulk-phase pinocytosis, is a mode of endocytosis in which small particles are brought into the cell, forming an invagination (folding inward), and then suspended within small vesicles)
Now what substances use exocytosis: secretory cells that produce digestive enzymes, hormones, mucus, etc
- nerve cells release neurotransmitters
If we were to describe transcytosis we'd say: that it is an active process that uses endo and exocytosis to move vesicles in - thru- and out of cells
Because we reviewed the above material it becomes more apparent that what we see in endocytosis, where the cell engulfs some of its extracellular fluid (ECF) including material dissolved or suspended in it. A portion of the plasma membrane is invaginated (folded inward), coated with molecules of the protein clathrin, and pinched off forming a membrane-bounded vesicle called an endosome.
In other words, sometimes the cell needs to move molecules that are simply too large or too numerous for protein channels/pumps, or even plasmodesmata/gap junctions (depending on the cell type of course). This is when the cell relies on special processes called endocytosis and exocytosis. As the name suggests (endo = into, cyto = cell, sis= process), endocytosis is used to import large molecules or large quantities of molecules into the cell. The molecules gather on the cell membrane, outside the cell, and the cell membrane begins to bulge inward, eventually pinching off. The molecules are now inside the cell, enclosed within a vesicle derived from the cell membrane itself. Exocytosis functions in a similar way, except the cell is expelling molecules. A vacuole or vesicle within the cell will fuse with the cell membrane and force its contents out of the cell.
The reverse of endocytosis is exocytosis, where the molecules are exported out of the cell. A vesicle, filled with the molecules to be removed the cell, travels to the plasma membrane and merges with it. The molecules spill out of the cell and are successfully exported from the cell.
