Endocytosis and exocytosis are both cellular means of transporting substances across a cell membrane via the use of a vesicle. However, the processes differ in the direction in which the substances move across the cell membrane. During endocytosis, substances are brought into the cell (“endo” = in). During exocytosis, substances exit the cell (“exo” = exit).
Phagocytosis and pinocytosis are two variations of endocytosis. Phagocytosis is commonly called “cell eating”, whereas pinocytosis is commonly known as “cell drinking”. Phagocytosis and pinocytosis are both explained below.
- During phagocytosis, cells engulf solid particle by surrounding the solid substance with a vesicle that is called a phagosome. The phagosome is then brought into the cell. A lysosome attaches to the phagosome so that its contents can be “digested” and used for energy. Substances that are not used within the phagosome are removed from the cell via exocytosis. Phagocytes are organisms that undergo phagocytosis. Amoebas and white blood cells are two examples of phagocytes.
- Pinocytosis is similar to phagocytosis, except the material that is being ingested by a cell is a liquid instead of a solid. Again, the liquid is surrounded by an invagination of a cell’s membrane. The invagination then pinches off to form a vesicle that is brought within the cell. Body cells take in enzymes and hormones via the process of pinocytosis. Cells that have microvilli that are found along the intestinal wall also undergo pinocytosis in order to intake nutrients.
During exocytosis, an intracellular vesicle that contains waste migrates towards the cell membrane. Once the vesicle reaches the cell membrane, it fuses with the cell membrane. This fusion allows the contents of the vesicle to be released outside of the cell.