ABSTRACT
Exocytosis is the secretion of proteins out of the cell across the plasma membrane into the extracellular space. Proteins destined to be secreted are synthesized on ribosomes bound to the rough endoplasmic reticulum (RER) membrane and are then transported in membrane-bound vesicles to the Golgi apparatus where they are sorted and packaged up into secretory vesicles. Endocytosis is the uptake of macromolecules from the extracellular space into the cell across the plasma membrane via the formation of an intracellular vesicle pinching off from the plasma membrane. Endocytosis can be divided into three distinct types depending on the size of the ingested macromolecule and whether specific cell surface receptors are involved. These three processes are: phagocytosis, pinocytosis and receptor-mediated endocytosis. Receptor-mediated endocytosis is the selective uptake of macromolecules from the extracellular fluid via clathrin-coated pits and vesicles. Phagocytosis is the ingestion of large particles such as bacteria and cell debris via large endocytic vesicles called phagosomes.
