Endocytosis
Endocytosis
Endocytosis, the ingestion of material by cells, is a collective term that describes three similar processes, phagocytosis, potocytosis, and receptor-mediated endocytosis (Figure 3-20). They are pathways for specifically internalizing solid particles, small molecules and ions, and macromolecules, respectively. All require energy and thus may be considered forms of active transport.
Phagocytosis, which literally means “cell eating,” is a common method of feeding among protozoa and lower metazoa. It is also the way in which white blood cells (leukocytes) engulf cellular debris and uninvited microbes in the blood. By phagocytosis, an area of the cell membrane, coated internally with actin-myosin, forms a pocket that engulfs the solid material. The membrane-enclosed vesicle then detaches from the cell surface and moves into the cytoplasm where its contents are digested by intracellular enzymes.
Potocytosis is similar to phagocytosis except that small areas of the surface membrane are invaginated into cells to form tiny vesicles. The invaginated pits and vesicles are called caveolae (ka-veeo-lee). Specific binding receptors for the molecule or ion to be internalized are concentrated on the cell surface of caveolae. Potocytosis apparently functions for intake of at least some vitamins, and similar mechanisms may be important in translocating substances from one side of a cell to the other (see “exocytosis,” following) and internalizing signal molecules, such as some hormones or growth factors.
In phagocytosis, potocytosis, and receptormediated endocytosis some amount of extracellular fluid is necessarily trapped in the vesicle and nonspecifically brought within the cell.We describe this as bulkphase endocytosis, and because it is nonspecific, the process corresponds roughly to what we have called traditionally pinocytosis, or “cell drinking.”Actually, potocytosis also means “cell-drinking” but was coined to distinguish internalization of specific small molecules or ions.
Receptor-mediated endocytosis is a specific mechanism for bringing large molecules within the cell. Proteins of the plasma membrane specifically bind particular molecules (referred to as ligands in this process), which may be present in the extracellular fluid in very low concentrations. The invaginations of the cell surface that bear the receptors are coated within the cell with a protein called clathrin; hence, they are described as clathrin-coated pits. As a clathrincoated pit with its receptor-bound ligand invaginates and is brought within the cell, it is uncoated, the receptor and the ligand are dissociated, and the receptor and membrane material are recycled back to the surface membrane. Some important proteins and peptide hormones are brought into cells in this manner.
Exocytosis
Just as materials can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can fuse with the plasma membrane and extrude its contents to the surrounding medium. This is the process of exocytosis. This process occurs in various cells to remove undigestible residues of substances brought in by endocytosis, to secrete substances such as hormones (Figure 3-10), and to transport a substance completely across a cellular barrier, as we just mentioned. For example, a substance may be picked up on one side of the wall of a blood vessel by potocytosis, moved across the cell, and released by exocytosis.
Endocytosis, the ingestion of material by cells, is a collective term that describes three similar processes, phagocytosis, potocytosis, and receptor-mediated endocytosis (Figure 3-20). They are pathways for specifically internalizing solid particles, small molecules and ions, and macromolecules, respectively. All require energy and thus may be considered forms of active transport.
Phagocytosis, which literally means “cell eating,” is a common method of feeding among protozoa and lower metazoa. It is also the way in which white blood cells (leukocytes) engulf cellular debris and uninvited microbes in the blood. By phagocytosis, an area of the cell membrane, coated internally with actin-myosin, forms a pocket that engulfs the solid material. The membrane-enclosed vesicle then detaches from the cell surface and moves into the cytoplasm where its contents are digested by intracellular enzymes.
Potocytosis is similar to phagocytosis except that small areas of the surface membrane are invaginated into cells to form tiny vesicles. The invaginated pits and vesicles are called caveolae (ka-veeo-lee). Specific binding receptors for the molecule or ion to be internalized are concentrated on the cell surface of caveolae. Potocytosis apparently functions for intake of at least some vitamins, and similar mechanisms may be important in translocating substances from one side of a cell to the other (see “exocytosis,” following) and internalizing signal molecules, such as some hormones or growth factors.
In phagocytosis, potocytosis, and receptormediated endocytosis some amount of extracellular fluid is necessarily trapped in the vesicle and nonspecifically brought within the cell.We describe this as bulkphase endocytosis, and because it is nonspecific, the process corresponds roughly to what we have called traditionally pinocytosis, or “cell drinking.”Actually, potocytosis also means “cell-drinking” but was coined to distinguish internalization of specific small molecules or ions.
Receptor-mediated endocytosis is a specific mechanism for bringing large molecules within the cell. Proteins of the plasma membrane specifically bind particular molecules (referred to as ligands in this process), which may be present in the extracellular fluid in very low concentrations. The invaginations of the cell surface that bear the receptors are coated within the cell with a protein called clathrin; hence, they are described as clathrin-coated pits. As a clathrincoated pit with its receptor-bound ligand invaginates and is brought within the cell, it is uncoated, the receptor and the ligand are dissociated, and the receptor and membrane material are recycled back to the surface membrane. Some important proteins and peptide hormones are brought into cells in this manner.
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| Figure 3-20 Three types of endocytosis. In phagocytosis the cell membrane binds to a large particle and extends to engulf it. In potocytosis small areas of cell membrane, bearing specific receptors for a small molecule or ion, invaginate to form caveolae. Receptor-mediated endocytosis is a mechanism for selective uptake of large molecules in clathrin-coated pits. Binding of the ligand to the receptor on the surface membrane stimulates invagination of pits. |
Exocytosis
Just as materials can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can fuse with the plasma membrane and extrude its contents to the surrounding medium. This is the process of exocytosis. This process occurs in various cells to remove undigestible residues of substances brought in by endocytosis, to secrete substances such as hormones (Figure 3-10), and to transport a substance completely across a cellular barrier, as we just mentioned. For example, a substance may be picked up on one side of the wall of a blood vessel by potocytosis, moved across the cell, and released by exocytosis.
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