Types of Endocytosis: Pinocytosis, Receptor-Mediated Endocytosis and Phagocytosis
1.Pinocytosis is occurs in leukocytes, kidney cells, intestinal epithelium, liver macrophages, and plant root cells. Actin filaments are associated with the margins of pinocytic vesicles and are believed to play some role in the vesicle’s invagination
2. Receptor-Mediated Endocytosis:
Here , binding of molecules (hormones, antibodies ,proteins, and lipids to specific receptors in the plasma membrane which are then internalized by the cell. The receptors that mediate the endocytosis are integral proteins that span the membrane. Substances bound by these receptors, called ligands. Ligand-receptor complexes move laterally in the membrane toward coated pits in which the complexes are then concentrated. Because lateral movement through the membrane by the ligand-receptor complex is quite rapid, a coated pit is encountered within a few seconds.Coated vesicles that have just detached from the plasma membrane move deeper into the cytoplasm, progressively shed their clathrin coats, and fuse with one another to form larger, smooth-surfaced vesicles called endosomes (sometimes receptosomes) As endosomes move even deeper into the cytoplasm, there is a progressive release of ligands from their receptors into the lumen of the vesicle. The endosome develops a tubular portion in which the membrane-bound but ligand-free receptors are concentrated and a vesicular portion containing free ligand. At this stage the endosome . The tubular portion of the endosome is subsequently dispatched to the plasma membrane, thereby recycling its ligand receptors, and the vesicular portion continues its journey deeper into the cell.If the endosome contains proteins, other macromolecules, or particles, it may fuse with a lysosome so that enzymatic digestion of the entrained material can ensue.
3. Phagocytosis:It involves the endocytosis of much larger quantities of material than either pinocytosis or receptor-mediated endocytosis. For example, entire ciliates, rotifers, or other microscopic organisms may be phagocytosed by an amoeba and enclosed within one or more vacuoles called phagosomes, food vacuoles, or food cups .White blood cells phagocytose hundreds of bacteria. The removal and destruction of old red blood cells in the liver, spleen, and bone marrow by reticuloendothelial cells in these organs also occurs by phagocytosis. Following phagocytosis, the phagosomes fuse with primary lysosomes in the cell. The hydrolytic enzymes from these lysosomes digest the engulfed material, converting it to a form that may be transported across the vacuolar membranes and into the cytosol.
1.Pinocytosis is occurs in leukocytes, kidney cells, intestinal epithelium, liver macrophages, and plant root cells. Actin filaments are associated with the margins of pinocytic vesicles and are believed to play some role in the vesicle’s invagination
2. Receptor-Mediated Endocytosis:
Here , binding of molecules (hormones, antibodies ,proteins, and lipids to specific receptors in the plasma membrane which are then internalized by the cell. The receptors that mediate the endocytosis are integral proteins that span the membrane. Substances bound by these receptors, called ligands. Ligand-receptor complexes move laterally in the membrane toward coated pits in which the complexes are then concentrated. Because lateral movement through the membrane by the ligand-receptor complex is quite rapid, a coated pit is encountered within a few seconds.Coated vesicles that have just detached from the plasma membrane move deeper into the cytoplasm, progressively shed their clathrin coats, and fuse with one another to form larger, smooth-surfaced vesicles called endosomes (sometimes receptosomes) As endosomes move even deeper into the cytoplasm, there is a progressive release of ligands from their receptors into the lumen of the vesicle. The endosome develops a tubular portion in which the membrane-bound but ligand-free receptors are concentrated and a vesicular portion containing free ligand. At this stage the endosome . The tubular portion of the endosome is subsequently dispatched to the plasma membrane, thereby recycling its ligand receptors, and the vesicular portion continues its journey deeper into the cell.If the endosome contains proteins, other macromolecules, or particles, it may fuse with a lysosome so that enzymatic digestion of the entrained material can ensue.
3. Phagocytosis:It involves the endocytosis of much larger quantities of material than either pinocytosis or receptor-mediated endocytosis. For example, entire ciliates, rotifers, or other microscopic organisms may be phagocytosed by an amoeba and enclosed within one or more vacuoles called phagosomes, food vacuoles, or food cups .White blood cells phagocytose hundreds of bacteria. The removal and destruction of old red blood cells in the liver, spleen, and bone marrow by reticuloendothelial cells in these organs also occurs by phagocytosis. Following phagocytosis, the phagosomes fuse with primary lysosomes in the cell. The hydrolytic enzymes from these lysosomes digest the engulfed material, converting it to a form that may be transported across the vacuolar membranes and into the cytosol.
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