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Endocytosis

Endocytosis

The ability to internalize material from outside the cell is important for several cellular processes including the ingestion of essential nutrients, removal of dead or damaged cells from the body, and defense against microorganisms. Eukaryotic cells internalize fluid, large and small molecules, and even other cells from their surroundings by a process called endocytosis. During endocytosis, the plasma membrane of the cell forms a pocket around the material to be internalized. The pocket closes and then separates from the inside surface of the plasma membrane to form a membrane-enclosed bubble, or vesicle, containing the ingested material.

There are two main types of endocytosis that are distinguished by the size of the vesicle formed and the cellular machinery involved. Pinocytosis (cell drinking) describes the internalization of extracellular fluid and small macromolecules by means of small vesicles. Phagocytosis (cell eating) describes the ingestion of large particles such as cell debris and whole microorganisms by means of large vesicles. While all eukaryotic cells are continually ingesting fluid and molecules by pinocytosis, only specialized phagocytic cells ingest large particles.

Specialized Phagocytic Cells Engulf Large Particles

Phagocytosis begins with the extension of large, handlike projections from the plasma membrane. The projections surround the particle and fuse together so that the particle is completely engulfed in a large vesicle within the cell called a phagosome. Inside the cell, the phagosome fuses with another membranous organelle called a lysosome, forming a single membranous organelle and mixing their contents in the process. Lysosomes, acting as the "stomach" of the cell, carry digestive enzymes that break down all types of biological molecules. Consequently, after a phagosome fuses with a lysosome, the digestive enzymes break down the ingested material into small molecules that are transported into the cytosol and made available for cell use. Many single-celled organisms like amoebas and ciliates use phagocytosis as a means to acquire food. In multicellular animals, only specialized types of cells use phagocytosis. For example, in humans, specialized white blood cells called macrophages use phagocytosis to defend the body against infection by engulfing invading microorganisms and to remove cell debris from the body by ingesting damaged or old cells.

All Eukaryotic Cells Constantly Ingest Fluid and Molecules by Pinocytosis

In contrast to phagocytosis, pinocytosis begins with small, convex pits on the cell surface that collect material or fluid to be internalized. The convex pits expand into the interior of the cell forming small vesicles that pinch from the inside of the plasma membrane. All eukaryotic cells have a continuous stream of vesicles budding from the plasma membrane. The constant removal of membrane from the plasma membrane would quickly deplete the plasma membrane if not for the balancing effects of another continual process called exocytosis. Exocytosis is the process by which vesicles from inside the cell fuse with the plasma membrane to secrete material and fluid. So, pinocytosis brings fluid and material into the cell and removes membrane from the plasma membrane, while exocytosis expels fluid and mate-rial from the cell while adding membrane to the plasma membrane. Thus, the two processes work together to continuously recycle the plasma membrane.

The most thoroughly understood form of pinocytosis is receptor-mediated endocytosis. Receptor-mediated endocytosis selectively internalizes specific molecules that are found in low concentrations in the extracellular space, such as hormones , growth factors, antibodies, iron, enzymes, vitamins, and cholesterol. The specific molecules to be internalized bind to proteins called receptors on the outside surface of the cell. Receptors are proteins that are embedded in the plasma membrane with portions of the protein extending outside the cell to form a binding site for a specific molecule. Once molecules bind to their receptors, the receptors move within the plasma membrane and become concentrated in small depressions called clathrin-coated pits. Clathrin-coated pits are formed when many clathrin protein molecules interact with each other to form a convex, basketlike structure on the inside of the plasma membrane that molds the membrane into a pit. The coated pits then progressively invaginate, or form inward, to form clathrin-coated vesicles that pinch off the plasma membrane into the cytoplasm . Hence, the clathrin-coated vesicles carry the receptor proteins taken from the plasma membrane and their bound molecules taken from the extracellular space.

Once a clathrin-coated vesicle pinches from the inner surface of the plasma membrane, the clathrin coat is removed. The "uncoated" vesicle, still carrying receptor proteins and their bound cargo molecules, fuses with another membranous organelle called an endosome. Endosomes function as "sorting stations." In an endosome, molecules are sorted and packaged into new vesicles for transportation to various locations within the cell.

Receptors brought into the cell by receptor-mediated endocytosis have one of several fates after unloading their cargo and leaving the endosome: (1) they can be recycled back to the same area of plasma membrane from which they came; (2) they can be transported to another region of the plasma membrane; or (3) they can be transported to the lysosome where they are degraded. Thus, in contrast to phagocytosis, not all material brought into the cell by receptor-mediated endocytosis ends up in the lysosome for digestion.

Endocytosis of Cholesterol

Receptor-mediated endocytosis was discovered by Michael Brown and Joseph Goldstein, who were investigating the internalization of cholesterol by cells from the bloodstream. Brown and Goldstein won the Nobel Prize in medicine in 1985 for their discovery. Cholesterol, a type of lipid , is insoluble and is transported in the bloodstream bound to protein in particles called lowdensity lipoproteins (LDL). The LDL particles bind to LDL receptors on cell surfaces, and the LDL/receptor complexes are concentrated into clathrincoated pits. The clathrin-coated pits develop into clathrin-coated vesicles that move into the interior of the cell, lose their clathrin coats, and fuse with an endosome. The acidic environment of the endosome causes the LDL particles to detach from the LDL receptors, and the two are sorted from each other. The LDL particles are transported to the lysosome where they are broken down by digestive enzymes; the cholesterol is released into the cytosol where it is used in the synthesis of new membrane. The LDL receptors are packaged into membrane vesicles that travel back to and fuse with the plasma membrane (via exocytosis) so that the receptors once again face the exterior of the cell and can pick up more LDL particles to start the cycle again.

see also Exocytosis; Hormones; Lipids; Lysosomes; Membrane Proteins

Cynthia K. Damer and Scott N. Daigle

Bibliography

Aderem, Alan, and David M. Underhill. "Mechanisms of Phagocytosis in Macrophages." Annual Review of Immunology 17 (1999): 593623.

Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Publishing, 2000.

Mukherjee, Sushmita, Richik N. Ghosh, and Frederick R. Maxfield. "Endocytosis." Physiological Reviews 77 (1997): 759803.

Schmid, Sandra L. "Clathrin-Coated Vesicle Formation and Protein Sorting: An Integrated Process." Annual Review of Biochemistry 66 (1997): 511548.

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endocytosis

endocytosis (ĕn´dōsītō´səs), in biology, process by which substances are taken into the cell. When the cell membrane comes into contact with a suitable food, a portion of the cell cytoplasm surges forward to meet and surround the material and a depression forms within the cell wall. The depression deepens and the movement of the cytoplasm continues until the food is completely engulfed in a pocket called a vessicle. The vessicle then drifts further into the body of the cell where it meets and fuses with a lysosome, a vessicle normally found in the cell that contains digestive enzymes known as acid hydrolases. The food is then broken down into molecules and ions that are suitable for the cell's use. There are two types of endocytosis: pinocytosis, the engulfing and digestion of dissolved substances, and phagocytosis, the engulfing and digestion of microscopically visible particles. Phagocytosis is the process by which many protozoans obtain most of their food supply. It is also the process through which specialized cells in animals eliminate foreign matter, such as infecting microorganisms, as part of the body's defense system (see blood; immunity). The various phagocytic cells in higher animals are derived from relatively unspecialized cells called stem cells that are either fixed within a network of supporting (reticular) cells and fibers of the spleen, thymus, and bone marrow, or that wander freely throughout body tissues. Many phagocytic cells respond chemically to substances produced by foreign bodies or by degenerating tissue by moving toward the substances, a mechanism known as chemotaxis. When a particle of the proper charge or chemical composition adheres to the cell surface, the cell cytoplasm moves so that it finally surrounds the particle and traps it within a cytoplasmic vacuole. Various enzymes are then secreted into the vacuole to digest the foreign substance. In higher animals each phagocyte can ingest about 5 to 25 invading bacterial cells. Phagocytosis often precedes production of antibodies by the body, but some species of bacteria cannot be phagocytized unless specific antibody is already present. Although phagocytosis is an effective response to infection, some organisms, such as the bacteria causing brucellosis and tuberculosis, can survive for years within the descendant cells of the phagocytes that ingested them. The process of phagocytosis was first described in the late 19th cent. by the Russian zoologist Élie Metchnikoff.

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endocytosis

endocytosis The process by which materials enter a cell without passing through the plasma membrane. The membrane folds around material outside the cell, resulting in the formation of a saclike vesicle into which the material is incorporated. This vesicle is then pinched off from the cell surface so that it lies within the cell (see also endosome). Both phagocytosis and pinocytosis are forms of endocytosis. In receptor-mediated endocytosis, cells selectively take in substances (e.g. hormones, low-density lipoproteins) that bind to receptors on the cell surface. Compare exocytosis.

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endocytosis

endocytosis The mechanism by which a cell engulfs large material by the invagination of the cell membrane to form a vesicle or vacuole. See also PHAGOCYTOSIS;PINOCYTOSIS.

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"endocytosis." A Dictionary of Zoology. . Encyclopedia.com. 21 Aug. 2017 <http://www.encyclopedia.com>.

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endocytosis

endocytosis The entry of particles or fluids into a cell by their enclosure in an invagination of the cell membrane which is then detached.

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endocytosis

endocytosis The mechanism by which a cell engulfs large material by the invagination of the cell membrane to form a vesicle or vacuole.

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