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In order to function properly and remain healthy, cells must absorb certain nutrients and other substances. Many of the molecules necessary for cell health, including proteins and other nutrients, are too large to pass through the cell membrane, so the cell depends on a process called endocytosis to absorb these substances. Endocytosis allows the cell to surround vital molecules with its cell membrane, thus absorbing them into the body of the cell.
During endocytosis, a portion of the cell membrane surrounds the substance to be absorbed, forming a pocket that projects into the interior of the cell. This membrane then closes off around the substance, creating a vesicle called an endosome or phagosome that then migrates into the cytoplasm of the cell. The endosome delivers its contents to another structure within the cell called a lysosome. Endosomes and lysosomes fuse together, allowing a digestive process to occur within the lysosome. A similar process, called exocytosis, occurs in reverse to excrete substances from the interior of the cell.
Depending on the type of substance being absorbed, endocytosis can be referred to by different terms. Phagocytosis involves cell absorption of a solid material, and in pinocytosis, the cell absorbs a liquid. Macropinocytosis absorbs liquid, as well, but also often brings in solid material at the same time. Many viruses exploit macropinocytosis in order to enter cells. Viruses stimulate the cell to absorb extracellular liquids, then travel into the cell along with the liquid.
Clathrin-mediated endocytosis uses specialized receptors to attract and absorb specific materials such as lipoproteins and antibodies. In clathrin-mediated endocytosis, the cell forms a pit, or a caveolae, on the exterior membrane. This pit is coated with receptors. When the pit becomes filled with the lipoproteins or other molecules that are intended to attach to the existing receptors, the pitted area forms a vesicle and is absorbed into the cell. Most cells have caveolae on their membrane surfaces, but some, most notably neurons, or nerve cells, have none.
Endocytosis also serves as a defense mechanism against bacteria by absorbing bacteria into the cell, then destroying them within the liposome. Bacteria such as salmonella bacteria short-circuit this process by producing a protein that makes it impossible for the phagosome to fuse with the liposome. Tuberculosis spreads through cells in a similar manner. Under normal conditions, the phagosome is coated with tryptophan-aspartate-containing protein, which is removed when the phagosome and lysosome fuse. The tuberculosis bacterium prevents the coat protein from being removed so the phagosome cannot fuse to the lysosome.