Secretion
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Secretion is the process of segregating, elaborating, and releasing chemicals from a cell.
Eukaryotic Cells have a highly evolved process of secretion. Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum. As they are synthesized, these proteins translocate into the ER lumen, where molecular chaperones aid protein folding. Another set of chaperones helps to sort the new proteins and direct them into compartmentalized vesicles. Mutations are usually identified here and retrotranslocated by an unknown mechanism to the cytosol, where they are degraded by a proteasome. The vesicles containing the properly-folded proteins then enter the Golgi apparatus.
In the Golgi apparatus, the proteins are glycosylated as needed and further posttranslational modifications, including cleavage and functionalization, may occur. The proteins are then moved into secretory vesicles which travel along the cytoskeleton to the edge of the cell. More modification can occur in the secretory vesicles (for example insulin is cleaved from proinsulin in the secretory vesicles).
Eventually, the vesicle fuses with the cell membrane in a process called exocytosis, dumping its contents into the cell's environment.
Strict biochemical control is maintained over this sequence by usage of a pH gradient: the pH of the cytosol is 7.4, the ER's pH is 7.0, and the cis-golgi has a pH of 6.5. Secretory vesicles have pHs ranging between 5.0 and 6.0; some secretory vesicles evolove into lysosomes, which have a pH of 4.8.
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