Secretion

Secretion is the process of segregating, elaborating, and releasing something.

Eukaryotic Cells have a highly evolved process of secretion. Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum, translocated across a pore in the membrane barrier to the ER's lumen, where they are intercepted by a gaggle of chaperones which assist in folding the protein correctly. Then another set of chaperones aids in the sorting and direction of proteins to compartmentalized vesicles which reach the Golgi Bodies. Mutations are usually identified here and retrotranslocated by an unknown mechanism to the cytosol where they are degraded by the proteasome.

In the Golgi apparatus, the proteins are glycosylated as needed and further posttranslational modifications, including cleavage or functionalization, occurs. Proteins which are exported are then moved into secretory vesicles, which run 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, an exocytotic event occurs which culminates in secretion.

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 4.0.