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Secretion is the movement of material from one point to another, e.g. secreted chemical substance from a cell or gland. In contrast, excretion, it is the removal of certain substances or waste products from a cell or organism. The classical mechanism of cell secretion is via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell.
Secretion in bacterial species means the transport or translocation of effector molecules for example: proteins, enzymes or toxins (such as cholera toxin in pathogenic bacteria for example Vibrio cholerae) from across the interior (cytoplasm or cytosol) of a bacterial cell to its exterior. Secretion is a very important mechanism in bacterial functioning and operation in their natural surrounding environment for adaptation and survival.
Secretion in eukaryotic cells
Eukaryotic cells, including human cells, have a highly evolved process of secretion. Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum (ER). As they are synthesized, these proteins translocate into the ER lumen, where they are glycosylated and where molecular chaperones aid protein folding. Misfolded proteins are usually identified here and retrotranslocated by ER-associated degradation 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 glycosylation of the proteins is modified 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, there is vesicle fusion with the cell membrane at a structure called the porosome, in a process called exocytosis, dumping its contents out of 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 evolve into lysosomes, which have a pH of 4.8.
There are many proteins like FGF1 (aFGF), FGF2 (bFGF), interleukin-1 (IL1) etc. which do not have a signal sequence. They do not use the classical ER-golgi pathway. These are secreted through various nonclassical pathways.
At least four nonclassical (unconventional) protein secretion pathways have been described. They include 1) direct translocation of proteins across the plasma membrane likely through membrane transporters, 2) blebbing, 3) lysosomal secretion, and 4) release via exosomes derived from multivesicular bodies. In addition, proteins can be released from cells by mechanical or physiological wounding and through nonlethal, transient oncotic pores in the plasma membrane induced by washing cells with serum-free media or buffers.
Secretion in human tissues
Many human cell types have the ability to be secretory cells. They have a well-developed endoplasmic reticulum and Golgi apparatus to fulfill their function. Tissues in humans that produce secretions include the gastrointestinal tract which secretes digestive enzymes and gastric acid, the lung which secretes surfactants, and sebaceous glands which secrete sebum to lubricate the skin and hair. Meibomian glands in the eyelid secrete sebum to lubricate and protect the eye.