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Secretory Vesicles, Exocytosis, pimocytosis and endocytosis


Under appropriate stimuli, the secretory vesicles or vacuoles move towards and fuse with the plasma membrane. This movement is created by cytoplasmic contractile elements; the microtubule system. The inner membrane of the vesicle fuses with outer plasma membrane, while cytoplasmic side of vesicle fuses with cytoplasmic side of plasma membrane. Thus the contents of vesicles are externalized. 

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This process is called exocytosis or reverse pinocytosis. Release of trypsinogen by pancreatic acinar cells; release of insulin by beta cells of Langerhans and release of acetyl choline by presynaptic cholinergic nerves are examples of exocytosis. Often, hormones are the signal for exocytosis, which leads to calcium ion changes, triggerring the exocytosis.

Endocytosis

Endocytosis is the mechanism by which cells internalize extracellular macromolecules, to form an endocytic vesicle. This requires energy in the form of ATP as well as calcium ions in the extracellular fluid. Cytoplasmic contractile elements take part in this movement. In general, plasma membrane is invaginated, enclosing the matter. This forms the endocytic vesicle. The endocytosis may be pinocytosis or phagocytosis or receptor mediated endocytosis

Pinocytosis

Pinocytosis literally means ‘drinking by the cell'. Cells take up fluid by this method. The fluid phase pinocytosis is a nonselective process.

Receptor Mediated Endocytosis




The selective or adsorptive pinocytosis is receptor mediated; also called as absorptive pinocytosis. Low Density Lipoprotein (LDL) is a good example. LDL binds to the LDL receptor and the complex is later internalized. The cytoplasmic side of these vesicles are coated with filaments; mainly composed of Clathrin. These are called Clathrin coated pits. After the LDL-receptor complex is internalized, the receptor molecules are released back to cell surface; but the LDL is degraded by lysosomal enzymes. 

Several hormones are also taken up by the cells by receptor-mediated mechanism. The protein, Dynamin which has GTPase activity, is necessary for the internalisation of clathrin coated pits. Many viruses get attached to their specific receptors on the cell membranes. Examples are Influenza virus, Hepatitis B virus, polio virus and HIV. They are taken up by caveolae mediated processes. Caveolae mediated endocytosis is also known as potocytosis.

Phagocytosis

The term is derived from the Greek word "phagein" which means to eat. It is the engulfment of large particles such as bacteria by macrophages and granulocytes. They extend pseudopodia and surround the particles to form phagosomes. Phagosomes later fuse with lysosomes to form phagolysosomes, inside which the particles are digested. An active macrophage can ingest 25% of their volume per hour. In this process, 3% of plasma membrane is internalized per minute. The biochemical events accompanying phagocytosis is described as respiratory burst


Secretory Vesicles, Exocytosis, pimocytosis and endocytosis Secretory Vesicles, Exocytosis, pimocytosis and endocytosis Reviewed by gafacom on August 31, 2019 Rating: 5

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