Vesicular Transport

The membraneprotein, clathrin is involved in receptor mediated endocytosis. 

Clathrin-mediated endocytosis occurs at membrane indentations where the protein clathrin accumulates. Clathrin molecules have the shape of triskelions, with three “legs” radiating from a central hub. As endocytosis progresses, the clathrin molecules form a geometric array that surrounds the endocytotic vesicle. At the neck of the vesicle, the GTP binding protein dynamin is involved, either directly or indirectly, in pinching off the vesicle.

Once the complete vesicle is formed, the clathrin falls off and the three-legged proteins recycle to form another vesicle. The vesicle fuses with and dumps its contents into an early endosome. From the early endosome, a new vesicle can bud off and return to the cell membrane. Alternatively, the early endosome can become a late endosome and fuse with a lysosome in which the contents are digested by the lysosomal proteases. 

Clathrin-mediated endocytosis is responsible for the internalization of many receptors and the ligands bound to them—including, for example, nerve growth factor (NGF) and low-density lipoproteins. It also plays a major role in synaptic function.

Also Know:

In exocytosis the cytoplasmic sides of two membranes fuse, whereas in endocytosis two non cytoplasmic sides fuse.

Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 2. Overview of Cellular Physiology in Medical Physiology. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong’s Review of Medical Physiology, 24e.

Clathrin-mediated endocytosis occurs at membrane indentations where the protein clathrin accumulates. Clathrin molecules have the shape of triskelions, with three “legs” radiating from a central hub. As endocytosis progresses, the clathrin molecules form a geometric array that surrounds the endocytotic vesicle.

The membrane of the zymogen granule fuses with the apical cell membrane to release the proenzymes synthesized by the pancreatic acinar cells into the acinar lumen. This is exocytosis.

• Clathrin-mediated endocytosis occurs at membrane indentations where the protein clathrin accumulates. It has shape of triskelions, with three legs radiating from a central hub. 
• Clathrin-mediated endocytosis is responsible for the internalization of many receptors and the ligands bound to them. Eg nerve growth factor and LDL. It also plays a major role in synaptic function.

Preventing the flow of Ca2+ into the cell would prevent the release of transmitter, because Ca2+ initiates the intracellular events leading to the docking of the vesicle to its binding site on the active zone. Although Ca2+ normally enters the cell through voltage-operated channels that are opened by the depolarization of the nerve terminal that occurs as the action potential propagates along the nerve axon, release of transmitter will not occur if Ca2+ does not enter the nerve terminal.

The flow of Na+ into the nerve terminal would depolarize the membrane and open Ca2+ channels, leading to Ca2+ entry and exocytosis. However, Na+ entry does not directly stimulate exocytosis. K+ does not affect the nerve terminal membrane.

C i.e. Ca++

A i.e. Extrusion of cell bound vesicles

A i.e. Receptor mediated endocytosis

Fate: Most endocytic vesicles fuse with primary lysosomes (early endosome) to form secondary lysosomes which contain hydrolytic enzymes. & therefore are specialized orgenelle for intracellular disposal.

* ClathrinQ: Clathrin molecules have the shape of triskelion (three legs radiating from central hub). As endocytosis progresses, the clathrin molecule form a geometric array that surrounds the endolytic vesicle. Once complete vesicle is formed, it falls off & recycle to form another vesicle.

B i.e. Endocytosis C i.e. Exocytosis

The process of vesicular transport as shown in the picture represents Exocytosis.

• Exocytosis – Process by which a cell transports secretory products through the cytoplasm to the plasma membrane. 

• Secretory products are packaged into transport vesicles (membrane-bound spheres).

• Simple terms, “Extrusion of cell-bound vesicles” / “Expulsion of cell contents”