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.

• Receptor-mediated endocytosis is a  process by which cells internalize receptor-bound ligands.
• Clathrin-mediated endocytosis is so far the best-characterized pathway by which cells internalize specific molecules and modulate the expression level of membrane-bound receptors. 
• Clathrin-mediated endocytosis occurs at membrane indentations where the protein clathrin accumulates. 
  • It has the 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++

Ans. A. i.e. Extrusion of cell-bound vesicles
 Exocytosis/emiocytosis/reverse pinocytosis:

• The reverse of endocytosis.
• Process of extrusion of secretory granules from the cell.
• On contact of the secretory vesicle with the cell membrane, vesicular fusing & subsequent vesicular content extrusion takes place à Extruded as secretory granules.

• Carrier protein involved – No
• The energy required – Yes
• Concentration gradient – N/A

Mechanisms:
1. Non-constitutive/regulated pathway:

• Processing & storage before the release occurs.
• Eg: Release of mature hormones from the Golgi apparatus.

2. Constitutive pathway:

• Direct release without storage/processing occurs.
• Eg: Neurotransmitter release from presynaptic 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.

VESICULAR TRANSPORT

• Transports macromolecules/large protein molecules in & out of cells, transported as a vesicle

3 mechanisms –

• Endocytosis
• Exotoxsis.
• Transcytosis

1. Endocytosis

• The cell takes contents in (ie., from ECF to cytoplasm).

Steps:

• On contact with large molecules, the cell membrane invaginates forming vesicle, including macromolecule.
• Vesicle is pinched within cell & restoring cell membrane.
• Requires energy, Ca⁺⁺ & contractile elements in cell.

II) Exocytosis/emiocytosis/reverse pinocytosis:

• The reverse of endocytosis.
• Process of extrusion of secretory granules from the cell.
• On contact of the secretory vesicle with the cell membrane, vesicular fusing & subsequent vesicular content extrusion takes place and extruded as secretory granules.

• Carrier protein involved – No
• The energy required – Yes
• Concentration gradient – N/A

• Exocytosis is a reverse of endocytosis.
• This process is involved when secretory granules are extruded from the cell.
• When secretory vesicle comes in contact with the inside of the cell membrane, the membrane of the vesicle fuses with the cell membrane and then there is extrusion of the contents of the vesicle from the cells as secretory granules.
• Similar to endocytosis, exocytosis requires Ca2+ and energy.
• Exocytosis is also called emiocytosis or reverse pinocytosis.