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Exocytosis is the process by which a large amount of molecules are released; thus it is a form of bulk transport. Exocytosis occurs via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structures at the cell plasma membrane, where secretory vesicles transiently dock and fuse to ...
When a vesicle is produced inside the cell and fuses with the plasma membrane to release its contents into the extracellular space, this process is known as exocytosis. In the reverse process, a region of the cell membrane will dimple inwards and eventually pinch off, enclosing a portion of the extracellular fluid to transport it into the cell.
Axon terminals are specialized to release neurotransmitters very rapidly by exocytosis. [1] Neurotransmitter molecules are packaged into synaptic vesicles called quanta that cluster beneath the axon terminal membrane on the presynaptic side (A) of a synapse.
There are two types of exocytosis: Constitutive secretion and Regulated secretion. In both of these types, a vesicle buds from the Golgi Apparatus and is shuttled to the plasma membrane, to be exocytosed from cell. Exocytosis of lysosomes commonly serves to repair damaged areas of the plasma membrane by replenishing the lipid bilayer. [5]
In both images neurons are stained with a somatodendritic marker, microtubule associated protein (red). In the right image, synaptic vesicles are stained in green (yellow where the green and red overlap). Scale bar = 25 μm. [3] Synaptic vesicles are relatively simple because only a limited number of proteins fit into a sphere of 40 nm diameter.
Consequently, exocytosis releases acetylcholine in packets that are called quanta. The acetylcholine quantum diffuses through the acetylcholinesterase meshwork, where the high local transmitter concentration occupies all of the binding sites on the enzyme in its path.
Kiss-and-run fusion is a type of synaptic vesicle release where the vesicle opens and closes transiently. In this form of exocytosis, the vesicle docks and transiently fuses at the presynaptic membrane and releases its neurotransmitters across the synapse, after which the vesicle can then be reused.
In other words, bulk transport is a type of transport which involves the transport of large amount of substance like lipid droplets and solid food particles across plasma membrane by utilising energy. Special processes are involved in the transport of such large quantities of materials, which include endocytosis and exocytosis.