Exocytosis is a means of membrane transportation that expels the intracellular material out of the cell. The transport of the substance is mediated by the vesicles that eliminate the cell debris and releases specific proteins, enzymes, hormones etc. outside the cell. It is a type of active transport mechanism that makes the use of ATP.
The examples of exocytosis include the release of enzymes into the blood, neurotransmitter into the synaptic gap, and plasma membrane proteins out of the cell.
Exocytosis can define as the cellular mechanism, where the transport vesicles incorporate with the cell membrane and eliminate the materials out of the cell via constitutive, regulatory and lysosome mediated secretory pathway. Thus, exocytosis merely refers as the process that expels the biomolecules and metabolic waste out of the cell to the extracellular space.
Pathways of Exocytosis
Generally, exocytosis can be mediated by the given three pathways:
Constitutive secretory pathway
This kind of mechanism that can be carried out by all cells. It operates continuously to deliver the newly generated lipids, proteins etc. from the Golgi complex to the bilayer cell membrane.
Regulated secretory pathway
The specialized cells carry out this pathway by secreting specific hormones, proteins, enzymes, neurotransmitters etc. from the cytosol to the extracellular space. Regulated secretory pathway requires extracellular signals for the exudation of substances out of the cell. It only occurs in the secretory cells, not in all the cell types.
The specific substance will expel out of the cell when it is stimulated by the extracellular signals. In this kind of pathway, the secretory vesicles not completely incorporate with the cell membrane, it only fuses enough to expel the contents out of the cell. This kind of secretory vesicle reform and goes back to the cytosol once they deliver the material out of the cell.
Lysosome mediated secretory pathway
This pathway involves the incorporation of the lysosomes with the bilayer cell membrane. Lysosome mediated secretory pathway functions to exclude the undigested cellular debris out of the cell. A lysosome appears as a small vesicle that contains acid hydrolase enzyme, which function is to break down the waste materials, microbes etc. These carry the cellular debris to the bilayer cell membrane and fuse to expel the contents out to the extracellular matrix.
- The transportation in exocytosis occurs from the interior of the cell, i.e. cytosol to the exterior or extracellular space of the cell.
- Mechanism of exocytosis plays a significant role in the exudation of the metabolic waste out of the cell, chemical messaging between the cells etc.
- The vesicle forms through the process of exocytosis refer to as “Exocytic-vesicle”.
- The Golgi body, presynaptic neurons are the regions of the cell, which processes the formation of exocytic-vesicle.
- Exocytic vesicles require precursors released by the endoplasmic reticulum, which later processed by the Golgi body. Finally, the secretory vesicle buds off through the Trans face of the Golgi body.
- There are three pathways, namely constitutive, regulated and lysosome mediated secretory pathways that are the three primary mechanisms of exocytosis.
Steps of Exocytosis
Generally, exocytosis involves five steps for the expulsion of the intracellular substance.
Vesicle trafficking: It is the first step where the vesicles are transported to the cell membrane along with the microtubules of the cytoskeleton. Motor proteins, kinesins, dyneins and myosins mediate the movement of secretory vesicles.
Tethering: Once the vesicle reaches the cell membrane, it tends to contact with the cell membrane.
Docking: It is the third step, which involves the attachment between the cell membrane and the vesicle membrane. During this step, the phospholipid bilayer of both the membranes begins to merge.
Priming: It is the step that only occurs in the regulated exocytosis. Priming involves specific modifications in the molecules of the cell membrane, for the signalling process to stimulate exocytosis.
Fusion: Two types of fusion occur in exocytosis.
- Complete fusion: Here, the vesicle membrane entirely incorporates with the cell membrane. It makes the use of ATP that helps in the separation and fusion of the phospholipid membrane. The fusion results in a fusion pore that expels the content out of the cell membrane.
- Kiss and run fusion: It’s the temporary fusion of the vesicle with the cell membrane and expels the contents out of the fusion pore. After the release of substances to the extracellular environment, the vesicle detaches from the bilayer cell membrane.
Exocytosis in Pancreas: When the blood sugar level becomes low, the islet alpha cells of pancreas release glucagon hormone via exocytosis. Then the glucagon hormone activates the hepatic cells of the liver to facilitate the transformation of stored glycogen into glucose. Then the glucose released by the liver cells finally goes into the bloodstream until it reaches a normal range.
Formation of Secretory Vesicle
The secretory vesicle forms by the protein packaging organelle, i.e. Golgi complex. The Golgi body releases the secretory vesicle from its Trans face. The release of a secretory vesicle can be calcium-dependent or independent.
The calcium-dependent cycle is a regulated secretory pathway that requires specialized cells for the release of vesicular contents. In contrast to this, the cells carrying out a constitutive pathway are calcium-independent. Once, the vesicles releases from the Golgi complex, it tends to move near to the cell membrane. Finally, there will be a fusion between the vesicle membrane and the cell membrane, and the contents will go out into the extracellular space.