What is exocytosis? Exocytosis mechanism

What is exocytosis?

  • As compared to endocytosis, exocytosis is a process that is used to use energy to move materials from inside the cell to the outside of the cell. Therefore, it is a kind of active transport mechanism and the opposite of endocytosis. Generally, in this exocytosis mechanism, a special vesicle attached to the cell membrane and containing the cell particles will expel the cell contents to the outside of the cell.
  • This mechanism has been used in the elimination of cellular debris, transport of hormones and proteins for the transmission of chemical signals between cells and the construction of the cell membrane. The vesicles involved in exocytosis are made up of Golgi bodies, endosomes, and presynaptic neurons. These vesicles fuse with the cell membrane and, depending on their function, can be complete or temporary vesicles.
  • Normally, after endocytic pinocytosis and phagocytosis, the cell membrane is damaged and as a result, the exocytic process works to repair the cell membrane by transporting proteins and lipids to the membrane for repair mechanisms.
  • It is also the end-point mechanism for the transport of protein complexes and their packaging to their destinations and their insertion into the cell membrane.

Exocytosis mechanism

In summary, the mechanism of exocytosis includes:

  • The transport of cellular vesicles with molecules from the interior of the cell to the cell membrane.
  • The vesicle then attaches itself to the cell membrane.
  • When the vesicles fuse with the cell membrane, it allows the contents of the vesicles to be released outside the cell.

The Exocytotic Vesicle

  • The exocytotic vesicle contains protein elements derived from Golgi complexes, which are receptors for proteins and lipids synthesized in the endoplasmic reticulum.
  • The function of Golgi complexes is to classify and modify proteins and store them in the secretory vesicle that sprouts on the trans side of the Golgi apparatus.
  • However, not all vesicles that fuse with the cell membrane come into contact with the Golgi apparatus; some form from the early endosome, a membrane sac located in the cell cytoplasm. The vesicle fuses with the early endosome by endocytosis of the cell membrane, classifies internalized elements into proteins, lipids, and microbes, and guides them to their destinations via transport vesicles. The transport vesicles then sprout from the early endosome, transporting proteins and lipids to the cell membrane and waste products to the lysosomes, where they break down.
  • Another type of vesicle that does not arise from Golgi complexes is the one found at the synaptic ends of neurons.

There are three pathways involved in the exocytotic process.

  1. Constitutive exocytosis
  2. Regulated exocytosis
  3. Lysosome mediated exocytosis

Constructive exocytosis

With this type of exocytosis, membrane proteins and lipids are released to the cell membrane, and substances are also transported out of the cell. This is the most common way that all cells in the body do it.

Regulated Exocytosis

  • This mechanism is common in secretory cells, whose function is to store hormones, digestive enzymes, and neurotransmitters.
  • The secretion of these products must be triggered by extracellular signals to initiate the formation of secretory vesicles, which fuse for a long time with the cell membrane to allow the release of cell contents from the cell to the outside. After birth, the vesicles reform and return to the cytoplasm.
  • In general, this mechanism depends on extracellular signaling for the release of its contents to the outside of the cell.

Lysosome mediated Exocytosis

  • This process involves the fusion of cellular vesicles with cellular lysosomes. Lysosomes contain digestive enzymes and hydrolase enzymes, whose functions include breaking down cellular debris, microorganisms, and cellular debris. The lysosome transports the degraded elements to the cell membrane, where it fuses with the cell membrane and releases its elements into the extracellular cell matrix.

Of the three signaling pathways discussed, constructive exocytosis is the regular exocytotic mechanism that occurs in four stages, while regulatory exocytosis occurs in five stages. The steps include:

  • Vesicle transport: Cellular vesicles are transported through the microtubules of the cytoskeleton to the cell membrane using motor proteins such as kinesins, dyneins, and myosins.
  • Attachment: When cell vesicles reach the cell membrane, they are attracted to the cell membrane and eventually come into contact with each other.
  • Coupling: this is the connection of the vesicles to the cell membrane, which initiates the fusion of the phospholipids of the vesicular membrane with those of the cell membrane.
  • Onset: This step occurs in regulated exocytosis rather than constitutive exocytosis. Some changes take place in some of the elements of the cell membrane for exocytosis that aid the signaling processes that trigger exocytosis.
  • Fusion: Two types of fusion are involved in exocytosis, namely complete fusion and temporary fusion (kiss and run). Complete fusion involves the complete fusion of the vesicular membrane with the lipid cell membrane using energy (ATP) and they also use energy to separate both the vesicular membrane and the cell membranes after the contents are released. During fusion or temporary fusion, the vesicle temporarily fuses with the cell membrane to allow the formation of the fusion pore to release the cellular contents out of the cell. The vesicle is then detached from the cell membrane, where it reforms and returns to the cell cytoplasm.

Exocytosis Examples

  • A good example of exocytosis in the transport of glucagon from the pancreas of the islets of Langerhans to the liver, where it is broken down into glycogen, which in turn is broken down into glucose, which can be easily reabsorbed. The glucose is then released into the bloodstream.
  • In the islets of Langerhans, glucagon and insulin are stored in the secretory vesicles of the pancreas. When circulating glucose levels are low, glucagon is secreted by islet alpha cells, transported by secretory vesicles to liver cells, where it is released by exocytosis for further processing and use.
  • The pancreas also releases certain digestive enzymes through exocytosis.
  • Other exocytotic processes include synaptic vesicle exocytosis, in which a neurotransmitter-filled synaptic vesicle in the presynaptic neuron fuses with the presynaptic membrane and releases neurotransmitters into the synaptic cleft (the space between neurons). Neurotransmitters can bind to receptors on the postsynaptic neuron.

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What is exocytosis? Exocytosis mechanism

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