- Exocytosis is a type of process, in which involves the movement of materials from the inside of a cell to the exterior of the cell by the use of energy.
- Exocytosis is a type of active transport because to perform this process energy is required.
- It is a vital process of plant and animal cells as it performs the opposite function of endocytosis.
- In endocytosis process, the substances enter into the cell from the external environment.
- During the exocytosis, membrane-bound vesicles carrying cellular molecules are moved into the cell membrane. The vesicles combine with the cell membrane and discharge their contents to the exterior of the cell.
- Cell use Exocytosis for the removal of waste materials, transport hormones, and proteins, for chemical signaling between cells and the construction of the cell membrane.
- The Golgi bodies, endosomes, and presynaptic neurons help in the formation of vesicles which are involved in exocytosis process.
- The vesicles combine with the cell membrane and they can be complete or temporary vesicles, depending on their function.
- Normally the cell membrane gets damaged after the endocytic pinocytosis and phagocytosis, hence the exocytic process performs to repair the cell membrane by moving proteins and lipids to the membrane for repair mechanisms.
- It is also the end-point mechanism for the transportation of protein complexes and packaging them in their destined sites and insertion to the cell membrane.
Basic Mechanism of Exocytosis
- The first step of Exocytosis involves the transportation of vesicles containing molecules from the inside of the cell to the cell membrane.
- After that, the vesicle gets attached to the cell membrane.
- During the fusion of the vesicle membrane with the cell membrane, the vesicle contents are released outside the cell.
- The Exocytotic vesicles containing protein are typically originated from an organelle known as the Golgi apparatus, or Golgi complex, where newly synthesized proteins and lipids are transferred from the endoplasmic reticulum for modification and sorting.
- Once processed, the products are contained within secretory vesicles, which bud from the trans face of the Golgi apparatus.
- There are other vesicles which are fused with the cell membrane, they do not come directly from the Golgi apparatus, instead, they are formed from early endosomes, which are membrane sacs found in the cytoplasm.
- Early endosomes combine with the vesicles and are internalized by endocytosis of the cell membrane. These endosomes order the internalized material (proteins, lipids, microbes, etc.) and direct the materials to their own destinations.
- The Transport vesicles bud off from the early endosomes and send the waste material onto lysosomes for degradation while returning proteins and lipids to the cell membrane.
- Other examples of vesicles which are not derived from Golgi complexes are, the Vesicles located at synaptic terminals in neurons.
Types of Exocytosis
There are present three pathways within the Exocytosis processes such as;
- Constitutive exocytosis
- Regulated exocytosis
- Lysosome mediated exocytosis
- In constitutive exocytosis, involve the transfers of membrane proteins and lipids to the cell’s surface and to discharge materials to the cell’s exterior.
- It involves the regular secretion of molecules and this action is performed by all cells.
- This exocytosis process relies on the presence of extracellular signals for the expulsion of substances within vesicles.
- It commonly takes place within secretory cells and not in all cell types. These secretory cells mainly store hormones, neurotransmitters, and digestive enzymes.
- The secretion of these products from the secretory cells must be triggered by extracellular signals to start the development of secretory vesicles, which combine with the cell membrane for a long time to support the release of the cell contents out of the cell, into the exterior. After delivery, the vesicles are reformed and returned to the cytoplasm.
Lysosome mediated exocytosis
- This pathway involves the fusion of vesicles with lysosomes.
- Lysosomes contain different types of acid hydrolase enzymes which involve in the breakdown of waste materials, microbes, and cellular debris.
- The digested material is carried out by the lysosomes to the cell membrane and they get to fuse with the membrane, after that they release their contents into the extracellular matrix.
Steps Involve in Exocytosis
The constitutive exocytosis process takes place in four steps while the regulated exocytosis involves five steps. All of these steps are discussed below;
- Trafficking/vesicle trafficking: During this step involves the transfers of Vesicles to the cell membrane along with microtubules of the cytoskeleton and this process is powered by the motor proteins kinesins, dyneins, and myosins.
- Tethering: After transferring to the cell membrane, it is linked to and pulled into contact with the cell membrane.
- Docking: In this step, the vesicle membrane is get attached to the cell membrane. The attachment process involves the merging of phospholipid bilayers of the both vesicle membrane and cell membrane.
- Priming: This step takes place during regulated exocytosis. It involves specific modifications of certain cell membrane molecules which are required for signaling processes that will trigger the exocytosis process to take place.
- Fusion: There are involving two types of fusion that are taking place in exocytosis such as; complete fusion and kiss-and-run fusion. During complete fusion, a fusion occurs between the vesicle membrane and cell membrane as a result a fusion pore is formed, which permits the contents of the vesicle to be discharged as the vesicle becomes part of the cell membrane.ATP provides the energy which is required to separate and fuse the lipid membranes. During the kiss-and-run fusion, a fusion pore is formed when the vesicle temporarily fuses with the cell membrane. After that, it releases the contents through the fusion pore to the exterior of the cell. Then, the vesicle pulls away from the cell membrane and reforms before retreating to the interior of the cell.
Examples of Exocytosis
There are present numerous examples of Exocytosis, all of them are discussed below;
Exocytosis in the Pancreas
- The islets of Langerhans in the pancreas produce the hormones insulin and glucagon which are stored in secretory granules and released by exocytosis when signals are received.
- If the glucose concentration in the blood is started increase, insulin will be released from islet beta cells and it will trigger the cells and tissues to take up glucose from the blood.
- If the glucose concentrations become low, glucagon is secreted from islet alpha cells. This will lead the liver to convert stored glycogen to glucose. Glucose is then released into the blood causing blood-glucose levels to rise.
Exocytosis in Neurons
- The Synaptic vesicle exocytosis takes place within the neurons of the nervous system.
- The communication system of Nerve cells involves the passage of electrical or chemical (neurotransmitters) signals from one neuron to the next. The transmission of these Neurotransmitters is accomplished by the exocytosis process.
- The synaptic vesicles help to transport these chemical messages from nerve to nerve. Synaptic vesicles are basically a type of membranous sacs which are formed by endocytosis of the plasma membrane at presynaptic nerve terminals.