Exocytosis is the process of releasing substances from inside the cell to the outside. This complex process occurs in several steps. A distinction is made between constitutive exocytosis and stimulated exocytosis.
What is exocytosis?
Exocytosis is the process of releasing substances from inside the cell to the outside. Figure shows cell interior with components. The discharge of substances from the cell is defined as exocytosis. The process is very fast and compact. Substances are released from the intracellular space, i.e. from inside the cell, into the intercellular space, outside the cell. Exactly the opposite is the case with endocytosis. Since by-products and waste products generated by metabolic processes of the cell cannot be permanently stored inside the cell, these products are removed. The Golgi apparatus (cell organelle that lies immediately to the nucleus and is responsible for protein adjustment) laces itself off with the waste products filled vesicles. These are the exosomes. These prevent the waste products from coming into contact with the cytoplasm (cell plasma). Preventing this is particularly important because by-products that are no longer needed could cause damage when they collide with cell organelles. When the exosome encounters the cell membrane, it fuses with it and empties the contents into the exterior of the cell.
Function and task
Exocytosis plays a major role not only in the removal of sheep substances from the cell. Exocytosis also plays an important role in the cell-controlled removal of and in the release of hormones and neurotransmitters (biochemical messenger for the transmission of information from one nerve to another). Two major types of exocytosis are distinguished: constitutive exocytosis is the process by which membrane proteins are integrated into the cell membrane and the biomembrane (separating layer between cell compartments) is renewed or expanded. The process is referred to as cell membrane biogenesis. Constitutive exocytosis is particularly important in the cells of supporting and connective tissues, as proteins are also released to the outside. In stimulated exocytosis, the specific stimulus is a hormone. It is located at a receptor (a cell’s target molecule sensitive to specific stimuli) on the cell surface and triggers a signal inside the cell. This plays an important role in the release of hormones into the blood and in the release of digestive secretions into the food mush in the digestive system. An important example of stimulated exocytosis is insulin release. Insulin release is a process of exocytosis. Insulin is produced in the pancreas. The secretion is stimulated by increased glucose levels and also by free fatty acids and amino acids. The beta cells produce more adenosine triphosphate and this leads to a blockade of potassium-dependent channels. Insulin secretion is activated as calcium ions from the extracellular space enter the beta cells. The insulin vesicles fuse with the cell membrane of the beta cell and empty outward. Insulin secretion has begun. The insulin ensures a balanced blood glucose level. If this process is disturbed, there is a risk of diabetes. Part of the sperm, which consists of the secretion of the prostate, is also related to exocytosis. The produced secretion is transported out of the cells to the ureter by exocytosis. For hormone secretion, exocytosis takes a special place. The process of hormone release is analogous. The triggering signal is an electrical impulse in the releasing cell. The hormone, such as epinephrine, is released into the bloodstream after being released into the tissue environment. It triggers a different response, depending on the target organ. Neurotransmitters, along with hormones, are also an important product of exocytosis. They transmit the electrical nerve impulse between nerve cells. To date, there are a large number of neurotransmitters that have a positive effect on the human body. Probably the most important neurotransmitter of the peripheral nervous system is acetylcholine. This neurotransmitter enables the transmission of nerve impulses to the muscles. If the system gets out of kilter, symptoms of Parkinson’s disease, for example, can be triggered by a lack of dopamine in the brain. Glutamate plays an important role in the brain.This messenger substance is necessary for the control of movement, for sensory perception and also for memory. Thus, in Alzheimer’s patients, the release and uptake of glutamate is impaired.
Diseases and disorders
Exocytosis of neurotransmitters can be prevented by toxins in the body. For example, toxins from the bacteria of tetanus have a toxic effect. This results in convulsions and paralysis. The inherited metabolic disease cystic fibrosis is also a cause of improper exocytosis. The affected cells cannot penetrate into the surrounding tissue. As a result, the bronchial secretion, the secretion of the pancreas, the bile and the internal genital organs become viscous and dysfunctions occur in the affected organs. Generally, the viruses leave the host cell by exocytosis and therefore infect foreign cells. To stop the multiplication of the viruses, antivirals are taken. These are drugs that inhibit multiplication. To prevent many diseases, it is now possible to be vaccinated. Vaccination prepares the immune system to fight off the pathogens. The immune system recognizes foreign structures and forms antibodies. In the case of very complicated viruses, such as the HI virus (HIV) or hepatitis C, this is not yet possible. Since viruses can change in an unforeseeable time, the development of a vaccine is very difficult. The growing knowledge about the functions of neurotransmitters also provides starting points for the development of effective drugs, such as for depression.
