Endocytosis is the uptake of liquid or solid substances by a cell. In this process, phagocytosis is used by the cell to take up solid particles, whereas pinocytosis is used to internalize dissolved molecules.
What is endocytosis?
Endocytosis is the uptake of liquid or solid substances by a cell. Eukaryotic cells have a semipermeable cell membrane that only a few particles can pass through. In order for macromolecules to enter the cell, the membrane must be penetrated. Endocytosis allows particles to be taken up from the extracellular space, and there are two different forms of endocytosis – pinocytosis and phagocytosis. Endocytosis is mainly used to take up fluids and macromolecules, eliminate pathogens and maintain metabolism. In addition, endocytosis plays an important role in relaying extracellular signals.
Function and role
Endocytosis is used to take larger particles, macromolecules, and molecules into the cell, which occurs by means of transport vesicles. After the signaling molecules are bound to the surface of the cell, the cell membrane is indented, enclosing the absorbed cargo. A vesicle called an endosome forms inside the cell. Thousands of these vesicles then transport the cargo through a cell, where it is either recycled or degraded. Endocytosis allows for controlled uptake, and also plays an essential role in immune response, tissue and cell development, cell communication, and signal transduction. Furthermore, it is also involved in neuronal signal transduction. Endocytosis can be used to repel microorganisms; however, it is also possible for viruses or unwanted microorganisms to enter the cell by the endocytic route. Overall, two different forms of endocytosis can be distinguished: pinocytosis and phagocytosis. By means of phagocytosis, larger particles can be internalized, including, for example, macrophages or leukocytes, also called phagocytes. Phagocytosis is primarily used for food intake and disposes of degenerated cells and extracellular debris. Phagocytosis is mediated by the Fc receptor, which recognizes particles labeled with IgG molecules. Phagocytosis is also referred to as “foreign body uptake” because the cell incorporates foreign material. This ability is possessed by unicellular or few-celled eukaryotes, which include algae or fungi, for example. Phagocytosis allows the body to fight exogenous antigens, such as bacteria. The MHC-II receptors remember the particles that have been fragmented so that they can be repelled if they are attacked again. In the human body, there are several types of cells capable of phagocytosis. These include:
- Dendritic cells
- Macrophages contained in the tissue
- Monocytes
- Granulocytes
The process of phagocytosis is closely related to immunity in humans. Therefore, cells that have the ability to undergo phagocytosis play an important role in the defense against viral or bacterial diseases. Pinocytosis involves the uptake of extracellular fluid, and within a very short time the cell internalizes the extracellular fluid and the substances dissolved in it. This process is also known as fluid phase endocytosis. In eukaryotic cells, there are four distinct forms of pinocytosis: macropinocytosis, clathrin-dependent endocytosis, caveolae-mediated endocytosis, and clathrin- and caveolae-independent endocytosis. In macropinocytosis, fusion of the plasma membrane with the long membrane protrusions occurs, entrapping much extracellular fluid. By means of clathrin-dependent endocytosis, extracellular molecules are internalized. This allows continuous uptake of important substances such as iron. Caveolae are invaginations of the plasma membrane that have the shape of a bottle and fulfill numerous functions in the cell. For example, they are responsible for signal transduction. However, caveolae are internalized very slowly in cells, so that large amounts of extracellular fluid are not taken up by caveolea-mediated endocytosis.Clathrin-independent mechanisms are found in neuroendocrine cells and in neurons, where they are involved in the reuptake of proteins into the plasma membrane.
Diseases and disorders
Endocytosis is a cellular process by which signals are transmitted and food is absorbed. If a disruption of this process occurs, diseases may result. Several diseases can be attributed to a defect in membrane transport, including tumors, infections, and neurogenerative diseases. For example, mutations in Rab family genes cause Charcot-Marie-Tooth neuropathy. This syndrome is a disease of the peripheral nervous system in which the ability to walk is limited. Foot deformities occur and muscles fatigue very quickly. Muscle atrophy may also occur in the feet and lower legs or in the forearms and hands. In addition, the nerve conduction velocity is reduced and the affected persons suffer from sensory disturbances. Muscle reflexes are weakened or absent altogether, and skeletal deformities may also occur in the course of life. Endocytosis is also disturbed in Huntington’s disease. Huntington’s is also a neurodegenerative disease in which nerve cells die and symptoms such as dementia, movement disorders or changes in character occur. Huntington’s is an inherited disease caused by the protein huntingtin. In affected individuals, the base triplet CAG occurs up to 250 times, whereas in healthy individuals it occurs only 9 to 35 times. The first symptoms usually become apparent between the ages of 30 and 40, although the disease can last up to 20 years and ultimately has a fatal course.
