Eukaryotic cells have a semipermeable cell membrane that only a few particles can pass through. In order for macromolecules to get into the cell, the membrane has to be penetrated. Particles from the extracellular space can be taken up by endocytosis, whereby there are two different forms of endocytosis – pinocytosis and phagocytosis.

With the help of endocytosis, mainly liquids and macromolecules are absorbed, pathogens are eliminated and the metabolism is maintained. In addition, endocytosis plays an important role in the transmission of extracellular signals.

Function & task

With the help of endocytosis, larger particles, macromolecules and molecules are taken up into the cell, which happens by means of transport vesicles. After the signaling molecules have been bound to the surface of the cell, the cell membrane is invaginated and the picked-up charge is enclosed. A vesicle called an endosome forms inside the cell. Thousands of these vesicles then carry the cargo through a cell, either recycling it or breaking it down.Controlled uptake is possible through endocytosis, and it also plays an essential role in the immune response , tissue and cell development, cell communication and signal transduction . It is also involved in neuronal signal transmission.

Microorganisms can be repelled with the help of endocytosis, but it is also possible for viruses or undesirable microorganisms to enter the cell via the endocytic route.

Overall, two different forms of endocytosis can be distinguished: pinocytosis and phagocytosis. Larger particles can be internalized by means of phagocytosis, including, for example, macrophages or leukocytes , also called phagocytes.

Phagocytosis is primarily used for food intake and disposes of degenerated cells and extracellular deposits. Phagocytosis is mediated by the Fc receptor, which recognizes particles labeled with IgG molecules.

Phagocytosis is also known as “foreign body uptake” because the cell incorporates foreign material. This ability is possessed by unicellular or few-celled eukaryotes, which include, for example, algae or fungi.

With the help of phagocytosis, the body can fight exogenous antigens, such as bacteria . The MHC-II receptors remember the particles that have been broken up so that they can be repelled in the event of another infestation.

In the human body there are different types of cells capable of phagocytosis. These include:

• dendritic Zellen
• macrophages , which are contained in tissues
• monocytes
• granulocytes

The process of phagocytosis is closely related to human immunity . Therefore, cells that have the ability to phagocytosis play an important role in the defense against viral or bacterial diseases .

Extracellular fluid is taken up by pinocytosis, with the cell internalizing the extracellular fluid and the substances dissolved in it within a very short time. This process is also known as fluid phase endocytosis.

There are four distinct forms of pinocytosis in eukaryotic cells: macropinocytosis, clathrin-dependent endocytosis, caveolae-mediated endocytosis, and clathrin- and caveolae-independent endocytosis.

During macropinocytosis, the plasma membrane fuses with the long membrane protuberances, which traps a large amount of extracellular fluid.

Extracellular molecules are internalized by clathrin-dependent endocytosis. As a result, important substances such as iron can be continuously absorbed.

Caveolae are invaginations in 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 the 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 & Ailments

Endocytosis is a cellular process through which signals are transmitted and food is absorbed. If this process is disrupted, disease can result.A number of diseases can be traced back to a defect in membrane transport , including, for example, tumors , infections or neurodegenerative diseases. Charcot-Marie-Tooth neuropathy , for example, is caused by mutations in the genes of the Rab family .

This syndrome is a peripheral nervous system disorder in which the ability to walk is impaired. Foot deformities occur and the muscles tire very quickly. Muscle wasting can also occur in the feet and lower legs , or in the forearms and hands . In addition, the nerve conduction speed is reduced and those affected suffer from sensory disturbances . Muscle reflexes are weakened or absent altogether, and skeletal deformities can also occur over 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 personality changes occur.

Huntington’s is an inherited disease caused by the protein huntingtin. The CAG base triplet occurs up to 250 times in those affected, whereas it occurs only 9 to 35 times in healthy people. The first symptoms usually become noticeable between the ages of 30 and 40, although the disease can last up to 20 years and is ultimately fatal.