The term pinocytosis derives from the Greek word “pinein,” which translates to the German verb “to drink,” and “kytos,” which means “cavity” or “cell.” Cells take up liquids (pinocytosis) and solids (phagocytosis) in the form of small vesicles from their surrounding medium.
What is pinocytosis?
Cells take up liquids (pinocytosis) and solids (phagocytosis) in the form of small vesicles from their surrounding medium. Two synonyms of the term are cell drinking and hydrophatocytosis. Pinocytosis takes up liquid and phagocytosis takes up solid components from the extracellular space. Both processes are grouped under the generic term endocytosis. The uptake of fluid into the cytoplasm occurs in the form of small vesicles that are only 150 nm in diameter. The liquid or dissolved components are absorbed into the cystol of the cells. Membrane extensions enclose the substance to be taken up, where it is absorbed as vesicles into the cystol of the cell. There it is enzymatically processed. The absorbed substances are incorporated into the metabolism of the cells. Pinocytosis plays an important role in the uptake of protein-bound lipids into hepatocytes and enterocytes. In the reversal process, the absorbed fluids are released from the cell. This is a biological process that belongs to the field of cell biology (cytology).
Function and task
Endocytosis allows macromolecules and larger particles to be taken up into the cells. This process occurs through transport vesicles (small vesicles). The signaling molecules are bound to the cell surface, the cell membrane is indented, and the ingested contents are trapped. An endosome in the form of a vesicle is formed in the cell. Thousands of these small vesicles now transport the cargo through the cell, either being recycled or degraded. Through this cellular process, controlled uptake of fluids and solids is possible. Thus, pinocytosis plays an important role in cell and tissue development, immune response, cell communication and signal transduction. Furthermore, it is also involved in signal transduction in neuronal areas. Microorganisms can be repelled, although it cannot be ruled out that harmful microorganisms and viruses can enter cells through the endocytic transmission pathway. Phagocytosis internalizes larger particles, for example leukocytes and macrophages. It disposes of degenerate cells, extacellular debris, and serves as a food scavenger. Pinocytosis involves extracellular fluid uptake along with the solutes in it. Subsequently, the fluid and solutes are internalized. Medicine also refers to this process as fluid phase endocytosis. Eukaryotic cells recognize four different types of pinocytosis: clathrin-dependent endocytosis, macropinocytosis, caveolae- and clathrin-independent endocytosis, and caveolae-mediated endocytosis. In macropinonocytosis, plasma membranes fuse with long membrane protrusions to include a larger amount of extracellular fluid. Clathrin-dependent endocytosis internalizes extracellular molecules. Substances important to the human organism, such as iron, are taken up through this process. A caveolae is an invagination of the plasma membranes in the shape of a bottle. It performs many functions within the cell and is responsible for signal transduction. Internalization of caveolae within cells is slow. For this reason, caveolae-mediated endocytosis takes up only small amounts of extracellular fluid. Clathin-independent mechanisms are located in neurons and neuroendocrine cells, where they support the reuptake of proteins into the plasma membrane. With the reversal process of exocytosis, the vesicle is released from the cell again. The word component “exo” means “out.” The membrane of a vesicle fuses with a cell membrane, allowing the substance taken up by the vesicle to exit. This process is stimulated by certain membrane receptors. Some cells are able to internalize 25 percent of their membrane, always returning the same amount of membrane. This process occurs mostly through clathrin-coated vesicles that fuse with endosomes.Lipid membranes form the basis for the vesicles that seal off a lumen (clear width of a cell). By constricting cell compartments, the vesicles migrate to their destination to fuse with the cell membrane. Proteins assist in this process by pinching off a vesicle from a flat membrane.
Diseases and disorders
In pinocytosis, ingested food is first absorbed by droplets into an environmental medium in the food vacuole. Digestion of the ingested food begins by lysosomes (vesicles) containing digestive enzymes based on a fusion between the membrane and food vacuole. Pinocytosis transfers the digested food from the digestive vacuole into the plasma of the cell. Food residues that cannot be digested are transported to the cell membranes by means of a defecation vacuole and emptied to the outside. If this process is disturbed, various diseases can occur that are attributed to a defect in membrane transport. For example, dementia, Alzheimer’s disease, metabolic disorders, elevated cholesterol levels, neurological disorders (impaired muscle reflexes, sensory disturbances), Huntington’s disease (neuronal cell death), changes in temperament, and various types of physical and mental disabilities, such as Charcot-Marie-Tooth neuropathy syndrome, may be related to defective pinocytosis.
