Pinocytosis Definition

• Pino, which means “to drink” and cytosis which means “related to the cell” are the two terms that make up the phrase ‘Pinocytosis’.
• This is an endocytosis which involves the passage of minute particles floating in extracellular fluid via cell membrane pores.
• It is a general method of internalizing fluid and dissolved nutrients and occurs continuously in a majority of cells.
• The transport of many small molecules through a fluid is the focus of this process, which is also known as fluid endocytosis or bulk-phase endocytosis.
• Once within the cells, the chemicals form vesicles that are later joined with endosomes for different metabolic activities.

Process/ Steps of Pinocytosis

• The presence of the targeted molecule in the extracellular fluid triggers the pinocytosis process.
• The chemical, which at this stage serves as an inducer, attaches to the cell membrane, starting the process of pinocytosis.
• Proteins, sugar, ions, and other molecules can be among the molecules. Pinocytosis mostly pertains to the absorption of fat in humans.
• Although pinocytosis also includes molecules attaching to cell membrane receptors, in contrast to various receptor-mediated endocytosis processes, the receptor is not specific to a single type of molecule.
• The broad breakdown of the stages involved in pinocytosis is as follows:
  • • With commencement, the pinocytosis process gets underway. The chemicals attach to the cell membrane receptor and deliver instructions to the cell membrane for the subsequent procedure.
    • The extracellular fluid is then enveloped by tiny, open-ended pockets or folds in the cell membrane in order to be absorbed. Additionally, the extracellular fluid’s molecules are drawn into the pockets.
    • At the open end of the pocket, the cell membrane starts to join, forming a full invagination.
    • The cell membrane then pinches off at the pocket portion, becoming what is now referred to as a vesicle. The vesicle is a cell membrane-derived organelle that encloses the fluid carrying the required chemicals. The cytoplasm may be moved about inside the vesicles without impairing the cell’s regular operations.
    • The vesicles either travel toward the opposite end of the cell membrane for exocytosis or merge with the endosome depending on the function of the molecules.
• In the first instances, the vesicles in the cells join with the endosomes to hydrolyze the particles into smaller molecules that the cell can utilize.
• The extracellular fluid may also be expelled from the cell through exocytosis if the vesicles merely travel to the other end of the cell membrane. As membrane components are recycled back into the cell membrane, this process is crucial to maintaining the cell’s size.
• Occasionally, the molecules may be sent into the cytoplasm right away after the vesicle has formed.

Types of Pinocytosis

• The size of the molecules that need to be transported or the method of vesicle production are used to categorize pinocytosis.
• Pinocytosis can be divided into two categories based on the size of the molecules:

Macropinocytosis

• The molecules that need to be transported during macropinocytosis are rather big.
• The length of the newly generated vesicles may be between one and two micrometers.
• Large invaginations or pockets are created for the entrance of the molecules during macropinocytosis.
• Additionally, during macropinocytosis, ruffles are generated in the cell membrane.
• As the cell’s cytoskeleton rearranges the actin filaments in the cell membrane, these ruffles are produced.
• Macropinosomes, the vesicles produced by this process, develop in the cytoplasm and either join lysosomes or go toward the cell membrane to be recycled.
• White blood cells like macrophages and dendritic cells frequently go through this procedure.

Micropinocytosis

Smaller molecules must be transported during micropinocytosis.

The length of the vesicles that are generated is around 0.1 μm.

On the cell membrane, small depressions are created for the entrance of the molecules.

A typical instance of the micropinocytosis that forms in the blood vessel epithelium is caveolin-mediated pinocytosis.

The following categories are classified according to the receptors engaged in the process and the mechanism of vesicle formation:

• Clathrin-mediated pinocytosis
• Caveolin-mediated pinocytosis
• Clathrin- and caveolin- independent pinocytosis

Functions of Pinocytosis

• Pinocytosis is a type of active transport that is essential for cellular activities such as nutrition intake, waste elimination, and signal transduction.
• Diverse unicellular species use pinocytosis to absorb nutrients such as some sugars, the majority of amino acids, organic acids, and several inorganic ions.
• In higher species, pinocytosis is essential for the transfer of dissolved compounds such as lipids and vitamins.
• Due to the fact that pinocytosis is a non-specific absorption of molecules, it enables the simultaneous transport of a huge number of distinct molecules.
• Pinocytosis is also involved in the removal of waste products from cells, such as the removal of water and waste items from the kidney cells into the urine.
• Pinocytosis is utilised by immune system cells such as macrophages and dendritic cells to detect the presence of antigens in the extracellular fluid.

Examples of Pinocytosis

• In order to absorb nutrients from the gastrointestinal tract’s lumen, pinocytosis is seen in the small intestine’s microvilli.
• Similar to this, it is seen in the kidney’s duct cells while urine is being produced.
• The human egg in the female reproductive system uses pinocytosis to take in nutrients before fertilizations.
• Pinocytosis is used by unicellular organisms, such as amoebas, to absorb dissolved nutrients and water.
• The majority of cells in the body also engage in pinocytosis in order to recycle cell membrane components and preserve cell size.

References

Friedman, M. (2008). Principles and models of biological transport. Springer.

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