Pinocytosis is a process where cells absorb small particles and dissolved fluids into themselves through the formation of a cavity called the intussusception. It is a type of endocytosis, different from phagocytosis, and involves the creation of small, uniform vesicles. Pinocytosis occurs continuously and requires the replacement of the cell membrane through exocytosis. Harmful microorganisms can also use pinocytosis to enter cells and take over their metabolism.
Pinocytosis is a process by which cells absorb small particles into themselves. These are usually substances that dissolve in a fluid and are found immediately around the cell. A cell is surrounded by an enclosing barrier known as a membrane and, during pinocytosis, forms a cavity called the intussusception. Invagination deepens and closes around external substances. This means that the substances inside the intussusception are brought into the cell.
When cells consume substances and carry them inside the cell, this is known as endocytosis. There are two main types of endocytosis, known as phagocytosis and pinocytosis. Phagocytosis refers to a process that is more similar to eating, where large particles are taken inside the cell. It could be other dead cells or microbes. They are contained in a small cell membrane bubble known as a vesicle, which is relatively large in the case of phagocytosis.
Unlike phagocytosis, pinocytosis involves the consumption of small particles and dissolved fluids, and a pinocytotic vesicle is typically smaller than a phagocytotic one. While most human cells use pinocytosis continuously to absorb substances dissolved in the fluid, phagocytosis tends to be used only by specialized cells. In addition to being small, pinocytotic vesicles are generally uniform in size, unlike phagocytotic vesicles which must enlarge until they can accommodate a microbe or cell. Since pinocytosis occurs continuously and involves introducing parts of the cell membrane into the cell, the membrane must be replaced in order for the cell to remain intact. This is accomplished by a process known as exocytosis, which is the reverse of endocytosis, and by which substances are moved out of cells.
There are several mechanisms for pinocytosis. Certain sections of the cell membrane form specialized regions called clathrin-coated dimples. As these pits form, they invaginate to create vesicles containing fluid from outside the cell. Cell membranes also contain small cavities known as caveolae, which are thought to collect proteins inside them when they invaginate. The vesicles that are created transport the substances to the compartments inside the cell or to the membrane on the other side.
Harmful microorganisms are able to use pinocytosis for their own purposes. Sometimes, viruses enter cells inside caveola vesicles. They are then able to release their genetic information into the cell, taking over its metabolism and directing it to produce more virus particles.
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