Pinocytosis is a process by which cells take small particles into themselves. These are usually substances which are dissolved in fluid and are found immediately around the cell. A cell is surrounded by an enclosing barrier known as a membrane and, during pinocytosis, this forms a hollow called an invagination. The invagination deepens and closes around the outside substances. This means that the substances inside the invagination are brought into the cell.
When cells consume substances and bring 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 which is more like eating, where large particles are taken inside the cell. These could be other, dead cells or microbes. They are contained in a small bubble of cell membrane known as a vesicle, which is relatively large in the case of phagocytosis.
Unlike phagocytosis, pinocytosis involves the consumption of small dissolved particles and fluid, and a pinocytotic vesicle is typically smaller than a phagocytotic one. While most human cells use pinocytosis continually in order to take in substances dissolved in fluid, phagocytosis tends to be used only by specialized cells. As well as being small, pinocytotic vesicles are usually of a uniform size, unlike phagocytotic vesicles which must enlarge until they can take in a microbe or cell. Since pinocytosis takes place continuously and involves taking parts of the cell membrane into the cell, the membrane has to be replaced for the cell to remain intact. This is achieved by a process known as exocytosis, which is the reverse of endocytosis, and by which substances are moved out of cells.
A number of different mechanisms for pinocytosis exist. Certain sections of the cell membrane form specialized regions called clathrin-coated pits. Almost as soon as they are formed, these pits 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 which are created transport substances to 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 gain entry to cells inside caveolae vesicles. They are then able to release their own genetic information into the cell, taking over its metabolism and directing it to manufacture more virus particles.