Endocytosis is a process whereby cells absorb material (molecules such as proteins) from outside by engulfing it with their cell membrane. It is used by all cells of the body because most substances important to them are polar and consist of big molecules, and thus cannot pass through the hydrophobic plasma membrane. Endocytosis is the opposite of exocytosis, and always involves the formation of a vesicle from part of the cell membrane.
The absorption of material from the outside environment of the cell is commonly divided into two processes, phagocytosis and pinocytosis.
- Phagocytosis (literally, cell-eating) is the process by which cells ingest large objects, such as cells which have turned into apoptosis or bacteria and virusses. The membrane folds around the object, and the object is sealed off into a large vacuole known as phagosome. Phagocytosis is not considered to be an endocytic process.
- Pinocytosis (literally, cell-drinking) is a synonym for endocytosis. This process is concerned with the uptake of solutes and single molecules such as proteins.
There are three types of endocytosis namely macropinocytosis, clathrin-mediated endocytosis and caveolar endocytosis.
- Macropinocytosis is the invagination of the cell membrane to form a pocket (vesicle) filled with extracellular fluid (and molecules within it). The filling of the pocket occurs in non-specific manner. The vesicle then travels into the cytosol and fuses with other vesicles such as endosomes and lysosomes.
- Clathrin-mediated endocytosis is the specific uptake of large extracellular molecules such as proteins, membrane localized receptors and ion-channels. Uptake of extracellular proteins is prompted after the binding of the protein to a receptor on the cell membrane. These receptors are associated with the cytosolic protein clathrin which initiates the formation of a vesicle by forming a crystaline coat on the membrane of the vesicle.
- Caveolae consist of the protein caveolin-1, cholesterol and glycosphingolipids. Caveolae form flask shaped pits into the membrane that resemble the shape of a cave (hence the name caveolae). Uptake of extracellurar particles is also specifically mediated via receptors present in caveolae.