Endocytosis is a form of active transport in which a cell transports molecules (such as proteins) into the cell (endo- + cytosis) by engulfing them in an energy-using process.
Endocytosis and its counterpart, exocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane by passive means.
Endocytosis pathways can be subdivided into four categories: namely, clathrin-mediated endocytosis, caveolae, macropinocytosis, and phagocytosis.
Clathrin-mediated endocytosis is mediated by small (approx. 100 nm in diameter) vesicles that have a morphologically characteristic coat made up of a complex of proteins that are mainly associated with the cytosolic protein clathrin.
Caveolae are the most common reported non-clathrin-coated plasma membrane buds, which exist on the surface of many, but not all cell types.
Macropinocytosis, which usually occurs from highly ruffled regions of the plasma membrane, is the invagination of the cell membrane to form a pocket, which then pinches off into the cell to form a vesicle (0.5–5 µm in diameter) filled with a large volume of extracellular fluid and molecules within it (equivalent to ~100 CCVs).
Phagocytosis is the process by which cells bind and internalize particulate matter larger than around 0.75 µm in diameter, such as small-sized dust particles, cell debris, micro-organisms and apoptotic cells.
Components of Endocytic Pathway
The endocytic pathway of mammalian cells consists of distinct membrane compartments, which internalize molecules from the plasma membrane and recycle them back to the surface (as in early endosomes and recycling endosomes), or sort them to degradation (as in late endosomes and lysosomes).