Endocytosis

Endocytosis is a form of active transport in which a cell transports molecules (such as proteins) into the cell ( + 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 includes pinocytosis (cell drinking) and phagocytosis (cell eating).

The term was proposed by De Duve in 1963.Phagocytosis was discovered by Élie Metchnikoff.

Endocytosis pathways can be subdivided into four categories: namely, receptor-mediated endocytosis, caveolae, macropinocytosis, and phagocytosis.

More recent experiments have suggested that these morphological descriptions of endocytic events may be inadequate, and a more appropriate method of classification may be based upon the clathrin-dependence of particular pathways, with multiple subtypes of clathrin-dependent and clathrin-independent endocytosis. Mechanistic insight into non-phagocytic, clathrin-independent endocytosis has been lacking, but a recent study has shown how Graf1 regulates a highly prevalent clathrin-independent endocytic pathway known as the CLIC/GEEC 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). The principal components of the endocytic pathway are:

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