Autophagy

The term auto (self) phage (to eat) literally refers to a process whereby the cell digests its own contents. This process is most often refered to as " cellular turnover." Whatever it is called, it refers to a very important, normal cellular process. Many important cellular constituents , such as proteins, organelles and membranes, are in a constant state of flux. That is they are constantly being synthesized and degraded. Indeed, an interuption of this process can lead to a disease condition such as Tay Sach's disease.

In the diagram below a cellular constituent such as a mitochondria fuses with a primary lysosome. The enzymes of the lysosome digest the macromolecules and the digestion products can be resused by the cell or ejected by and exocytotic process.

Heterophagy

The term hetero (different) phagy (to eat) refers to a process whereby lysosmes aid in the intracellular digestion of material gathered from outside of the cell by some kind of endocytotic mechanism. The endocytotically ingested material is sequested in a membrane bounded vesicle.

In the diagram above bacteria are ingested by phagocytosis. The ingested bacteruia are packaged in a phagocytic vesicle. This vesicle fuses with a primary lysosome forming a secondary lysosome. The enzymes of the lysosome digest the bacterial macromolecules.