Delivery Pathways to the Lysosome

Based on the type of cargo, the cell uses four specialized delivery pathways to the lysosome— endocytosis, phagocytosis, macropinocytosis, and autophagy.

In endocytosis, receptors on the plasma membrane bind the target molecule, which can be a nutrient, a protein, or some other type of soluble substance.

A portion of the plasma membrane bends inward to surround the target finally detaching from the plasma membrane to form a transport vesicle.

Cells such as macrophages and neutrophils follow a similar pathway called phagocytosis to engulf cell debris, foreign particles, or microbes, like bacteria or protozoa.  A phagocyte traps a receptor-bound particle inside a vesicle called a phagosome.

In macropinocytosis, part of the cell membrane folds inward to create tear-shaped pockets surrounding the material being taken into the cell. There are no receptors involved; therefore, the process is non-specific.

Lastly, in autophagy, the material to be digested is sequestered in a double-membrane organelle called an autophagosome.

The outer membrane of the autophagosome contains protein markers that target it for delivery to the lysosome.

In all four pathways, the vesicles ultimately fuse with the lysosome and release their contents for degradation by lysosomal enzymes.