Also, there are two different types of secretory pathways:

  1. Constitutive pathway; where vesicles continuously form and carry proteins from the Golgi to the cell surface. Example: mucus or digestive enzymes that are continually produced for release.
  2. Regulated pathway; in which proteins are confined into vesicles that are stored in the cell until they are secreted in response to specific signals. For example: insulin, produced by beta cells of the pancreas, is stored in dense secretory granules until the need arises for them to be released.


Finally for this chapter, we’ll discuss the quality of lysosomes since they are pretty vital intracellular compartments that filter and digest compounds for the cell.

They literally eat things which disturb the natural order of a cell. They pick up foreign invaders such as bacteria, macromolecules and old organelles, then break them into smaller pieces that can be used again. If they pick up a really harmful invader, they will eat up and expel its remains out of the cell, where the debris will be removed.

Lysosomes have a unique surrounding membrane that contains acid hydrolase enzymes. Lysosome’s pH (an acidity measurement unit) is about 5 (anything less than 7, which is water’s acidity is considered acidic) maintained by membrane bound H+ ATPase (an transmembrane protein that acts as an enzyme which pumps protons. More of in next chapter).

Materials from different places take different pathways to lysosomes that fuse into the lysosome (endocytosis) for digestion. Lysosomes can also digest mitochondria and other organelles as well. This happens when the organelle is enclosed by an additional double membrane creating an autophagosome which then fuses with the lysosome.

Autophagosome formation. Click on image for credit.

The specialized digestive enzymes and membrane proteins of the lysosomes are synthesized in the ER and transported to the Golgi apparatus to the trans-Golgi network.

Ubiquitin is a protein signal for endosome sorting of membrane proteins which leads the cargo to the lysosome for degradation.