17.18 : Lysosomes

Lysosomes are membrane-enclosed spherical sacs derived from the Golgi apparatus. The most important function of the lysosome is degrading macromolecules and biological polymers that are released during membrane trafficking events such as the secretory, endocytic, autophagic, and phagocytic pathways. The degradation is carried out by several hydrolytic enzymes active in an acidic environment of the lysosomal lumen. These acid hydrolases are involved in cellular processes such as cell signaling, energy metabolism, restoration of the plasma membrane, counting discharged materials.

Newly formed lysosomes derived from the inner Golgi sacs, called primary lysosomes, are dynamic and can divide or fuse with macromolecules, other subcellular organelles, and the plasma membrane. Once the primary lysosomes fuse with substances to be degraded, they become secondary lysosomes. Depending on the pathway they are involved in, they become phagosomes, endo-lysosome, digestive vacuoles, and autophagosomes. There is no clear distinction between them other than their involvement in different membrane traffic pathways. Thus, they represent morphologically diverse organelles, defined by a common function of degrading intracellular material.

Secretory lysosomes, a specialized set of lysosomes, have features of both conventional lysosomes and secretory granules. Due to this, they are abundant in some cell types, such as macrophages, hematopoietic cells, and melanocytes. Secretory lysosomes secrete antimicrobial proteins and cytokines in addition to lysosomal enzymes. Hence, they are involved in pigmentation, coagulation, wound repair, and immunologic functions.

Plant cells contain vacuoles filled with hydrolytic enzymes similar to lysosomes but are functionally diverse. In addition to degradation, they have a role in storing both nutrients and waste products and controlling turgor pressure.