The following points highlight the top four types of digestive function in lysosomes. The types are: 1. Degradation of Foreign Matter Introduced into the Cell 2. Destruction of Defective Cellular Organelles 3. Complete Destruction of Cell 4. Hydrolytic Enzymes.
Digestive Function: Type # 1.
Degradation of Foreign Matter Introduced into the Cell:
Foreign materials are introduced within the cell by a process of endocytosis. Primary lysosomes fuse with the vesicles so produced (endosomes), and the materials present in the endosome are digested by the hydrolytic enzymes present in the lysosome.
The digested materials are absorbed into the cytoplasm but the undigested substances remain inside the secondary lysosome; such a lysosome is called residual body. The undigested matter is excreted from the cell through exocytosis (Fig. 2.11).
Polymorphonuclear leukocytes possess some “specific granules” in addition to lysosomes; these granules also contain hydrolytic enzymes. When a bacterium is ingested through phagocytosis, the specific granules fuse with the vesicle containing the bacterium, and release their enzymes to begin the degradative process.
Then the lysosomes fuse with such vesicles and release a large amount of hydrolytic enzymes. As a result, the bacterium is digested, but sometimes, the leukocytes also die in this process.
Digestive Function: Type # 2.
Destruction of Defective Cellular Organelles:
Degradation of unwanted defective cellular organelles is achieved by lysosomes; it is called autophagy. The unwanted/defective organelle is first enclosed by smooth ER membrane to form a vesicle called outophagic vacuole.
Now a primary lysosome fuses with the vacuole and forms a secondary lysosome (Fig. 2.11). The digestion of the organelle occurs due to the hydrolytic enzymes. The residual part of the organelle can be observed within autophagic lysosomes.
Digestive Function: Type # 3.
Complete Destruction of Cell:
Complete destruction of cell by its own lysosomes is called autolysis. During the development, a selective destruction of cells is required for the normal development of organs. In the cells to be digested, membrane of the primary lysosomes breaks and hydrolytic enzymes are released within the cell (Fig. 2.11). This leads to the digestion of the cell components.
Digestive Function: Type # 4.
Hydrolytic Enzymes:
Sometimes, hydrolytic enzymes are secreted outside the cell in the extracellular space (Fig. 2.11). For example, the sperm cell releases the hydrolytic enzymes on the surface of egg to enable penetration by the sperm head into the egg cytoplasm. This may be regarded as extracellular digestion.
Human diseases due to defective hydrolytic enzymes in lysosomes: Due to the production of defective hydrolytic enzymes, the substrates accumulate in lysosome which is harmful to the organism.
For example, deficiency of β-glucosidase in human causes storage of large amount of glycogen in the lysosomes. Such individuals die at an early age due to an accumulation of large amounts of glycogen in liver, heart and muscles.
In the case of deficiency of the enzyme β-N- acetyl-hexosaminidase, ganglioside is accumulated resulting in mental retardation, blindness and muscular weakness (infantile amaurotic idiocy). Similarly, defects in the enzyme α-galactosidase, leads to an accumulation of ceramidetrihexoside that causes skin rash and kidney failure (Fabry’s disease). These diseases are called lysosomal storage diseases.