Describe the biogenesis and function of lysosomes. What is the function of lysos

Question

Describe the biogenesis and function of lysosomes. What is the function of lysosomes? Why must their contents be compartmentalized? Why must the lumen of lysosomes be regulated at an acidic pH? How are newly synthesized hydrolases sorted from other proteins and targeted to lysosomes? Describe the role of lysosomes in some specialized cell types. Trace the movement of a phagocytosed particle in a phagocytic cell. What is the difference between a heterophagic and an autophagic lysosome? Discuss the processes of autophagy and extracellular digestion, and outline their roles in specialized cells.

Explanation / Answer

1. Describe the biogenesis and function of lysosomes.

Lysosomes are single membrane organelles containing hydrolases active at acidic pH. In mammalian cells they account for 1 to 15% of cell volume and of cell protein, and in fungal and plant cells its 30 to 90% of cell volume, depending on cell type and physiological condition. Morphologically they are vesicular to complex tubular lattice structures.

2. What is the function of lysosomes?

Lysosomes are involved in the degradation of both extracellular and intracellular proteins as well as other macromolecules. This function depends on their cell type and nutritional conditions.

3. Why must their contents be compartmentalized?

4. Why must the lumen of lysosomes be regulated at an acidic pH?

Lysosome compartments are filled with hydrolytic enzymes which include about 40 types such as proteases, nucleases, glycosidases, lipases, phospholipases, phosphatases, and sulfatases. All these are acid hydrolases, which need an acid environment for their optimal activity. For this, the pH within the lysosomal lumen is maintained between 5.0 and 5.5. In this way, the contents of the cytosol are doubly protected against acid hydrolases. The membrane of the lysosome normally keeps the digestive enzymes out of the cytosols. The acidic pH is maintained by a multisubunit, proton-pumping ATPase in the lysosomal membrane of mammalian cells and in the vacuolar membrane of yeast.

5. How are newly synthesized hydrolases sorted from other proteins and targeted to lysosomes?

The lysosomal hydrolases are synthesized in the rough ER and transported through the Golgi apparatus to the trans Golgi network with the transport vesicles. The vesicles incorporate the lysosomal proteins and exclude the other proteins for other locations.The lysosomal hydrolases carry a unique marker mannose 6-phosphate (M6P), which are linked specifically to the N-linked oligosaccharides of lysosomal enzymes when they pass through the lumen of the cis Golgi network. The transmembrane M6P receptor proteins in the trans Golgi network recognizes the M6P groups and bind to lysosomal hydrolases on the lumenal side of the membrane and to adaptins on the cytosolic side for assembling clathrin coats. Thus the hydrolases are packed into clathrin-coated vesicles and subsequently the contents are delivered to a late endosome, which later develop into lysosomes.

6. Describe the role of lysosomes in some specialized cell types.

In liver and kidney, lysosomes are the major protein degraders, but in skeletal muscle and lymphocytes, they are the minor protein degraders. Lysosome mediated proteolysis is activated during nutritional deprivation also. This pathway is functional to some extent in all eukaryotic cells except in mature red blood cells.

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