Lysosomes are cytoplasmic organelles and are different from others due to their morphology. These were isolated as a separate component for the first time by de duve (1949). Lysosomes (lyso = splitting; soma = body) are found in most eukaryotic cells. Any foreign object that gains entry into the cell is immediately engulfed by the lysosome and is completely broken into simple digestible pieces. The process is known as phagocytosis (eating process of a cell).
They are most abundant in those animal cells which exhibit phagocytic activity. They are bounded by a single membrane and are simple sacs rich in acid phosphatase and several other hydrolytic enzymes. These enzymes are synthesized on RER and are further processed in the Golgi apparatus. The processed enzymes are budded of as Golgi vesicles and are called as primary lysosomes processed enzymes are budded off as Golgi vesicles and are called as primary lysosomes (fig. 4.11). lysosomes contain those enzymes which can digest the phagocytosed food particles.
Fig. 4.11. :lysosomes protect the cells from invading organisms or any other foreign object, (food) which are engulfed in the cell as phagocytic vacuoles. These fuse with primary lysosomes to form digestive vacuole (secondary lysosome) in which various lysosomal enzymes digest various components of the vacuole. Some times, under abnormal circumstances, e.g. starvation, or as a normal physiological process the parts of the cell are engulfed by primary lysosomes and digested to generate energy. The lysosomes which eat parts of their own cell are known as autophagosomes. The digestive vaculoles and autophagosomes are also known as secondary lysosomes.
They are also involved in the autophagy (self eating). During this process some old, worn out parts of cell, such as old mitochondria are digested. In this way, materials of cell may be recycled and cell may be renewed. Their enzymes can also result in degeneration of cell, as may occur during some developmental processes. Lysosomes also release enzymes for extra cellular digestion.
Several congenital diseases have been found to be due to accumulation within the cell of substances such as glycogen or various glycolipids. These are also called storage diseases and are produced by a mutation that effect one of the lysosomal enzymes involved in the catabolism of a certain substance. For example, in glycogenosis type II disease, the liver and muscle appear filled with glycogen within membrane bound organelles. In this disease, an enzyme that degrades glycogen to glucose, is absent. About twenty such diseases are known these days, which are because of absence of a particular enzyme. For example tay-sach’s disease is because of absence of an enzyme that is involved in the catabolism of lipids. Accumulation of lipids in brain cells lead to mental retardation and even death.