Lysosome (cell biology)

● LYSOSOMES 

1. Lysosomes were discovered by de Duve in 1949 biochemically in rat liver homogenate by ultracentrifugation. 

2. Lysosomes are generally found in the cytoplasm of animal cells. 

3. The term lysosome was introduced by de Duve in 1955 (Gr. lysis, dissolution; soma, body). 

4. de Duve (1963) also coined the name "suicide bags" to lysosomes as they contain hydrolytic enzymes. Lysosomes are covered by one lipoprotein unit membrane (double membranes are absent). 

5. Lysosomes exhibit polymorphism. The different types of lysosomes are: 

• Primary lysosome     • Phagocytic vacuole 

• Autophagic vacuole  • Residual body

6. The simplest chemical definition of a lysosome is 'a body rich in acid hydrolase'. 

7. There are about 50 lysosomal hydrolases which are able to digest most biological substances. 

8. Most hydrolytic enzymes of lysosomes function at acidic pH, approximately 4.6, which is maintained by a proton pump that accumulates H+ inside the lysosome.

9. In the cell, digestive enzymes are contained in lysosomes. 

10. Lysosomes are involved in intracellular digestion and are responsible for breakdown of parts of the cell and foreign particles in the cells. 

Lysosomes and Autophagy 

11. Under conditions of starvation or aging, the cell digests its own organelles by lysosomal enzymes and this process is called autophagy or autodigestion.

12. Autolysis is related to lysosomes and when a cell dies, the lysosome membrane breaks and the enzymes are released resulting in the digestion of cellular materials. 

Origin

 13. Electron microscopy has revealed that the Golgi complex is involved in the formation of primary lysosome. 

MICROBODIES

 1. The term 'microbodies' was introduced by Rhodin (1954). 

2. Two types of microbodies related to lysosomes are peroxisomes and glyoxysomes. 

(a) Peroxisomes 

3.  Peroxisomes were discovered by Tolbert and Yamazaki in 1969. 

4. Peroxisomes are round to oval bodies that have a diameter ranging between 0.1 to 0.5 µm.

5. Like lysosome, peroxisome is limited by a single mem- "brane. 

Peroxisomes contain many enzymes for peroxide bio- synthesis 

1. Two important enzymes in peroxisomes are urate oxidase and catalase. 

L Peroxisomes contain oxidising enzynies and promote gluconeogenesis in animals. . 

In mesophyll ells of leaves, peroxisomes interact with mitochondria and chloroplast to take part in photorespira- tion. 

During photorespiration, H2O, is formed and broken down into water and oxygen with the help of catalase, 

L Peroxisomes protect cells from toxic effect of H,O2 . 

2 Urate oxidase play a role in the catabolism of purines. 

(b) Glyoxysomes 

3. Glyoxysomes are reported fromn the endosperm of ger- minating seeds by Beevers (1969). 

4. Glyoxysomes usually occur in fat-rich plant cells and are associated with triglyceride metabolism through glyoxylate cycle . 

5. Glyoxysomes serve as enzymatic sites for reactions includ- ing the conversion of stored fatty acids to carbohydrates. 

6. The function of glyoxysomes is gluconeo genesis and they can convert triglycerides into carbohydrates.