Lysosome are important cellular organelles commonly known as “suicide bags” or “disposal units” of the cell. They play a crucial role in intracellular digestion and cellular defense mechanisms.
Discovery and Structure
Lysosomes were discovered by Christian de Duve in 1955 and were later named and observed under the electron microscope by Novikoff in 1956. Structurally, lysosomes are small membrane-bound vesicles, measuring about 0.2–0.8 µm in diameter. Each lysosome is surrounded by a single membrane and is formed by the Golgi apparatus.
Enzymatic Composition
Lysosomes contain nearly 40 types of hydrolytic enzymes, collectively known as acid hydrolases. These enzymes function optimally in an acidic environment and include:
- Nucleases
- Proteases
- Phosphatases
- Sulphatases
Due to the presence of these enzymes, lysosomes are capable of digesting almost all organic substances except cellulose.
Distribution
Lysosomes are found in almost all eukaryotic cells but are most abundant in phagocytic cells such as white blood cells (WBCs) and osteoclasts, where active digestion and defense functions are required.
Polymorphism of Lysosomes
- Lysosomes exhibit polymorphism, meaning they exist in different forms depending on their function:
- Primary lysosomes – newly formed lysosomes containing digestive enzymes.
- Secondary lysosomes – formed by fusion of primary lysosomes with phagosomes; also called digestive vacuoles or heterophagosomes.
- Residual lysosomes – contain undigested materials and may undergo ephagy.
- Autophagic vacuoles – involved in autophagy or autolysis, where damaged or useless cell parts are digested.
Lysosomes in Plant Cells
In many plant cells, the function of lysosomes is performed by sphaerosomes and vacuoles, as typical lysosomes are less prominent in plants.
Lysosomal Membrane Stability
The lysosomal membrane is stabilized by substances such as cortisone, cortisol, antihistamines, heparin, chloroquine, and certain types of cholesterol. However, the membrane becomes fragile in:
- Absence of oxygen
- Excess vitamin A and vitamin E
- Presence of progesterone, testosterone, bile salts
- Exposure to high-energy radiations
Clinical Importance
Failure of lysosomal function, due to absence of specific hydrolytic enzymes or defective exocytosis, leads to accumulation of residual bodies in cells. This can cause diseases such as:
- Hepatitis
- Polynephritis
Hurler’s disease, characterized by bone deformities due to accumulation of mucopolysaccharides (glycosaminoglycans) caused by enzyme deficiency.
Role in Fruit Ripening
Lysosomal enzymes also influence fruit ripening and rotting. Fruit rotting can be slowed down by inhibiting the enzyme polygalacturonase, which is responsible for cell wall degradation.
Conclusion
Lysosomes are vital organelles responsible for digestion, recycling, defense, and maintenance of cellular health. Any disturbance in their structure or function can lead to serious metabolic disorders, highlighting their importance in normal cellular physiology.