A Lysosome is a small cytoplasmic membrane bound vesicle found in many types of animal cells that contains hydrolytic enzymes, which play an important role in the degradation of material ingested by phagocytosis and endocytosis. The word Lysosome was derived from the Greek word lysis (loosening) and soma (body). The limiting membrane of lysosomes protects the remainder of the cell from the effects of the enzymes which when released into the cytosol would digest or lyse the cell leading to Autolysis such as it occurs normally during such diverse events as regression of the mesonephros during kidney development and regression of mammary tissue after cessation of lactation. Increased lysosomal activity also occurs during the regression of some tumors.
What are Lysosomes
The lysosome is the suicidal bag of cells (because a break in their membranes would release their digestive enzymes leading to the destruction of the cell) this happens normally in programmed cell death (or apoptosis ). It is the site of intracellular digestion of extracellular and intracellular components. It is bounded by a single membrane and contains lytic enzymes that are involved in digestion of ingested macromolecules and turnover of intracellular components. A lysosome that contains undigested wastes is called a residual body. Residual bodies may eliminate their waste by exocytosis, or the wastes may accumulate within the cell as the cell ages.
Who discovered lysosomes?
Lysosomes were first discovered by and described for the first time as a new cell organelles by the Belgian Biochemist called de Duve in the year 1955.
Types of Lysosomes
- Primary Lysosomes: these are newly synthesized lysosomes, released at the Golgi complex, that have not engaged in digestive activities.
- Secondary Lysosomes: these are vacuolar structures that represent sites of past or current lysosomal activity and include heterophagic vacuoles, residual bodies, and cytolysosomes.
Steps in the formation of Lysosomes
- Acid hydrolase enzymes synthesized in Endoplasmic Reticulum reach the Golgi complex where they are packed into vesicles. The enzymes in these vesicles are inactive because of the lack of an acidic medium. These newly formed vesicles are called primary lysosomes or Golgi hydrolase vesicles.
- These primary lysosomes then fuse with other vesicles derived from cell membrane (endosomes). The endosomes possess the membrane proteins necessary for producing an acidic medium. The product formed by fusion of the two vesicles is an endolysosome or secondary lysosome.
- Hydrogen ions are pumped into the vesicle to create an acid environment which then activates the enzymes and a mature lysosome is formed.
- In a similar manner lysosomes may also fuse with pinocytotic vesicles to form structures with numerous small vesicles within them these structures so formed are called multivesicular bodies (the function of which is not yet known).
Lysosome Functions (What does the Lysosome do?)
- Lysosomes are useful in helping to defend the body against microorganisms and germs as the fuse with phagosomes during Phagocytosis
- Lysosomes are important in helping to degrade proteins, carbohydrates, lipids and nucleotides.
- Degradation of wornout organelles
- Removal of excess of secretory products
- Secretion of perforin, granzymes, melanin and serotonin
- Dysfunction of the lysosomes leads to lysosomal storage diseases
- The acrosome, located at the head of the spermatozoa, is a specialized lysosome and is probably involved in some way in the penetration of ovum by the sperm.
- There are good evidences that the metamorphosis of tadpoles to frogs, the regression of the tadpoles tail is accomplished by the lysosomal digestion of the tail cells.
Lysosomes are small tiny and dense membrane-bound vesicles found in the cytosol of the cytoplasm, measuring 0.2 to 0.55 m in diameter with mean diameter of approximately 0.4 (their sizes vary in between that of microsomes and mitochondria). Lysosomes are surrounded by a single layer lipoprotein membrane. The pH inside the lysosomes is lower than that of the cytosol and the pH needed for optimal activity of the lysosomal enzymes is around 5. The marker enzyme for detecting lysosomes is the Acid phosphatase it is the easiest to detect with the Electron Micrograph by histochemical staining.
Lysosomes are located in the cytoplasm of animal cells. They are usually found scattered in the cytoplasm as small vesicles containing hydrolytic enzymes that are usually released when the fused with Phagosomes of phagocytes to form Phagolysosomes.
Lysosomal Enzymes (Enzymes of Lysosomes)
- Proteolytic enzymes: examples include Cathepsins (Proteinase), Collagenase and Elastase
- Nucleic acid hydrolysing enzymes examples include Ribonucleases and Deoxyribonucleases
- Lipid hydrolysing enzymes such as Lipases, Phospholipases and Fatty acyl esterases
- Carbohydrate splitting enzymes such as glucosidase, galactosidase, Hyaluronidase and Aryl sulphatase, etc.
- Other enzymes such as Acid phosphatase and Catalase etc.
More than 50 lysosomal enzymes have been identified; since they are active at acidic pH, lysosomal enzymes often are referred to as acid hydrolases. Essentially there are about 30 to 40 Lysosomal enzymes that are largely hydrolytic in nature. As long as the lysosomal membrane is intact, the encapsulated lysosomal enzymes can act only locally, but when the membrane is ruptured, the enzymes are released into the cytoplasm and can hydrolyze external substrates.
Small amounts of lysosomal enzymes that leak out of lysosomes into the cytosol normally get inactivated due to the absence of acidic pH. However, large leakages cause destruction of the cell. The medical importance of this explains what happens in gout where phagocytes ingest uric acid crystals and such ingestion triggers the extracellular release of lysosomal enzymes that contribute to the inflammatory response observed in the joints in Gout.
Do Plant cells have Lysosome?
Plant cells do not have Lysosome, in fact one of the enzymes that distinguishes plant cells from animal cells is the Lysosome which is found only in animal cells but not in plant cells. The only animal cell that does not contain lysosome is the red blood cell (erythrocyte), apart from this, all animal cells have lysosomes.