Some of the main functions of Lysosomes are as follows:
1. Intracellular digestion:
The word lysosome is derived from (lyso lytic or digestive; and soma body) thus helping in digestion.
Pinocytic vacuoles formed as a result of absorption of fluid substance into cell or phagocytic vacuoles formed by absorption of solid particles into cell, carry protein material to lysosomal region.
These foreign proteins may undergo digestion within cell as a result of endocytosis. Endocytosis includes the processes of phagocytosis (Gr., phagein, to eat), pinocytosis (Gr., pinein, to drink) and micropinocytosis.
Phagocytosis and pinocytosis are active mechanisms in which cell requires energy for their operation. During phagocytosis by leucocytes oxygen consumption, glucose uptake and glycogen breakdown all increase significantly.
In endocytosis contraction of microfilaments of actin and myosin present in the peripheral cytoplasm occurs. It causes plasma membrane to invaginate and form the endocytic vacuole. Ingested particles enclosed in membranes derived from the plasma membrane and forming vacuoles are sometimes celled phagosomes.
After the entrance of a particle or large body into the cell by endocytosis and the formation of a phagosome, the membranes of the phagosome and a lysosome may fuse to form a single large vacuole. Within this vacuole the lysosomal enzymes begin the process of digestion of the foreign material.
Initially the lysosome, known as a primary lysosome containing the complex of enzymes in an inactive state, but after fusion with the phagosome produces a secondary lysosome with a different morphology and active enzymes.
After enzymatic digestion the digested material diffuses out into the hyaloplasm of the cell. Some material may remain in the enlarged lysosome vacuole. This remnant vacuole is the residual body, since it contains the residue of digestive process. During starvation also, lysosomes digest stored food materials, i.e., proteins, lipids and glycogen of cytoplasm and provide energy required by the cell.
Digestion of proteins usually ends at the level of dipeptide, which can pass through the membrane and be further digested into amino acids.
Carbohydrates are usually hydrolysed to monosaccharides which are easily released. However, disaccharides and polysaccharides (cellobiose, inulin or dextrin) are not digested and remain within the lysosome.
Sucrose may be taken into macrophages by pinocytosis, but it is not hydrolyzed and remains trapped in secondary lysosomes.
Digestion of intracellular substances or autophagy:
Many cellular components, such as mitochondria, are constantly being removed from the cell by lysosomal system. Cytoplasmic organelles become surrounded by membranes of smooth endoplasmic reticulum, forming vacuoles, then lysosomal enzymes are discharged into autophagic vacuoles and the organelles are digested. Autophagy is a general property of eukaryotic cells. It is related to the normal renovation and turnover of cellular components.
The digestion of mitochondria or other cell structures, such as elements of ER, provides a source of energy for these cells. After the digestion of cell structure, the autophagic vacuoles may become residual bodies.
2. Removal of dead cells:
Hirsch and Cohn (1964) told that lysosomes help in the removal of dead cells in tissues such as white blood cells with engulfed bacterium in blood, cells in the outer layer of skin and mucous membrane linings of the body.
Lysosomal membrane ruptures in these cells, releasing enzymes into body of cell, so that whole cell may be digested. Lysosomes contain a sufficient complement of enzymes to digest most types of biological or organic materials and the digestive process (autolysis) occurs quite rapidly in dead cells. This process of tissue degeneration (necrosis) is due to this lysosomal activity.
3. Role in metamorphosis:
Recently lysosome’s role has been discovered in the metamorphosis of frog. Disappearance of tail from tadpole larva of frog is due to lysosomal activity (action of cathepsins present in lysosomes) as described by Weber.
4. Help in protein synthesis:
Novikoff and Essner (1960) have suggested the possible role of lysosomes in protein synthesis. Recently, the author (Dr. Singh 1972), has correlated lysosomal activity with the protein synthesis. In liver and pancreas of some birds, lysosomes seem to be more active and developed as reported by Singh (1972), showing possible relationship with cell metabolism.
5. Help in fertilization:
During fertilization, sperms head secrete some lysosomal enzymes which help in the penetration of sperm into vitelline layer of ovum. Acrosome contains protease and hyaluronidase and abundant acid phosphatase. Hyaluronidase disperse the cells around the oocyte and protease digests the zona pellucida making a channel through which sperm nucleus penetrates.
6. Role in osteogenesis:
It has been argued that formation of bone cells and also their destruction depends upon lysosomal activity. Likewise, ageing of cells and parthenogenetic development are related with the lysosome activity.
The osteoclasts (multinucleated cells) which remove bone, do so by the release of lysosomal enzymes which degrade the organic matrix. This process is activated by the parathyroid hormone.
7. Malfunctioning of lysosomes:
Lysosomal malfunction may lead to diseases, for example, when glycogen taken up by lysosomes is not digested (Pompe’s disease). Ruptures of lysosomes in skin cells exposed to direct sun light leads to pathological changes following sunburn. The enzymes liberated by these lysosomes kill cells in the epidermis, causing blistering and later to ‘peeling’ of a layer of epidermis.
8. Autolysis in cartilage and bone tissue:
The excess of vitamin A causes cell poisoning. It disrupts the lysosomal membrane, causing release of enzymes into the cell and producing autolysis in cartilage and bone tissue.