Lysosomes: Structure and Functions
Lysosomes (Laso = dissolving; some = body) are membrane-bound spherical organelles containing lysosomal enzymes, that are hydrolytic in nature. Important enzymes present in the lysosome are acid hydrolases, proteases, lipases and acid phosphatases. These enzymes are capable of digesting organic molecules like lipids, proteins nucleic acids and polysaccharides under acidic conditions. So these are called “Suicidal Bags“.
De dive first described there in 1949. Almost all cells contain lysosomes but their number if much increased in the cells involved in the process of phagocytosis except erythrocytes.
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Types of Lysosomes and its formation
There are two main types of lysosomes i.e. Primary Lysosomes and Secondary Lysosomes.
Formation of Primary lysosomes
Enzymes present in the lysosomes are synthesized in rough endoplasmic reticulum and are then transported to the cytoplasm by transport vesicles into Golgi apparatus through Cis-Golgi or forming a face. These enzymes are further processed in Golgi apparatus and then budded off from Trans–Golgi or maturing face of Golgi apparatus in the form of the primary lysosome.
Enzymes are sometimes synthesized by ribosomes, transported through endoplasmic reticulum into Golgi apparatus and from trans Golgi, they are punched of as vesicles, the primary lysosomes.
They are formed when primary lysosomes fuse with phagocytic vesicles, thus exposing the contents of the vesicle to lysosomal enzymes. These enzymes present in the primary lysosomes digest the food presentation and the soluble substances are diffused into the cytoplasm of the cell. Undigested material containing vacuole known as the residual body is expelled out by exocytosis.
Other forms of Lysosomes
- Phagosomes: When the primary lysosome fuses with a specialized white blood cell, the phagocytes, an activated phagosome or phagocytic vesicles formed. They fight against a pathogen by engulfing them very rapidly than the ordinary phagocytes.
- Autophagic vacuoles: During starvation or after the destruction of cell components especially the liver cells and cells destroyed during metamorphosis fuse with the primary lysosome to form autophagic vacuoles or autolysosomes.
Functions of Lysosomes
Any foreign object (pathogens) that gains entry into the cell is immediately engulfed by lysosome and is completely broken into simple digestible pieces. This process is called phagocytosis.
They are involved in intracellular digestion since they have enzymes to digest the phagocytosed food particles present in food vacuoles.
They also help in extracellular digestion by releasing enzymes.
Sometimes enzymes of primary lysosome are released from the cell. This occurs during the replacement of cartilage by bone during development. Similarly, the matrix of bone may be broken down during the remodeling of bone that can occur in response to injury, new stresses and so on.
Autophagy: (Self Eating):
It is the process by which unwanted structures like damaged mitochondria etc within the cell are removed. Unwanted structures are first enclosed by a single membrane, usually derived from smooth endoplasmic reticulum. Then this structure fuses with a primary lysosome to form secondary lysosome called autophagic vacuole in which unwanted material is digested.
It is the self-destruction of a cell by release of the contents of lysosomes within the cell. It is a normal event in some differentiation processes and may occur throughout a tissue, as when a tadpole tail is reabsorbed during metamorphosis. It also occurs after cells die. Sometimes it occurs as a result of certain lysosomal diseases or after cell damage.
Recycling of important constituents:
As a result of phagocytosis and digestion of different components, the lysosomes help in the recycling of important components of the cytoplasm.