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(cell biology) Organelle containing a large range of digestive enzymes used primarily for digestion and removal of excess or worn-out organelles, food particles, and engulfed viruses or bacteria
Lysosome – an organelle?
Organelle literally means “little organs”. As the body is composed of various organs, the cell, too, has “little organs” that perform special functions. Some references are strict in their definition of an organelle. An organelle is a structure surrounded by two lipid bilayers. In this regard, nucleus, mitochondria, and chloroplast (plastid) are regarded as organelles because their membrane consists of two lipid bilayers whereas ribosomes and nucleosomes are not. In the same way, lysosomes and vacuoles, would not qualify as an organelle because they are single-membrane-bounded cytoplasmic structures. Other references, though, are less restrictive. An organelle is one that acts as a specialized subunit inside the cell that performs a specific function. In this case, there are two types of organelles: (1) membrane-bound organelles (included are double-membraned and single-membraned cytoplasmic structures) and (2) non-membrane-bound organelles. Examples of membrane-bound organelles are nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, lysosomes, and vacuoles. Examples of non-membrane-bound organelles are ribosomes, spliceosome, vault, proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, photosystem I, ATP synthase, nucleosome, centriole, microtubule-organizing center, cytoskeleton, flagellum, nucleolus, stress granule, etc.
Lysosomes were discovered by the Belgian cytologist Christian de Duve in 1949.
Lysosomes are spherical vesicles with various digestive (hydrolytic) enzymes. These digestive enzymes are lysosomal enzymes that are synthesized in the rough endoplasmic reticulum. Next, they undergo post-translational modifications by adding mannose 6-phosphate as a label in the Golgi apparatus. Finally, they are imported as vesicles by budding off from the Golgi apparatus. The enzymes inside the vesicles will be used primarily for digestion and removal of excess or worn-out organelles, food particles, and engulfed viruses or bacteria. The lumen of the lysosomes has a pH ranging from 4.5 to 5.0, which is optimal for the enzymes for hydrolysis. The lysosome is able to maintain this optimal pH by pumping in protons (H+ ions) from the cytosol via its proton pumps and chloride ion channels.
Lysosomes belong to the GERL system in which apart from lysosomes includes the Golgi apparatus and the endoplasmic reticulum. The GERL system is associated with biological processes such as endocytosis and exocytosis.
Types of Lysosomes
There are two major types of lysosomes according to their development stages: (1) primary lysosomes and (2) secondary lysosomes. The primary lysosome is a newly formed lysosome and has not been exposed yet to the material to be digested. When it fuses with a vacuole containing the material, it is now referred to as secondary lysosome.1
One of the main functions of lysosomes is associated with the digestion of macromolecules from phagocytosis, endocytosis, and autophagy, and digestion of bacteria and other waste materials. The lysosomes, therefore, act as the waste disposal system of the cell. Apart from this though they are also involved in the repair of damage to the plasma membrane by acting as a membrane patch, and apoptosis (e.g. digesting web from the fingers of a 3- to 6-month-old fetus). Often, they are referred to as “suicide-bags” because of their role in autolysis. Its other functions are cell signaling and energy metabolism.
Lysosomes digest materials as catalyzed by hydrolytic enzymes located in their membrane and lumen. The first step is endocytosis wherein the material enters the cell through the cell membrane in the form of a food vacuole. Next, the lysosome fuses with the food vacuole releasing its hydrolytic enzymes into the food vacuole. Lastly, the hydrolytic enzymes digest the materials inside. The vesicle, then, shrinks, forming small dense membrane-bound vesicles. The so-called residual bodies refer to the remnants of digested materials. 1
Pathobiology and Genetics
Lysosomal enzymes are encoded by the genes in the nucleus. Mutations in these genes could lead to genetic disorders since these mutations could result in dysfunctional lysosomal enzymes. When this occurs, metabolic disorders could ensue. Without a functional lysosomal enzyme, certain materials (metabolites, waste materials, etc.) tend to accumulate and could affect the normal, proper metabolism. This is the underlying cause of Tay-Sachs disease. Tay-Sachs disease is a disease characterized by neurodegeneration, developmental disability, or even early death. It is caused by a deficiency in a functional hexosaminidase A resulting in the accumulation of GM2 gangliosides in neurons. Other metabolic disorders caused by a mutation resulting in dysfunctional or a deficiency of functional lysosomal enzymes are Farber disease, Krabbe disease, galactosialidosis, gangliosides, alpha-galactosidase (e.g. Fabry disease, Schindler disease, etc.), beta-galactosidase, GM2 gangliosidosis (e.g. Sandhoff disease, Tay-Sachs disease, etc.), glucocerebroside (e.g. Gaucher disease), sphingomyelinase (e.g. lysosomal acid lipase deficiency), sulfatidosis, mucopolysaccharidosis, mucolipidosis, lipidosis (e.g. neuronal ceroid lipofuscinosis, Wolman disease, etc.), cholesterol ester storage disease, lysosomal transport disease, glycogen storage disease, etc. They are collectively called lysosomal storage disease.
- Greek words lysis, which means dissolution or destruction, and soma, which means body
- HISTOLOGY BIOL 4000 LECTURE 4. (2019). Retrieved August 21, 2019, from Auburn.edu website: http://www.auburn.edu/academic/classes/zy/hist0509/html/Lec03Bnotes-the_cell2.html
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