Lysosome Lect-7

Lysosomes They form the digestive system of the cell. They break down all non-functional or harmful complex macromolecules into simple molecules which can be reused by the cell

Composition: They are unit membrane bounded digestive spherical organelles. Lysosomes contain concentrated hydrolytic enzymes include (protease, lipase, carbohydrases, and acidic phosphatase); about forty types of hydrolytic enzymes. They are active in acidic environment at pH 5.0. They function to break down of proteins, nucleic acids, oligosaccharides and phospholipids. They are present in all cells except mature erythrocytes.

Types of lysosomes: 1. Primary lysosome   It is a small vesicle that arises from Trans - cisternae of Golgi-complex and contains homogenous material. It carries concentrated hydrolytic enzymes, but still not engaged in the digestive process. Its fusion with membrane bounded endosome, phagosome or autophagosome containing material to be digested results in formation of secondary lysosome.

2. Secondary lysosome:   It is a larger than primary lysosome and composed of membrane bounded vesicle containing heterogenous material . It results from fusion of primary lysosomes and membrane bounded vesicle or endosome that contains material to be digested (ex. Bacterial cell.).

The intracellular functions of lysosomes 1- Digestion of complex molecules. 2- Digestion of organelles that lost their ability to work in the cell. 3- Break down of ingested foreign materials. 4- Lysosomes are responsible for cellular death.  

Apoptosis It is a normal process of programmed cell death due to release of the lysosomal enzymes into the cytosol. Removal of cells forming webs between the fingers in developing hand and removal of the upper layer of skin and intestinal wall are examples for apoptosis.

Hypoxia – oxygen deficiency increases the cellular acidity that causes releasing of lysosomal enzyme and digestion of cell contents and death. Lysosomal Storage Disease - Hereditary deficiency of lysosomal enzymes leads to ac-cumulation of lipid materials of cellular membranes in nerve cells which results in mental retardation and death. This disease is called Tay-Sach,s disease. And Lysosomal glycogen storage disease in which there is abnormal accumulation of glycogen.

Sources of the Lysosomal Digested Materials The followings are the different sources for materials that are digested by lysosomes. Materials may be taken from outside the cell or found inside the cell :   1. Phagocytosis. Engulfing of large foreign solid particles such as bacterial cells results in formation of membrane encapsulated phagosome containing the foreign materials. Fusion of primary lysosome with the phagosome results in formation of secondary lysosome or digestive vacuole.

2. Pinocytosis.   It is the process of internalization of fluid by membrane–bounded vesicles – pinocytotic vesicle or endosome. Fusion of the endosome with primary lysosome results in formation of multivesicular body or secondary lysosome or digestive vacuole.

4. Receptor –Mediated Endocytosis. It is the process of internalization of specific legends into the cell within Clathrin –coated vesicles. Then the cytoplasmic coated vesicles lose their coats and transform to endosomes. Fusion of the primary lysosome with the endosome results in formation of secondary lysosome or digestive vacuole.  

The intracellular degradation of materials starts generally by fusion of endosome, phagosome or autophagosome with primary lysosome that results in: 1. Formation of digestive vacuole or secondary lysosomes. 2. The useful break down product leaves the digestive vacuole into the cytoplasm while the undigested material condenses within the vacuole to form residual body. 3. Reduction of plasma membrane during endocytosis is neutralized by increase of plasma membrane during exocytosis

Peroxisomes or Microbodies They have the following characters: 1- Small spherical to ovoid membrane –bound vesicles. 2- Self replicating organelles; they increase in size and undergo fission to form new peroxi- somes. 3- Present in almost all animal cells. (they are most prominent in cells of liver and renal tubules) 4- Peroxisome contains more than forty oxidative enzymes.

Functions of peroxisomes 1- They catalyzed the long fatty acid chains into acetyl Co-enzyme A, and Hydrogen ions that combines with oxygen ions to form hydrogen peroxide (H2 O2).   2- Acetyl Co-enzyme A is used for cellular metabolism, while hydrogen peroxide is able to detoxify various noxious (e.g: ethanol), and kill microorganisms. Peroxisomes are extensive in liver and kidney . Defect in enzymes of peroxisomes can result in metabolic disorder associated with storage of abnormal lipids in some cells(brain , adrenal).

Proteasomes They are small organelles composed of protein complexes. They are responsible for the following mechanisms for cytosolic proteolysis of specific types of proteins . Proteasomes have functions like: 1. Degradation of proteins that act in metabolic regulation 2. Elimination of denatured, damaged, or malformed proteins 3. Cleavage of antigenic proteins into small polypeptide fragments (epitopes) in antigen presenting cells (APCs).

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