Lysosomes | MANISH MEVADA | NEET BIOLOGY

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CELL : STRUCTURE AND FUNCTIONS

Lysosomes

• They were discovered accidently by a Belgian scientist, Christian de Duve, in 1955 through fractionation technique. 
• The organelles were observed under electron microscope by Novikoff ( 1956 ) . He also coined the term, lysosomes. 
• Lysosomes ( Gk . lysis- digestive or loose , soma- body ) are small vesicles which are bounded by a single membrane and contain hydrolytic enzymes in the form of minute crystalline or semicrystalline granules of 5-8 nm. 
• About 50 enzymes have been recorded to occur in them. 
• All the enzymes do not occur in the same lysosome but there are different sets of enzymes in different types of lysosomes. 
• The important enzymes are acid phosphatases, sulphatases, proteases, peptidases, nucleases, lipases and carbohydrases. 
• They are also called acid hydrolases because these digestive enzymes usually function in acidic medium or pH of 4–5. 
• Acidic conditions are maintained inside the lysosomes by pumping of H + or protons into them. 
• The covering membrane of lysosomes keeps the hydrolytic enzymes out of contact from the cellular contents. 
• It is itself protected from them by high glycosylation of its proteins and lipids. 
• The covering membrane becomes fragile in the absence of the oxygen, or the presence of excess of vitamins A and E, male and female hormones, bile salts, carcinogens, silica, asbestos particles, heat, many drugs, X - rays and ultra - violet rays. 
• The membrane is protected from these agencies by cortisone, cortisol, chloroquine and a type of cholesterol. 
• Lysosomes are called suicide bags because of the presence of a large number of digestive enzymes or acid hydrolases in them. 
• Only a thin membrane separates the destructive enzymes from the rest of the cell. 
• If the membrane happens to get broken, the various cellular constituents would undergo lysis.
• Lysosomes are generally rounded but can be irregular (e.g. , root tip cells) in outline. 
• The diameter varies from 0.2-0.8 pm but sometimes it may grow to a very large size ( upto 5 um in leucocytes, kidney cells , etc. ). 
• The interior may be almost solid or differentiated into outer denser region and a central less dense mass with granular content. 
• Lysosomes occur in all animal cells with the exception of red blood corpuscles. 
• In plants and fungi, their function is taken over by vacuoles. 
• In animals, lysosomes are abundant in leucocytes, macrophages, Kupffer's cells and similar cells with phagocytic activity. 
• Lysosomes are believed to be formed by the joint activity of endoplasmic reticulumendosomes and Golgi complex ( GERL system).
• The precursors of hydrolytic mostly synthesised at the rough endoplasmic reticulum. 
• The latter transfers them to the forming face of Golgi complex either directly or from smooth endoplasmic reticulum through its vesicles. 
• In Golgi complex the precursors are changed to enzymes. 
• The enzymes are then packed in larger vesicles which are pinched off from the maturing face. 
• Golgian vesicles are joined by endosomes to produce lysosomes (Machamer, 1993). 
• Lysosomes do not normally burst in the cytoplasm. 
• All materials which are to be acted upon by lysosome enzymes must enter them. 
• Rather the mateials are usually enclosed inside vacuoles and the vacuoles fuse with the lysosomes for digestion of materials. 
• Lysosomes take part in intracellular digestion of various types of materials of endogenous or exogenous origin. 
• Ex tracellular digestion can be performed by them under certain conditions. 
• They help in removing various toxic substances including carcinogensLysosomes pass through various stages in the same cell. 
• The phenomenon is called poly morphism or existence of more than one morphologi cal form. 
• Depending upon their morphology and function, there are four types of lysosomes - pri mary, secondary, residual bodies and autophagic vacuoles.

Primary Lysosomes 

• They are newly pinched off vesicles from the Golgi apparatus which generally fuse with some endosomes to become fully functional. 
• The primary lyso somes are small in size. 
• They contain hydrolytic enzymes in the form of granules. 

Secondary Lysosomes 

• They are also called heterophagosomes or digestive vacuoles 
• A secondary lysosome is formed by the fusion of food containing phagosome with lysosome (having hydrolytic or digestive enzymes). Digestion occurs.
• The digested food passes out into the cytoplasm. 
• Finally, the secondary lysosome is left with undigested food . 

Residual Bodies ( Residual or Tertiary Lysosomes )

• They are those lysosomes in which only indigestible food materials have been left.
• The residual bodies or lysosomes pass outwardly and fuse with the plasma membrane to throw out the debris into external envi ronment by exocytosis or ephagy.
• Sometimes, residual bodies remain inside the cells due to 
• failure of exocytosis and 
• absence of some hydrolytic enzymes.
• This leads to patho logical diseases (storage diseases) like hepatitis, Pompe's disease, Hurler's disease, Tay - Sachs disease and polynephritis. 
• Ageing is also due to them. 
• Lipofuschin pigment granules are actually residual bodies 

Autophagic Vacuoles ( Autophagosomes , Autolysosomes )

• They are produced by the fusion of a number of primary lysosomes around worn out or degenerate intracellular organelles. 
• The latter are first wrapped over by one or two membranes from endoplasmic reticulum (Dunn, 1990) before being recognised by lysosomes. 
• The cell debris is digested. 
• The phenomenon is also called autophagy or autodigestion. 
• It helps in disposal of cell debris . The worn out, aged or injured cells are also disposed of similarly ( apoptosis ). 
• Therefore, lysosomes are also called disposal bags or disposal units. 
• The digested products are made available to the cell for new synthesis.
• Lysosomes are,therefore, also known as recycling centres. 
• Besides removing worn out organelles, old or diseased cells, the autophagic vacuoles are also used removing internal obstructions. Autophagic vacuoles provide nourishment during starvation (de Duve , 1967). 

Autolysis

• It is self destruction of a cell, tissue or organ with the help of lysosomes. 
• Lysosomes performing autolysis do not enclose the structures to be broken down. Instead, they themselves burst to release the digestive enzymes. 
• Autolysis occurs in ageing, dead or diseased cells. 
• The disappearance of larval organs metamorphosis e.g. ,tail in frog) is due to autolysis.

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Manish Mevada

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