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Lysosomes are electron microscopic, vesicular structures of the cytoplasm, bounded by a single membrane (lipoproteinous) which are involved in intracellular digestive activities, contains hydrolytic enzymes, so called lysosomes.

Discovery

(i) These were first discovered by a Belgian biochemist, Christian de Duve (1955) in the liver cells and were earlier named pericanalicular dense bodies.

(ii) Terms Lysosome was given by Novikoff under the study of electron microscope.

(iii) Matile (1964) was first to demonstrate their presence in plants, particularly in the fungus Neurospora. Polymorphism in lysosomes were described by De Robertis et. al (1971).

Occurrence : These are absent from the prokaryotes but are present in all eukaryotic animal cells except mammalian RBCs. They have been recorded in fungi, Euglena, cotton and pea seeds.

Shape : These are generally spherical in shape but are irregular in plant root tip cells.

Size : Size range is \[0.2-0.8\,\,\mu m\] while size is \[0.5\,\,\mu \,m\,\,(500nm).\]

Types of lysosomes : On the basis of their contents, four types of lysosomes are recognised.

(1) Primary Lysosomes : A newly formed lysosome contains enzymes only. It is called the primary lysosomes. Its enzymes are probably in an inactive state.

(2) Secondary Lysosomes : When some material to be digested enters a primary lysosome, the latter is named the secondary lysosome, or phagolysosome or digestive vacuole, or heterophagosome.

(3) Tertiary lysosomes/Residual bodies : A secondary lysosome containing indigestible matter is known as the residual bodies or tertiary lysosome. The latter meets the cell by exocytosis (ephagy).

(4) Autophagosomes/Autolysosomes : A cell may digest its own organelles, such as mitochondria, ER. This process is called autophagy. These are formed of primary lysosomes. The acid hydrolases of lysosomes digest the organelles thus, it is called autophagosome. The lysosome are sometimes called disposal units/suicidal bags. Sometime they get burst and causes the distruction of cell or tissue.

Chemical composition : Matrix of primary lysosome is formed of hydrolases, which is involved in hydrolysis or polymeric compounds, that operate in acidic medium at pH 5, so called acid hydrolases. Upto now 50 types of enzyme have been reported. These are as :

Proteases (cathepsin and collagenase), Nucleases (DNAse and RNAse), Glycosidases (\[\beta -\]galactosidase, \[\beta -\]glucoronidase), Phosphatases (ATPase, acid phosphatase /marker enzyme).

Functions

(1) Lysosomes of sperms provide enzyme for breaking limiting membrane of egg e.g., hyaluronidase enzyme.

(2) Lysosomes functions as trigger of cell division or initiate cell division by digesting repressor molecules.

(3) Nucleases (DNAse) of lysosomes may cause gene mutations which may cause disease like leukemia or blood cancer (partial deletion of 21^st chromosome).

(4) Sometimes residual bodies accumulate inside the cells leading to storage diseases e.g., a glycogen storage disease called Pompe’s disease, polynephritis Hurler’s disease (deformed bones due to accumulation of mucopolysaccharides).

(5) Lysosomes also engulf the carcinogens.