Synonyms:
It is also called “suicidal bag”, “scavenger of cell”, “atom bomb of cell”, “demolition squad”, “disposal unit” of cell.
Discovery:
Belgian biochemist Christian de Duve (1955) accidentally discovered Lysosomes from rat liver cells for which he was awarded Nobel prize in 1974. Novikoff (1956) first observed lysosomes under EM. Lie also coined the term ‘lysosomes’. (Gk. lysis digestive or loose, soma—body).
Occurrence:
Lysosomes are common in animal cells and protozoans. These are absent in prokaryotes, mature mammalian RBCs, yeasts and plant cells. However, meristematic cells, root tip cells of maize, neurospora (fungi) contain lysosomes. Generally in plant cells lysosomal activities are carried out by vacuoles, spherosomes and aleurone grains which together known as plant lysosomes.
Number:
About 15-20 lysosomes are found in a cell. However, they are abundant in phagocytic cells (e.g., leukocytes, macrophages, kuffer’s cells etc.), injured and degenerating cells.’
Structure:
Lysosomes are single membrane bounded digestive vesicles that are budded off from TGN (Trans-Golgi network). They vary in size from 0.2 – 0.8 µn. Lysosomes contain about 50 different hydrolytic enzymes that can digest proteins, lipids, polysaccharides, DNA and RNA. All the lysosomal enzymes are optimally active at acidic pH (4.6 – 5.0) and hence called as acid hydrolases.
The lysosomal membranes contain proton pumps (H+– AT Pases) that concentrate protons from cytosol to maintain their acidic internal pH. The acid dependence of lysosomal enzymes protects the digestion of cytoplasmic contents even if the leakage occurs. The membranes of lysosomes are strengthened by some biochemical called membrane stabilizers e.g., cholesterol, agents that destabilize the lysosomal membranes arc called membrane labilizers, e.g., bile salts, steroid hormones, carcinogens, silica, excess fat soluble vitamins, X-rays, UV-rays etc.
Types of Lysosomes (Polymorphism):
The lysosomes change the nature of their enzyme contents at different times in the same cell. On this basis, four types of lysosomes exist in animal and plant cells (Fig. 3.33).
(i) Primary Lysosomes or Storage Granules:
These are newly-formed lysosomes that are pinched off from TGN. They contain enzymes in an inactive state.
(ii) Secondary Lysosomes (Hetero-phagosomes or Digestive vacuoles):
These are formed by the fusion of a primary lysosome with a phagosome or pinosome. The proton pumps make their internal pH acidic which activates the hydrolytic enzymes. Secondary lysosomes are the sites of digestion analogous to gut.
(iii) Autophagosomes (Autolysosomes or Autophagic vacuoles):
These are fused in two steps- first an unwanted organelle enclosed in a membranous vacuole derived from SER which then fuses with a lysosome, and its contents are digested. This process of self destruction is called autophagy or auto-digestion, which is important for disposal of unwanted organelles and diseased cells, starvation management and remodeling of tissues during differentiation.
(iv) Residual bodies or Telolysosomes:
These are secondary lysosomes or autolysosomes with indigestible materials. They expel their contents to the outside through exocytosis or ephagy. Failure of ephagy or absence of some lysosomal enzymes causes storage diseases.