Lysosomes were discovered by Christian de Duve. These are small vesicles which are bounded by a single membrane and contain hydrolytic enzymes. Depending upon their morphology and function, these are of four types :
(1) Primary lysosomes : They are newly pinched off vesicles from the golgi apparatus. These contain hydrolytic enzymes in the form of granules.
(2) Secondary lysosomes : These are also called heterophagosomes or digestive vacuoles. It is formed by the fusion of food containing phagosomes with lysosomes. After digestion, the digested food passes out into cytoplasm. This lysosomes is left with undigested food.
(3) Residual bodies : They are lysosomes in which only indigestible food materials have been left. These bodies pass outwardly and further into external environment by exocytosis or ephagy. Sometimes, these remain inside the cells and this leads to pathological diseases like hepatitis and polynephritis.
(4) Autophagic vacuoles : They are produced by the
fusion of a. number of primary lysosomes around worn out or degenerated intracellular organelles. The latter are digested. The phenomenon is also called autophagy or autodigestion. It helps in disposable of cell debris. Thus, lysosomes are also called as disposal bags.