Lysosomes | Structure, Definition, Function & Diagram
Lysosomes are simple tiny spherical sac-like structures evenly distributed in the cytoplasm. These are formed by the process of packaging in the Golai apparatus. They are bounded by a singie membrane. They are rich in hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases).
Optimally active at the acidic pH: These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acid. Acidic conditions are maintained inside the lysosomes by pumping of H ions into them.
This organelle shows polymorphism : On the basis of morphology, their contents and functions, lysosomes are divided into following four forms :
(i) Primary lysosomes : These are small, vesicle-like newly formed structures produced from the golgi apparatus, at trans face. Primary lysosomes contain inactive enzymes.
(ii) Secondary lysosomes : These are also called heterophagosomes or digestive vacuoles. Secondary lysosomes are formed when phagosomes fuse with already existing primary lysosomes. These contain the enzymes against the material be digested.
(iii) Residual bodies : These are secondary lysosomes containing undigested substances. Residual bodies pass outwardly, come in contact with plasmalemma and throw their contents to the outside through ephagy or exocytosis. However, in certain cells the residual bodies do not discharge their contents to the outside. Instead, they load the cells and bring about ageing, e.g., liver cells, muscle cells. Polynephritis may occur due to absence of ephagy from residual bodies. A number of other diseases are linked to malfunctioning of lysosomes — arthritic joints, gout and lung fibrosis. Some 20 genetic or congenital diseases occur in human beings due to deficiency of certain lysosomal enzymes, e.g., Hunter’s syndrome, Niemann-Pick disease, Farber’s disease.
(iv) Autophagic vacuoles: They are formed by union of many primary lysosomes around old or dead organelles which surround them with vacuolar membrane and digest them by autolysis or autodigestion. These are also called suicidal bags. The disappearance of larval organs during metamorphosis (e.g., tail in frog) is due to autolysis.