Lysosome – Ultrasturucture and Polymorphism
Christian de Duve first observed these organelles in animal cells in 1949, by cell fractionation studies and Novikoff (1960) suggested that they are derived from pinocytic vesicles.
These are single membrane bound small vesicular structures of 0.2 – 0.5 mm in diameter or larger, rich in hydrolyzing enzymes (acid hydrolases) and you can find lysosomes pictures below.
One important property of lysosomes is their stability in the living cell. The enzymes are enclosed by a membrane and are not readily available to the substrate. This socalled latency of the lysosomal enzymes is due to the presence of the membrane.
The membrane is resistant to the enzymes that it encloses, and the entire process of digestion is carried out within the lysosome. In this way, it protects the rest of the cell from the destructive effect of the enzymes, and its stability is of fundamental importance to the normal function of the cell. Below is the description of a lysosome.
Ultrastrucuture of Lysosomes
Each lysosome is a small vesicle surrounded by a single membrane and contains about 50 strong hydrolytic enzymes (acid hydrolases), which are capable of digesting or breaking down all powerful biological substances.
Hydrolytic enzymes of lysosomes act at an acidic pH. The acidic condition is maintained by pumping protons into the interior of lysosomes. Some of the hydrolytic enzymes are acid phosphatases, acid ribonucleases, acid deoxyribonucleases, cathepsins, glycosidases, etc.
Lysosomal membrane is generally strengthened by cholesterol, cortisone, heparin etc. They are called membrane stabilizers. Excess of liposoluble vitamins, steroidal sex hormones, bile salts, X-rays and UV rays make the lysosomal membrane fragile. They are called membrane destabilizers.
The organelle shows polymorphism. According to the current interpretation, the polymorphism is the result of the association of primary lysosomes with the different materials that are phagocytized by the cell.
On the basis of morphology, their contents and functions, lysosomes are divided into following four forms:
Polymorphism in lysosomes
(i) Primary lysosomes:
These are small, vesicle-like newly formed structures produced from the Golgi apparatus function, at trans face. Primary lysosomes contain inactive enzymes.
(ii) Secondary lysosomes
These are also called heterophagosomes or disgestive vacuoles, which are formed when phagosomes fuse with already existing primary lysosomes. These contain the enzymes against the material to be digested and this is the lysosomes function.
(iii) Residual bodies
These are formed from digestive and autophagic vacuoles which contain only undigested materials. Residual bodies pass outwardly, come in contact with plasmalemma and throw their contents to the outside through ephagy or exocytosis.
(iv) Autophagic vacuoles
They are formed by union of many primary lysosomes around old or dead organelles, surround them with vacuolar membrane and digest them by autolysis or autodigestion.
Now that you have learnt all about lysosomes, you can further read other related topics which will be helpful in your medical entrance exams.