The word ‘lysosome’ is derived from two Greek words ‘lysis’ and ‘soma’ where the word ‘lysis’ means digestive and ‘soma’ means body. Hence, in simple words, we can say that lysosome is a structure which helps in digestion. Lysosomes were first discovered by de Duve in 1955 A.D. He found them in the liver cells of rats. These are present in all animal cells and in protists, fungi etc. They are more common in macrophages. They are mostly found in liver cells, White Blood Cells (W.B.C.), pancreas etc. which perform digestive functions.
Lysosomes are bound by single membrane. They contain many digestive enzymes which are known as acid hydrolases. These enzymes are capable of hydrolyzing or digesting all classes of macromolecules. Lysosomes are formed directly from Endoplasmic Reticulum (E.R.) and Golgi bodies (G.B.). The enzymes are synthesized by Endoplasmic Reticulum and transported to Golgi bodies. Then the enzymes are concentrated and then packed into vesicles which form primary lysosomes.
On the basis of morphological contents and functions, lysosomes can be categorized into four types which are listed and described below:
- Primary Lysosomes: Primary lysosomes are small bodies which are just pinched off or separated from Golgi bodies. They enclose enzymetic contents which are usually secreted by Endoplasmic Reticulum.
- Digestive Vacuoles or Phagosomes (Secondary Lysosomes): When the primary lysosome fuses with other vacuoles containing extracellular or intracellular materials, it is called secondary lysosome. Within secondary lysosome, the material is hydrolyzed by enzymes and then it is digested.
- Residual Lysosomes (Tertiary Lysosomes): In secondary lysosomes, food materials are digested. The digested food materials are absorbed into cytoplasm and undigested materials i.e., residue are left. The lysosomes having residue are called residual bodies or residual lysosomes. They move near to the cell surface membrane and they throw the contents by exocytosis process.
- Autophagic Vacuoles (Autolysosomes or Autophagosomes): When a lysosome contains a part of its one cell and digests it, it is called autophagic vacuole. It is a part of normal activity by which old, damaged and functionless organelles are digested.
Functions of Lysosomes
The major functions of Lysosomes are listed below:
- Digestion of large extracellular particles like food materials, that enter the cell is done by lysosomes.
- Digestion of intracellular substances like cell organelles during starvation to provide energy is carried out by lysosomes. This process is also known as autolysis. There can be self-destruction by the release of the contents of lysosomes within the cell. So, lysosomes are called the suicidal bags of the cell.
- Digestion of harmful substances is done by lysosomes.
- Lysosomes actively participate in the digestion of a cell as a whole during pathological conditions when the cells die due to attack of foreign bodies or lack of oxygen or poisoning etc. They digest damaged, old and functionless cells.
- Lysosomes even help in the digestion of substances outside the cell. Lysosomes move near to the cell surface membrane and discharge their contents to digest the materials.