They were first of all observed by Christian de Duve in 1955.
The lysosomes are tiny vesicles which are bounded by single membrane and are made from proteins and lipids.
They are found in different organisms like in most of the animal cells and only in a few plant cells like yeast, fungi and in green unicellular organisms.
They are absent in bacteria and mature mammalian erythrocytes.
They are originated from endoplasmic reticulum or from the vesicles of the Golgi complex.
They contain many enzymes about 40 kinds of enzymes that are involved in intracellular digestion.
The enzyme includes proteases that helps in protein digestion, nucleases, glycosidases (helping in digestion of polysaccharides), lipases, phosphatases (phosphate linked compounds) and sulphatases (sulphate linked compounds).
The internal pH of the lysosome is about 5, which is two units lower than that of the cytoplasm.
So, the hydrolytic enzymes within the lysosomes function optimally at this pH.
These hydrolytic enzymes are nonspecific in their activity and could potentially destroy key cellular macromolecules if not contained.
Types of Lysosomes
The lysosomes are grouped into four types. They are:
i) Primary lysosome: These are newly formed small bodies which contain hydrolytic enzymes in inactive state and are the pinched off vesicle from the golgi apparatus.
ii) Secondary lysosome: The primary lysosome fuses with the vacuoles containing extracellular or intracellular substances and form secondary lysosome. They are also known as heterophagosomes or digestive vacuoles.
iii) Residual body: The lysosomes with undigested bodies are called residual bodies. These are also called tertiary lysosomes.
iv) Autophagic vacuole: Lysosomes are able to digest cell organelles due to which they are known as autophagic lysosomes.
They digest the particles that enter the cell like food particles.
In some pathological conditions, the lysosomes start to digest the various cell organelles and cause ultimately the death of the cell which is known by the process autolysis or cellular autophagy.
For this reason, the lysosomes are also known as suicidal bags.
They digest disease causing microorganisms.
They also digest worn-out and the dead cells.
The lysosomes of sperms discharge their enzymes and digest the limiting membrane of the ovum at the time of fertilization.
Lysosomes are involved in formation of bone cells as well as their destruction.
Ageing of cells as well as parthenogenetic development are also related with lysosomal activity.
Lysosomes have been found to be involved in protein synthesis in case of some birds.