Lysosomes are membranous vesicles discovered by Belgian biochemist Christian ­de Duve in 1955. Presence of lysosomes in Neurospora, an ascomycetous fungus was reported by P.Matile in 1964. Besides their common occurrence in animal cells, lysosomes are also found in plant cells. Lysosomes are tiny bag like structures with an average diameter of 0.23-0.5u. Their form, size and density are extremely variable. Lysosomes are mostly present in secretory cells like pancreatic cells, spleen cells, leucocytes, etc. of animals and in meristematic tissues of plants. Lysosomes generally originate from Golgi cisternae or from the tubules of endoplasmic reticulum.

The lumen of the lysosome is filled with a finely granular digestive fluid rich in acid hydrolases. As many as 50 hydrolytic enzymes have been reported in lysosomes. Lysosomes differ from mi­tochondria in that they lack oxidative enzymes. Acid phosphatases, acid ribonucleases, acid deoxyribonucleases, etc., are some of the hydrolases found in lysosomes.

Four types of lysosomes are recog­nised on the basis of their morphology, internal contents and function. The first type is primary lysosome and the other three types are grouped together as secondary lysosomes.

The primary lysosome or storage granule is a small vesicle like struc­ture produced from Golgi apparatus. It ­contains powerful lytic enzymes. The enzymes are synthesised in the endoplasmic reticulum and are stored in the primary lysosomes. The secondary lysosomes include-


a. Digestive vacuole:

These are small bodies, containing foreign materials engulfed by the cell. Digestion, or breaking down of the foreign body, takes place within the lysosome.

b. Residual bodies:

The lysosome may form a secondary body left with unhydrolysed residues due to incomplete digestion. The residues thus, formed may remain inside the lysosome or may be discharged out.


c. Autophagic vacuole:

Very often the lysosomes release the hydro­lase enzymes in damaged or age­ing cells to digest them. The phe­nomenon is called autolysis (self eating) and the lysosomes are au­tophagic vacuoles or autophagosomes. Due to this autophagic nature of lysosomes, they are also known as suicidal bags of cell.

Lysosomal enzymes may also be released out­side the cell to break down the extra cellular materials. The lysosomes of sperm re­lease enzymes which dissolve the protein coating of the ovum. Lysosomes are also believed to initiate the mitotic process. Besides in­ternal digestion, lysosomes are helpful in digestion of external particles. Lysosomes of leucocytes digest proteins, bacteria and viruses.