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.