Short Essay on Lysosomes of Structural Cell

Lysosomes are polymorphic organelles of membranous bags of hydrolytic enzymes used for the controlled intracellular digestion of macromolecules.

Lysosomes were discovered by Belgian biochemist Christian de Duve in 1955, though lysosomal enzymatic studies in liver homogenates were carried out in 1949. Lysosomes were identified by electron microscopic cytochemistry by Novikoff in 1956. Some 50 enzymes are known to be contained in different types of lysosomes.

They are all hydrolytic enzymes including proteases, nucleases, glycosidase, lipases, phospholipases, phosphates and sulfatases. All these enzymes are acid hydrolases optimally active near pH. Of 5 maintained within these organelles.

The pH of near 5 is maintained inside the lysosomes by a proton pump present in the membrane of lysosome that pumps or protons from cytoplasm into the lysosome.

The membranes of lysosomes are impermeable to the hydrolytic enzymes contained in them. If, however, leakage of enzymes occurs due to accidental damage to lysosomal membrane and these enzymes are released to cytoplasm, the higher pH value of the cytoplasm renders the enzymes inactive. Due to the presence of this large number of hydrolases, lysosomes are called as “Suicidal bags”.

Lysosomes are found in all eukaryotic cells. Some cells like pancreatic cells, liver cells, spleen cells, leucocytes, meristematic cells of plants are used for study of lysosomes.

The inter communicative network of Golgi complex, ER, nuclear membrane and lysosomes form the endomembrane system. Some regions of endomembrane system constituting more of Golgi complex are often called as GERL (Golgi-ER-Lysosome Complex). Liver lysosomes are spherical bodies / vesicles of 0.5 mm in diameter with an electron dense core that is packed with hydrolases which constitute 60% of the core by weight.

The lysosomes are formed from the Trans most Golgi as enzyme packed vesicles called as primary lysosomes or protolysosomes. Lysosomal enzymes are synthesized by ribosome attached to ER. These enzymes then loaded inside transport vesicles move to Golgi from ER. These enzymes have one oligosaccharide attached to it.

So all lysosomal enzymes are glycoproteins. The oligosaccharide part of the lysosomal enzyme has one Mannose – Phosphate residue as a marker and this marker is essential for packaging of lysosomal enzymes into Golgian vesicles.

Golgi a vesicles are clathrin (a type of protein) coated vesicles which fuse with endosomes to produce lysosomes. Two general classes of lysosomes are usually distinguished. Primary lysosomes – which are newly formed and, therefore, not yet encountered the substrate for digestion and secondary lysosomes or heterophagosomes which are membranous sacs of diverse morphology that contain substrates and hydrolytic enzymes.

Secondary lysosomes result from repeated fusion of primary lysosomes with a variety of membrane bound substrates.

The secondary lysosomes, therefore, present a variety of morphology depending upon the internalization and packaging of different substrates. So lysosomes are polymorphic – large ones resulting from phagocytosis, small ones from endocytosis and so on.

Secondary lysosomes can be of different types: (i) Large digestive vacuoles result from phagocytosis of large bodies like bacteria (ii) multi vesicular bodies are membranous bags containing numerous vesicles of ~50 nm in diameter and (iii) autophagic values or autophagosomes or cytolysosomes are lysosomal structures containing intracellular membranous organelles such as mitochondria or secretory vesicles for digestion.

Lysosomal disorder results in Hurler’s disease in man where bone deformities develop. This is due to accumulation of large amount of mucopolysaccharides in the cell due to absence of lysosomal activities.

Functions:

1. Digestion and turnover of intra and extra cellular constituents.

2. Programmed cell death during embryogenesis.

3. Digestion of phagocytosed micro organisms.

4. Principal site of cholesterol assimilation from endocytosed serum lipoprotein.

5. Removal of carcinogens from cells.

6. Help in bone formation from cartilage as it causes breakdown of existing matrix so that it can be replaced by new one.