Functions of Lysosomes

The lysosome is essentially a bag containing digestive enzymes. The enzymes may be released within the cell itself or outside the cell. Lysosomes of dead or injured cells rupture together and release the enzymes.

1. Extacellular digestion

Lysosomal enzymes may be released outside the cell where they take part in the hydrolysis of extracellular material. Digestion of the extracellular matrix is a two-way process involving both extracellular and intracellular digestion. The primary lysosomes secrete hyrolases into the extracellular matrix by exocytosis, resulting in partial degradation of the matrix (extracellular digestion). The incomplete products of digestion are absorbed into the cell by endocytosis (phagocytosis). Digestion is then completed in the secondary lysosomes (intracellular digestion).

2. Intracellular digestion

Digestion of substances within the cell is called intracellular digestion.
Autophagy is intracellular digestion in which enzymes produced by the cell break down molecules and pieces of cytoplasmic material from the cell itself.
Hetrophagy is the intake of exogenous material into the cell and the subsequent breakdown of this material by enzymes. The bulk intake of exogenous material is called endocytosis.

3. Hormone secretion

There is evidence that lysosomal hydrolases are involved in the secretory activity of some cells. They function in modifying products synthesized by the cell before secretion and also in the regulation of secretion. Thus the secretory process in under feedback control from the target endocrine glands.

4. Fertilization

During the sperm releases hydrolytic enzymes from the acrosome vesicle. These enzymes help in the penetration of the sperm through the envelopes of the eggs. Acrosome vesicles contain several enzymes, including hyaluronidase and proteases, which are also found in lysosomes.

5. Development of processes

A large amount of larval tissue is destroyed during metamorphosis of insects and amphibians. This destruction takes place in an organized and sequential manner. The products of tissue digestion are utilized in the synthesized of tissue of the adult forms. The lysosomes secrete their enzymes into the cell, resulting in lysis or cellular digestion of the cell.

6. Lysosomes and disease

Malfunctioning of the lysosomes resulting in tissue damage, and disease could be caused in several ways.
(i) Increase release of hydrolases. There may be increased and uncontrolled autophagy resulting in excessive lytic activity.
(ii) Intracellular release of hydrolases. Hydrolases may leak into the cytoplasm from the lysosomes as a result of permeability changes in lysosomal membranes. This could lead to damage of various cell structures, resulting in disorganization and even death of the cell.
(iii) Extracellular release of hydrolyses. There may be excessive release of lysosomal hydrolyses outside the cell, resulting in the break down of the matrix in connective tissue.
(iv) Accumulation of indigestible material within lysosomes. Indigestible material may accumulate in the cell under certain pathological conditions.