Lysosomes are the membrane-bound vesicular organelles found throughout the cytoplasm. The lysosomes are formed by Golgi apparatus. The enzymes synthesized in rough endoplasmic reticulum are processed and packed in the form of small vesicles in the Golgi apparatus. Then, these vesicles are pinched off from Golgi apparatus and become the lysosomes.

Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane. The membrane is formed by a bilayered lipid material. It has many small granules which contain hydrolytic enzymes.

Types of Lysosomes

Lysosomes are of two types:

1. Primary lysosome : Primary lysosome is pinched off from Golgi apparatus. It is inactive in spite of having hydrolytic enzymes.

2. Secondary lysosome : Secondary lysosome is the active lysosome. It is formed by the fusion of a primary lysosome with phagosome or endosome (see below).

Functions of Lysosomes

Lysosomes are often called 'garbage system' of the cell because of their degradation activity. About 50 different hydrolytic enzymes, known as acid hydroxylases are present in the lysosomes, through which lysosomes execute their functions.

Important lysosomal enzymes

1. Proteases, which hydrolyze the proteins into amino acids.

2. Lipases, which hydrolyze the lipids into fatty acids and glycerides.

3. Amylases, which hydrolyze the polysaccharides into glucose.

4. Nucleases, which hydrolyze the nucleic acids into mononucleotides.

Mechanism of lysosomal function

Lysosomal functions involve two mechanisms:

1. Heterophagy: Digestion of extracellular materials engulfed by the cell via endocytosis.

2. Autophagy: Digestion and degradation of intracellular materials such as worn-out cytoplasmic organelles.

Specific Functions of Lysosomes

1. Degradation of macromolecules

Macromolecules are engulfed by the cell by means of endocytosis (Chapter 3). The macromolecules such as bacteria, engulfed by the cell via phagocytosis are called phagosomes or vacuoles. The other macromolecules taken inside via pinocytosis or receptor-mediated endocytosis are called endosomes.

The primary lysosome fuses with the phagosome or endosome to form the secondary lysosome. Secondary lysosome becomes acidic and the lysosomal enzymes are activated. Bacteria and the other macromolecules are digested and degraded by these enzymes. Secondary lysosome containing these degraded waste products moves through cytoplasm and fuses with cell membrane. Now the waste products are eliminated by exocytosis.

2. Degradation of worn-out organelles

The rough endoplasmic reticulum wraps itself around the worn-out organelles such as mitochondria and form autophagosomes. One primary

lysosome fuse with one autophagosome and form the secondary lysosome. Enzymes in the secondary lysosome are activated. Now, these enzymes digest the contents of autophagosome.

3. Removal of excess secretory products in the cells

Lysosomes in the cells of the secretory glands remove the excess secretory products by degrading the secretory granules.

4. Secretory function: Secretory lysosomes

Lysosomes having secretory function called secretory lysosomes are found in some of the cells, particularly in cells of immune system. The conventional lysosomes are modified into secretory lysosomes by combining with secretory granules (which contain the particular secretory product of the cell).

Examples of secretory lysosomes:

a. Lysosomes in the cytotoxic T lymphocytes and natural killer (NK) cells secrete perforin and granzymes, which destroy both viral-infected cells and tumor cells. Perforin is a pore-forming protein that initiates cell death. Granzymes belong to the family of serine proteases (enzymes that dislodge the peptide bonds of the proteins) and cause the cell death by apoptosis.

b. Secretory lysosomes of melanocytes secrete melanin.

c. Secretory lysosomes of mast cells secrete serotonin, which is a vasoconstrictor substance and inflammatory mediator.