Lysosomes: Definition, Structure, Functions

Lysosomes are tiny vascular membrane-bound vesicles involved in intracellular digestion. They contain a variety of hydrolytic enzymes that remain active under acidic conditions.

Greek words “Lysis = digestive or break down, and soma = body“.

During electron microscopic studies in the early 1950s, rounded dense bodies were observed in rat liver cells. These bodies were initially described as “perinuclear dense bodies“. Later in 1955, Belgian cytologist C. de Duve renamed these organelles “lysosomes“.

Definition of Lysosomes

Lysosomes are small sac-like membrane-bound cytoplasmic cell organelles that contain hydrolytic enzymes for cellular digestion.

Lysosome diagram
Figure: Lysosome and its different parts

Occurrence

Lysosomes occur mainly in most animal and few plant cells (e.g.-Onion seeds, Barley seeds, Corn seedlings, Yeast, and Neurospora).

They are absent in bacteria and mature mammalian erythrocytes. Leukocytes, especially granulocytes are rich in lysosomes. Few lysosomes occur in muscle cells or in acinar cells of the pancreas.

Shape and size

The shape and size of lysosomes are variable. Morphologically they can be compared with Amoeba and white blood cells (W.B.C.). Lysosomes are normally 0.2 to 0.5 μm in size.

Since the size and shape of lysosomes vary from cell to cell and time to time (polymorphic nature), their identification becomes difficult.

Structure of Lysosomes

Like other cytoplasmic complexes, lysosomes are round sac-like structures that remain filled with dense material and digestive enzymes.

Each lysosome consists of mainly two parts- a limiting membrane and an inner dense mass or matrix.

Limiting Membrane or Lysosomal Membrane

This is a single-unit membrane on the outside of the lysosome and is composed of lipoproteins. The limiting membrane contains different types of acidic and highly glycosylated integral proteins, Which protect the limiting membrane from the action of various enzymes in the lysosome (lysosomal enzymes).

The presence of protein transporters (like- H+ ATPase) in the lysosomal membrane can be observed that control the high protein concentration in the lysosome.

Permeability of Lysosomal Membrane

The lysosomal membrane functions as a semi-permeable membrane (the lysosomal membrane is impermeable to the substrate of the enzymes contained in the lysosome). As a result, the substrate of the lysosomal enzymes (the enzymes present in the lysosome) cannot enter the lysosome.

Significantly the presence of certain substances (Vitamin A, Vitamin B, Vitamin K, Progesterone, Testosterone, etc.) called labializes, cause instability of the lysosomal membrane. Other substances (cholesterol, cortisol, phenothiazine, etc.) called stabilizers help maintain the membrane’s stability.

Inner Dense Mass or Matrix

The granular heterogeneous ground substances (may be solid or very dense) that are present inside the limiting membrane are called the matrix. solid or of very dense contents.

Some lysosomes have a very dense outer zone and a less dense inner zone. Some others have cavities or vacuoles within the granular material.

Lysosomal Enzymes

A lysosome may contain up to 40 types of hydrolytic enzymes (e.g.- proteases, nucleases, lipases, glycosidases, etc.). All lysosomal enzymes are acid hydrolyses (function more efficiently under sightly acidic medium), optimally active at the pH 5.0 maintained within lysosomes.

All these enzymes of the lysosomes are enclosed within a single lipoprotein membrane. Some important lysosomal enzymes are:

  1. Nucleases:
    • Acid ribonuclease
    • Acid deoxyribonuclease
  2. Proteases and peptidases:
    • Cathepsin A, B, C, D, and E
    • Collagenase
    • Peptidases
  3. Phosphatases:
    • Acid phosphatase
    • Phosphodiesterase
  4. Enzymes acting on oligosaccharide chains of glycoproteins and glycolipids:
    • β-galactosidase
    • β-Glucosidase
    • α-Glucosidase
    • α-Mannosidase
    • Acetylhexosaminidase
    • Sialidase
  5. Enzymes acting on glycosaminoglycans:
    • Lysozyme
    • Hyaluronidase
    • β-Glucuronidase
  6. Enzymes acting on lipids:
    • Phospholipase
    • Esterase

Polymorphism of Lysosomes

The lysosomes have no specific shapes. Even within the same cell, they have different shapes. Due to their extremely dynamic nature, lysosomes exhibit polymorphism in their morphology.

The polymorphism of lysosomes depends on the type of substance that has been absorbed into the cell by phagocytosis (variation in contents of lysosomes with different stages of digestion).

Generally, lysosomes can be traced in four forms- Primary lysosomes, Secondary lysosomes, Autophagosomes, and Residual bodies.

Polymorphism of lysosomes
Figure: Polymorphism of lysosomes

Primary Lysosomes

Primary lysosomes are newly formed organelles having a single unit membrane containing enzymes in inactive forms. The primary lysosome secretes its enzymes out of the cell or stores them inside itself.

This type of lysosome does not involve in digestion. The primary lysosomes are also called storage granules, protolysosomes, or virgin lysosomes.

Secondary Lysosomes

Secondary lysosomes or heterophagosomes are formed by the fusion of primary lysosomes with endosomes (e.g. phagosomes and pinosomes) or cytoplasmic vacuoles containing extracellular substances. This type of lysosome is involved in digestion by the enzymatic activities of the hydrolytic enzymes.

The secondary lysosomes are also called digestive vacuoles, heterophagic vacuoles, heterolysosomes, or phagolysosomes.

Autophagosomes

The autophagic vacuoles or autophagosomes are formed when the cell feeds on its intracellular organelles such as the Mitochondria, Endoplasmic reticulum, or Golgi apparatus by the process of autophagy (autodigestion). In such cases, the lysosomes are concentrated around the intracellular organelles and digest them ultimately.

The autophagosomes are also known as cytolysosomes or autolysosomes. These are actually a type of secondary lysosomes.

Residual Bodies

After complete digestion in heterophagosomes and autophagosomes, some substances (like- grains, amorphous masses, ferritin-like or myelin figures) remain indigestible. Lysosomal membranes characterized by the presence of undigested materials are called residual bodies.

They are also known as telolysosomes or dense bodies.

Functions of Lysosomes

The functions of lysosomes are as follows:

1. Intracellular Digestion

Lysosomes take part in intracellular digestion through the processes of heterophagy and autophagy.

In heterophagy, the extracellular materials present within the heterophagosomes are digested by the lysosomal enzymes. Food digestion in protozoans, sponges, and coelenterates is a common example of heterophagy. Also, heterophagy develops a defense mechanism in these animals.

In autophagy, various intracellular organelles or materials in the autophagosomes are digested by the lysosomal enzymes. Autophagy mainly occurs during critical cell conditions such as pathological conditions, starvation, etc. It is also known as autolysis.

With the help of the lysosomal enzymes, the lysosome can digest or destroy its own cell organelles, which leads to the death of the cell (autolysis). Hence, the lysosome is called the “suicide bag” of the cell.

Autolysis helps in the regression of the uterus following delivery, the regression of mammary glands following weaning, and so on.

Both heterophagy and autophagy may occur simultaneously within the same lysosome. Such a lysosome is called the ambilysosome.

2. Extracellular Digestion

Lysosomal enzymes help in extracellular digestion. Some cells release lysosomal enzymes to the outside through exocytosis for the digestion of extracellular materials.

During the remodeling of bones, the osteoclasts of bone undergo extracellular digestion of the bone matrix. Also, saprophytic fungi obtain their nutrition through extracellular digestion.

3. Role in Metamorphosis

Recently, the role of the lysosome in the metamorphosis of frogs has been discovered. The disintegration of the tail and gills of tadpole larvae of frogs is due to the activity of the lysosomal enzymes cathepsins (protease enzymes).

4. Help in Fertilization

During fertilization, the acrosome (giant lysosome) of the sperm head secretes lysosomal enzymes called lysins. Lysins dissolve the limiting membrane around the egg.

In this way, a channel is formed, through which the sperm nucleus penetrates into the egg.

5. Removal of Dead Cells

Lysosomes also help in the removal of dead cells from tissues such as cells in the outer layer of skin, white blood cells with engulfed bacteria in the blood, and mucous membrane linings of the body (Hirsch and Cohn; 1964).

6. Lysosomal Storage Diseases

Lysosomal storage diseases are genetic diseases caused by the accumulation of undigested materials (polysaccharides, lipids) due to deficiencies in single lysosomal enzymes.

There are about 50 types of lysosomal storage diseases known in humans, e.g., Pompe disease, Tay-Sach’s disease, Gaucher’s disease, Hunter’s syndrome, etc.

7. Other Lysosomal Functions

Lysosomes also serve a few other functions, such as:

  • Thyroxin Formation: Hydrolysis of thyroglobulin produces thyroxine in the thyroid, which occurs under the influence of lysosomes.
  • Role in Cell Division: Lysosomes stimulate mitotic cell division by overcoming agents that cause repression of mitosis.
  • Role in Osteogenesis: Lysosomal enzymes play an important role during the formation of bone cells from cartilage.

References

  1. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.
  2. Cooper, G. M. (2000, January 1). Lysosomes – the Cell – NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK9953/
  3. Bonam, S. R., Wang, F., & Muller, S. (2019, September 2). Lysosomes as a Therapeutic Target – Nature Reviews Drug Discovery. Nature. https://www.nature.com/articles/s41573-019-0036-1

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