Lysosomes - Definition, Types, Significance, Functions

Lysosomes are cell organelles that are also known as "suicide bags" or "cell recycling centers" of the cell. Lysosomes function in cellular waste disposal and programmed cell death (apoptosis). Lysosomes are eukaryotic membrane-bound cell organelles that appear small and spherical.

Lysosomes arise from the Golgi apparatus. They are the reservoirs of digestive enzymes. In this article, we will study in detail lysosome structure and function, diagram, and more.

Lysosome Definition

Lysosome is a membrane-bound organelle present in cells. Lysosomes are often referred to as the "garbage disposal" or "sucidal bag" of the cell because their primary function is to break down and digest various cellular waste materials.

What are Lysosomes?

Lysosomes are eukaryotic cell organelles that help in the digestion and removal of cellular waste and debris. They are formed by the joint activity of the endoplasmic reticulum and Golgi complex. All materials that have to be acted upon by lysosome enzymes are usually enclosed inside vacuoles and the vacuoles fuse with the lysosomes for the digestion of materials.

Thus, lysosomes take part in the intracellular digestion of various types of materials of endogenous or exogenous origin, extracellular digestion can be performed by them under certain conditions. Lysosomes pass through various stages of development in the same cell and this phenomenon is called polymorphism (the existence of more than one morphological form).

Location of Lysosome

They are found in the cell cytoplasm of eukaryotic cells. In animals, lysosomes are present in huge amounts in leucocytes, macrophages, Kupffer's cells, and similar cells with phagocytic activity. They are present in fungi, Euglena, root tip cells of maize, cotton, and pea seeds. In plants, the function of lysosomes is shared by phagosomes, aleurone grains, and vacuoles.

Why Lysosomes are Known as Suicidal Bags?

They contain hydrolytic enzymes like proteases, lipases, and nucleases that are capable of breaking down all types of biological polymers (e.g. proteins, nucleic acids, carbohydrates, and lipids) that enter the cell or are no longer useful to the cell. When a cell dies, the lysosomes release some enzymes which digest the cell debris, and even it can break its proteins by the process of autolysis. Thus, they are referred to as "suicide bags".

For more information read: Why Lysosomes are Known as Suicidal Bags?

Lysosome Structure

Lysosomes are spherical and are enclosed by a single-layered membrane. The interior of lysosomes may be almost solid or differentiated into an outer denser region and a central less dense mass with granular content called the lumen. The interior of the lysosome contains a variety of hydrolytic enzymes, known as lysosomal enzymes, which are responsible for breaking down and digesting various cellular components, including damaged organelles, waste materials, and foreign substances.

Within the lysosome, there is an acidic environment maintained by specialized pumps that regulate the pH between 4.5 and 2.0. Their size may vary with each other and are responsible for various cellular processes like energy metabolism, cell signaling, etc.

Lysosome Diagram

Given below diagram of lysosome shows hydrolytic enzymes present in the single membrane structure.

Where are Lysosomal Enzymes Made?

Lysosomal enzymes are synthesized and produced in the endoplasmic reticulum (ER). The endoplasmic reticulum is involved in various cellular processes, including protein synthesis, folding, and modification. In the endoplasmic reticulum, lysosomal enzymes undergo specific post-translational modifications, such as glycosylation and phosphorylation, during their synthesis.

These enzymes are then transported to the Golgi apparatus for further processing and packaging into lysosomes. These lysosomes then fuse with endosomes or autophagosomes containing cellular materials for digestion, break down various molecules, and maintain cellular homeostasis.

Enzymes of Lysosomes

Lysosome consists of around 45 enzymes which are categorized into 6 different classes, which are;

1. Protease: Digest proteins.
2. Nuclease: Digests nucleic acids.
3. Lipase: Digest lipids
4. Phosphatase: Releases phosphates from organic substances.
5. Glycosidase: Breaks glycosidic bonds.
6. Sulphatase: Releases sulfates from organic substances.

Lysosome in Plant Cell

Lysosomes are not present in plant cells. Plant cells have specialized organelles called vacuoles, that function similarly to lysosomes in animal cells. Vacuoles are large membrane-bound structures that help in cellular processes. They act as storage compartments for various nutrients, such as sugars, ions, and pigments. Vacuoles also serve as waste disposal units. More importantly, vacuoles contribute to maintaining turgor pressure, providing structural support to plant cells and tissues.

Types of Lysosomes

There are four types of lysosomes depending on their morphology and function.

1. Primary Lysosomes: They have newly pinched-off vesicles from the Golgi apparatus. The primary lysosomes are small in size and contain inactive enzymes.
2. Secondary Lysosomes: Also called hetero-phagosome or digestive vacuole. A secondary lysosome is formed by the fusion of a food-containing phagosome with a lysosome. In secondary lysosomes, the enzymes become activated and start breaking down the enclosed materials.
3. Residual Bodies (Residual or Tertiary Lysosomes): They are those lysosomes in which only indigestible food materials have been left.
4. Autophagic Vacuoles (Autophagosome, Autolysosomes): They are produced by the fusion of several primary lysosomes around worn-out or degenerated and digested intercellular organelles. The phenomenon also called autophagy or autodigestion, helps in the disposal of cell debris. Therefore, lysosomes are also called disposal bags or disposal units.

Significance of Lysosomes

The significance of Lysosomes is as follows:

• In WBC or leucocytes: Cells of leucocytes digest foreign proteins, bacteria, and viruses.
• In autophagy: During starvation, the lysosomes digest stored food contents such as proteins, fats, and glycogen of the cytoplasm and transport the necessary amount of energy to the cell.
• In metamorphosis (Frog): During the transformation of a tadpole into a frog, the embryonic tissues such as gills and tail are digested by the lysosomes and utilized by other body cells.
• In fertilization: The lysosomal enzymes present in the acrosome of sperm cells digest the limiting membrane of the ovum (egg). Thus, the sperm can enter the ovum and start fertilization.
• Cellular processes: They are essential for various cellular processes, including waste disposal, recycling of cellular components, and programmed cell death.

Functions of Lysosomes

The function of the lysosome is as follows:

• They take part in the digestion of food obtained through phagosomes known as intercellular digestion.
• Lysosomes release their enzymes to the outside through exocytosis to perform extracellular digestion.
• Lysosomes destroy the obstructing structures.
• By breaking down cellular components, lysosomes help recycle nutrients and biomolecules back into the cell. This recycling process is crucial for energy conservation and the synthesis of essential molecules.
• In sperms, lysosomes provide enzymes for breaking the limiting membrane of eggs.
• Lysosomes destroy the engulfed material, aiding in the body's defense against infections.
• Leucocyte granules are derived from lysosomes.
• Lysosomes cause the breakdown of aging and dead cells.

Lysosomal Diseases

Lysosomal diseases, also known as lysosomal storage disorders (LSDs), is an inherited metabolic disorder that results from deficiencies in lysosomal enzymes or transport proteins. It leads to the accumulation of undigested substances within lysosomes. As a result, cellular waste products, lipids, and complex molecules cannot be properly broken down and recycled, causing progressive damage to tissues and organs. Lysosomal disease symptoms include neurological, skeletal, and visceral manifestations. Examples of Lysosomal diseases are Gaucher disease and Tay-Sachs disease.

Conclusion - Lysosomes

In conclusion, lysosomes, often called "suicide bags" are crucial organelles responsible for cellular waste disposal and programmed cell death. Lysosomes arise from the Golgi apparatus. Lysosomes contains digestive enzymes that play important roles in intracellular digestion and maintaining cellular homeostasis. Understanding lysosomes' structure, function, and significance sheds light on their vital contributions to cellular processes and the implications of lysosomal diseases in human health.

Also Read:

• Difference Between Lysosomes and Ribosomes
• Cell Organelles – Structure, Types and their Functions
• Endomembrane System – Overview, Structure, and Functions