Lysosomes: Definition, Types, Examples, Diagram, Function

Lysosome meaning: A lysosome is defined as a cell organelle having an acidic pH and harbours hydrolytic enzymes. The combination means the organelle’s role is to degrade and break down components of the cell. In this article, lysosomes, historical background, lysosome structure, lysosomes being known as suicidal bags, types of lysosomes, the function of the lysosome, and formation of lysosomal enzymes, lysosomal storage disorders, lysosomes in a plant cell, biogenesis and maintenance of lysosomes, and applications and relevance are discussed. Lysosomes are a topic of the chapter Cell: The Unit of Life in Biology.

What are Lysosomes?

Lysosomes are membrane-bound vesicles localised in eukaryotic, acclimated as the garbage disposal system of the cell. They include different hydrolytic enzymes that are capable of degrading macromolecules like protein, lipids, nucleic acids, and carbohydrates into smaller and simpler units. Lysosomes are vital in digestive processes in the cell, waste disposal as well as recycling of cellular products.

Overview of lysosomes as membrane-bound organelles: Lysosomes are formed by a membrane that surrounds several hydrolytic enzymes that break down macromolecules into molecules. They are necessary for the digestion and the destructive and recycling processes of cells.

Importance in cellular digestion and waste management: Lysosomes are essential in the recycling of the components of the cells; they maintain the balance in cells. They recycle worn-out organelle, pathogen, and debris within the cell, to maintain the cell’s optimum efficiency and hygiene. Also, lysosomes involve themselves in activities such as autophagy in which damaged organelles are broken down and their parts used for energy and other cellular processes.

Historical Background

Discovery by Christian de Duve: Lysosomes have been identified since the mid-1950s by a Belgian cytologist by the name of Christian de Duve. De Duve spotted membrane-bound vesicles holding strong hydrolytic enzymes in cells, he called them lysosomes. He made a discovery that changed the principle of cellular biology and explained how a cell gets its nutrients and disposes of waste products.

Lysosome Structure

The structure of the lysosome is described below-

General Structure

Description of lysosome membrane and contents: Lysosomes are single membrane-bound organelles that are bounded by a lipid bilayer which creates a distinction between the inside of the lysosome and the rest of the cytoplasm. The lysosome has different proteins in its membrane pore that are responsible for importing macromolecules headed to the lysosome.

Hydrolytic enzymes and acidic environment: The lysosomal contents contain a broad spectrum of hydrolytic enzymes which comprise proteases, lipases, nucleases, and glycosidases. These enzymes are optimally active in the acidic environment that characterizes the lysosome which has a pH of about 4. 5 to 5. 0. The pH of the lysosome is acidic, this is facilitated by a proton pump in the lysosomal membrane that translocates hydrogen ions (protons) into the lysosome thus creating a such environment that favours enzyme function.

Also read-

• MCQ on Cell: The Unit of Life
• Concept: Cell: The Unit of Life
• Plasma Membrane
• Cell Wall

Lysosome Diagram

Why are Lysosomes known as Suicidal Bags?

Through this process of analysis or self-degradation, lysosomes are popular suicidal bags. This occurs under various circumstances most especially when the lysosomal membranes are disrupted and thus release the hydrolytic enzymes into the cytoplasm. As such, they can lyse the cell and break down the cellular material leading to death. This process is quite paramount in situations where cellular stress or injury occurs as; this results in the rupture of the lysosome leading to cell damage and ultimately death. Therefore, the nickname “suicidal bags” refers to the dangerous destructive capability of lysosomes if their membranes are interrupted.

Types of Lysosomes

The different lysosomal types are discussed below-

Primary lysosomes

The primary lysosomes are the new-forming lysosomes, which contain inactive hydrolytic enzymes. These enzymes are biosynthesized in the endoplasmic reticulum and then transported to the Golgi apparatus where they are modified before being shipped to the lysosomes.

Secondary lysosomes

Some of the structures that arise from the fusion of primary lysosomes with other vesicles are known as secondary lysosomes. On fusion, the hydrolytic enzymes in the primary lysosomes get activated break down the engulfed materials, and discharge their elements for re-cycling or other uses.

Autophagosomes

‘Autophagosomes’ are the vesicles that are formed during autophagy through which cellular components like damaged organelles or protein aggregates are enveloped within the double membrane structure. These autophagosomes later fuse with the lysosomes to form the so-called autolysosomes where the engulfed material is degraded by the enzymes of lysosomes.

Endolysosomes

Endolysosomes are vesicles derived from the fusion of endosomes with lysosomes of the cell. Endosomes are small spherical structures that are still surrounded by a membrane and play a part in sorting and transporting materials within the cell inclusive of the degradable ones. Endosomes that transport the internalized material fuse with lysosomes to form endolysosomes through which the endocytosed materials are degraded by lysosomal enzymes.

Residual Bodies

These are the lysosomes with undigested food material. They fuse with the plasma membrane and remove the debris through exocytosis.

Function of Lysosmes

Following describe the functional aspect of lysosomes-

Intracellular Digestion

Breakdown of macromolecules: Lysosomes act as the sewerage system of the cell with the function to degrade all types of macromolecules like proteins, lipids, nucleic acids, and carbohydrates into their basic components.

Role in recycling cellular components: Lysosomes also act as garbage disposal where damaged organelles, total cell debris, and other unwanted materials get degraded. It also aids the cell in reclaiming and recycling hard-earned nutrients and components for use in the construction of new cellular structures.

Autophagy

Process of self-digestion: Autophagy is a process in which cellular components include organelles and proteins, which are no longer functional to be exchanged by a double membrane called autophagosomes. These autophagosomes then fuse with lysosomes and form what are referred to as autolysosomes within which the enclosed materials are then broken down by lysosomal enzymes.

Role in removing damaged organelles and proteins: This is a quality control since the organelles and proteins must be evaluated and disposed of to give way for the new ones if they are dysfunctional or have formed aggregates. This process is crucial for the sustenance of a healthy and stable internal environment within the cell’s cytoplasm and for avoiding the buildup of hazardous by-products of cellular metabolic activity.

Pathogen Destruction

Role in the immune response: Lysosomes help the cell in its defence mechanisms as they digest antibodies and any other foreign material like bacteria and viruses taken in by the cell. Cells including macrophages and neutrophils recognize pathogens and engulf them into the phagosomes that later on fuse with lysosomes thus forming phagolysosomes where the pathogens are destroyed.

Mechanism of destroying bacteria and viruses: Proteases and nucleases that are present in lysosomes break down the proteins and nucleic acids of the pathogens and make them non-functional. This process is important in the protection of the cell against microorganisms’ intrusions and general immune system response.

Apoptosis

Involvement in programmed cell death: Lysosomes actively participate in apoptosis also known as the process of programmed cell death through the release of hydrolytic enzymes into the cytoplasm of the cell and hence in degrading almost all cellular organelles culminating in the death of the cell.

Significance in development and disease: Lysosome-dependent apoptosis is crucial in the process of embryonic development, tissue remodelling, and the clearance of unhealthy, damaged, or infected cells. Dysregulation of lysosomal-dependent apoptosis leads to diseases such as cancer, neurodegenerative diseases, and autoimmune diseases.

Where are Lysosomal Enzymes made?

Lysosomal enzymes are synthesized mainly in the ER, which is a system of intracellular membranes that play the role of synthesizing and modifying proteins. Once synthesized in the ER, these enzymes are translocated to the Golgi apparatus which is another cell organelle where the enzymes are processed and matured further. Within the Golgi apparatus, the lysosomal enzymes undergo several chemical changes, one of which is glycosylation which involves the addition of specific carbohydrate residues.

Lysosomal Storage Disorders

The various aspects of lysosomal storage disorders are explained below-

Overview of Lysosomal Storage Diseases (LSDs)

Genetic basis and enzyme deficiencies: Lysosomal storage disorders are a group of genetic diseases that are caused by the reduced activity of lysosomal enzymes; thus, the substrates fail to degrade within the lysosomes.

Some of the LSDs are Gaucher; Tay-Sachs; Pompe.

Symptoms and Diagnosis

Clinical features of major LSDs: LSDs have different clinical manifestations depending on the sort of enzyme defect and the collectable substrate. However, typical signs for affected dogs may be a delay in development, neurological degeneration, changes in the structure of the skeleton, enlargement of certain organs, and progressive dysfunction of the organs.

Diagnostic methods: Thus, the diagnosis of LSDs may include clinical examination, identification of the metabolite, and/or molecular analysis.

Treatment and Management

Enzyme replacement therapy: ERT is widespread for LSD management as it involves the administration of recombinant enzymes that can compensate for the lack of activity of deficient enzymes in patients’ cells. It involves the use of recombinant proteins that are given through the veins to supplement deficient enzymes hence minimizing the level of substrate in the body.

Gene therapy and other emerging treatments: Currently, gene therapy, stem cell transplantation, and substrate reduction therapy are developing treatments for LSDs that target the source of the disease or attempt to minimize the occurrence of substrate formation. For gene therapy, genetic copies of the mutated gene are introduced into the affected tissues to replace the non-functional enzyme. Stem cell transplantation is a means of trying to acquire healthy cells, which can produce functional lysosomal enzymes.

Summarising the key LSDs, symptoms, and treatments

The symptoms and treatment options for LSDs are given below-

Lysosome in Plant Cell

In plant cells, the structures functionally related to lysosomes are termed as the lytic vacuoles or simply vacuoles. Whereas in the animal cells lysosomes are strictly involved in cellular digestion and waste disposal, vacuoles in the plant cells have many other functions.

Phagosomes of plant cells can be compared to the lysosomes in animal cells as they are involved in the process of intracellular digestion and are equipped with hydrolytic enzymes that degrade macromolecules. They are involved in cellular waste disposal since they break and recycle other cellular structures and products.

Biogenesis and Maintenance of Lysosomes

The process of Biogenesis and Maintenance of Lysosomes is described below-

Formation of Lysosomes

Origin from Golgi apparatus and endosomes: Lysosomes are obtained through the Golgi apparatus and endosomes. The newly formed lysosomal enzymes have to be retrogradely transported with the help of mannose-6-phosphate (M6P) receptors in the Golgi apparatus. These enzymes are then incorporated within structures known as primary lysosomes, which in turn pinch off from the Golgi apparatus.

Primary lysosomes combine with endosomes thereby forming the secondary lysosomes or endosome-lysosomes. Endosomes host materials that have been brought in from the cell surface and when these two come together with the primary lysosome then we have substrates for digestion in lysosomes.

Role of mannose-6-phosphate (M6P) tagging: For the targeting of lysosomal enzymes to the lysosomes, the process of mannose-6-phosphate (M6P) tagging is very important. In the Golgi complex, the lysosomal enzymes are modified with the M6P group which helps in targeting and shuttling the enzymes to lysosomes.

Regulation of Lysosomal Function

Role of transcription factors: User-defined transcription factors such as transcription factor EB (TFEB) are critical in controlling the process of the formation of lysosomes and even their functionality. TFEB targets the genes that are involved in the formation of the lysosome, the autophagy process as well as the activity of lysosomal enzymes.

Mechanisms ensuring lysosomal enzyme activity and membrane integrity: There are various ways through which the lysosomes operate effectively and the integrity of its membrane is preserved. The lysosomal enzymes are active only within the lysosomes since they are activated at the acidic pH within lysosomes where they degrade the macromolecules.

Applications and Relevance

Role in understanding and treating diseases: Lysosomes are used significantly in the diagnosis and treatment of various diseases, especially Lysosomal storage diseases. Studying the lysosomal function and the dysfunctions has given me an understanding of LSDs and the attempt to control them for instance through ERT and gene therapy.

Use in drug delivery systems: Lysosomes are employed in drug delivery vehicles to increase the efficiency of delivery of the molecules of interest to specific regions of the target cells. Liposomes and nanoparticles can also be designed to release the drug in the lysosomal matrix, that has a low phys and enzymatic environment.

Importance for students and researchers: Lysosomes are integral components that exist in cellular and molecular biology structures because they are involved in degradation, recycling, and signalling mechanisms. Knowledge of the role of lysosomes and their pathophysiology is crucial to the students and investigators focusing on cell biology and metabolism and degradation pathways as well as for those interested in the mechanisms of lysosomal diseases.

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