What are Lysosomes?
Lysosomes are membrane-boundand dense granular structures that contain hydrolytic enzymes that are responsible for digestion in extracellular and intracellular cells. The term “lysosome” is composed of two words “lysis” which means breakdown, and “soma” which means body. It is a crucial cell organelle, responsible for inter- and extracellular breakdown of various substances.
They are most often found in animal cells, but they are only found in a few smaller plant species ( the saprophytic and slime molds). Lysosomes are found in abundance throughout the cells’ cytoplasm. In mammals, they are found in all cells, except in RBCs. They are located in large numbers in cells that are linked to enzymatic reactions, such as pancreatic cells, liver kidney cells, leucocytes, spleen cells, macrophages, and so on.
Structure of Lysosomes
- Lysosomes lack any distinctive shape or shape i.e. they are pleomorphic.
- They’re mostly globular, or granular .
- It measures 0.2-0.5 millimeters in size. It is enclosed by a single, single-lipoprotein membrane distinct in its composition.
- The membrane is high glycosylated lysosomal associated membrane proteins (LAMP) and Lysosomal integral membrane proteins (LIMP).
- LAMPs and LIMPs create a coating on the surface of the membrane
- They shield the membrane from attacks by the various hydrolytic enzymes that are stored inside.
- The lysosomal membrane is equipped with an hydrogen proton pump that helps maintain pH conditions for the enzyme. acidic medium , which is maintained via the proton pumps, which is able to pump H+ into the lumen ensures the efficiency of the enzymes in the lysosomal.
- In this membrane is an organelle that is home to enzymes that are crystallized.
Lysosomal Enzymes
For the degradation of extracellular and intracellular materials and intracellular material, lysosomes are filled with hydrolases, which are enzymes that break down extracellular and intracellular material. There are about 40 kinds of enzymes that are classified into the following major kinds, which are:
- Proteases are enzymes that digest proteins.
- Lipases, which digest the lipids
- Amylase is a digestive enzyme that digests carbohydrates.
- Nucleases, enzymes that process nucleic acids
- Monoesters of phosphoric acid
The group of enzymes is known as hydrolases that cause the cleavage of substrates via adding water molecules. The majority of enzymes in the lysosomal family function inside the acidic media.
Types of Lysosomes
- Primary Lysosomes: Primary Lysosomes are small sac-like structures that house enzymes produced through the rough endoplasmic Reticulum. Also known as storage granules that store enzymes.
- Secondary Lysosomes: Developed through the fusion of primary Lysosome and Phosomes. Contain engulfed material plus enzymes. The material is gradually processed.
Functions of Lysosomes
Lysosomes perform two main functions:
- Intracellular Digestion: In order to digest food the lysosome membrane joins with the vacuole for food and releases the enzymes within. The food digested is then absorbed across the vacuole’s membrane and is absorbed into the cell, where it can be used as growth and energy.
- Autolytic Action: Cell organelles that have to be removed are covered by vesicles and vacuoles through the process of autophagy. This results in an autophagosome. Autophagosomes are destroyed by the action the lysosomal enzymes.
Lysosomes are involved in a variety of processes. important roles include:
A. Heterophagy
The introduction into cells of exogenous matter via phagocytosis or piocytosis as well as the digesting of food particles after the formation of a vacuole and Lysosome.
B. Autophagy
Normal physiological process that is responsible for degeneration of cell structures within the body. It is vital to ensure homeostasis, normal functioning through degrading proteins, and regenerating organelles destroyed by cells, and the formation of new cells.
c. Extracellular Digestion
Primary lysosomes release hydrolases externally through exocytosis, resulting in the degradation of extracellular material.
Eg. Saprophytic fungi
D. Autolysis
It is the process of killing of a whole set of cells through the degeneration of the membrane of the lysosomal. It happens during amphibian and metamorphosis of insects.
e. Fertilization
The acrosome inside the head of the sperm is a massive lysosome, which breaks and releases enzymes onto the egg’s surface. This facilitates the sperm to enter the egg through digestion of the egg’s membrane.
F. as Janitors in the Cell
Lysosomes clear out ‘junk’ which could build up inside cells aiding in the prevention of diseases.
Development of Lysosomes
Lysosomes form through the union of vesicles in the Golgi complex and endosomes. Endosomes are vesicles created by endocytosis when an area of plasma membrane gets cut off and is then internalized by cells. Through this process, extracellular matter is absorbed by cells. As endosomes grow and become mature, they are known in the late endosomes. Endosomes that are late in maturation fuse with transport vesicles of the Golgi which contain acid hydrolases. Once they are fused, these endosomes develop into Lysosomes.
Different enzymes are present in Lysosomes
| Sr. No | Enzymes | Substrate |
|---|---|---|
| 1 | Phosphates | |
| A- Acid phosphatase | Most phosphomonoesters | |
| B- Acid phosphodiesterase | Oligonucleotides and phosphodiesterase | |
| 2 | Nucleases | |
| A- Acid ribonuclease | RNA | |
| B- Acid deoxyribonuclease | DNA | |
| 3 | Polysaccharides/ mucopolysaccharides hydrolyzing enzymes | |
| A- β-Galactosidase | Galactosides | |
| B- α–Glucosidase | Glycogen | |
| C- α–Mannosidase | Mannosides, glycoproteins | |
| D- β- Glucoronidase | Polysaccharides and mucopolysaccharides | |
| E- Lysozymes | Bacterial cell walls and mucopolysaccharides | |
| F- Hyaluronidase | Hyaluronic acids, chondroitin sulfates | |
| H- Arylsulphatase | Organic sulfates | |
| 4 | Proteases | |
| A- Cathepsin(s) | Proteins | |
| B- Collagenase | Collagen | |
| C- Peptidase | Peptides | |
| 5 | Lipid degrading enzymes | |
| A- Esterase | Fatty acyl esters | |
| B- Phospholipase | Phospholipids | |
| 6 | Sulfatases | |
| A- Arylsulfatase(A, B & G) | O- and N-Sulfate esters | |
| B- Glucosamine (N-acetyl)-6-Sulfatase/GNS | Glycosaminoglycans | |
| C- Iduronate 2-Sulfatase/IDS | O- and N-Sulfate esters |
Lysosomal Disease/Lysosomal Storage Disorders
Certain metabolic disorders that are inherited can result in problems with the functioning of the lysosomes. These conditions are known as Lysosomal Storage Disorders, or LSDs. There are about 50 distinct LSDs. Each kind of LSD is extremely rare and occurs in less than one per 100,000 births. However in the aggregate, LSDs occur in 1 of 5,000 to 10,000. LSDs are typically seen when an individual is lacking in a specific enzyme that is responsible for breaking down large molecules such as proteins or the lipids. As the enzyme isn’t functioning large molecules, they are unable to be broken down and eventually they accumulate inside the cell, and eventually end up killing it.
The majority of LSDs are passed down through an autosomal recessive manner. This means it could be concealed by a duplicate of an allele, but without being affected by the change (a dominant allele) and can be caused by a mutation in an autosomal chromosomes which include all chromosomes, with the exception of the sex chromosomes X as well as The Y. The disease Tay-Sachs is one of the most well-known LSD which is recessively acquired. Due to the lack of function in the enzyme called hexosaminidase glycolipids build up inside the brain, causing problems with normal brain function. The result is that nerve cells disintegrate, causing mental and physical functioning to decrease. The cure isn’t there for this, and usually death occurs around the age of four.
Certain LSDs are X-linked and occur due to an X-linked mutation that affects the X chromosome. One of these LSD has been identified as Fabry disease. It is extremely rare it occurs in 1 of 40,000-20,000 live births. People with Fabry disease are deficient in the enzyme alpha galactosidase A, which causes the glycolipid globotriaosylceramide to build up within the body. The symptoms include fatigue and burning pains in the extremities and full bodily pains, hearing loss nausea, kidney and cardiac complications as well as papules on your skin, which are known as angiokeratomas. The mutation that triggers Fabry disease is found on the X-chromosome. However women with just 1 copy of this affected gene may also experience symptoms. Because men have only one the X chromosomes the symptoms of a woman’s X chromosome tend to be more serious. The average life expectancy for people with this condition within the United States is 58.2 for males and 75.4 for females.
Lysosomes in the Plant Cell
Lysosomes can be found in eukaryotic animal cells . They serve as a means of breaking down cell debris. For plants, the function of lysosomes are performed by vacuoles in accordance with what traditional cell biology dictates.
However, recent findings indicate that the role of vacuoles is very similar to the function of a lysosome within animal cells. Further, studies suggest that vacuoles have hydrolytic enzymes that are similar to ones found in animal cells.
Botanists have been challenging the definition of vacuoles. Also, it caused a debate that pits the definition of two organelles in the cellular system according to some who claim that both have similar functions.
Why are Lysosomes known as Suicidal Bags?
As mentioned earlier Lysosomes are the organelles that eliminate waste from the cell, processing unwanted substances and removing them on the outside of the cell and from waste constituents within the cell.
Sometimes, digestive enzymes can cause damage to the lysosomes which can lead to cells to cease to exist. This is known as autolysis “auto” refers to “self” as well as “lysis” is “the breakdown of the cell due to the destruction of the cells’ membrane”.
Therefore, lysosomes can be described for their role as “Suicidal Bags” of the cell.
How does the Lysosome work?
Lysosomes’ primary function is digestion and elimination of waste. Foreign particles or cell debris are brought into the cell by this process called endocytosis. The process of endocytosis takes place when the cell membrane folds over itself (invagination) and forms an invacuole or pouch surrounding the contents of the outside and then bringing them inside the cell.
However the discarded wastes and other substances that originate inside the cell are processed by autophagocytosis, also known as autophagy. Autophagy is a process that involves the breakdown or disassembly of cell components by an organic, controlled mechanism.
Where is Lysosomal Enzymes produced?
Lysosomes are made up of more than 50 enzymes. They are made within the endoplasmic reticulum, which is a rough structure.
After being synthesized, the enzymes are absorbed by the Golgi system in small sacs or vesicles which later join larger acidic vesicles.
The specific enzymes created for lysosomes mix with mannose 6-phosphate, a molecule and are then fixed to form acidified vesicles.
