Lysosomes

 Lysosomes

The word "lysosome" means “dissolving body” or “digestive body”. Lysosomes are small organelles found in eukaryotic cells, especially in animal cells. The main functions of lysosomes are to break and dissolve cellular contents and sometimes kill the cell. Hence, lysosomes are also known as the “suicidal bags” or “digestive system” of the cells.  

Lysosomes Structure

Lysosomes are simple, round membranous structures containing a huge number of destructive enzymes that can break, dissolve and destroy materials.

Lysosomal membrane

The membrane of a lysosome is called the lysosomal membrane. It is so specially designed that the lysosomal enzymes cannot digest it. There are special proteins in the lysosomal membrane which are called LAMPs (Lysosomal associated membrane proteins) and LIMPs (Lysosomal integral membrane proteins).

LAMPs are present on the internal side of the lysosomal membrane and LIMPs are embedded in the lysosomal membrane.  These proteins do not provide attachment sites to the lysosomal enzymes so they cannot be able to attach to the membrane and the membrane is not destroyed and stays safe.  This is important for the maintenance of the internal environment of the lysosomes.

Lysosomal Enzymes

Lysosomal enzymes are acid hydrolases that perform the function of hydrolysis. These enzymes work in an acidic environment. The acidic environment is maintained by special proteins called proton pumps present in the lysosomal membrane. These proteins pump hydrogen ions from the cytoplasm into the lysosomes by utilizing ATP. As more hydrogens enter the lysosomes, the pH inside lysosomes becomes 4-5 which is good for the proper functioning of lysosomal enzymes. 

Types of Enzymes Present in Lysosomes

There are more than 40 types of enzymes present in lysosomes. Some particular enzymes are proteases. Lipases, nucleases, glycosidases, acid phosphatases, etc. These enzymes have important functions as follows:

• Proteases; breakdown proteins
• Lipases; breakdown lipids
• Nucleases; breakdown DNA and RNA
• Glycosidases; degrade carbohydrates
• Acid phosphatases; breakdown phosphate esters

How are Lysosomes Synthesized in the Cells?

The ribosomes present on surface of endoplasmic reticulum synthesize proteins for lysosomes. These proteins undergo modifications and move to the Golgi bodies for further modifications. Golgi bodies then release primary vesicles of lysosomes. The primary lysosomes combine with the harmful substances in the cells such as bacteria, viruses and start digesting them by release enzymes on their surface. These lysosomes which are involved in the breakage of harmful substances are called secondary lysosomes.

Lysosomes Functions

Endocytosis

When a particle enters the cell by binding to a receptor, the cell gets a signal and takes the particle inside by making it a vesicle. This vesicle is now called an endosome. The lysosome attaches with it and breaks down its contents i.e. lipids, proteins or carbohydrates. The lysosome then sends these broken contents such as fatty acids, amino acids or glucose molecules into the cytoplasm where they are used for various purposes. The lysosomes also send these molecules out of the cells. 

Phagocytosis

The main function of lysosomes is phagocytosis. When any pathogen enters the body, it is destroyed and phagocytosed by white blood cells such as macrophages, dendritic cells, and neutrophiles. The number of lysosomes in White blood cells is higher compared to other cells in the body. During phagocytosis, the white blood cells engulf the pathogens and vesicles are formed which are usually termed as phagosmes. These phagosomes are attached by lysosomes which break down their contents and send the antigens to the B lymphocytes due to which an immune response is generated.

Autophagy

Autophagy means “eating itself”. Some organelles in the cells like mitochondria become useless sometimes so they must be destroyed or eliminated from the cells otherwise they create disturbance in the functions of other organelles. These organelles are destroyed by lysosomal walls. During autophagy, a lysosomal wall is formed around the organelle and an autosome is generated. Lysosomes release their enzymes on the organelle due to which the contents of the organelle are broken down into smaller particles which are then utilized by the cells for various cellular functions. 

Autolysis

Another important function of lysosomes is autolysis. Autolysis means “the destruction of own cell”. Sometimes, a cell becomes extensively damaged or its genetic material becomes useless. The signal of cell damage goes to lysosomes and lysosomes release all their enzymes into the cytoplasm due to which cell destruction occurs. The best example of autolysis is metamorphosis.

Extracellular Digestion

Extracellular digestion is also a function of lysosomes. Some processes have to be performed outside the cell e.g. bone repair. Osteoblasts are the cells that repair the bones by releasing enzymes of lysosomes onto the bone tissues.

Fertilization

In sperm cells, there is a special type of lysosome called acrosome which helps sperm penetration into the egg cells during fertilization.