Lysosome and Its Functions

Lysosomes are single membrane-bound cytoplasmic organelles of most cells filled with a wide variety of hydrolytic enzymes that are involved in intracellular digestion.  The term “Lysosome” comes from the Greek word ‘lysis’, to separate and ‘soma’ body.  Sometimes it can be described as the stomach of the cell. In 1950, Belgian Cytologist Christian Rene de Duve isolated a type of particles from the animal cell and named it lysosome. In 1965, Novikoff clearly identified the lysosome in rat live cells through the electron microscope.

It is a small round or irregular vesicle which is found in most animal cells except in RBC and a few plant cells. They are abundant in epithelial cells of intestine, liver, and kidney and usually remain distributed evenly in the cytoplasm.

Generally, Lysosomes are rounded in shape but may be irregular. The size of the lysosome usually ranges from 0.1-1.2 µm but they may be up to 5.0 µm or more.  The number varies in different cells. The secretary cells like liver, pancreas, spleen, etc. contain more lysosomes. Lysosomes originate from the endoplasmic reticulum (ER) and Golgi complex.

The membrane of lysosome is made up of protein, lipid, and trace element of carbohydrate. They contain vacuolated, granular, dense homogeneous materials which rich in acid phosphatase and other hydrolytic enzymes (about 60 different enzymes) with more than 60 membrane proteins. They also contain some stack of membranous structure which is known as myelin figure. These hydrolytic enzymes are able to digest most biological substances. Lysosomes can separate the enzymes from the cellular contents. These enzymes are so powerful and sometimes they can cause severe damage to the cell. For this reason, lysosome is called “suicidal bags or suicidal sacs”.

Types of Lysosomes

Lysosomes are four types on the basis of their functions such as:

1. Primary Lysosome: It is also known as storage granule. It is a small sac-like body which is formed from the maturing face of the Golgi bodies. This type of lysosome also occurs as monocytes or granulocytes. They are surrounded by a single layer of a phospholipid which contains acid hydrolases. Generally, the majority of the primary granules combines with phagosomes and finally forms secondary lysosomes.

2. Heterophagosome: It is also known as digestive vacuole or secondary lysosome.  They are formed by the fusion of a primary lysosome with phagosomes. The phagosomes are membrane-bound vesicles which contain foreign materials. It is formed by the fusion of food containing phagosome with lysosome during the phagocytosis or pinocytosis. Ultimately, heterophagosome is left with undigested food.

3. Residual body: Lysosome with undigested food materials or debris are known as the residual body. The undigested material is ejected from the cell and combines with the plasma membrane by exocytosis or ephagy into the external environment. Sometimes, these types of lysosome left inside the cells due to malfunction of exocytosis and lack of hydrolytic enzymes which lead to many diseases such as hepatitis, Tay-Sachs disease, polynephritis, Pompe’s disease, and Hurler’s disease.

4. Autophagosome: It is also known as autophagic vacuole. An autophagosome is a sphere-shaped body which is covered by a double layer membrane. It is a special type of lysosome which contains some part of the cell in the process of auto-digestion such as various cell organelles like mitochondria or portion of the endoplasmic reticulum(ER). After forming autophagosomes, they release cytoplasmic components to the lysosomes. The outside membrane of the autophagosome combines with a lysosome to develop an autolysosome.  Autophagosome`s formation is controlled by genes. The size of the autophagosome varies from species to species. In yeast, the size ranges from 500-900 nm while in mammals; it is larger in size which ranges from 500-1500 nm. In the embryonic fibroblasts, hepatocytes and embryonic stem cells, autophagosomes are ring-shaped structures and seen under a light microscope. 

Functions of Lysosomes