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:
- 1Primary lysosome;
- 2Heterophagosome;
- 3Residual body;
- 4Autophagosome;
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. A single layer of a phospholipid surround the primary lysosome and it contain enzymes such as hydrolases. Typically, phagosomes and most of the primary granules unite together and finally produce secondary lysosomes.
2. Heterophagosome:
A primary lysosome and phagosome fusion to form heterophagosome. It is also called as secondary lysosome or digestive vacuole. This type of phagosome is bounded by membrane with foreign particles. During the phagocytosis or pinocytosis, it is formed by making fusion with lysosome and food containing phagosome.
Functions of Lysosome
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
- Lysosomes also take part in cellular homeostasis, energy metabolism, and cell signaling.
- Lysosomes perform an important role in the act of protein synthesis.
- Lysosomal enzymes released from sperm play a role in fertilization. In this case, enzymes help the sperm to penetrate into the ovum.
- Lysosomes protect the cell from the bad effects of the dying cells.
- They breakdown discarded macromolecules or died cells to make new cells and organelles.
- Lysosomes take part in the repair of the cell membrane.
- It plays an important role in the immune system. In this case, it makes defense different against antigen and various disease-causing agents for example bacteria, viruses, etc.
- Lysosomes release digestive enzymes which destroy own cells during the illness of cells.
- Lysosomes act like garbage disposals for removing waste materials from the cells.
- Lysosomes release many enzymes which can break down different types of biological polymers such as Nucleic acids, carbohydrates, proteins, and lipids.
- Lysosomes have more than 50 degradative enzymes which can hydrolyze DNA, RNA, proteins, carbohydrates, and lipids.
- Lysosomes digest the stored carbohydrate (glycogen), protein, and lipid of the cytoplasm during the starvation period and in this way, they discharge energy to the cells.
- A large number of lysosomes are present in the epithelial cells of the thyroid gland which help in the secretion of thyroid hormones.
- Lysosomes start to digest the various organelles of the cells under certain pathological conditions.
- During the cell division, it helps to break down the nuclear and cellular membrane.
- They help to produce keratin within the cells.