What are the lysosomes? Describe their origin structure and function. Describe the full information of lysosomes?
In this article we will discuss about lysosomes:-(1) explanation of lysosomes ,(2) structure of lysosomes, (3) function of the lysosomes.
Explanation of lysosomes
Structure of lysosomes
Function of lysosomes
Lysosomes (Lyso=Digestive, soma=body) are dense, spherical cytoplasmic organelles which contain digestive enzymes. They are found only in eukaryotic animal cells and absent from plant cells. However, the large central vacuole of plant cells may function as lysosomes. Lysosomes are lytic bodies as they contain hydrolytic enzymes. Christian de Duve (1955) appropriately named these organelles as lysosomes. So its name indicates its function. Chemically, lysosome is a single-membrane-bound body which is capable of containing a large number of hydrolytic enzymes inside it. When the membrane ruptures, the enzymes engulf the cell or cause autolysis. Lysosomes are abundant in cells with phagocytic activity. The cells of liver, spleen, thyroid and brain are particularly rich in lysosomes. The size of lysosomes varies from 0.25 to 0.50 µ but some of them may be as large as 5 u. The lysosomal enzymes are most active at an optimal pH of 5.0.
Lysosomes are formed in two ways. They are formed either by budding off from the maturing face of the golgi complex or they may develop directly from the surface of ER. Lysosome consists of a single limiting membrane, a densely granulated stroma and a large vacuole which may contain more than 50 enzymes. The important enzymes found in lysosomes are phosphatase, deoxyribonuclease, lipase, phospholipase, B-galactosidase, hyalonuridase, amino-peptidase, ATPase etc. It is remarkable that the lipo-protein monolayer holds all the enzymes inside. This is possible due to certain substances called stabilizers (cholesterol, Vit. E, cortisol, heparin etc.) which have a stabilizing effect on the membrane. The membrane permeability is affected by certain other substances called labilizers (Vit A, Vit B, progesterone, B-estradiol etc.) which helps in the release of enzymes from the lysosomes. The net effect of the action of labilizers and stabilizars is the stability of the lysosome in the living cell.
The lysosomes occur in most animal and few plant cells. They are absent in bacteria and mature mammalian erythrocytes. Few lysosomes occur in muscle cells or in acinar cells of the pancreas. Leucocytes, especially granulocytes are a particularly rich source of lysosomes Their lysosomes are so large-sized that they can be observed under the light microscope. Lysosomes are also numerous in epithelial cells of absorptive, secretory and excretory organs (e.g.. intestine, liver, kidney etc.). They occur in abundance in the epithelial cells of lungs and uterus. Lastly. phagocytic cells and cells of reticuloendothelial system (eg, bone marrow, spleen and liver) are also rich in lysosomes.
The lysosomes are round vacuolar structures which remain filled with dense material and are bounded by single unit membrane. Their shape and density vary greatly. Lysosomes are 02 to 05 in size. Since, size and shape of lysosomes vary from cell to cell and time to time (ie they are polymorphic), their identification becomes difficult. However, on the basis of the following three criteria, a cellular entity can be identified as a lysosome: (1) It should be bound by a limiting membra (2) It should contain two or more acid hydrolases; and (3) It should demonstrate the property of enzyme latency when treated in a way that adversely affects organelle’s membrane structure.
Lysosomes are polymorphic i.e., they exist in more than one form. Electron microscopic studies of lysosomes have revealed the following four distinct forms.
1. Primary lysosomes:
These are small and newly formed from the maturing face of the golgi complex. The digestive enzymes present in the primary lysosomes have not yet participated in digestive events.
2. Secondary lysosomes:
These are formed by the fusion of primary lysosomes with food vacuoles or phagosomes. The activated hydrolytic enzymes digest the food inside these vacuoles. These are of two types :(1)heterophagosomes, and (2)autophagosomes. When the primary lysosomes fuse with the food-containing vesicles from outside the cell, they form heterophagosomes. On the otherhand, when they fuse with vesicles from the cell itself, they form autophagosomes.
3. Residual lysosomes:
The hydrolytic enzymes complete digestion in the secondary lysosomes. If the digestive process is incomplete, it results in the formation of residual bodies called residual lysosome. The undigested residues are usually eliminated by exocytosis. But sometimes, they remain inside the cell and bring about harmful effect (aging) to the body.
4.Autophagic vacuoles :
Autophagy is the process by which unwanted, undigested, harmful materials are removed by the autophagic vacuoles (autophagosome). Autolysis is the process by which the cell substance may be dissolved or the cell may be killed inside autophagic vacuoles.
1.Lysosomes contain digestive enzymes. They bring about intracellular digestion or autolysis of the cell. Therefore, lysosomes are commonly called the ‘suicide sacs’ or ‘suicide bags’ of the cell.
2. Lysosomes are phagocytic in nature. They kill bacteria and other foreign materials.
3. Aging, degenerating and dead cells are eliminated by autolysis.
4. Some biologically inactive compounds are split in the presence of lysosomal enzymes.
5. When large molecules enter into the cell, they fuse with lysosomes for digestion. The residues are removed by exocytosis.
6.During prolonged starvation, the stored food such as fats and carbohydrates are digested by lysosomal enzymes to provide energy.
7. Some lysosomal enzymes have been corelated with certain pathological conditions such as fever, hypertension, hepatitis, joint pain etc.
8 Lysosomal enzymes bring about the histolysis of the tail of amphibian tadple during metam orphosis.
9.Acrosomes are specialized and differentiated lysosomes which contain hyalorunidase enzyme to break through the gelatinous material surrounding the egg.
10. Tissue degeneration or necrosis is attributed to lysosomal activities.