Lysosomes Notes

Lysosomes are tiny membrane-bound vesicular structures of cytoplasm that perform intracellular digestion of cells.

First reported by de Duve.

Origin:

Some workers suggested that lysosomes originated from the Pinacytotic vacuole, develop from the plasma membrane.
Others believe that the lysosome has a Golgi origin.
Navy koff in 1965, suggested that lysosome originated from granular ER.

Occurrence:

Usually, lysosomes occur in most animal and plant cells.
The animal cell which is secretory in function such as pancreatic cells, liver cells, spleen cells, and kidney cells contain a large number of lysosomes.

Morphology (Shape and Size):

The shape of lysosomes are spherical or ovoid dense bodies but maybe irregular as in meristematic cells of the root.
Size vary from 0.2- 0.8 micrometer.
Exceptionally large in mammalian kidneys up to 5 micrometers.

Structure of Lysosomes:

Lysosomes are round vascular structures filled with dense material or acid phosphatase or acid hydrolases.

The lysosome contains two-part:

limiting membrane: Bounded by a single lipoprotein membrane. The membrane is impermeable to the substrate of enzymes contained in lysosomes.
Inner dense mass: This includes mass that may be solid or of very dense content. Some lysosomes have a very dense outer zone and a less dense inner zone. Some others have cavities or vacuoles within the inner Granular material usually they are supposed to possess dense content than mitochondria.

Enzymes of Lysosomes:

At least 40 lysosomal enzymes have been located in a variety of tissue types. A particular lysosome may contain only a few or even a single species of the enzyme.

The important enzymes of lysosomes are:

Sulphatases: helps in the digestion of sulfate.

Proteases: help in the digestion of protein.

Nucleases: help in the digestion of nucleic acid.

Lipases: help in the digestion of lipid.

Amylase: helps in the digestion of carbohydrates and starch.

Lysosomes maintain a pH of about 5 in their interior for the activity of acid hydrolases.
Hydrolytic enzymes work best in an acidic environment.

Release of Enzymes:

Certain substrates called Labilizer like progesterone, testosterone, vitamin A, E, UV radiation causes instability of the lysosomal membrane leading to the release of enzymes from the lysosome.

Other substances called stabilizers like cholesterol, cortisol has a stabilizing action on the membrane.

The mechanism by which Labilizer affects the membrane is not known.

The fat-soluble substances are probably acted by changing the fluidity of the liquid part of the membrane.

A Labilizer might increase the permeability of the lysosomal membrane to a small solute likes sucrose. The osmotic swelling that results might completely disrupt the membrane.

Molecular weight and molecular structure play an important part in the permeability of the lysosomal membrane. Compounds having a molecular weight higher than 200 do not diffuse through the membrane.

Due to the limited permeability of the lysosomal membrane, lysosomal hydrolases do not have direct access to cellular components. This prevents uncontrolled digestion of the cell contents by the lysosomal enzymes.

Koining has suggested that the enzymes are kept in an inert condition through electrostatic binding to the acid group in the lipoprotein matrix of the membrane.

The release of lysosomal enzymes may be brought about by extremes of ph and certain critical levels of cations like Ag+, Hg++, and Cu++.

Types of lysosomes:

Lysosomes is polymorphic structure and four types of lysosome have been observed.

Primary lysosome: Small sac-like body having many enzymes. Either secreted directly by granular ER or by cisternae of Golgi complex
Secondary lysosome: Phagosome formed by phagocytosis or Pinocytosis fuse with primary lysosome and forms secondary lysosome.
Residual bodies: It is a final particle-containing indigestible material due to the lack of a certain enzyme in the lysosome.
Autophagic vacuoles Autophagosome: It is a special case in which lysosomes digest a part of cells such as mitochondria and ER etc by the process of autophagy. In such cases:

Primary lysosomes are concentrated around the intracellular organelles and digest them ultimately.

Autophagic vacuoles are formed in special pathological and physiological conditions.

The function of Lysosomes:

Important functions of Lysosomes are as follows:-

1. Extracellular digestion of large particles: lysosome digest the food contents of phagosome Or pinosomes.

2. Intracellular digestion of Substances: During starvation, the lysosome digests the stored food contained. Example: protein-lipid and carbohydrate of cytoplasm.

3. Autolysis: In certain pathological conditions, the lysosome starts to digest various organelles of cells known as autolysis hence lysosome is called a suicidal bag.

4. Fertilization: Lysosome of certain cells such as the sperm discharge their enzyme outside the cell during fertilization.

The lysosomal enzyme digests the limiting membrane of the ovum and makes the penetrating path for sperm in the ovum.

5. Hormonal Secretions: There is evidence that lysosomal hydrolase is involved in the secretory activity of some cells. Their function is in modifying products synthesized by the cell before secretion and also in regulation of secretion.

6. Removal of Dead Cells: Hirrach and Cohn in 1964 told that lysosome help in the removal of dead cells in tissue.

7. Role in metamorphosis: Disappearances of a tale from tadpole larva of frog is due to lysosomal activity that is action or Cathepsins in the lysosome.

8. Help in Protein Synthesize: Recently Dr. Singh in 1972 has co-related lysosomal activity with protein synthesis.

9. Role in Osteogenesis: It has been suggested that the formation of bone cells and also their distribution depends upon lysosomal activity.

10. Malfunctioning of Lysosomes: Malfunctioning of lysosomes resulting in tissue damage and disease could be caused in several ways which are shown in the following tissue.

11. Autolysis in cartilage and bone tissue: Excess of vitamin A causes cell poisoning it disturbs the lysosomal membrane causing the release of enzymes into cells and producing autolysis in cartilage and bone tissue.

Disease:

Tay Sachs: Occurs due to the accumulation of lipids in the brain cells which leads to mental retardation.