Lysosomes and perioxysomes

Lysosomes : Lysosomes, found in the cytoplasm of animal cells , are spherical vesicles bounded by a single membrane . They are smell membrane bound vesicles in the cytoplasm containing destructive enzymes called hydrolases. These enzymes are capable of breaking down a variety of particulate materials and bring about intracellular digestion of cells.

Discovery and occurrence: The lysosomes were first reported by the Belgian cytologist and biochemist christain de Duve in 1995. In 1956, Novikofff observed lysosomes in the cell under the electron microscope. The lysosomes occur practically in all animal cells and a few cell types such as the mammalian red blood cells lack lysosomes The lysosomes also occur in the protists and also been found in fungi and plants Prokaryotic cells are without lysosomes.

Morphology: The lysosomes are generally spherical in form. Certain meristematic cells of plant roots, however, have irregular lysosomes. The lysosomes usually range from 0.25 to 0.8micrometer in size but may be exceptionally large in leucocytes and phagocytes. They remain filled with a dense material and acid phosphate enzymes.

Chemical composition: 40 different enzymes/hydrolases have been reported from the lysosomes. All enzymes are enclosed within a membrane. Most of the enzymes present are nucleases, phosphatases, lipases, proteases and sulphatases. All enzymes present in a lysosomes require the presence of an acid for their action , commonly known as acid hydrolyses. Enzymes are inactive at a neutral pH. Lysosome is an acidic compartment of pH 5.

Polymorphism: Following type of lysosomes have been observed in different cells and different stages in the same cell: Primary lysosomes Secondary Lysosomes Residual bodies Autophagic vacuoles

Primary lysosomes: They are newly formed lysosomes. They are formed as vesicles by pinching off from Golgi complex. Its enzymes are particularly in inactive state.

Secondary lysosomes: When some material to be digested enters a primary lysosome, the latter is named as secondary lysosome or digestive vacuole. Fusion activates the lysosome enzymes, which start breakdown of the incoming biological molecules. Lysosomes fuse only with the vesicles containing materials to be digested. The membranes of the lysosome and he vesicles having material for digestion have some sort of recognition system which facilitates their fusion.

Residual bodies: In a secondary lysosome, the enzymes digest the incoming materials. The products of digestion pass through the lysosome membrane into the cytoplasmic matrix for use as a source of nutrition or energy. Indigestible matter left inside the lysosome remains in the secondary lysosome. A secondary lysosome left with indigestible matter is known as residual bodies.

Autophagic vacuoles A cell may digest its own organelles such as mitochondria and endoplasmic reticulum. This process is called autophagy. Primary lysosomes fuse together about the damaged or unwanted organelles, forming a large sac known as autophagic vacuole. The enzymes of the lysosomes digest the organelles.

Function of lysosomes Digestion of useful materials: The food particles are digested by the cells in the vacuoles. Digestion of harmful substances: The foreign particles, such as viruses, bacteria and toxic substances are disposed by hydrolysing them . Renewal of cells and organelles: The old and worn out cells and cell organelles are broken down to make the component molecules available for formation of new cells and cell organelles. Digestion of unwanted materials: The dead cells that accumulate at the sites of injury are destroyed in some white blood cells.

Peroxisomes Peroxisomes are specialized organelles inside of almost all eukaryotic cells. As is the case with most organelles, they are there to carry out a specific task. They are bound only by a single membrane, in contrast to many other double membrane-bound organelles. Peroxisomes are organelles that contain oxidative enzymes, such as D-amino acid oxidase, ureate oxidase, and catalase.

They are self replicating like mitochondria. Peroxisomes function to rid the body of toxic substances like hydrogen peroxide, or other metabolites. They are a major site of oxygen utilization and are numerous in the liver where toxic byproducts are going to accumulate.

Functions of peroxisomes The main function of a Peroxisomes is to transfer hydrogen from different compounds and materials to oxygen in order to create hydrogen peroxide (H2O2). This occurs in most cells, but especially in liver cells. Hydrogen peroxide is itself toxic, but there are other specialized parts of peroxisomes that take the hydrogen peroxide and convert it into harmless water. The peroxisomes can take in, create, then destroy toxic materials all in one place.

Some peroxisomes also help break apart fatty acids using oxygen so that they can be used as fuel for respiration that takes place in the mitochondria. This is useful for us because it helps along our cells absorption of nutrients. peroxisomes bud off from the endoplasmic reticulum, not the Golgi apparatus .

Function of peroxisomes in liver cells Breakdown (by oxidation) of excess fatty acids. Breakdown of hydrogen peroxide (H2O2), a potentially dangerous product of fatty-acid oxidation. It is catalyzed by the enzyme catalase. Participates in the synthesis of cholesterol. One of the enzymes involved, HMG-CoA reductase, is the target of the popular cholesterol-lowering "statins". Participates in the synthesis of bile acids. Participates in the synthesis of the lipids used to make myelin. Breakdown of excess purines (AMP, GMP) to uric acid.

Peroxisomes are also present in plant cells where they participate is such functions as symbiotic nitrogen fixation photorespiration

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