Lysosome Lysosome Lysosomes were discovered by the Belgian

Lysosome

Lysosome • Lysosomes were discovered by the Belgian cytologist Christian de Duve in 1949. • The name lysosome derives from the Greek words lysis, which means dissolution or destruction, and soma, which means body. • They are frequently nicknamed "suicide-bags" or "suicide-sacs" by cell biologists due to their role in autolysis • They are created by the addition of hydrolytic enzymes to early endosomes from the Golgi apparatus.

Lysosome • Lysosomes are large, spherical organelles that contain enzymes (acid hydrolases). • They are found in animal cells, while in plant cells the same roles are performed by the vacuole. • They break up food so it is easier to digest. • They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. • The membrane around a lysosome allows the digestive enzymes to work at the 4. 5 p. H they require. • Lysosomes fuse with vacuoles and dispense their enzymes into the vacuoles, digesting their contents

Lysosome

Lysosome structure • The size of lysosomes varies from 0. 1– 1. 2 μm. • The interior p. H of the lysosomes is acidic compared to the slightly alkaline cytosol (p. H 7. 2). • The lysosome maintains this p. H differential by pumping protons (H+ ions) from the cytosol across the membrane via proton pumps and chloride ion channels. • The lysosomal membrane protects the cytosol, and therefore the rest of the cell, from the degradative enzymes within the lysosome. • The cell is additionally protected from any lysosomal acid hydrolases that leak into the cytosol as these enzymes are p. H-sensitive and function less well in the alkaline environment of the cytosol.

Primary & secondary lysosomes • There are primary and secondary lysosomes • Primary lysosomes are formed by the Golgi apparatus where hydrolytic enzymes are incorporated • Lysosome's produced at the golgi apparatus where they fuse with phagosomes to become secondary lysosome's. • Phagosomes fuse with lysosomes and work as one digestive vacuole. • Lysosomal enzymes are released into this vacuole in order to digest the bacteria or other materials. • Small molecules which are the result leave the vacuole through its membrane and are used to make new molecules. • The indigestible materials are deposited outside the cell

Primary & secondary lysosomes

Residual bodies • In lysosomal digestion, residual bodies are vesicles containing indigestible materials. • Residual bodies are either secreted by the cell via exocytosis or they become lipofuscin granules that remain in the cytosol indefinitely. • • Longer-living cells like neurons and muscle cells usually have a higher concentration of lipofuscin than other more rapidly-proliferating cells.

The Functions of Lysosomes • Lysosomes are the cells' garbage disposal system. • They are used for the digestion of macromolecules from: A- phagocytosis B- endocytosis C- autophagy

Autophagy 1) When cells are faced with an inadequate supply of nutrients in their extracellular fluid (ECF), they may begin to cannibalize some of their internal organelles for re-use of their components. 2) when some of the organelles become old, for example, when a mitochondria comes to the end of its ten-day life, it is engulfed by membrane derived from the endoplasmic reticulum. 3) when microbes that have invaded the cytoplasm or for the digestion of some intracellular materials like proteins.

Autophagy • This phenomenon, called autophagy, involves: formation of a double membrane within the cell which envelops the materials to be degraded into a vesicle called an autophagosome. • The autophagosome then fuses with a lysosome forming an autolysosome whose hydrolytic enzymes degrade the materials • Autophagy may also lead to autophagic cell death, a form of programmed self-destruction, or autolysis, of the cell, which means that the cell is digesting itself

Enzymes • Some important enzymes found within lysosomes include: • Lipase, which digests lipids • Amylase, which digest carbohydrates (e. g. , sugars) • Proteases, which digest proteins • Nucleases, which digest nucleic acids • phosphoric acid monoesters. • Lysosomal enzymes are synthesized in the cytosol and the endoplasmic reticulum

Genetic disorder • A group of genetic disorders caused by defective lysosomal enzymes demonstrates the importance of lysosomes. These are called lysosomal storage diseases, these disorders are characterized by the harmful accumulation of undigested substances. • The accumulated materials damage or kill the affected cells, resulting in skeletal or muscular defects, mental retardation, or even death.

Lysosome membrane • The lysosome, has been shown to initiate a cell death pathway. • Lysosomal membrane permeabilization (LMP) causes the release of cathepsins and other hydrolases from the lysosomal lumen to the cytosol. • LMP is a potentially lethal event because the ectopic presence of lysosomal proteases in the cytosol causes digestion of vital proteins and the activation of additional hydrolases • Resulting in cytochrome c release and apoptosome-dependent caspase activation. • Massive LMP often results in cell death without caspase activation; this cell death may adopt a subapoptotic or necrotic appearance

Microbody

Microbody • A microbody is a cytoplasmic organelle of a more or less globular shape that comprises degradative enzymes bound within a single membrane. • Microbodies are specialized as containers for metabolic activity. Types include: 1) peroxisomes, 2) glyoxisomes, 3) glycosomes 4) Woronin bodies.

Peroxisome • Peroxisomes are organelles from the microbody family and are present in almost all eukaryotic cells. • They participate in the metabolism of fatty acids and many other metabolites. • Peroxisomes harbor enzymes that clear the cell from toxic peroxides. • Peroxisomes are bound by a single membrane that separates their contents from the cytosol and contain membrane proteins critical for various functions, such as importing proteins into the organelles and aiding in proliferation. Peroxisomes can replicate by enlarging and then dividing

Function • Peroxisomes contain oxidative enzymes, such as catalase, D-amino acid oxidase, and uric acid oxidase. • However the last enzyme is absent in humans, explaining the disease known as gout, caused by the accumulation of uric acid. • Certain enzymes within the peroxisome, by using molecular oxygen, remove hydrogen atoms from specific organic substrates (labeled as R), in an oxidative reaction, producing hydrogen peroxide (H 2 O 2, itself toxic): • RH 2 + O 2 = R + H 2 O 2 • Catalase, another enzyme in the peroxisome, in turn uses this H 2 O 2 to oxidize other substrates, including phenols, formic acid, formaldehyde, and alcohol, by means of the peroxidation reaction: • H 2 O 2 + R`H 2 = R` + 2 H 2 O

• Thus eliminating the poisonous hydrogen peroxide in the process. • This reaction is important in liver and kidney cells, where the peroxisomes detoxify various toxic substances that enter the blood. • In addition, when excess H 2 O 2 accumulates in the cell, catalase converts it to H 2 O through this reaction: • 2 H 2 O 2 = 2 H 2 O + O 2 • A major function of the peroxisome is the breakdown of fatty acid molecules, in a process called beta-oxidation. • In this process, the fatty acids are broken down two carbons at a time, converted to Acetyl-Co. A, which is then transported back to the cytosol for further use.

Glyoxysome • Glyoxysomes are specialized peroxisomes found in plants (particularly in the fat storage tissues of germinating seeds) and also in filamentous fungi. • As in all peroxisomes, in glyoxysomes the fatty acids are hydrolyzed to acetyl-Co. A by peroxisomal β-odixation enzymes. • Besides peroxisomal functions, glyoxysomes possess additionally the key enzymes of glyoxylate cycle (isocitrate lyase and malate synthase) which complete the glyoxylate cycle bypass. • Additionally, glyoxysomes possess the enzymes to produce intermediate products for the synthesis of sugars by gluconeogenesis. • The seedling uses these sugars synthesized from fats until it is mature enough to produce them by photosynthesis

Glyoxylate cycle • The glyoxylate cycle is an anabolic metabolic pathway occurring in plants, and several microorganisms, such as E. coli and yeast. • The glyoxylate cycle allows these organisms to use fats for the synthesis of carbohydrates. • Fatty acids from lipids are commonly used as an energy source by vertebrates via degradation by beta oxidation into acetate molecules. • This acetate, bound to the active thiol group of coenzyme A, enters the citric acid cycle (TCA cycle) where it is fully oxidized to carbon dioxide. • To utilize acetate from fat for biosynthesis of carbohydrates, the glyoxylate cycle, is used. The net result of the glyoxylate cycle is therefore the production of glucose from fatty acids

Glycosome • The glycosome is a membrane-enclosed organelle that contains the glycolytic enzymes. • It is found in a few species of protozoa, most notably in the human pathogenic trypanosomes, which can cause sleeping sickness and Chagas's disease, and Leishmania. • The organelle is bounded by a single membrane and contains a dense proteinaceous matrix. • It is believed to have evolved from the peroxisome and contain glycolysis enzymes

Woronin body • A Woronin body (named after the Russian botanist Mikhail Stepanovich Woronin) is a peroxisomederived, dense core microbody with a double membrane. It is a special organelles found only in filamentous fungi • One established function of Woronin bodies is the plugging of the septal pores after hyphal wounding, which restricts the loss of cytoplasm to the sites of injury