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First stage 14/12/2015 Biology Lec-6 انعام.د Lysosomes (Intracellular digestion centers) The lysosomes ( intercellular digestion centers), are round or vacuolar structures which filled with dense material and are bounded by single unit membrane. Size and shape of lysosomes vary from cell to cell and time to time. They contain at least 60 hydrolytic destructive enzymes. These enzymes work under acidic medium and hence they are collectively called as acidic hydrolases. A lysosome may contain one or more of these enzymes: proteases, lipases, phosphatase, glycosidases, nucleases and sulphatases. They digest excess or worn out organelles, food particles, and engulfed viruses or bacteria. The membrane surrounding a lysosome prevents the digestive enzymes inside from destroying the cell. Lysosomes fuse with vacuoles and dispense their enzymes into the vacuoles, digesting their contents. At pH 4.8, the interior of the lysosomes is more acidic than the cytosol (pH 7.2) . The hydrolytic enzymes in lysosomes, are produced in the endoplasmic reticulum, and transported and processed through the Golgi apparatus. The Golgi apparatus produces lysosomes by budding. Each acid hydrolase is then target to a lysosome by phosphorylation. The lysosomes are generally divided into two categories, primary and secondary lysosomes and the latter is further divided into 3 types: (1) a- Heterophagosomes: also called as secondary lysosomes. They contain acid hydrolyases. A digestive vacuole called phagosome, is formed as a result of pinocytosis (cell drinking) or phagocytosis (cell eating). The phagosome fuses with the primary lysosom so as to form the secondary lysosome or heterophagy. b- Residual bodies: they contain indigestible material. They are formed when the digestion is incomplete, perhaps due to the absence of some enzymes. ), large quantities of residual bodies accumulate and are referred to as lipofuscin, or age pigment. c- Autophagosomes: also called autophagic vacuoles or cytolysosomes. They contain some part of the cell in the process of digestion such as ER, mitochondria etc. digestion of these intracellular components is described as cellular autophagy. Lysosomes (2) Ribosome Ribosome are small, dense-electron particles in cells that assembles proteins. Ribosomes are composed of 65% ribosomal RNA and 35% ribosomal proteins .Ribosomes consist of two subunits a small subunit and a large subunit that bind together and work as one to translate the mRNA into a polypeptide chain during protein synthesis . There are different type of ribosome;70s ribosome, 55s ribosome and 80s ribosome. Peroxisomes Peroxisomes are small membrane-bound , self-replicating organelles found in nearly all eukaryotic cells ,that function to rid the cell of toxic substances . They have a single lipid bilayer membrane that separates their contents from the cytosol. (3) Unlike lysosomes, which are formed in the secretory pathway, peroxisomes usually self-replicate by enlarging and then dividing, although there is some indication that new ones may be formed directly. Peroxisomes contain oxidative enzymes, such as catalase, Damina acid oxidase and ureate oxidase. 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 (H2O2, itself). RH2 + O2 R+ H2O2 The name peroxisome refers to the hydrogen peroxide produced as a by-product of the activities of the oxidative enzymes in the microbody. Hydrogen peroxide is dangerous of cells because of its violent chemical reactivity. However peroxisomes also contain the enzyme catalase, which breaks down hydrogen peroxide into harmless water and oxygen and uses this H2O2 to oxidize other substrates, including phenols, formic acids, formaldehyde and alcohol, by means of the peroxidation reaction: H2O2 + RH2 R+ 2H2O 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 H2O2 accumulates in the cell, catalase converts it to H2O through this reaction: 2H2O2 2H2O+ O2 (4) A major function of the peroxisome is the breakdown of fatty acid molecules, in a process called beta-oxidation. Vacuoles and Vesicles Vacuoles are membrane -bound organelles within some eukaryotic cells that can serve a variety of secretory, excretory, and storage functions. In general, vacuole functions include 1- Removing unwanted structural debris 2- Isolating materials that might be harmful or a threat to the cell. 3-Containing waste products. 4-Maintaining internal hydrostatic pressure within the cell 5-Enabling the cell to change its' shape. Vesicles are basic tool of the cell for organizing metabolism, transport, enzyme storage, as well as being chemical reaction chambers. Many vesicles are made in Golgi apparatus ,but also in the endoplasmic reticulum ,or are made from the plasma membrane. (5)