Cell Biology Lec. 2 Dr: Buthaina H. Al-Sabawi Date:13/12/2011 Cell Biology Lec. 2 Dr: Buthaina H. Al-Sabawi Date:13/12/2011 Lysosomes (Intracellular digestion centers)

Lysosome( intercellular didestion centers), membrane- bounded digestive vesicles, are also components of the endomembrane system that arise from the Golgi apparatus. They contain at least 60 hydrolytic enzymes, including proteases, lipases, phospholipases, glycosidases and nucleases. Lysosome( intercellular didestion centers), membrane- bounded digestive vesicles, are also components of the endomembrane system that arise from the Golgi apparatus. They contain at least 60 hydrolytic enzymes, including proteases, lipases, phospholipases, glycosidases and nucleases. 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. 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 ebndoplasmic reticulum, and transported and processed through the Golgi apparatus. The Golgi apparatus produces lysosomes by budding.each acid hydrolase is then targed to a lysosome by phosphorylation. The hydrolytic enzymes in lysosomes, are produced in the ebndoplasmic reticulum, and transported and processed through the Golgi apparatus. The Golgi apparatus produces lysosomes by budding.each acid hydrolase is then targed to a lysosome by phosphorylation.

Functions: The lysosomes are used for the digestion of macromolecules from phagocytosis (ingestion of other dying cells or larger extracellular material), endocytosis, and autophagy (where old or unneeded organelles or proteins, or microbes which have invaded the cytoplasm are delivered to the lysosome). Autophagy may also lead to autophagic cell death, a form of programmed self-destruction, or hydrolysis, of the cell. Which means that the cell is digesting itself. The lysosomes are used for the digestion of macromolecules from phagocytosis (ingestion of other dying cells or larger extracellular material), endocytosis, and autophagy (where old or unneeded organelles or proteins, or microbes which have invaded the cytoplasm are delivered to the lysosome). Autophagy may also lead to autophagic cell death, a form of programmed self-destruction, or hydrolysis, of the cell. Which means that the cell is digesting itself.

Lysosomes also do much of the cellular digestion required to digest tails of tadpoles and to remove the web from the fingers of 3-6 month old fetus. This process of programmed cell death is called apoptosis Lysosomes that have not entered into digestive event are identified as primary lysosomes. lysosomes can digest materials taken into the cell from its environment, a process called heterophagy.digestion follows, and the composite structure is now termed a secondary lysosomes called phagosomes, or phagolysosomes.

After digestion of the contents of the secondary lysosome, nutrients diffuse through the lysosomal limiting membrane and enter the cytosol. Indigestible compounds are retained within the vacuoles, which are now called residual bodies. In some long-lived cells (e.g, neurons, heart, muscle, hepatocytes), large quantities of residual bodies accumulate and are referred to as lipofuscin, or age pigment. In some long-lived cells (e.g, neurons, heart, muscle, hepatocytes), large quantities of residual bodies accumulate and are referred to as lipofuscin, or age pigment. Another function of lysosomes concerns the turnover of cytoplasmic organelles. The resulting secondary lysosomes are known as autophagosomes. Another function of lysosomes concerns the turnover of cytoplasmic organelles. The resulting secondary lysosomes are known as autophagosomes. In some cases, primary lysosomes release their contents extracellularly (osteoclasts).

Lysosomal storage disease: Lysosomal storage disease are caused by the accumulation of macromolecules( proteins, polysaccharides, lipids) in the lysosomes because of a genetic failure to manufacture an enzymes needed for their breakdown. Neurons of the central nervous system are particularly susceptible to damage. Lysosomal storage disease are caused by the accumulation of macromolecules( proteins, polysaccharides, lipids) in the lysosomes because of a genetic failure to manufacture an enzymes needed for their breakdown. Neurons of the central nervous system are particularly susceptible to damage. Most of these diseases are caused by the inheritance of two defective alleles of the gene encoding one of the hydrolytic enzymes. Example; Tay-Sachs disease & Gauchers' disease-both caused by a failure to produce an enzyme needed to breakdown sphingolipids (fatty acid derivatives found in all cell membrane). Most of these diseases are caused by the inheritance of two defective alleles of the gene encoding one of the hydrolytic enzymes. Example; Tay-Sachs disease & Gauchers' disease-both caused by a failure to produce an enzyme needed to breakdown sphingolipids (fatty acid derivatives found in all cell membrane).

Ribosome Ribosome are small, dense-electron particles incells that assembles proteins. Ribosomes are about 20*30 nm in size and are composed of 65% ribosomal RNA and 35% ribosomal proteins ( almost 80 different proteins) (known as a Ribonucleoprotein or RNP). It translates messenger RNA ( mRNA) to build a polypeptid chain (e.g., a protein) using amino acids delivered by Transfes RNA ( tRNA) Ribosomes are now classified as “ ribozymes. 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. Usually in bacterial cells, several ribosomes are working parallel on a single mRNA, forming what we call a polyribosome or polysome.

Peroxisomes Peroxisomes are ubiquitous organelles in eukaryotes that function to rid the cell of toxic substances. They have a single lipid bilayer membrane that separates their contents from the cytosol. 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

Function: Peroxisomes contain oxidative enzymes, such as catalase, D-amina acid and uric acid 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 (H 2 O 2, itself) RH 2 + O 2 R+ H 2 O 2 The name peroxisme 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 H 2 O 2 to oxidize other substrates, including phenols, formic acids, formaldehyde and alcohol, by means of the peroxidation reaction: H 2 O 2 + RH 2 R+ 2H 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. About 25% of the ethanol we drink is oxidized to acetaldehyde in this way. In addition, when excess H 2 O 2 accumulates in the cell, catalase converts it to H 2 O through this reaction: 2H 2 O 2 2H 2 O+ O 2 A major function of the peroxisome is the breakdown of fatty acid molecules, in a process called beta-oxidation.

Vacuoles Vacuoles are membrane -bound compartments 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-Maintaining an acidic internal pH

6-Exporting unwanted substances from the cell. 7-Enabling the cell to change shape. Vesicles In cell biology, a vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. Vesicles store, transport, or digest cellular products and waste. Vesicles are a basic tool of the cell for organizing metabolism, transport, enzyme storage, as well as being chemical reaction chambers. Many vesicles are made in the Golgi apparatus, but also in the endoplasmic reticulum, or are made from parts of the plasma membrane.