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BIOL 130: Oxford Tutorial COMPLETE NOTES

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Department
Biology
Course
BIOL 130
Professor
Niels C Bols
Semester
Fall

Description
Phagocytosis What mechanisms do professional phagocytes have to kill bacteria and how do some bacteria get around them?  Phagocytosis is the process where materials are taken into the cell  It is also known as “cell-eating” because that’s what the process resembles  In this process, the phagocyte, such as macrophages or neutrophils (which are types of white blood cells in mammals) recognize invading organisms (such as pathogens) as well as damaged or dying cells and debris  Once recognized, receptors on the surface of the phagocyte bind to the receptor sites on the invading organism which initiates the phagocytic process  Through actin polymerization, folds in the cell membrane known as pseudopodia (literally “false feet”) stretch and envelop the material, forming a vacuole, or phagosome, which then pinches off on the inside of the cell membrane as the folds of the cell membrane fuse back together  Once inside the cell, a lysosome containing lysosomal enzymes from the Golgi Complex fuses with the phagosome, forming a phagolysosome  Here, there are a few different ways in which the material can be digested or destroyed: o They may be destroyed by lysosomal enzymes when the lysosome fuses with the phagosome o Since the lysosome has a low pH, the acidic environment may kill the bacteria o Oxygen free radicals generated within the lumen of the phagosome may also kill the bacteria  However, this isn’t always the case; some bacteria have ways of hijacking the phagocytic process to survive  I’ll mention 3 different examples or cases: o Bacteria responsible for tuberculosis (mycobacterium tuberculosis) is taken into the cytoplasm as usual, but inhibits the fusion of the lysosome with the phagosome that’s enclosing the bacteria. Instead, the bacteria multiplies within the cell o Bacteria responsible for Q fever (Coxiella burnetii) is also taken into the cell and becomes enclosed in a phagosome, but although the lysosome fuses with the phagosome, neither the acidic environment of the phagolysosome nor the lysosomal enzymes are able to destroy the bacteria o Bacteria responsible for meningitis (Listeria monocytogenes), on the other hand, produces proteins that destroy the integrity of the lysosomal membrane, which allows the bacteria to escape to the cell’s cytosol Cholesterol Explain how cholesterol is taken up by animal cells, such as human cells, and how cholesterol contributes to the formation of atherosclerotic plaques.  Cholesterol is a hydrophobic molecule that are packed in low density lipoprotein particles (LDLs) which carry the cholesterol in the blood from the liver to body cells  At the core, each LDL contains around 1500 cholesterol molecules esterified to long- chain fatty acids  A protein and phospholipid layer surrounds the cholesterol molecules  Protein portion, called apolipoprotein B-100 binds specifically to LDL receptors on the surface of the cell (plasma membrane)  Once the protein has bonded to the LDL receptors, clathrin molecules coat the membrane, causing the membrane coat to bend and invaginate, forming a vesicle containing LDL particles as well as the receptors which pinches off inside the cell  Once inside the cell, the clathrin coat is disassembled, and the vesicle fuses with the endosome, which is a compartment that first receives the material  Due to the low pH of the endosome, the LDL receptors release their cargo, and the empty receptors are recycled back to the plasma membrane  LDL particles are delivered to a lysosome containing hydrolytic enzymes to diges
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