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SQ03_Lect_7_8_ans - BioMI 290 10 February 2008 Study...

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BioMI 290 10 February 2008 Study Questions #3 Lectures 7, 8 (Answers) Part I: CELL ENVELOPE STRUCTURE 1) At right is another diagram of a prokaryotic cell envelope. a) Does this show a Gram (+) or Gram (-) type cell envelope? How can you tell? Gram (-) type cell envelope because it has an outer membrane b) Describe the process a small molecule like a sugar would go through to get from the outside to inside the cell. Consider the following questions: Which layers are permeable to it? It can get through the porins in the outer membrane, and through the thin peptidoglycan cell wall Which layers require proteins for passage? The cytoplasmic membrane requires a protein transporter Are these transport processes specific or general? Porins are general channels open to molecules that are the correct size; transporters are usually very specific to one type of molecule c) Where would you expect to find the following proteins (point it out on the diagram)? enzymes that break huge polymers like cellulose into smaller units like sugars outside the outer membrane (huge molecules like cellulose cannot get through the porins in the outer membrane) enzymes that synthesize crosslinks between tetrapeptides in the periplasmic space, where the cell wall is built enzymes that allow specific nutrients into the cell in the cytoplasmic membrane, which is the only real semi-permeable barrier. enzymes that bind to DNA during replication in the cell cytoplasm, where the DNA is. d) How would you have to change this diagram to make it look like a Gram (+) cell envelope? Take away the outer membrane, make a thicker cell wall, and add teichoic acids and lipoteichoic acids e) Look at the Bacterial Domain of the SSU rRNA phylogenetic tree. Do most cells have this type of envelope or do they have the Gram (+) type? Which do you think evolved first? Why? Most cells in the Bacterial Domain have this type of Gram (-) envelope. Only one line has the Gram (+) cell envelope. Therefore, even though the Gram (+) type is often considered to be “less complex" the wider distribution of the Gram (-) type suggests that it probably evolved earlier. 2) Explain why certain antibiotics like penicillin, vancomycin, which inhibit peptidoglycan synthesis, do not harm human cells. Why would these antibiotics not harm Archaeal cells either? Penicillin and vancomycin attack or inhibit synthesis of peptidoglycan, which
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BioMI 290 10 February 2008 eukaryotes do not have. Archaea also do not have true peptidoglycan, although some have a pseudopeptidoglycan, which is chemically different. Thus, these antibiotics do not affect pseudopeptidoglycan synthesis. 3) List the salient structural and chemical components of Gram-negative and Gram- positive cell walls of Bacteria that can be used to distinguish them from the cell walls of all other organisms. Gram-negative:
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SQ03_Lect_7_8_ans - BioMI 290 10 February 2008 Study...

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