MICR 4423 EXAM 2 QUESTIONS AND ANSWERS
Why is the bacterial cell wall a good target for antibiotic action? There are 2 criteria it
meets. - Answers - It is essential to bacterial cells and it is only in bacterial cells, not
animal cells.
How do gram-positive and gram-negative cells typically differ with respect to their cell
envelope? - Answers - Gram-positive cells have a thick peptidoglycan layer and no
outer membrane. Gram-negative cells have a thin peptidoglycan layer and an outer
membrane composed of LPS.
Why are gram-negative organisms typically more resistant to antibiotics? - Answers -
Gram-negative organisms have three layer: an LPS outer membrane, a cell wall, and
the inner cytoplasmic membrane. It is difficult for antibiotics, especially large antibiotics,
to penetrate the outer LPS membrane.
Describe the structure of peptidoglycan. Which parts are relatively flexible and which
parts are not? - Answers - Peptidoglycan is a single molecule composed of glycan
chains interspersed with peptide cross-bridges. The glycan chains are inflexible and the
peptide cross-bridges are flexible.
What is the function of the bacterial cell wall? - Answers - Retains turgor pressure to
keep water from rushing into the cell. Cofers shape to the cell.
How many molecules compose the cell wall surrounding an individual bacterium? -
Answers - One molecule.
Describe a peptidoglycan precursor molecule in terms of the number and types of
smaller molecular groups that it is made from. You can draw a diagram. - Answers -
NAM and NAG connected, with the NAM attached to a pentapeptide chain with L-
alanine, Gamma-D-glutamate, DAP (Diaminopimelic acid), and D-alanine, D-alanine.
Which part of a PG precursor is used to build glycan strands? Peptide cross-bridges?
What are the names of the enzymes that polymerize glycan strands and peptide cross-
bridges, respectively? - Answers - NAM and the NAG are used to build glycan strands.
The pentapeptide group is used to build peptide cross-bridges, typically, the D-alanine
of one pentapeptide and the DAP (diaminopimelic acid) of another. The enzymes that
polymerase glycan strands are transglycosylases and the enzymes that polymerize
peptide cross-bridges are transpeptidases.
What part of a PG precursor is used to link it to the existing PG wall structure? -
Answers - The pentapeptide.
Where are PG precursors located when they are used by enzymes to build new cell wall
material? What is the whole precursor-carrier molecule called? - Answers - They are
, attached to the outer face of the cytoplasmic membrane with a diphosphate group and
bactoprenol. The whole precursor molecule is call lipid II.
Why do bacterial cells have to have carefully coordinated enzymatic activities on the cell
wall in order to grow? - Answers - Bond-breaking and bond-making are both required for
bacterial cell walls to grow because bonds in existing material have to be broken in
order to incorporate new material. However, if too many bonds are broken the cell will
not be able to resist turgor pressure and will lyse.
For a rod-shaped bacterial cell, cell wall growth can be divided into two distinct phases.
What are they? Which of these is the only phase for a spherical cell? - Answers -
Elongation and division are the two distinct phases. Division is the only phase for a
spherical cell.
What is the name of the lipid carrier upon which PG precursors are built and
transported? - Answers - Bactoprenol is the name of the lipid carrier upon which PG
precursors are built and transported. (55 carbon chain- greasy boy)
Which positions on pentapeptides are typically linked together to form peptide cross-
bridges in PG? - Answers - Pentapeptides are typically linked together by 3-position
DAP (Diaminopimelic acid) of one pentapeptide and the 4-position D-alanine of another.
Why might rod shape be favorable to a bacterial cell? - Answers - The surface area to
volume ratio of a rod shaped cell does not change as the cell elongates. In spherical
shaped cells, when cell growth occurs, the volume increases at a greater rate than the
surface area. This difference causes a rod shape to be favorable for bacterial cells
because it helps the cell wall resist turgor pressure while maintaining a larger volume.
What type of fabric resembles the PG meshwork? - Answers - Spandex
What would happen if a bacterial cell had a shortage of precursor molecules? - Answers
- It would not be able to synthesize lipid II and would run out of new materials to add to
the cell wall. This would cause the cell to stop growing because the cell wall would not
be able to expand without Lipid II.
Name/draw the steps involved in PG synthesis, beginning with GlcNAc and ending with
Lipid IIon the extracellular face of the membrane. - Answers - NAG is turned into NAM
by MurAB. NAM gets the tripeptide from MurCDE. MurF converts to the pentapeptide
form, which is linked to the bactoprenol carrier by MraY to form lipid I. MurG converts
lipid I to lipid II by adding the NAG. It is flipped out by MurJ (flippase). It is polymerized
by transglycosylase and transpeptidase activity leaving behind bactoprenol
diphosphate. Bactoprenol diphosphate is converted to bactoprenol monophosphate by
phosphatase activity. Then the bactoprenol monophosphate is flipped back to the inner
side of the cytoplasmic membrane by a different flippase to be reloaded with the
pentapeptide precursor.
Why is the bacterial cell wall a good target for antibiotic action? There are 2 criteria it
meets. - Answers - It is essential to bacterial cells and it is only in bacterial cells, not
animal cells.
How do gram-positive and gram-negative cells typically differ with respect to their cell
envelope? - Answers - Gram-positive cells have a thick peptidoglycan layer and no
outer membrane. Gram-negative cells have a thin peptidoglycan layer and an outer
membrane composed of LPS.
Why are gram-negative organisms typically more resistant to antibiotics? - Answers -
Gram-negative organisms have three layer: an LPS outer membrane, a cell wall, and
the inner cytoplasmic membrane. It is difficult for antibiotics, especially large antibiotics,
to penetrate the outer LPS membrane.
Describe the structure of peptidoglycan. Which parts are relatively flexible and which
parts are not? - Answers - Peptidoglycan is a single molecule composed of glycan
chains interspersed with peptide cross-bridges. The glycan chains are inflexible and the
peptide cross-bridges are flexible.
What is the function of the bacterial cell wall? - Answers - Retains turgor pressure to
keep water from rushing into the cell. Cofers shape to the cell.
How many molecules compose the cell wall surrounding an individual bacterium? -
Answers - One molecule.
Describe a peptidoglycan precursor molecule in terms of the number and types of
smaller molecular groups that it is made from. You can draw a diagram. - Answers -
NAM and NAG connected, with the NAM attached to a pentapeptide chain with L-
alanine, Gamma-D-glutamate, DAP (Diaminopimelic acid), and D-alanine, D-alanine.
Which part of a PG precursor is used to build glycan strands? Peptide cross-bridges?
What are the names of the enzymes that polymerize glycan strands and peptide cross-
bridges, respectively? - Answers - NAM and the NAG are used to build glycan strands.
The pentapeptide group is used to build peptide cross-bridges, typically, the D-alanine
of one pentapeptide and the DAP (diaminopimelic acid) of another. The enzymes that
polymerase glycan strands are transglycosylases and the enzymes that polymerize
peptide cross-bridges are transpeptidases.
What part of a PG precursor is used to link it to the existing PG wall structure? -
Answers - The pentapeptide.
Where are PG precursors located when they are used by enzymes to build new cell wall
material? What is the whole precursor-carrier molecule called? - Answers - They are
, attached to the outer face of the cytoplasmic membrane with a diphosphate group and
bactoprenol. The whole precursor molecule is call lipid II.
Why do bacterial cells have to have carefully coordinated enzymatic activities on the cell
wall in order to grow? - Answers - Bond-breaking and bond-making are both required for
bacterial cell walls to grow because bonds in existing material have to be broken in
order to incorporate new material. However, if too many bonds are broken the cell will
not be able to resist turgor pressure and will lyse.
For a rod-shaped bacterial cell, cell wall growth can be divided into two distinct phases.
What are they? Which of these is the only phase for a spherical cell? - Answers -
Elongation and division are the two distinct phases. Division is the only phase for a
spherical cell.
What is the name of the lipid carrier upon which PG precursors are built and
transported? - Answers - Bactoprenol is the name of the lipid carrier upon which PG
precursors are built and transported. (55 carbon chain- greasy boy)
Which positions on pentapeptides are typically linked together to form peptide cross-
bridges in PG? - Answers - Pentapeptides are typically linked together by 3-position
DAP (Diaminopimelic acid) of one pentapeptide and the 4-position D-alanine of another.
Why might rod shape be favorable to a bacterial cell? - Answers - The surface area to
volume ratio of a rod shaped cell does not change as the cell elongates. In spherical
shaped cells, when cell growth occurs, the volume increases at a greater rate than the
surface area. This difference causes a rod shape to be favorable for bacterial cells
because it helps the cell wall resist turgor pressure while maintaining a larger volume.
What type of fabric resembles the PG meshwork? - Answers - Spandex
What would happen if a bacterial cell had a shortage of precursor molecules? - Answers
- It would not be able to synthesize lipid II and would run out of new materials to add to
the cell wall. This would cause the cell to stop growing because the cell wall would not
be able to expand without Lipid II.
Name/draw the steps involved in PG synthesis, beginning with GlcNAc and ending with
Lipid IIon the extracellular face of the membrane. - Answers - NAG is turned into NAM
by MurAB. NAM gets the tripeptide from MurCDE. MurF converts to the pentapeptide
form, which is linked to the bactoprenol carrier by MraY to form lipid I. MurG converts
lipid I to lipid II by adding the NAG. It is flipped out by MurJ (flippase). It is polymerized
by transglycosylase and transpeptidase activity leaving behind bactoprenol
diphosphate. Bactoprenol diphosphate is converted to bactoprenol monophosphate by
phosphatase activity. Then the bactoprenol monophosphate is flipped back to the inner
side of the cytoplasmic membrane by a different flippase to be reloaded with the
pentapeptide precursor.
