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Chapter 2

BIOL240 Chapter Notes - Chapter 2: Aureus, Magnetospirillum, Polysaccharide

Course Code
Laura A Sauder

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2.1 Fact Check
1. What are the most common bacterial shapes?
Common bacterial shapes include spherical (cocci), rods (bacilli), curved rods (vibrio) and
spiral (spirilla). The terms in parentheses refer to the terms used by scientists to describe
bacteria with these shapes.
2. Describe the various multicellular arrangements formed by some bacteria as a result of their
cellular division.
Bacillus and streptococcus can form long chains of cells because they do not fully separate
after cell division. Other bacteria, like staphylococci form clusters that look like bunches of
grapes. Hyphae are long chains of cells formed by some bacteria that can rise above or
penetrate below the main group of cells, not unlike strands of hair or tree roots.
Cyanobacteria can form long, smooth chains of cells that are contained within a
polysaccharide sheath.
3. What are the general size ranges of bacteria?
Most bacteria are between 0.5 µm and 5 µm in length or diameter. However some bacteria
fall greatly outside of this general range.
2.2 Fact Check
1. What is the nucleoid and how is it different from the eukaryal nucleus?
The nucleoid is a large mass in the cell cytoplasm comprising the bacterial chromosome
coated with various proteins and RNA molecules undergoing synthesis. Unlike the eukaryal
nucleus, the nucleoid/bacterial chromosome is not bounded by its own membrane.
2. Describe the strategies used by bacterial cells to package the large chromosome into a
“manageable form.”
The nucleoid is condensed into a manageable form by its interactions with positively
charged ions, its association with positively charged proteins and the fact that it is
supercoiled due to the action of various topoisomerases.
3. What additional components are contained within the cytoplasm of bacterial cells?
The cytoplasm is an aqueous (water-based) mix of RNA (mRNA and tRNA), ribosomes,
and proteins. Many of these proteins are enzymes that carry out metabolic functions while
others serve structural purposes.
4. What are inclusion bodies and what is their role in the bacterial cell?
Inclusion bodies are large granules that serve as storage of particular nutrients including
carbon, nitrogen, or phosphorus.
2.3 Fact Check
1. How are cytoskeletal-like proteins involved in magnetotaxis?
The MamZ protein is similar to eukaryal actin and the MreB protein of bacteria. This
protein has been found in filaments associated with magnetosomes (using a green
fluorescent protein tagged version of the MamZ protein). Another protein, MamJ, may act
as a tether that links magnetosomes to the MamZ filaments. Mutant Magnetospirillum
bacteria that lack either the MamZ or MamJ proteins produce magnetosomes that fail to
line up properly.
2. Describe the functions of the FtsZ protein and the Z-ring in bacterial cells.
The FtsZ protein is related to the eukaryal cytoskeleton protein tubulin. FtsZ protein
monomers polymerize together to form a ring on the inner face of the cytoplasmic
membrane at the point where the cell will divide. This ring interacts with membrane
proteins that direct the synthesis of new cell wall. The FtsZ ring can contract, getting
smaller through the release of FtsZ monomers, which in turn leads to the cell wall
constricting at the point of cell division.
3. What is the purpose of the MreB protein in bacteria?
The MreB protein is evolutionarily related to actin and it plays a role in defining the shape
of non-spherical bacteria. MreB polymers form bands underlying the plasma membrane
(in the cytosol) and appear to control or guide the synthesis of the cell wall to generate a
cylindrical shaped cell.
4. What role do ParM proteins play in bacterial cell division?
ParM proteins helps copies of plasmid DNA molecules move to the opposite ends of the
bacterial cell. In this way each newly formed cell inherits a copy of the plasmid after cell
2.4 Fact Check
1. What are the key components of the bacterial plasma membrane and what are its functions?
The plasma membrane is primarily a bilayer formed by a mixture of phospholipids and, in
a small subset of bacterial species, cholesterol-like molecules called hopanoids. The lipid
bilayer serves as a permeability barrier that prevents charged or large polar compounds
from entering the cell. Small polar molecules (like water) and uncharged compounds (like
CO2, O2, and N2) can diffuse freely across the membrane.
Approximately half of the membrane is composed of protein. These proteins perform many
functions including the transport of various molecules (nurtients or wastes) either into or
out of the cell. Some membrane proteins form “secretion systems” (like the SecYEG
channel) that are involved in the export of proteins from the cytoplasm to the outside of the
cell. Other proteins are involved in sensory systems that can detect conditions present in
the outside environment. Still others are involved in energy capture by creating gradients
of ions or other components across the membrane while different proteins utilize this
gradient to move the cell, transport molecules, or generate ATP.
2. Describe the structure and function of the bacterial cell wall.
The cell wall is a rigid structure that gives the bacterial cell its shape and protects the cell
from both mechanical and osmotic lysis. It is a net-like structure composed of
“peptidoglycan,” a complex polymer composed of glycan (carbohydrate) strands made of
repeats of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Most NAM
residues are attached to short “wall peptides” that can be crosslinked (by peptide bonds) to
wall peptides contained on other NAM residues from other glycan strands, hence
generating the net-like structure. In several bacteria (like E. coli) wall peptides are linked
directly to one another, but in other species they are connected through a peptide
interbridge. The amino acid composition of the wall and interbridge peptides can vary
between bacterial species.
3. Differentiate between Gram-positive and Gram-negative bacterial cell envelopes.
The Gram-positive cell envelope primarily consists of a very thick cell wall (peptidoglycan).
Intermingled with the peptidoglycan are other polymers including teichoic acids, anionic
polymers that are either tethered to the cell membrane (lipoteichoic acids) or to the
peptidoglycan (wall teichoic acids).
In Gram-negative bacteria the peptidoglycan layer is quite thin compared to that of the
Gram-positive bacteria. These bacteria instead possess an additional membrane outside of
the layer of peptidoglycan (the outer membrane), which contains a unique type of lipid
called lipopolysaccharide (LPS). The space bounded by the inner and outer membranes is
called the “periplasm. The peptidoglycan layer is found in the periplasm.
4. Describe the role of porins and TonB-dependent receptors in Gram-negative bacteria.
The outer membrane of Gram-negative bacteria presents a barrier to the transfer of
nutrients and waste between the bacterial cell and the environment. To facilitate diffusion
of these molecules, the outer membrane has proteins called “porins,” which serve as
channels that permit the diffusion of small molecules (generally less than 600 Da in size)
across the membrane. Molecules that utilize porins to transit the membrane do so through
passive diffusion. Some nutrients (like iron or vitamin B) have difficulty crossing the
membrane or are present in very low concentrations in the environment. To ensure their
uptake Gram-negative bacteria produce high affinity receptors on their surface that bind
these nutrients and deliver them to the periplasm. This type of transport process is “active”
instead of “passive” – it requires energy from the proton motive force to drive movement of
the TonB protein to help pull the receptor and its target substrate into the periplasm.
5. What is the type III secretion pathway?
The type III secretion pathway enables the transport of proteins across the inner and outer
membrane simultaneously. It is composed of several proteins that combine to make a
structure that resembles a syringe that crosses both membranes and the peptidoglycan
layer. Proteins synthesized in the cytoplasm enter a pore at the base of the syringe and are
actively secreted into the environment or into another cell (a “host” cell). Type III secretion
systems share considerable homology to the structures that make bacterial flagella.
2.5 Fact Check