Chapter Overview
A synopsis of the bacterial cell
● The plasma membrane and transport
● The cell wall and other outer layers
● The nucleoid: structure and expression
● How bacterial cells divide
● Specialized structures, including pili &
stalks
● Bacterial flagella and chemotaxis
●
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Figure 3.1
2
The Bacterial Cell: An Overview
Cytoplasm = Consists of a gel-like network
Cell membrane = Encloses the cytoplasm
Cell wall = Covers the cell membrane
Nucleoid = Non-membrane-bound area of
the cytoplasm that contains the
chromosome in the form of looped coils
Flagellum = External helical filament whose
rotary motor propels the cell
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Biochemical Composition of
Bacteria
All cells share common chemical
components.
- Water
- Essential ions
- Small organic molecules
- Macromolecules
Cell composition varies with species, growth
Figure 1.1
phase, and environmental
conditions.
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Table 3.1
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The Cell Membrane
Figure 3.7
6
Plasma membrane
Phospholipid bilayer: Contain a hydrophilic head and
hydrophobic tail
The hydrophobic head is composed of glycerol and
phosphate or other polar molecule
Tail contains fatty acid or hydrocarbon chains of differing
chain length and chemistry
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8
Membrane Constituents
Membranes have
approximately equal parts
of phospholipids and
proteins.
A phospholipid consists of
glycerol with ester links to
two fatty acids and a
phosphoryl head group.
- May have side chain
Figure 3.8
9
Plasma membrane is a selective
permeable barrier allows
passage of some molecules into
the cell and not all
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Membrane Constituents
Membrane proteins serve numerous functions,
including:
•
•
•
•
•
•
Structural support
Detection of environmental signals
Secretion of virulence factors and
communication signals
Ions and nutrient uptake proteins
Proteins for ATP synthesis
Photosythetic pigments, etc
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Transport across the Cell Membrane
Selective transport is essential for survival.
- Small uncharged molecules, such as O2
and CO2, easily permeate the membrane by
diffusion.
- Water tends to diffuse across the
membrane in a process called osmosis.
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Polar molecules and charged molecules
require transport through specific protein
transporters.
Figure 3.10
- Passive transport =
Molecules move along
their concentration
gradient
- Active transport =
Molecules move against
their concentration
gradient
- Requires energy
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Transport across the Cell Membrane
 Osmosis:
Movement of water across the
membrane
 Simple diffusion: Movement of a solute from
an area of high concentration to an area of
low concentration.
 Facilitative diffusion: Solute combines with
a transporter protein in the membrane.
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Membrane Transport
•
•
•
•
Passive diffusion (water, ions, O2, CO2)
Facilitated diffusion (protein carrier)
Active transporters (requires energy)
Group translocation (requires energy)
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Movement across membranes
16
Figure 4.17
Movement across membranes
•
Active transport of substances requires a
transporter protein and ATP.
•
Group translocation of substances requires a
transporter protein and PEP (phosphoenol
pyruvate) as an energy source.
17
Membrane Lipids
Phospholipids vary with respect to their
phosphoryl head groups
& their fatty acid side chains.
Figure 3.11
Figure 3.12
18
Membranes also include planar molecules that fill
gaps between fatty acid chains.
In eukaryotic membranes, the reinforcing agents
are sterols, such as cholesterol.
In bacteria, the same function is filled by
hopanoids, or hopanes.
Figure 3.13
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Archaea have the most extreme variations in
phospholipid side-chain structures.
- Ether links between glycerol and fatty acids
- Hydrocarbon chains are branched terpenoids.
Figure 3.14
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The Cell Wall
The cell wall confers shape and rigidity to the
cell, and helps it withstand turgor pressure.
The bacterial cell wall, or the sacculus,
consists of a single interlinked molecule.
Figure 3.16
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The Cell Wall
•Resists turgor pressure
•Provides shape and rigidity
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•N-acetyl glucosamine (G)
•N-acetyl muramic acid (M)
•β-1,4 glycosidic
•Peptide cross-link
23
Peptidoglycan Structure
Most bacterial cell walls are made up of
peptidoglycan (or murein).
The molecule consists of:
- Long polymers of two disaccharides called
N-acetylglucosamine and N-acetylmuramic
acid
- The peptides form cross-bridges
connecting the parallel glycan strands.
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Figure 3.17
25
Gram-Positive and
Gram-Negative Bacteria
Most bacteria have additional envelope layers that
provide structural support and protection.
Envelope composition defines:
- Gram-positive bacteria (thick cell wall)
- Example: The phylum Firmicutes
- Gram-negative bacteria (thin cell wall)
- Example: The phylum Proteobacteria
Figure 1.1
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27
Gram-Positive Cell Envelope
Capsule (not all species)
- Made of polysaccharides
S-Layer (not all species)
- Made of protein or
glycoproteins
Thick cell wall
- Amino acid cross-links in
peptidoglycan
- Teichoic acids for strength
Plasma membrane
Figure 3.18a
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Gram-Positive cell walls
• Teichoicacids:
• Lipoteichoic acid links to plasma membrane
• Wall teichoic acid links to peptidoglycan
• May regulate movement of cations
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Figure 4.13b
Gram-Negative Outer Membrane
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Figure 4.13c
31
Atypical Cell Walls
• Mycoplasmas
• Lack cell walls
• Sterols in plasma membrane
• Archaea
• Wall-less, or
• Walls of pseudomurein (lack NAM and D amino
acids)
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Mycobacterial Cell Envelopes
Mycobacterium tuberculosis
and
M. leprae have very
complex cell envelopes.
- Include unusual
membrane lipids (mycolic
acids) and unusual
sugars
(arabinogalactans)
Figure 3.21
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Gram-Negative Outer Membrane
The thin peptidoglycan
layer consists of one
or two sheets.
-Covered by an outer
membrane, which
confers defensive
abilities and toxigenic
properties on many
pathogens
Figure 3.18b
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Figure 3.22
35
Eukaryotic Microbes
Eukaryotic microbes possess their own
structures to avoid osmotic shock.
- Algae form cell walls of cellulose.
- Fungi form cell walls of chitin.
- Diatoms form exoskeletons of silicate.
- Paramecia possess a contractile vacuole
to pump water out of the cell.
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Eukaryotic Microbes
Figure 3.25
37
Gram positive vs Gram negative
Thick peptidoglycan &
suseptible to penicillin
 No outer membrane
present.
 Teichoic acid present
 No lipopolysaccharide
present.
 Porins absent

Single or a thin layer
of peptidoglycan
 Outer membrane
present
 Teichoic acid absent
 Lipopolysaccharide
present.
 Porins present

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