Chapter 3
Lecture Outline
Cell Structure and Function
Basic Structure of a Prokarytic Cell
Ribosomes (+)
Fimbria (+/-)
Additional layer (+/-)
Pili (+/-)
Nucleoid (+)
Plasmid (+/-)
Inclusion body (+/-)
Cell membrane (+)
Flagella (+/-)
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Cell wall (mostly
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The Bacterial Cell

Cytoplasm surrounded by envelope
 Cytoplasm

contains DNA in form of nucleoid
Envelope has lipid membrane boundary
 Plus
structural cell wall
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Cross Section through a GramNegative Bacterium
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Cell Membranes

Made of lipid bilayer
 Double
layer of
phospholipids
Separate cytoplasm
from outside world
 Proteins embedded in membrane

 Anchor
membranes to envelope
 Sense the outside world
 Transport materials into cell
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Functions of the Cell Membrane

Permeability barrier




Prevents leakage of cytoplasmic metabolites into the
environment
Selective permeability also prevents diffusion of most solutes
Active transport
Structural support


Anchor for membrane proteins
Anchor for cell appendages






Pili
Filaments
Flagella
Detection of environmental signals
Secretion of virulence factors and communication signals
Site of energy conversion (ATP generation)
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Differences between Prokaryotic
and Eukaryotic Membranes
Phospholipid composition
 Absence of sterols in prokaryotic
membranes

 In
many bacteria hopanoids instead
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Differences between
Bacteria and Archaea
Unlike Bacteria (and Eukarya), in which
ester linkages bond fatty acids to glycerol,
Archaea contain ether-linked lipids
 No fatty acids but isoprene (5 carbon
hydrocarbon) in Archaea
 Some Archaea have lipid monolayers

Ester
O
H2C-O-C-R
Ether
H2C-O-C-R
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Bacterial Cell Envelope

Bacterial cell wall

Peptidoglycan
 Two major classes

Originally differentiated based on staining characteristic

Gram positive
 Thick peptidoglycanl ayer, retains the dye crystal violet

Gram negative
 Thin peptidoglycan layer, does not retain the dye crystal violet

Additional layers



S-layer in gram positive bacteria only
Outer membrane in gram negative bacteria only
Slime layers or capsule
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Peptidoglycan

Strands of alternating repeats of N-acetylglucosamine
and N-acetylmuramic acid cross-linked between strands
by short peptides

Via N-acetylmuramic acid
 4 amino acids

L-alanine, D-alanine, D-glutamic acid, and either L-lysine or meso-diaminopimelic acid
(some
species)
Lipoteichoic acids
(embedded in
cytoplasmic
membrane)
Periplasm
S-Layer






Composed of symmetrical
hexagonal protein subunits
Protection
Molecular sieve
First described in archaea
Recently discovered in
gram-positive bacteria
Why only recently?
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The Gram-Negative Envelope

Capsule (not all species)


Polysaccharide mainly
Outer Membrane

Lipopolysaccharide (LPS)

In outer leaflet only
OM
Outer membrane proteins
Lipoprotein




Connects peptidoglycan with
outer membrane
Thin cell wall

4-amino acid crosslinks
in peptidoglycan

Thick periplasm

Plasma membrane
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Gram-Negative
Envelope
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Lipopolysaccharide (LPS)





Highly variable Opolysaccharide side
chains
Core sugars
Conserved lipid A
(toxic)
Endotoxin
Fever inducing
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Capsule



Optional
Most exterior layer
Predominantly made from
organized carbohydrate


Protective


Sometimes made from protein
(e.g. Bacillus anthracis)
Antiphagocytic
Related structures which serve
more for attachment


Slime (loose net work)
Glycocalix (very fine network)
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Mycobacterial
Envelope Structure




Galactan
Arabinan
Outer bilayer with
phenolic glycolipids
Capsule with loosely
associated
phospholipids and
phenolic lipids
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Cytoplasmic contents
Cytoplasmic skeleton
 Nucleoid
 Plasmids (in some organisms)
 Ribosomes
 Specialized structures

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The Bacterial Cytoskeleton

Determines cell shape

CreS “Crescentin”
 Curves
inner side of crescent-shaped
bacteria

FtsZ
 Forms
a “Z ring” essential for cell
septation
 Analog to tubulin
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The Bacterial Nucleoid

Single loop of double-stranded DNA
 Single
molecule of DNA
 ~4x106 bp in many bacteria
 Compacted via supercoiling

Attached to cell envelope
 No
membrane separates
DNA from cytoplasm

Replicates once for each cell division
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Gene Expression
RNA Polymerase transcribes DNA to mRNA
 Ribosome translates RNA to Protein

 Processes
occur simultaneously
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Bacterial Ribosomes
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Intracellular Structures Related to
Photosynthesis

Cyanobacteria have thylakoids
 Extensively
folded inner membrane
 Contain chlorophyll
 Ancestors of chloroplasts

Carboxysomes fix carbon
 Use

energy to make sugar
Other bacterial photosynthetic pigments
 Purple
membranes in some
 Phycobilisome proteins collect light energy
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Storage Granules

Intracellular deposits of material
 Glycogen
(sugar) for energy
 PHB (fatty acid polymer) for energy
 Polyphosphate to store material
 Sulfur for disposal
Carboxysomes,
lipid energy-storage granules
Iridescent sulfur granules
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Cell Appendages



Proteins
Composed of identical subunit that differ among
species
Attachment, specialized pili for plasmid transfer
Pili
 pilin


Fimbria
Flagella
Attachment
Motility
 flagellin
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Cell Attachment
Essential for bacterial pathogenicity
 Fimbriae, pili


Stalks attach cells to surfaces
 Extension
of cell cytoplasm
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Cell Motility

Flagella
 Long,
helical protein filaments
 Attached at ends, or over whole cell
 Rotate to propel cell
 Proton passage drives rotation

Clockwise or counterclockwise
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Concept Quiz
Which one of these membranes is NOT
found in gram-negative bacteria?
a.
b.
c.
d.
plasma membrane
thylakoid membrane
nuclear membrane
outer membrane
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Concept Quiz
Peptidoglycan is composed primarily of
a.
b.
c.
sugars and amino acids
sugars and nucleic acids
nucleic acids and lipids
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Concept Quiz
An extension of the cytoplasm that attaches
bacteria to a surface is called a
a.
b.
c.
d.
pilus
flagellum
fimbrium
stalk
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