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Cell Structure Page 1 of 11
Prokaryotic Cell Structure
Prokaryotic cell metabolism is extremely diverse, but the underlying
structure of all bacteria is much more uniform. Just like there is a
lot of plant diversity, but at the cellular level plants are very
similar.
We will look at diversity at the morphological level, then
look at bacterial structure in detail.
Diversity slides
In class questions 2:
1. In general, why is classification of bacteria based on morphology
not very useful? Can you think of a better way to classify organisms?
2. Why classify organisms at all? If you discovered a new bacterium,
growing in Diet Pepsi, why bother to classify it at all? What is
gained by being able to classify it?
Best to compare prokaryotic structure to eukaryotic structure:
•Most obviously bacteria are much smaller
Both prokaryotes and eukaryotes have:
Cytoplasm
Cell membrane
DNA, RNA protein
Prokarotes have NO:
•Nucleus- DNA is in the cytoplasm packaged into the nucleoid
•Internal membrane systems (ER, Golgi structures, vacuoles, etc)
though there are exceptions
•Obvious
ctyoskeleton,
though
there
are
non-obvious
cytoskeleton like structures
•Mitochondria
Unlike eukaryotes, prokaryotes have:
•Unique structures that help them swim (flagella) and stick to
substrate (pili)
•Rigid cell walls different in structure from rigid plant cell
walls (however some bacteria lack a rigid cell wall)
•Some have an external membranes with complex associated
molecules call lipopolysaccharides (Gram -)
•Small ribosomes
•Unique, very tough and hard to kill, spores that form inside
the cells of some bacterial species (endospores)
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Cytoplasm (work way from inside to outside)
All prokaryotes have cytoplasm with
-70S ribosomes
-DNA not packaged into a membrane bound nucleus (chromosome is
usually circular, but not always, it is sometimes linear. Called
a nucleoid
-Prokaryotes have coupled transcription and translation because
transcription takes place in the cytoplasm, where ribosomes are.
(Draw picture)
Cell wall is
outside the cell membrane and here Gram+ and Grambacteria differ (more later):
The Nucleoid
The region of the cytoplasm that contains the bacterial chromosome.
E.G. The E. coli chromosome is 3.8 million basepairs and is
1,400 um long about 700x longer that the cell itself (2 um). It must
be carefully packaged to get it into the cell, but also it needs to
be replicated and genes need to be available for transcription when
needed. Nucleoid picts
The Ribosomes
Prokaryotic ribosomes are smaller than eukaryotic ribosomes (70S vs.
80S, where S = Svedberg Unit which measures rates of sedimentation,
bigger S=faster sedimentation=bigger protein generally)
Prokaryotic 70S: 30S(16S rRNA+21 proteins) and 50S(23S rRNA+ 5S rRNA
+34 proteins)
Eukaryotic 80S: 40S(18S rRNA + 30 proteins) and 60S(28S rRNA+ 28S
rRNA+50 proteins). XRay crystal pict
Many important antibiotics target the prokaryotic ribosome and do not
affect eukaryotic ribosomes:
tetracyline and its derivatives (doxycycline)
streptomycin
Cell Structure Page 3 of 11
neomycin
spectinomycin
erythromycin
chloramphenicol (all of these compounds are made by microbes to
kill or inhibit competing bacteria
Cell membranes
Gram-, Gram+ and Archea all differ in chemistry of cell membranes and
cell walls. We will look at these both, but membranes first.
Membrane lipids have polar and hydrophobic regins. This allows them
to orient to form the lipid bilayer that makes up most biological
membranes. Bacterial and eukaryotic lipids have ester linkages
lipid bilayer and H2O pict
A phospolipid found in prok and euks: phosphotidlyethanolamine
Fluid Mosaic Slide
Archeal lipids have ether linkages(more stable in H2O than ester
linkages) with branched hydrocarbons, eg phytanol:
Bilayer schematic
Archea in hot environments also make very long tetraether lipids,
which is essentially a bilayer in molecule
Tetra “Bilayer” schematic
Cell Structure Page 4 of 11
Slide of lipids compared
In class questions 3
1. It is thought that perhaps there was a self-maintaining organic
chemistry of lipids, amino acids, etc. on early earth, and that some
of the interacting chemicals were captured in lipid bubbles
(vesicles). Such capture is considered a very important step in the
evolution of life--why? In what important ways are enclosed
“metabolisms (interacting organic chemicals) different that
unenclosed metabolisms?
Cell Structure Page 5 of 11
Cell wall structure and synthesis
-Cell walls of both Gram+ and Gram- bacteria are composed of a
protein/sugar compostie called peptidoglycan.
-The peptide forms crosslinks that link together chains
made
of sugar.
- The glycan (sugar) chains are composed of repeating units
of N-acetylglucosamine (G) and N-acetylmuramic acid in long chains.
peptide chain pict
G-M with peptide
-Look at crosslinking reaction--it’s very important because it gives
bacterial cell walls their strengthf and is the site of action of
many our most important antibiotics (penicillin, cephalexin, etc.)
Crosslinking rxn
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M-G depiction of crosslinked strands
-Slides of peptidoglycan formation
-Peptidoglycan with missing crosslinks becomes weak and the cell
eventually ruptures because the cell-wall can no longer contain the
internal pressure of the cell.
-Note: penicillin and other antibiotics that inhibit peptidoglycan
formation are effective only against cells that GROWING and making
cell wall--because they inhibit cosslink FORMATION (they can’t take
apart crosslinks that are already formed).
-Gram+
cells
are
most
sensitve.
Therefore,
penicillin-type
antibiotics are given for Staph and Strep infections. (Note: many
strains of these are now resistant to most cell-wall disrupting
antibiotics)
Gram- cells are less sensitive because the large drugs cant’ easily
pass through the outer membrane.
Penicillin action slides
Cell Structure Page 7 of 11
Outer Membrane Structure
-Lies outside of the cell wall in Gram- bacteria (not found in Gram+
bacteria)
-The inner leaflet is made of phospholipids and the outer leaflet is
made of lipopolysaccharide (LPS- a very important molecule)
-The membrane is held by Braun’s lipoprotein to the peptidoglycan
cell wall.
Outer membrane structure
-The outer membrane is protective and keeps out large (often toxic)
molecules such as antibiotics, bile salts etc).
-The outer membrane contains porins which let in small molecules such
as •sugars ( ) •amino acids ( )
E. coli porins let in molecules of 600 daltons MW, or less:
OmpF
OmpC
OmpA
-Also present are larger, gated channels that let in nutrients like
vitamin B12
Slides of porins
-In lab you will do antibiotic sensitivity testing and will see that
Gram+ are generally more sensitive that Gram-.
Cell Structure Page 8 of 11
LPS
-Found in the outer leaflet of the OM
-The base is composed of 2 N-acetylglucosamines hooked to 6 lipid
molecules (called “lipid A’
-These are linked to a core polysaccharide. The lipd A + core is the
same in all strains of a species.
-Core is linked to a sugar chain called “O antigen”. This varies from
strain to strain in a species (slide of LPS structure)
•There are 167 different E. coli O-antigens
•O-antigens named O55, O111, O127 are associated with
infant diarrhea
•E. coli O157 is the strain that often makes the news
papers. It is associated with foods and particulary
virulent (it produces Shiga toxin). Its natural habitat is
the cow GI tract (guess how it gets into food)
-Functions of LPS
•To make the OM fairly impermeable
•To make the cell surface less visible to host immune systems
(important for pathogen and symbionts to colonize hosts)
•Acts as an adhesion factor
Related: Exopolysaccharides which
are found loosely associate with
the exterior of many (most?)
bacteria (Slides)
LPS drawing
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Pili and Fimbriae
-Hair-like protein extensions aht emminate from the cell surface
-Used for attachment (Type I fimbriae). For example, the E. coli that
commonly causes urinary tract infections has a type of Type I
fimbriae that recognizes glycoproteins with D-mannose on urinary
tract cells.
-Type IV fimbriae are used for attachment
twitching motility) in many organisms
and
motility
(called
Type IV motility
Picts and movie
Flagella
-whip-like extension in the cell surface used
obviously, but also can be used for attachment
for
movement
Types of flagellar arrangements
Structure and assembly slides. Assembly movie, motility movies
most
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Chemotaxis and directed movement
Many bacteria such as E. coli swim smoothly when their flagella turn
CCW and they tumble when the flagella rotate CW because the bundle of
flagella fall fly apart into separate filaments(slide)
-Bacteria are attracted to beneficial environments
-Food, O2, temperature, pH light can act as attractants
-They avoid detrimental environments
-Toxins, non-optiman pH or temperature act as repellants
This phenomenon is called “chemotaxis” and comes about by something
called biased random movement:
When moving toward attractants, or away from repellents bacteria
swimm smoothly (flagella move in a CCW direction)
When moving away from attractants, or toward repellents they
tumble more frequently
In class problem
Given this drawing, apply the following rules:
1. When moving toward attractant, runs are 3x longer
2. when not moving toward attractant runs stay the same
What happens in terms of how the bacterium moved with respect to the
attractant?
Cell Structure Page 11 of 11
Endospores
-Small, very resistant, form made by a limited group of bacteria,
mostly in the Firmicutes (Gram +). Best known are
Bacillus sp
Clostridium species
Because they are so hard to destroy and sometimes cause serious
illness (anthrax, tenanus, botulism) they are important and wellstudied.
-Live a long time in natural environments:
~100,000 years (maybe as long as 2.5 million years!)
-Resistant to heat
can boil them and they survive. To kill they need dry heat or
high temperatures and pressures together (autoclave, pressure
cooker, etc)
-UV resistant
-Resistant to disinfectants such as alcohol, bleach, etc. thought
they will eventually succumb.
-Sporulation is triggered by a lack of nutrients. This makes sensethe cell is preparing to shut down for a long time and endure bad
times. In a population that is undergoing sporulation, at most ~10%
will form spores.
-Germination occurs when conditions improve
Slide