Prokaryotes
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Prokaryotes
1. The World of Prokaryotes.
2. Structure, Function, and Reproduction of
Prokaryotes.
3. Nutritional and Metabolic Diversity.
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The History
•The story of over 3.5 billion years.
•Dominate the biosphere.
•Leave in almost all conditions.
•Essential to all life on Earth: decompose dead organisms
and return vital chemicals to the environment.
•Leave in symbiotic relationships (mitochondria and
chloroplasts evolved from prokaryotes).
•Although only 5000 species are known, the estimates are
in the range of about 400 000 to 4 million.
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Eukaryotic versus prokaryotic cells
Major differences
•The presence of nucleus;
•Internal membrane – subdivision into many different
organelles in eukaryotes;
•Simpler genome, separation of the genetic material –
DNA into nucleus in eukaryotes.
•Cell wall of prokaryotes has different composition and
structure
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The world of prokaryotes
Carl Woese from the University of Illinois first recognized
the distinction between bacteria and archaea
Archae:
Believed to have evolved from the earliest cells
Inhabit extreme environments
Bacteria:
Considered the more “modern” prokaryotes, having
evolved later
More numerous
Differ from Archae in structural, biochemical, and
physiological characteristics
That is how taxonomic level above kingdom called
domain has appeared.
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The three major lineages of life
Domains Eukarya and Archaea share a common
ancestor that lived more recently than the ancestor
common to archaea and bacteria.
Molecular studies support hypothesis that the archaea
are more closely related to eukaryotes than they are to
bacteria.
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Three Domains and Five Kingdoms
Domain Bacteria
Domain Eukarya
Domain Archaea
Protista
Monera
All
prokaryotic
organisms,
including
Bacteria
and
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Archaea
Plantae
Unicellular
Carry out
Fungi
eukaryotes
photosynthes
and simple
is
Defined
by
multicellula
r relatives nutritional
mode, they
absorb
nutrients after
decomposing
organic
material
Animalia
Multicellular
eukaryotes
that ingest
other
organisms
Structure, Function, and Reproduction
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Structure, Function, and Reproduction
Cocci – spherical prokaryotes (diplococci, streptococci,
staphylococci);
Bacilli – rod-shaped prokaryotes;
Helices – helical prokaryotes (includes spirilla and
spirochetes).
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Bacilli
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This image is copyright of Dennis Kunkel
Rod-Shaped Bacterium, hemorrhagic E.
coli, strain 0157:H7 (division) (SEM
x22,810).
Cocci
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This image is copyright of Dennis Kunkel
Coccoid-shaped Bacterium (causes skin
infections), Enterococcus faecium (SEM
x33,370).
Helices
Left, Borrelia burgdorferi, the organism that causes Lyme
disease;
Right, Treponema pallidum, the spirochete that causes the
venereal disease syphilis.
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Cell wall
structure
Cell walls of bacteria contain peptidoglycan instead of the
cellulose found in cell walls of plants and some algae.
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Cell wall function
Maintain the cell shape
Protect the cell
Prevent the cell from bursting in hypotonic environment
Differ in chemical composition and construction from the
cell walls of protists, fungi, and plants.
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Cell wall
Most bacterial walls contain peptidoglycan (ptg): polymers
of modified sugars cross-linked by short polypeptides.
Based on differences in their wall structure many members
of the domain Bacteria can be separated into two groups:
- gram-positive, have simpler walls, with a large amount of
ptg;
- gram-negative, more complex with less ptg, the outer
membrane contains lipopolysacharides, carbohydrates
bonded to lipids.
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Cell wall
The outer membrane is often toxic and protect the
pathogen against host defence and antibiotic penetration.
Lipopolysaccharides impede entry of drugs into the cells,
making gram-negative bacteria more resistant to
antibiotics
That is why, among disease-causing bacteria, gramnegative are more threatening.
Many penicillin-like antibiotics selectively target only
bacteria by preventing the cross-link in ptg.
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Cell wall
Many prokaryotes secrete sticky substances that form an
additional protection layer – capsule outside the cell wall.
Some prokaryotes adhere to one another or to substrate by
surface appendages called pili.
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Pili
Function in:
-attaching to surfaces or
to other prokaryotes;
-during conjugation help
to hold partners together
while DNA is
transferred.
E. coli with fimbriae
(TEM x17,250)
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Many prokaryotes are motile
Half of all eukaryotes are capable of directional movement.
Most common mechanism of movement is flagellar action.
Flagella may be scattered over entire surface or be either
from both ends or from one.
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Difference from eukaryotic flagella
Unique in structure and function; lack the “9+2” microtubular
structure and rotate rather than whip back and forth like
eukaryotic flagella
Not covered by extension of plasma membrane
Only 1/10 the width of eukaryotic flagella
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Many prokaryotes are motile
Second – spirochetes-type of motility:
bacteria has several helical filaments under the outer
membrane with the basal motor attached at one or other
end of the cell;
rotation of the filaments forces the flexible cell to move like
a corkscrew.
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Many prokaryotes are motile
Third, some prokaryotes secrete slimy chemicals and
move by gliding motion that may result from the presence
of flagellar motors that lack flagellar filaments.
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Many prokaryotes are motile
Many prokaryotes are capable of taxis,
movement toward or away from a
stimulus.
Chemotaxis – response to chemical
stimuli, toward food or oxygen (a
positive chemotaxis)
or away from some toxic substances (a
negative chemotaxis).
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The cellular and genomic organisation
Bacteria lack a nuclear membrane and membrane-bound
organelles.
Biochemical processes that normally occur in a
choloroplast or mitochondrion of eukaryotes will take place
in the cytoplasm of prokaryotes.
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The cellular and genomic organisation
Bacterial DNA is circular and arrayed in a region of the cell
known as the nucleoid.
Scattered within bacterial cytoplasm are numerous small
loops of DNA known as plasmids.
Bacterial genes are organized in by gene systems known
as operons.
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EM of
Neisseria
gonorrhoeae
Note the nucleoid region (n) where DNA is located as well
as the electron dense areas of the cytoplasm (dark areas)
on these two cells of Neisseria gonorrhoeae.
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Rapid growth and adaptation
Asexual reproduction – binary fission leads to the formation
of a colony of identical offspring.
Without meiosis prokaryotes loos an important source of
genetic variation.
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Rapid growth and adaptation
However, they have three different mechanisms of genetic
recombination:
- transformation – genes are taken from the surrounding
environment;
- conjugation – direct gene transfer from one prokaryote to
another;
- transduction – the gene transfer by viruses.
All these are unilateral DNA transfer.
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Rapid growth and adaptation
Mutation is the major source of genetic variation in
prokaryotes.
Because of the short generation time favourable mutation
would be propagated to a large number of the offspring.
Without limiting resources, the growth of prokaryotes is
geometrical 2 – 4 – 8 – 16 - …
They stand extreme conditions and many of them release
antibiotics to inhibit the growth of the other
microorganisms and to compete for space and nutrients.
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Endospore formation
Certain bacteria will form a spore within their cell membrane
(an endospore) that allows them to wait out deteriorating
environmental conditions.
Certain disease causing bacteria (such as the one that
causes the disease Anthrax) can be virulent (capable of
causing an infection) 1300 years after forming their
endospore!
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Nutritional and Metabolic Diversity
Nutrition for bacteria is the way of obtaining energy and
source of carbon for synthesis of organic compounds.
Phototrophs use light as energy source.
Chemotrophs obtain their energy from chemicals taken
from the environment.
Autotrophs use only inorganic CO2 as a carbon source.
Heterotrophs requires at least one organic nutrient –
glucose.
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Nutritional and Metabolic Diversity
Combination of
the phototroph-versus-chemotroph (energy source) and
autotroph-versus-heterotroph (carbon source)
allows grouping prokaryotes to four major modes of nutrition:
1. Photoautotrophs – photosynthetic organisms that use light
energy to synthesize organic compounds from CO2 –
cyanobacteria and plants.
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Nutritional and Metabolic Diversity
2. Chemoautotrophs – need only CO2 and obtain energy
by oxidizing inorganic substances (H2S, NH3, Fe2+) –
unique to prokaryotes.
3. Photoheterotrophs – use light energy to obtain their
carbon from organic form – unique to certain prokaryotes.
4. Chemoheterotrophs – consume organic molecules for
both energy and carbon – wide range of prokaryotes,
fungi, animals, plants.
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Chemoheterotrophs are the majority
This category includes
saprobes, decomposers that absorb their nutrients
from dead organic matter, and
parasites, which absorb their nutrients from body fluids
of living hosts.
Some of them are very strict in the requirements of the
conditions: fool complex of amino acids, several vitamins,
organic compounds – genus Lactobacillus;
some of them are less particular, like E. coli which can
grow basically on everything if only glucose is present.
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Nitrogen metabolism
Diverse prokaryotes are able to metabolise most
nitrogenous compounds.
Nitrosomonas convert NH3 into NO2+ .
Pseudomonas convert NO2+ or NO3+ into N2 gas.
Some cyanobacteria are able to use N2 from the air as a
source of nitrogen – nitrogen fixation (N2 to NH3).
They require CO2, N2, H2O and some minerals in order to
grow.
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Oxygen metabolism
Obligate aerobes use O2 for cellular respiration and can
not grow without it.
Facultative anaerobes use O2 but also can grow by
fermentation in anaerobic environment.
Obligate anaerobes are poisoned by O2.
Some of them live exclusively by fermentation, another
extract chemical energy by anaerobic respiration.
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Reading
Campbell et al. Biology. Ch. 27 (556-565)
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