Prokaryotes
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I.
Overview
A.
4000
species have been identified but many think there are several thousands more
that haven’t been identified.
B.
Structurally
and metabolically diverse
C.
Differences
1.Smaller
2.
No membrane bound organelles
3.
Cell walls but structure is different than plants.
4.
Simple genomes
5.
Different replication, protein synthesis and recombination
II.
Archae and bacteria are the two main branches of prokaryotic evolution
A.
Traditional
5-kingdom system emphasized prokaryotes vs. eukaryotes and looks at the
structural differences.
B.
Recent
research put the prokaryotes into two kingdoms.
1.
Archaebacteria
a.
Evolved from earliest cells
b.
Inhabit extreme environments that may resemble early habitats
2.
Eubacteria
a.
More modern
b.
More numerous
c.
Differ structurally, biochemically, and physiologically
C.
Carol
Woese proposed six-kingdom system.
1.
Two prokaryotic kingdoms
2.
Four eukaryotic kingdoms
D.
An eight
kingdom system has also been proposed.
E.
A three
domain system is another idea.
1. Domain
is higher than a kingdom
III.
The success of prokaryotic life is based on diverse adaptations of form
and function.
A.
Morphological
1.
Single celled
2.
Some are multicellular aggregates
3.
Diversity of shapes
a. Spheres - cocci
b. Rods - bacilli
c. Spirals – spirilla and
spirochetes
B.
Cell
surface
1.
Prokaryotes
have cell walls that
a.
Maintain the cell shape
b.
Protect the cell
c.
Prevent the cell from bursting in a hypotonic environment
d.
Often contain peptidoglycan
1. Modified polymers
cross-linked by short polypeptides
2. Exact composition varies
among species
3. Some antibiotics work by
preventing the cross-links.
2.
Gram stain
is used to distinguish between two groups of eubacteria that have structural
differences in the cell walls.
a. Gram-positive bacteria
1.
Have simple cell walls with large amounts of peptidoglycan.
2. Stain blue
b. Gram-negative bacteria
1. Have more complex cell
walls with smaller amounts of peptidoglycan
2. An outer
lipopolysaccharide-containing membrane over cell wall
3. Stain pink
4. More often disease
causing
5. Lipopolysaccharides are
often toxic and protect from host
6. Lipopolysaccharides
impede entry of drugs into cells
3.
Some have
a capsule
a.
A gelatinous secretion which gives cells additional protection
b.
Helps them adhere to hosts
c.
Helps adhere to form an aggregate
4.
Some use a
pili
a.
Surface appendages used for adherence to a host
b.
Transferring DNA during conjugation
C.
The
motility of Prokaryotes
1.
Flagella
a.
Do not contain the 9+2 arrangement
b.
Rotate rather than whip back and forth
c.
Not covered by and extension of the plasma membrane
d.
Uses flagellin to attach to another protein hook at the basal apparatus
e.
Basal apparatus has 36 different proteins arranged in rings.
2.
Filaments
which are characteristic of spirochetes or helical shaped bacteria
a.
Several filaments spiral around the cell inside the cell wall
b.
Similar to flagella in structure but rotate cell like a corkscrew
3.
Gliding
a.
Glide through a layer of slimy chemicals secreted by the organism
b.
Movement may result from flagellar motors that lack filaments
D.
Prokaryotic
movement is fairly random in homogenous environments but may become directional
in a heterogeneous mixture
1.
Taxis is
the movement to or away from a stimulus
a.
Phototaxis – moving from light
b.
Chemotaxis – moving from chemicals
c.
Magnetotaxis – moving from magnetism
d.
Movement toward a stimulus is positive taxis
e.
Movement away from a stimulus is negative taxis
2.
During
taxis, bacteria move by running and tumbling movements.
a.
Enabled by rotation of flagella either counterclockwise or clockwise
b.
Caused by flagella moving coordinately about each other or randomly
E.
Internal
Membrane organization
1.
Prokaryotes
lake diverse internal membranes.
2.
Some have
specialized membranes formed by invaginations of the plasma membranes.
a.
Infoldings of the plasma membrane function in cellular respiration
b.
Cyanobacteria have thylakoid membranes containing chlorophyll
F.
Prokaryotic
Genomes
1.
Has only
1/1000 of the DNA found in a eukaryotic cell.
2.
The
bacterial chromosome is also known as the genophore
a.
Double stranded and circular
b.
DNA is concentrated in the nucleoid region and not membrane-bound
c.
Has very little protein associated with it
3.
Many also
have plasmids which are smaller rings of DNA having supplemental genes for
functions such as resistance or metabolism of unusual nutrients.
a.
Replicate independently
b.
Can be transferred during conjugation
4.
Bacterial
DNA replication and translation are similar to that of prokaryotes.
a.
Bacterial ribosomes are smaller and have different protein and RNA
b.
Difference allows antibiotics to block protein synthesis
G.
Growth,
Reproduction, and Gene exchange.
1.
Neither
mitosis or meiosis occurs in prokaryotes.
a.
Reproduction is asexual by binary fission
b.
DNA synthesis is almost continuous
2.
Growth in
the number of cells is geometric in an environment with unlimited resources
a.
Generation is usually 1-3 hours although some take 20 minutes
b.
At high concentration of cells, growth slows due to waste and lack of food
c.
Competition is reduced by release of antibiotic chemicals
d.
Optimal growth requirements depend on species.
3.
Some
bacteria survive adverse environmental conditions and toxins by producing
endospores.
a.
Resistant cell that contains one chromosome copy surround by a wall
b.
Original cell replicates and surrounds one copy with a durable wall
c.
Some can survive boiling water
d.
May remain dormant for many years
4.
Prokaryotes
do not go through meiosis or syngamy but genetic recombination can take place
through three mechanisms.
a.
Transformation
b.
Transduction
c.
Conjugation
5.
Short
generation times allow prokaryotic populations to adapt to rapidly changing
environmental conditions.
a. New mutations are screened by natural selection quickly
b.
Resulted in current diversity
IV.
All major
types of nutrition and metabolism evolved among prokaryotes.
A.
Major
modes of nutrition
1.
Exhibit a
great diversity in how they obtain the necessary resources to synthesize organic
compounds.
2.
Some
obtain energy from light while others use chemicals.
3.
Some
utilize carbon dioxide while others require at least one organic nutrient as a
carbon source.
a.
Photoautotrophs
1. Use light energy to make
compounds from carbon dioxide.
b.
Chemoautotrophs
1. Require only CO2 as
source and oxidize H2S, NH3, and Fe2+
2. Unique to certain
prokaryotes
c.
Photoheterotrophs
1. Use light to generate
ATP from organic carbon source
2. Unique to prokaryotes
d.
Chemoheterotrophs
1. Must obtain organic
molecules for energy as a source of carbon
2. Found in most eukaryotes
as well.
B.
Nutritional
Diversity among chemoheterotrophs
1.
Most
bacteria are chemoheterotrophs and can be divided into two groups
a.
Saprobes
1. Decomposers that absorb nutrients from dead organics
b.
Parasites
1.
Absorb nutrients from body fluids of living hosts
2.
The
chemoheterotrophs are very diverse groups and some have very strict requirements
while others are versatile.
a.
Lactobacillus
will grow well only when the medium contains all 20 amino acids, several
vitamins, and other organic compounds.
b.
E. coli
will grow on a medium which contains only a single organic ingredient.
3.
Almost any
organic molecules can serve as a carbon source for some species.
a.
Some
bacteria are capable of degrading petroleum and are used to clean oil spills.
b.
Those
compounds that cannot be used as a carbon source by bacteria are considered
non-biodegradable.
C.
Nitrogen
metabolism
1.
While
eukaryotes can only use some forms of nitrogen to produce proteins and nucleic
acid, prokaryotes can metabolize most nitrogen compounds.
2.
Examples
a.
Some convert ammonia to nitrates
b. Some convert nitrates to atmospheric nitrogen
c.
Nitrogen fixation is unique to prokaryotes
d.
Cyanobacteria are self sufficient with nitrogen fixation
D.
Metabolic
relationships to oxygen
1.
Prokaryotes
differ in their growth response to the presence of oxygen.
2.
Obligate
aerobes
a.
Prokaryotes needing oxygen for cellular respiration
3.
Facultative
anaerobes
a.
Use oxygen when presence but can grow using fermentation
4.
Obligate
anaerobes
a.
Prokaryotes that are poisoned by oxygen
b.
Live exclusively by fermentation
c.
Use inorganic molecules as electron acceptors
V.
The evolution of prokaryotic metabolism was both cause and effect of
changing environments on Earth.
A.
Prokaryotes
evolved all forms of nutrition and most metabolic pathways eons before
eukaryotes arose.
B.
Evolution
of these capabilities were a response to the changing environment of the early
atmosphere.
C.
As these
new capabilities evolved, they changed the environment for subsequent
prokaryotic communities.
D.
The origin
of Glycolysis.
1.
The first
prokaryotes were probably chemoheterotrophs that absorbed free organic compounds
generated by abiotic synthesis.
2.
The
universal role of ATP implies that prokaryotes used that molecule for energy
very early in their evolution.
3.
As ATP
supplies were depleted, natural selection favored those prokaryotes that could
regenerate ATP from ADP and thus leading to step by step evolution of
Glycolysis.
4.
Glycolysis
is the only metabolic pathway common to all modern organisms and does not
require oxygen.
5.
Some
Archaebacteria that live by fermentation have forms of nutrition believed to be
similar to those of the original prokaryotes.
E.
The origin
of Electron Transport Chains and Chemiosmosis.
1.
Chemiosmotic
ATP synthesis probably evolved in early prokaryotes as it is a common mechanism
in all three domains.
2.
Early
prokaryotes may have used the transmembrane pumps to help regulate their
internal pH by expelling hydrogen ions produced by fermentation.
Energy would have been necessary to drive these pumps.
3.
ATP may
have been saved by the first electron transport chains by coupling oxidation of
organic acids to the transport of H+ out of the cell.
4.
Some
bacteria may have evolved electron transport chains so efficiently that more H=
was extruded than necessary for pH regulation.
These cells could then utilize the influx of H+ to reverse the proton
pump and generate ATP.
a.
Modern
bacteria use this form of energy metabolism called aerobic respiration.
F.
The origin
of Photosynthesis
1.
As the
supply of free ATP and abiotically produced organic molecules was depleted,
natural selection may have favored organisms that could make their own organic
molecules from inorganic resources.
2.
Light
absorbing pigments in the earliest prokaryotes may have provided protection to
the cells by absorbing excels light energy that could be harmful.
3.
These
energized pigments may have then been couples with electron transport systems to
power ATP synthesis.
4.
Bacteriorhodopsin
is a light energy capturing pigment in the membrane of extreme halophiles
5.
Components
of the electron transport chains that functioned in anaerobic respiration in
other prokaryotes may have been co-opted to also provide reducing power.
G.
Cyanobacteria,
the oxygen revolution, and the origins of cellular respiration
1.
Eventually
prokaryotes evolved that could use water as the electron source and thus evolved
Cyanobacteria which released oxygen.
2.
They
evolved about 2.5 –3.4 billion years ago.
3.
They lived
with other bacteria in colonies that resulted in the formation of the
stromatolites.
4.
Oxygen
release by photosynthesis may have first reacted with dissolved iron ions to
precipitate as iron oxide preventing accumulation of free oxygen.
5.
Precipitation
of iron oxide would have eventually depleted the supply of dissolved iron and
oxygen would have accumulated in the seas.
6.
As the
oxygen accumulated, it was released into the atmosphere.
7.
As it
accumulated, may species became extinct while others survived in anaerobic
environments.
8.
Aerobic
respiration may have originated as a modification of electron transport chains
used in photosynthesis.
VI.
Molecular systematics is leading to a phylogenic classification of
prokaryotes.
A.
The use of
molecular systematics has shown that prokaryotes diverged into the
Archaebacteria and Eubacteria lineages very early in prokaryotic evolution.
B.
Domain
Archae
1.Cell
walls lack petidoglycan.
2.Plasma membranes have a unique lipid composition.
3.
RNA
polymerase and ribosomal protein are more like those of eukaryotes than of
Eubacteria.
4.
Methogens
a.
Named for their unique form of energy metabolism
b.
Use hydrogen to reduce CO2 to methane and are strict anaerobes
c. Decomposers in marshes, swamps, and sewage treatments
d.
Important digestive system symbionts
5.
Extreme
halophiles
a.
Inhabit high salinity environments
b.
Some tolerate and others require this condition
c.
Have Bacteriorhodopsin to absorb light and pump out H+
d.
Pigment is responsible for purple-red color
6.
Extreme
thermophiles
a. Inhabit hot environments
b. Habitats are 60 – 80oC
c.
Inhabit deep sea hydrothermal vents
C.
Domain
bacteria
1.Proteobacteria
is the most diverse group of bacteria containing 3 main sub groups.
a.
Purple bacteria
1. Have bacterichlorophylls built into
membrane invaginations
2.
Extract electrons from water and release no oxygen
3. Obligate anaerobes found in
sediments
4. Many are flagellated
b.
Chemoautotrophic proteobacteria
1. Includes free-living and symbiotic
species
2. Play roles in the nitrogen cycle
c. Chemoheterotrophic proteobacteria
1. Enteric bacteria
2. Most are rod-shaped facultative
anaerobes
D.
Gram-positive
Eubacteria
1. Form endospores
2.
Photosynthetic members
3.
Mycoplasmas
a. Smallest of all known cells
b. Only eubacteria that lack cell walls
c. Common in soil and some are
pathogenic
E.
Actinomycetes
1. Soil bacteria that form branching colonies which resemble fungi
2. Many are important sources of antibiotics
3.
Cyanobacteria
a.
Photoautotrophs with photosynthesis
b.
Most inhabit fresh water, some marine, and some relate with fungi
c.
Cell walls are often thick and gelatinous
d.
Motile forms move by gliding
e.
Many are unicellular while others are colonial
4.
Spirochetes
a.
Helical cells that are long and thin
b.
Internal flagellar filaments function in corkscrew-like movements
c.
Chemoheterotrophs that are both free living and pathogens
5.
Chlamydias
a.
Obligate intracellular parasites of animals
b.
Obtain ATP from host cells
c.
Have gram-negative walls but lack peptidoglycan
d.
Chlamydia is most common cause of blindness and is an STD
VII.
Prokaryotes continue to have an enormous ecological impact.
A.
Prokaryotes
and Chemical cycles.
1.
Prokaryotes
are critical links in the recycling of chemical elements between the biological
and physical components of ecosystems.
2.
Decomposers
a.
Decompose dead organisms and waste of live organisms
b.
Return elements to the environments
3.
Autotrophic
bacteria
a. Fix carbon dioxide
b.
Support food chains
B.
Symbiotic
Bacteria
1.
Most
prokaryotes form associations with other organisms usually with other bacterial
species possessing complementary metabolisms.
2.
Symbiosis
is the ecological relationships between organisms of different species that are
in direct contact. Usually the
smaller organism is the symbiont and lives within the larger host.
3.
Three
categories of symbiosis
a. Mutualism
b. Commensalism
c. Parasitism
C.
Bacteria
and disease
1.
About half
of human disease is caused by bacteria.
2.
To cause a
disease, the bacteria must invade the host, evade or resist the host’s
internal defenses long enough to grow, and harm the host.
3.
Some
pathogens are opportunistic meaning they become pathogenic only when defenses
are weakened by other factors such as poor nutrition or other infections.
4.
Louis
Pasteur, Joseph Lister and others were linking disease to pathogenic microbes.
5.
In the
late 1800’s Robert Koch was the first to determine a direct connection between
specific bacteria and certain diseases.
6.
While
doing this he determine four criteria used as guidelines in medical
microbiology.
7.
Koch’s postulates
a.
Find the same pathogen in each diseased individual
b.
Isolate the pathogen and grow it in a pure culture
c.
Use cultured pathogen to induce the disease
d.
Isolate the same pathogen from experimental animal
8.
Some
pathogens cause growth and invasion of tissues by production of a toxin.
9.
The two
main types of toxins are
a.
Exotoxins
1. Proteins secreted by
bacteria cells
2. Can cause disease
without the organism itself being present
3. Among the most potent
poisons know.
4. Elicits specific
symptoms
b.
Endotoxins
1. Toxic component of outer
membranes in gram-negative bacteria
2. Induce general symptoms
of fever and aches
