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Chapter 27- Prokaryotes and
the Origin of Metabolism
Key Points:
The evolution of Prokaryotic metabolism was
both cause and effect of changing
environments on Earth
Molecular Systematics is leading to
phylogenetic classification of prokaryotes
Prokaryotes are indispensable links in the
recycling of chemical elements in ecosystems
Many Prokaryotes are symbiotic
Humans use Prokaryotes in research and
Evolving metabolism is a cause
and effect of changing
Nutrition and Metabolic Pathways
evolved before Prokaryotes
 Met with constantly changing physical
and biological environments
 The evolved metabolic characteristics,
in response to the change, in turn
effects the environment.
 Studied through molecular systematics,
comparisons to other prokaryotes, and
geological evidence.
Origins of Metabolism
 Similar
metabolic pathways in
prokaryotes indicate that metabolic
processes developed early from an
ancient ancestor
– Glycolysis and ATP
The First Prokaryotes that originated 3.5
to 4.0 billion years ago were anaerobes
Traditional Evolutionary
Earliest cells were chemoheterotrophs
 Absorbed free organic compounds such as
 This depleted supply of free ATP
 Natural Selection prefers those organism
that produce their own ATP
 Led to evolution of glycolysis and
generation of ATP by substrate
Modern Hypothesis
 Find
it unlikely that early Earth
produced as much free ATP to
support the chemoheterotrophs
 Chemoautotrophs instead of
 May have made energy through
reacting compounds of iron and
hydrogen sulfide
FeS + H2S = FeS2 + H2 + free
Early source of energy
 The free energy was used to split H2 into
protons and electrons to establish proton
 This gradient must have drive synthesis of
 Natural Selection prefers those cells that
can manipulate hydrogen and establish
electron transport chains
Origin of Photosynthesis
 In
early prokaryotes, light-absorbing
pigments may have absorbed excess
energy and coupled with membrane
proteins involved in ATP synthesis.
 Best seen in modern archaea known
as extreme halophiles
 Contain
a pigment known as
bacteriorhodopsin that absorbs light.
 It uses this energy to pump H+ ions
across the membrane and generate
 Drives the synthesis of ATP
 Simplest known form of
Cont. Origins of Photosynthesis
 In
some prokaryotes, pigments and
photosystems evolved to use light to
move electrons from H2S to NADP+.
 This allows the potential fixation of
 Don’t necessarily produce O2 yet
 bacteria
that could use H20 instead
of H2S as a source of electrons.
 Also known as blue-green Algae
 Their ability to make organic
compounds from H20 allows for O2
to be released
 Changed our world
Cyanobacteria and Oxygen
 Evolved
2.5 and 3.4 years ago
 Lived in marine ecosystems
 Geology- marine sediments 2.5
billion years old are iron oxide
 Thus, infer that O2 only entered the
atmosphere when all the dissolved
iron was percipitated
Origin of Cellular Respiration
 Increasing
oxygen levels caused
extinction of many prokaryotes.
 Some prokaryotes evolved into
organisms that could tolerate oxygen
 Some even used the oxidizing
property of O2 to pull electrons down
electron chains.
 Some bacteria gave up
photosynthesis to become solely
Molecular Phylogeny related to
the classification of prokaryotes
 Researchers
first realized that the
domains, Archaea and Eukarya are
different because each prokaryotic
domain has unique signature
 Signature sequences are taxonspecific base sequences at
comparable locations on the
ribosomal RNA or other nucleic acids.
Domain Archaea
 They
inhabit the more extreme
environments of Earth
 This would require them to adapt to
the new environmental situations,
and thus may have unique energy
metabolism .
 Researchers classify Archaea into
three groups: methanogens, extreme
halophiles, and extreme
Archaea- methanogens
Have unique form of metabolism, because
they use H2 to reduce CO2 into methane
 Are Anaerobes that are poisoned by
 Live in swamps and marshes, where
microbes have already used up all the
 Important decomposers in sewage
 Farmers found that they could use
methanogens to convert garbage and
Archaea- extreme halophiles
 Live
in saline places such as Great
Salt Lake and the Dead Sea.
 Some species are tolerant to salinity,
while other species actually require
high salinity to undergo metabolism
 Form a purple scum, a resultant of
Archaea- Extreme thermophiles
Live in hot environments
 Temperature in environments is ideally 60
to 80 degrees Celsius
 Sulfolobus is an extreme thermophile that
thrives in the hot sulfur springs of
 Others live at deep-sea hydrothermal
vents where water temperature is around
105 degrees Celsius
 James Lake of UCLA has a theory that
extreme thermophiles are prokaryotes
that are most closely related to
Domain Bacteria
Majority of all Prokaryotes
 Since Bacteria have diversified very long
ago, evolutionary connections between
different phylogenetic groups of Bacteria
have been hard to distinguish
 Through molecular systematics, scientists
have studied gene sequences to further
subdivide the Bacteria domain
 Recently, Domain Bacteria was subdivided
into Proteobacteria, Gram-Positive
Bacteria, Gram Positive Bacteria,
Cyanobacteria, and Spirochetes.
Ecological Impact of
Prokaryotes- recycling of
 Basic Principle- atoms in our bodies were
parts of the inorganic molecules such as
soil, air and water
 Therefore, to continue life on Earth, these
chemical elements need to be recycled
between biological (organic) and chemical
(inorganic) parts of ecosystem
 Thus, Bacteria such as decomposers are
required to convert carbon, nitrogen and
other elements essential to life between
biological and physical systems.
Many prokaryotes are symbiotic
Prokaryotes interact in groups, and rarely
by themselves
 Symbiosis is the ecological relationship
between different species that are in direct
 Sybionts ae the organisms involved in
 There are three categories of symbiosis:
mutualism, commensalism, and parasitism
Evolutionary context of
 Symbiosis
played a major role in
prokaryotic evolution and the origin
of early eukaryotes
 Mitochondria have their own DNA in
eukaryotic cells, so scientists believe
mitochondria were early prokaryotes
with a symbiotic relationship with the
Prokaryotes and Disease
For pathogens to affect the body, they
must resist all internal defenses of body
and harm the host
 Opportunistic pathogens are parasitic
prokaryotes that normally live inside the
host, but cause illness when the host’s
immune system is altered
 German physician, Robert Koch, proposed
the Koch Postulates, which lay the
guideless for modern medical microbiology
Koch Postulates
To propose that a certain pathogen is
related to a certain disease:
– Researcher must find the same pathogen in
each individual with the same disease
– Isolate pathogen from the diseased subject
and grow the microbe in a pure culture
– Induce disease in Experimental animals by
transferring pathogen from the culture to the
– Isolate the same pathogen from experimental
animals after disease/symptoms develops
There are certain circumstances where Koch
Postulates don’t work
Disease- Exotoxins
 Exotoxins
are proteins secreted by
 Produce really strong poisons, such
as botulism toxin, where a gram of
this sample can kill million people
 Linked to Botulism, Cholera, and
traveler's diarrhea
Disease- Endotoxins
 Endotoxins
are certain components
of the outer membranes of certain
bacteria that cause disease
 Related to Salmonella.