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Chapter 27
Prokaryotes and the Origins
of Metabolic Diversity
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview – They’re Almost Everywhere!
• Most prokaryotes are microscopic
– But what they lack in size they more than
make up for in numbers
• The number of prokaryotes in a single handful
of fertile soil
– Is greater than the number of people who have
ever lived
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The Adaptive Ability of Prokaryotes
• Prokaryotes thrive almost everywhere including
places too acidic, too salty, too cold, or too hot
for most other organisms
Figure 27.1
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Bacteria and Archaea
• Bacteria and Archaea are the two main
branches of prokaryotic evolution.
• Traditionally:
– Prokaryotes were classified as Monera
• New Scheme – 3 Domains:
– Bacteria & Archaea
– Archaea differ from bacteria structurally,
biochemically, and physiologically.
– Archaea tend to live in extreme environments.
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The Three Domains of Life
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Genetic Diversity of Prokaryotes
• Biologists are discovering
– That these organisms have an astonishing
genetic diversity
• Structural, functional, and genetic adaptations
contribute to prokaryotic success
• Most prokaryotes are unicellular
– Although some species form colonies
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Common Prokaryotic Shapes
• Prokaryotic cells have a variety of shapes
– The three most common of which are spheres (cocci),
rods (bacilli), and spirals (spirilla).
1 m
Figure 27.2a–c (a) Spherical (cocci)
2 m
(b) Rod-shaped (bacilli)
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5 m
(c) Spiral
Cell-Surface Structures
• One of the most important features of nearly all
prokaryotic cells
– Is their cell wall, which maintains cell shape,
provides physical protection, and prevents the
cell from bursting in a hypotonic environment
• Using a technique called the Gram stain
– Scientists can classify many bacterial
species into two groups based on cell wall
composition, Gram-positive and Gramnegative
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Gram Positive v/s Gram Negative Bacteria
Lipopolysaccharide
Cell wall
Peptidoglycan
layer
Cell wall
Outer
membrane
Peptidoglycan
layer
Plasma membrane
Plasma membrane
Protein
Protein
Grampositive
bacteria
Gramnegative
bacteria
20 m
(a) Gram-positive. Gram-positive bacteria have
a cell wall with a large amount of peptidoglycan
that traps the violet dye in the cytoplasm. The
alcohol rinse does not remove the violet dye,
which masks the added red dye.
Figure 27.3a, b
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(b) Gram-negative. Gram-negative bacteria have less
peptidoglycan, and it is located in a layer between the
plasma membrane and an outer membrane. The
violet dye is easily rinsed from the cytoplasm, and the
cell appears pink or red after the red dye is added.
Pathogenic Bacteria
• Among pathogenic bacteria, gram-negative
species are generally more threatening than
gram-positive species.
– They are commonly more resistant to
antibiotics than gram positive species, and the
lipopolysaccharides on their walls are often
toxic.
• Many antibiotics inhibit the synthesis of
peptidoglycan in bacteria and thus prevent the
formation of a functional cell wall – particularly
in gram-positive species.
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The Protective Capsule
• The cell wall of many prokaryotes is covered by
a capsule, a sticky layer of polysaccharide or
protein
200 nm
Capsule
Figure 27.4
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Attachment in Prokaryotes
• Some prokaryotes have fimbriae and pili which
allow them to stick to their substrate or other
individuals in a colony
Fimbriae
200 nm
Figure 27.5
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Motility
• Most motile bacteria propel themselves by flagella
which are structurally and functionally different from
eukaryotic flagella
Flagellum
Filament
50 nm
Cell wall
Hook
Basal apparatus
Figure 27.6
Plasma
membrane
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Taxis
• In a heterogeneous environment, many
bacteria exhibit taxis
– The ability to move toward or away from
certain stimuli
• Chemitaxis
• Phototaxis
• Magnetitaxis
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Internal and Genomic Organization
• Prokaryotic cells usually lack complex compartmentalization
although some prokaryotes do have specialized membranes
that perform metabolic functions
0.2 m
1 m
Respiratory
membrane
Thylakoid
membranes
Figure 27.7a, b
(a) Aerobic prokaryote
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(b) Photosynthetic prokaryote
The Prokaryotic Genome
• The typical prokaryotic genome is a ring of DNA
that is not surrounded by a membrane and that is
located in a nucleoid region
Chromosome
Figure 27.8
1 m
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Plasmids
• Some species of bacteria also have smaller
rings of DNA called plasmids
– Plasmids are separate from the bacterial
chromosome
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Reproduction and Adaptation
• Prokaryotes
reproduce quickly
by binary fission
– And can divide
every 1–3 hours
– No mitosis or
meiosis
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Reproduction and Adaptation
• Prokaryotes have three
mechanisms that transfer
genes between individuals:
–
Transformation
–
Transduction
–
Conjugation
• Rapid reproduction and
horizontal gene transfer
–
Facilitate the evolution
of prokaryotes to
changing environments
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Endospore
• Many prokaryotes form endospores which can
remain viable in harsh conditions for centuries
Endospore
0.3 m
Figure 27.9
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Major Nutritional Modes of Prokaryotes
• A great diversity of nutritional and metabolic
adaptations have evolved in prokaryotes
• Examples of all four models of nutrition are
found among prokaryotes
– Photoautotrophy
– Chemoautotrophy
– Photoheterotrophy
– Chemoheterotrophy
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Major Nutritional Modes of Prokaryotes
Table 27.1
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Nutritional Diversity Among Chemoheterotrophs
• The majority of known prokaryotes are
chemoheterotrophs:
– Saprobes (decomposers that absorb their
nutrients from dead organic matter)
– Parasites (absorb nutrients from the body
fluids of living hosts)
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Metabolic Relationships to Oxygen
• Prokaryotic metabolism also varies with
respect to oxygen
• Obligate aerobes
– Require oxygen
• Facultative anaerobes
– Can survive with or without oxygen
• Obligate anaerobes
– Are poisoned by oxygen
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Nitrogen Metabolism
• Prokaryotes can metabolize nitrogen in a
variety of ways
• In a process called nitrogen fixation some
prokaryotes convert atmospheric nitrogen (N2)
to ammonia (NH4)
– Nitrogen fixation is the only biological
mechanism that makes atmospheric nitrogen
available to organisms for incorporation into
organic compounds.
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Thermophiles
• Some archaea
– Live in extreme environments
• Extreme thermophiles
– Thrive in very hot environments
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Halophiles
• Extreme halophiles
– Live in high saline environments
Figure 27.14
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Mehtanogens
• Methanogens
– Live in swamps and marshes
– Produce methane as a waste product
– Important decomposers in sewage treatment
– Can convert garbage and dung to methane
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Prokaryotes and the Biosphere
• Prokaryotes play crucial roles in the biosphere
• Prokaryotes are so important to the biosphere
that if they were to disappear
– The prospects for any other life surviving
would be dim
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Chemical Recycling
• Prokaryotes play a major role
– In the continual recycling of chemical elements
between the living and nonliving components of the
environment in ecosystems
• Chemoheterotrophic prokaryotes function as
decomposers
– Breaking down corpses, dead vegetation, and waste
products
• Nitrogen-fixing prokaryotes**
– Add usable nitrogen to the environment
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Symbiotic Relationships
• Many prokaryotes
– Live with other organisms in symbiotic
relationships such as mutualism and
commensalism
Figure 27.15
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Prokaryotic Interactions
• Other types of prokaryotes
– Live inside hosts as parasites
• Prokaryotes have both harmful and beneficial
impacts on humans
• Some prokaryotes are human pathogens
– But many others have positive interactions with
humans
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Pathogenic Prokaryotes
• Prokaryotes cause about half of all human
diseases
– Lyme disease is an example
Figure 27.16
5 µm
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Pathogenenic Prokarytoes
• Pathogenic prokaryotes typically cause disease by
releasing exotoxins or endotoxins
– Exotoxins are proteins secreted by prokaryotes that
can produce disease symptoms even in the absence
of the bacterium:
• Botulism
• Cholera
– Endotoxins are components of the outer membranes
of gram-negative bacteria
• Salmonella
• Many pathogenic bacteria
–
Are potential weapons of bioterrorism
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Prokaryotes in Research and Technology
• Experiments using prokaryotes
– Have led to important advances in DNA
technology
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Bioremediation
• Prokaryotes are the principal agents in
bioremediation
–
The use of organisms to remove pollutants from the environment
–
Sewage treatment, oil spill clean ups, commercial products
Figure 27.17
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Human Uses of Prokaryotes
• Prokaryotes are also major tools in
– Mining
– The synthesis of vitamins
– Production of antibiotics, hormones, and other
products
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