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Chapter 27
• Prokaryotes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: They’re (Almost) Everywhere!
• Most prokaryotes are microscopic
– What they lack in size they more than make up
for in numbers
• Number of prokaryotes in a single handful of
fertile soil greater than the number of people
who have ever lived
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• Thrive almost everywhere
– Places too acidic, too salty, too cold, or too hot
for other organisms
Figure 27.1
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• Astonishing genetic diversity
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• Structural, functional, and genetic adaptations
contribute to prokaryotic success
• Most unicellular although some species form
colonies
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• Variety of shapes
• 3 most common spheres (cocci), rods
(bacilli), and spirals
1 m
Figure 27.2a–c (a) Spherical (cocci)
2 m
(b) Rod-shaped (bacilli)
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5 m
(c) Spiral
• Structure of cell wall is very important, which
maintains cell shape, provides physical
protection, and prevents the cell from bursting
in a hypotonic environment
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• Gram stain
– classifies bacteria into two groups based on cell
wall composition, Gram-positive and Gramnegative
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.
• Cell wall of many prokaryotes
– covered by a capsule, a sticky layer of
polysaccharide or protein
200 nm
Capsule
Figure 27.4
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• 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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• Most motile bacteria propel w/ flagella
– 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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• Many bacteria exhibit taxis
– ability to move toward or away from certain
stimuli
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• Prokaryotic cells (most)
– lack compartmentalization
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• 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
• Prokaryotic genome
– Ring of DNA, not surrounded by a membrane,
located in a nucleoid region
Chromosome
Figure 27.8
1 m
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• Some also have plasmids
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• Reproduce quickly by binary fission
– And can divide every 20 min. – 3 hours
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• Many form endospores
– Remain viable in harsh conditions for centuries
Endospore
0.3 m
Figure 27.9
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• Rapid reproduction and horizontal gene
transfer
– Evolution of prokaryotes to changing
environments
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• Diversity of nutritional and metabolic
adaptations
– Photoautotrophy
– Chemoautotrophy
– Photoheterotrophy
– Chemoheterotrophy
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• Nutritional modes
Table 27.1
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Metabolism w/ respect to O2
• Obligate aerobes
– Require O2
• Facultative anaerobes
– Can survive w/, w/o O2
• Obligate anaerobes
– Poisoned by O2
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Nitrogen Metabolism
• e.g. nitrogen fixation
– Convert atmospheric N2 to ammonia
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Cooperation
• Cyanobacterium Anabaena
– Photosynthetic cells and nitrogen-fixing cells
exchange metabolic products
Photosynthetic
cells
Heterocyst
20 m
Figure 27.10
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• A tentative phylogeny of some of the major taxa of
prokaryotes based on molecular systematics
Domain
Archaea
Domain Bacteria
Proteobacteria
Figure 27.12
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Universal ancestor
Domain
Eukarya
2.5 m
• Proteobacteria
1 m
Rhizobium (arrows) inside a
root cell of a legume (TEM)
0.5 m
Nitrosomonas (colorized TEM)
Fruiting bodies of
Chondromyces crocatus,
a myxobacterium (SEM)
5 m
10 m
Chromatium; the small
globules are sulfur wastes (LM)
2 m
Bdellovibrio bacteriophorus
Attacking a larger bacterium
(colorized TEM)
Figure 27.13
Helicobacter pylori (colorized TEM).
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2.5 m
• Chlamydias, spirochetes, Gram-positive
bacteria, and cyanobacteria
5 m
Chlamydia (arrows) inside an
animal cell (colorized TEM)
1 m
5 m
Leptospira, a spirochete
(colorized TEM)
50 m
Hundreds of mycoplasmas
Streptomyces, the source of
covering a human fibroblast cell
many antibiotics (colorized SEM) (colorized SEM)
Figure 27.13
Two species of Oscillatoria,
filamentous cyanobacteria (LM)
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Domain: Archaea
• Archaea share certain traits with bacteria
– And other traits
with eukaryotes
Table 27.2
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• Extreme thermophiles
– Thrive in very hot environments
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• Extreme halophiles
– Live in high saline environments
Figure 27.14
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• Methanogens
– Live in swamps and marshes
– Produce methane as a waste product
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• 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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• Major role in recycling of elements between the
living and nonliving components of ecosystems
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• Decomposers
– Break down corpses, dead vegetation, and
waste products
• Nitrogen-fixers
– Add usable nitrogen to the environment
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Symbiotic Relationships
• e.g. mutualism and commensalism
Figure 27.15
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• Some are parasites
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Pathogenic Prokaryotes
• Prokaryotes cause ~1/2 human diseases
– e.g. Lyme disease
Figure 27.16
5 µm
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• Typically cause disease by releasing exotoxins
or endotoxins
• Some pathogenic bacteria are potential
weapons of bioterrorism
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• Experiments using prokaryotes
– Have led to important advances in DNA
technology
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• Bioremediation
– Use of organisms to remove pollutants from
the environment
Figure 27.17
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• Also major tools in
– Mining
– Synthesis of vitamins
– Production of antibiotics, hormones, and other
products
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings