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Chapter 27
Prokaryotes
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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• 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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• Biologists are discovering
– That these organisms have an astonishing genetic
diversity
– Even two strains of the species Escherichia coli can be
very different from each other
– Can live in symbiosis with humans or cause disease
– Two domains: Archaea and Bacteria (Eubacteria)
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• Concept 27.1: Structural, functional, and
genetic adaptations contribute to prokaryotic
success
• Most prokaryotes are unicellular
– Although some species form colonies or aggregate on
occasion
– Generally 1-10 μm in diameter
– Genome (essential DNA) one large circle 1-10 x 106
bp
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• Prokaryotic cells have a variety of shapes
– The three most common of which are spheres (cocci),
rods (bacilli), and spirals (spirilli)
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
– Eukaryotes – made up of cellulose or chitin
– Procaryotes – contains peptidoglycan
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• Using a technique called the Gram stain
– Scientists can classify many bacterial species into two
groups based on cell wall composition, Gram-positive
and Gram-negative
Lipopolysaccharide
Cell wall
Outer
membrane
Cell wall
Peptidoglycan
layer
Plasma membrane
Peptidoglycan
layer
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.
• Gram positive – thick peptidoglycan cell wall
layer – holds on to violet dye
• Gram negative – thinner peptidoglycan cell
wall – does not hold onto violet dye, stains red
– Generally more pathogenic to people
– Double lipid bilayer protects against immune
system and can secrete toxic
lipopolysaccharides
• antibiotics - many prevent cell wall formation
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• 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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• 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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• In a heterogeneous environment, many
bacteria exhibit taxis
– The ability to move toward or away from
certain stimuli
– Can move toward oxygen and away from toxin
– Colony formation stimulated by taxis toward
each other (group formation)
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Internal and Genomic Organization
• Prokaryotic cells usually lack complex
compartmentalization
– Do not have organelles (nucleus, Golgi
apparatus, Rough ER, etc.)
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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
• 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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• Some species of bacteriam also have smaller
rings of DNA called plasmids
– Provide additional (though not essential) genes
that impart certain characteristics such as
antibiotic resistance
– Can be transferred between bacteria through a
process known as conjugation
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• DNA replication and transcription/translation is
very similar to eukaryotes except:
– No introns/exons and gene splicing
– Ribosomes are much smaller and different
– Erythromycin and Tetracyline can interfere with
procaryotic ribosomes without affecting eukaryotic
ribosomes
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Reproduction and Adaptation
• Prokaryotes reproduce quickly by binary fission
and can divide every 1–3 hours
– some divide every 20 minutes!
– However, they face constant competition and need for
substances to grow, thus population is limited
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• Many prokaryotes form endospores which can
remain viable in harsh conditions for centuries
Endospore
0.3 m
Figure 27.9
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• Rapid reproduction and horizontal gene
transfer facilitate the evolution of prokaryotes
to changing environments
– Can observe evoltuion and change in gene structure of
a population over several years of growth
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• Concept 27.2: 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 in prokaryotes
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Metabolic Relationships to Oxygen
• Prokaryotic metabolism
– Also varies with respect to oxygen
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1. Obligate aerobes
– Require oxygen
2. Facultative anaerobes
– Can survive with or without oxygen
3. 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 to ammonia
– Process is absolutely essential for the growth of most
plants that require N in the form of nitrates, nitrites, or
ammonia
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Metabolic Cooperation
• Cooperation between prokaryotes allows them
to use environmental resources they could not
use as individual cells
– One cell does X while other cells do Y
– Anabaena – filament cells do photosynthesis
while heterocysts carry out N-fixation
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• In the cyanobacterium Anabaena
– Photosynthetic cells and nitrogen-fixing cells exchange
metabolic products
Photosynthetic
cells
Heterocyst
20 m
Figure 27.10
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• In some prokaryotic species
1 m
– Metabolic cooperation occurs in surface-coating
colonies called biofilms (recruit others to form
colonies)
Figure 27.11
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• Concept 27.3: Molecular systematics is
illuminating prokaryotic phylogeny
• Until the late 20th century
– Systematists based prokaryotic taxonomy on
phenotypic criteria
• Applying molecular systematics to the
investigation of prokaryotic phylogeny
– Has produced dramatic results
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Lessons from Molecular Systematics
• Molecular systematics
– Is leading to a phylogenetic classification of
prokaryotes
– Is allowing systematists to identify major new clades
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• A tentative phylogeny of some of the major taxa of
prokaryotes based on molecular systematics
Domain
Archaea
Domain Bacteria
Proteobacteria
Universal ancestor
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Domain
Eukarya
Bacteria
• Diverse nutritional types
– Are scattered among the major groups of bacteria
• The two largest groups are
– The proteobacteria and the Gram-positive bacteria
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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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Archaea
• Archaea share certaintraits with bacteria
– And other traits
with eukaryotes
Table 27.2
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• Some archaea live in extreme environments
(extremophiles)
– Extreme thermophiles - thrive in very hot
environments
– Extreme halophiles – thrive in salty places
Figure 27.14
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– Methanogens - live in swamps and marshes
and produce methane as a waste product
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• Concept 27.4: 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
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• 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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• Other types of prokaryotes
– Live inside hosts as parasites
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• Concept 27.5: Prokaryotes have both harmful
and beneficial impacts on humans
• Some prokaryotes are human pathogens
– But many others have positive interactions with
humans, serving as tools in agriculture and
industry
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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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• Pathogenic prokaryotes typically cause disease
– By releasing exotoxins or endotoxins
– Many pathogenic bacteria are potential weapons of
bioterrorism (e.g. Bacillus anthracis, C. botulinum, etc.)
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Prokaryotes in Research and Technology
• Experiments using prokaryotes
– Have led to important advances in DNA
technology
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• Prokaryotes are the principal agents in
bioremediation
– The use of organisms to remove pollutants from the
environment
Figure 27.17
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• Prokaryotes are also major tools in
– Mining
– The synthesis of vitamins
– Genetic engineering - production of
antibiotics, hormones, and other products
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