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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
• Biologists are discovering that these organisms
have an astonishing genetic diversity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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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• Concept 27.1: Structural, functional, and
genetic adaptations contribute to prokaryotic
success
• Most prokaryotes are unicellular
– Although some species form colonies
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• Prokaryotic cells have a variety of shapes
– The three most common of which are 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
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lipopolysaccharide
Cell wall
Outer
membrane
Peptidoglycan
layer
Cell wall
Peptidoglycan
layer
Plasma membrane
Plasma membrane
Protein
Protein
Grampositive
bacteria
Gramnegative
bacteria
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(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.
• 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
– allows them to stick to their substrate or other
individuals in a colony
Fimbriae
200 nm
Figure 27.5
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Motility
• Most motile bacteria propel themselves by flagella
– structurally and functionally different from
eukaryotic flagella
• In a heterogeneous environment, many
bacteria exhibit taxis
– The ability to move toward or away from
certain stimuli
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Flagellum
Filament
50 nm
Cell wall
Hook
Basal apparatus
Plasma
membrane
Figure 27.6
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Internal and Genomic Organization
• Prokaryotic cells usually lack complex
compartmentalization
• 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
• Some species of bacteria also have smaller
rings of DNA called plasmids
Chromosome
Figure 27.8
1 m
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Reproduction and Adaptation
• Prokaryotes reproduce quickly by binary fission
and can divide every 1–3 hours
• Rapid reproduction and horizontal gene
transfer (via conjugation) facilitate the evolution
of prokaryotes to changing environments
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• Many prokaryotes form endospores
– Which can remain viable in harsh conditions
for centuries
Endospore
0.3 m
Figure 27.9
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings