Chapter 27

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 27Chapter 27

Prokaryotes


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


• Prokaryotes thrive almost everywhere

– Including places too acidic, too salty, too cold, or too hot for most other organisms

Figure 27.1


• 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)


• 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


• Prokaryotic cells have a variety of shapes

– The three most common of which are spheres (cocci), rods (bacilli), and spirals (spirilli)

1 m 2 m 5 m(a) Spherical (cocci) (b) Rod-shaped (bacilli) (c) SpiralFigure 27.2a–c


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


• 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

(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.

(b) Gram-negative. Gram-negative bacteria have less peptidoglycan, and it is located in a layer between theplasma 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.

Figure 27.3a, b

PeptidoglycanlayerCell wall

Plasma membrane

Protein

Gram-positivebacteria

20 m

OutermembranePeptidoglycanlayer

Plasma membrane

Cell wall

Lipopolysaccharide

Protein

Gram-negativebacteria


• 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


• The cell wall of many prokaryotes

– Is covered by a capsule, a sticky layer of polysaccharide or protein

200 nm

Capsule

Figure 27.4


• Some prokaryotes have fimbriae and pili

– Which allow them to stick to their substrate or other individuals in a colony

200 nm

Fimbriae

Figure 27.5


Motility

• Most motile bacteria propel themselves by flagella

– Which are structurally and functionally different from eukaryotic flagella

Flagellum

Filament

HookCell wall

Plasmamembrane

Basal apparatus

50 nm

Figure 27.6


• 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)


Internal and Genomic Organization

• Prokaryotic cells usually lack complex compartmentalization

– Do not have organelles (nucleus, Golgi apparatus, Rough ER, etc.)


• Some prokaryotes do have specialized membranes that perform metabolic functions

(a) Aerobic prokaryote (b) Photosynthetic prokaryote

0.2 m 1 m

Respiratorymembrane

Thylakoidmembranes

Figure 27.7a, b


• The typical prokaryotic genome

– Is a ring of DNA that is not surrounded by a membrane and that is located in a nucleoid region

Figure 27.81 m

Chromosome


• 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


• 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


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


• Many prokaryotes form endospores which can remain viable in harsh conditions for centuries

Endospore

0.3 mFigure 27.9


• 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


• 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


• Major nutritional modes in prokaryotes


Metabolic Relationships to Oxygen

• Prokaryotic metabolism

– Also varies with respect to oxygen


1. Obligate aerobes

– Require oxygen

2. Facultative anaerobes

– Can survive with or without oxygen

3. Obligate anaerobes

– Are poisoned by oxygen


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


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


• In the cyanobacterium Anabaena

– Photosynthetic cells and nitrogen-fixing cells exchange metabolic products

Photosyntheticcells

Heterocyst

20 mFigure 27.10


• In some prokaryotic species

– Metabolic cooperation occurs in surface-coating colonies called biofilms (recruit others to form colonies)

Figure 27.11

1 m


• 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


Lessons from Molecular Systematics

• Molecular systematics

– Is leading to a phylogenetic classification of prokaryotes

– Is allowing systematists to identify major new clades


• A tentative phylogeny of some of the major taxa of prokaryotes based on molecular systematics

Domain BacteriaDomainArchaea

DomainEukarya

Alp

ha

Be t

a

Ga m

ma

Ep s

il on

Del

taProteobacteria

Chl

am

y dia

s

Sp i

roch

ete

s

Cya

no

bac

t er i

a

Gra

m-p

osi

t ive

ba

c te

r ia

Ko r

arc

hae

ote

s

Eu r

y arc

hae

ote

s

Cre

na

rcha

eo

t es

Nan

oa

rch a

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t es

Eu k

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s

Universal ancestor


Bacteria

• Diverse nutritional types

– Are scattered among the major groups of bacteria

• The two largest groups are

– The proteobacteria and the Gram-positive bacteria


• Proteobacteria

Chromatium; the smallglobules are sulfur wastes (LM)

Fruiting bodies of Chondromyces crocatus, a myxobacterium (SEM)

Bdellovibrio bacteriophorusAttacking a larger bacterium

(colorized TEM)

2.5

m

1

m0

.5

m1

0 m

5

m

2

m

Figure 27.13

Rhizobium (arrows) inside a root cell of a legume (TEM)

Nitrosomonas (colorized TEM)

Chromatium; the smallglobules are sulfur wastes (LM)

Fruiting bodies of Chondromyces crocatus, a myxobacterium (SEM)

Bdellovibrio bacteriophorusAttacking a larger bacterium

(colorized TEM)

Helicobacter pylori (colorized TEM).


• Chlamydias, spirochetes, Gram-positive bacteria, and cyanobacteria

Chlamydia (arrows) inside an animal cell (colorized TEM)

Leptospira, a spirochete (colorized TEM)

Streptomyces, the source of many antibiotics (colorized SEM)

Two species of Oscillatoria, filamentous cyanobacteria (LM)

Hundreds of mycoplasmas covering a human fibroblast cell (colorized SEM)

2.5

m

5

m5

m

50

m

1

mFigure 27.13


Archaea

• Archaea share certaintraits with bacteria

– And other traits with eukaryotes

Table 27.2


• Some archaea live in extreme environments (extremophiles)

– Extreme thermophiles - thrive in very hot environments

– Extreme halophiles – thrive in salty places

Figure 27.14


– Methanogens - live in swamps and marshes and produce methane as a waste product


• 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


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


Symbiotic Relationships

• Many prokaryotes

– Live with other organisms in symbiotic relationships such as mutualism and commensalism

Figure 27.15


• Other types of prokaryotes

– Live inside hosts as parasites


• 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


Pathogenic Prokaryotes

• Prokaryotes cause about half of all human diseases

– Lyme disease is an example

5 µmFigure 27.16


• 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.)


Prokaryotes in Research and Technology

• Experiments using prokaryotes

– Have led to important advances in DNA technology


• Prokaryotes are the principal agents in bioremediation

– The use of organisms to remove pollutants from the environment

Figure 27.17


• Prokaryotes are also major tools in

– Mining

– The synthesis of vitamins

– Genetic engineering - production of antibiotics, hormones, and other products

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Chapter 27