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BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
Viruses, Bacteria, Archaea, and Protists:
The Diversity of Life 1
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.
21.1 Life’s Categories and
the Importance of Microbes
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Life’s Categories and the Importance of
Microbes
• All living things on Earth can be classified as
falling into one of three domains of life:
– Bacteria
– Archaea
– Eukarya
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Life’s Categories and the Importance of
Microbes
• All the members of Domains Bacteria and
Archaea are single-celled and microscopic.
• Domain Eukarya is further divided into four
kingdoms:
–
–
–
–
plants
animals
fungi
protists
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Amazing Diversity in the Living World
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Figure 21.1
Importance of Microbes
• Microbes—living things so small they cannot
be seen with the naked eye—are indispensable
to all life on Earth.
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Importance of Microbes
• Microbes produce more than half of Earth’s
atmospheric oxygen.
• The bacteria and archaea among them are
responsible for putting atmospheric nitrogen
into a form plants can use.
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Importance of Microbes
• Bacteria and fungi are the most important
decomposers of the natural world.
• They break down dead organic matter, such as
tree branches, and recycle the resulting
elements back into the Earth.
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Importance of Microbes
• Microbes live in all environments in which
larger life-forms exist.
• They are present in numbers so immense that
the weight or biomass of all microbes on Earth
exceeds the biomass of all larger life-forms.
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The Tree of Life
Domain Eukarya
Domain
Bacteria
Domain
Archaea
Kingdom
Protista
amoebae
Kingdom
Plantae
Kingdom
Animalia
Kingdom
Fungi
flowering
plants
grampositive
purple
bacteria
methane
producers
foraminifera
evergreens vertebrates
ferns
salt
lovers
cyanobacteria
Domain
Bacteria
hot acid
lovers
mushrooms
flagellates
invertebrates
dinoflagellates
mosses
diatoms
Domain
Archaea
yeast
Domain Eukarya
(Protists, Plants, Animals, Fungi)
Universal ancestor
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Figure 21.2
21.2 Viruses: Making a Living
by Hijacking Cells
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Viruses
• Viruses are noncellular replicating entities that
must invade living cells to carry out their
replication.
• Because viruses can carry out so few of life’s
basic processes on their own, most scientists do
not classify them as living things.
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HIV: The AIDS Virus
• The human immunodeficiency virus (HIV),
which causes AIDS, has two structures
common to all viruses: genetic material and a
protein coat, called a capsid, surrounding this
material.
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HIV: The AIDS Virus
• HIV also has one other structural element that
many viruses possess: a fatty membrane, called
an envelope, which surrounds the capsid.
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HIV: The AIDS Virus
Human immunodeficiency virus (HIV)
receptor (spike)
capsid
reverse transcriptase
Life cycle of HIV
genetic material
(2 strands RNA)
protease
integrase
HIV binds with
receptors
on T-cell.
envelope
Viral envelope
fuses with T-cell
membrane.
Capsid disintegrates:
viral RNA and enzymes
are released.
nucleus
receptor for HIV
viral RNA
New virus particle buds
off from cell and goes
on to infect more cells.
HIV’s reverse
transcriptase
synthesizes DNA
from viral RNA.
double-stranded
viral DNA
Cell membrane
becomes the envelope
for new HIV particle.
integrase
integrated
viral DNA
Integrase
splices viral
DNA into cell’s
DNA.
protease
viral components
Viral DNA and protease begin
turning out materials necessary
to produce viral clones.
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Figure 21.3
Viral Lifecycle
•
Most viruses carry out four steps in their life
cycle:
1. They get their genetic material inside a “host”
cell.
2. They turn out viral component parts.
3. They construct new virus particles from these
parts.
4. They move the new particles out of the cell, at
which point the particles go on to infect more
cells.
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Avian Flu
• Viruses cause a host of human illnesses.
• Health officials worldwide are now on high
alert merely because of the potential harm that
stands to come from one viral illness, avian flu.
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Avian Flu
• Scientists are watching to see if the virus that
causes avian flu, A (H5N1),will undergo a
genetic transformation that will allow it to
move easily between one human being and
another.
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Avian Flu
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Figure 21.5
Genetic Transformation
•
Viruses can undergo genetic transformations
through two different means:
1. Mutation to their DNA or RNA
2. Two different viruses can infect a single cell and
then exchange genetic sequences while within
the cell, thus producing a virus with different
properties.
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21.3 Bacteria: Masters of Every
Environment
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Bacteria
• Bacteria are microscopic, single-celled
organisms that are prokaryotes.
• Prokaryotes are organisms whose genetic
material is not contained within a nucleus.
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Bacteria
• Other defining features of bacteria are that they
have only a single organelle (the ribosome) and
reproduce asexually through a simple cell
splitting called binary fission.
• Millions of species of bacteria exist.
• Bacteria are metabolically far more diverse than
plants or animals.
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Different Shapes of Bacteria
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Figure 21.7
21.4 Intimate Strangers:
Humans and Bacteria
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Humans and Bacteria
• Bacteria live on and in human beings in great
numbers.
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Humans and Bacteria
• In the digestive tract, the relationship between
humans and many bacteria is one of mutualism:
a relationship between two organisms that
benefits both of them.
• Bacteria get food and habitat from this
relationship; human beings get an efficiently
functioning digestive system.
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Humans and Bacteria
scalp
nasal passages
about 200
species of
resident
bacteria in
mouth
digestive
tract
few resident
bacteria in
stomach
because of
its acidic pH
500-1,000
species of
resident
bacteria in
large intestine
rectum
skin
armpit
vagina
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Figure 21.8
21.5 Bacteria and Human Disease
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• Only a small proportion of bacteria are
pathogenic or disease causing, but these
bacteria are responsible for some of humanity’s
worst diseases.
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Bacteria and Human Disease
• A few pathogenic bacteria cause harm by
invading human cells, but bacteria generally do
their damage by releasing or leaving behind
harmful substances called toxins.
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Bacteria and Human Disease
• The primary human defense against pathogenic
bacteria is the class of drugs known as
antibiotics, defined as substances produced by
one microorganism that are toxic to another.
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Antibiotics
• The first antibiotic, penicillin, was developed in
the 1940s.
• Antibiotics work by exploiting the differences
between bacterial and human cells, such that
they kill bacteria while leaving human cells
unharmed.
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The Threat of Antibiotic Resistance
• The power of antibiotics is being threatened by
the emergence of antibiotic-resistant strains of
bacteria.
• These bacteria are evolving in greater numbers
because of an overuse of antibiotics in medicine
and agriculture.
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The Threat of Antibiotic Resistance
• One antibiotic-resistant bacterium, methicillinresistant Staphylococcus aureus (MRSA or
“mersa”), is being seen with increasing
frequency in the general public, in particular
among high school and college athletes.
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MRSA
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Figure 21.9
21.6 Archaea: From Marginal Player
to Center Stage
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Archaea
• Archaea were once thought to be a form of
bacteria but are now known to constitute their
own domain of life, standing beside Domains
Bacteria and Eukarya.
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Archaea
• Archaea are superficially similar to bacteria in
that they are single-celled prokaryotes that
reproduce through simple cell splitting.
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Archaea
• However, archaea are unique in the living
world at the level of the chemical structure of
their cells.
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Archaea
• This structural uniqueness is based on a genetic
uniqueness.
• Many of the genes found in archaea are unlike
the genes found in either bacteria or eukaryotes.
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Archaea and the Universal Tree of Life
Domain
Bacteria
Domain
Archaea
Domain Eukarya
(Protists, Plants, Animals, Fungi)
Universal
ancestor
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Figure 21.10
Archaea and Their Habitats
• Archaea exist in large numbers in some
common environments.
• They make up 40 percent of the microbial life
in large portions of the world’s oceans.
• They are seen in large numbers in common soil,
where they join bacteria in carrying out one
phase of the nitrogen-fixing process.
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Extremophiles
• Many species of archaea live in extreme
environments and thus are extremophiles:
organisms that grow optimally in environments
whose conditions would kill most other
organisms.
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Extremophiles
• Three large classes of extremophiles are:
– Thermophiles—organisms that live in extremely
hot environments.
– Halophiles—organisms that live in extremely salty
environments.
– Anaerobes—organisms that can either do without
oxygen or that actually are poisoned by it.
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Extremophiles
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Figure 21.11
Extremophiles
• Today, pharmaceutical and biotechnology firms
are “prospecting” for novel extremophiles in
their home environments.
• They intend to develop commercial products
from the enzymes these organisms produce.
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21.7 Protists: Pioneers in Diversifying Life
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Protists
• A protist is a eukaryotic organism that does not
have all the defining features of a plant, an
animal, or a fungus.
• This unsatisfactory definition stems from the
fact that the term protist doesn’t refer to a
single evolutionary grouping.
• Instead, it is used as a label for several different
evolutionary lines of organisms, many of which
are only distantly related.
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Protists
Bacteria
Nucleariid amoeba
Red algae
Green sulfur bacteria
Plants
Fungi
Choanoflagellates
Animals
Eukarya
Cyanobacteria
Methanococcus
Protista
Thermoplasma
Archaea
Protists can be as
different from each
other as animals are
from plants
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Figure 21.13
Protists
• Protists are mostly microscopic.
• All of them live in environments that are at
least moist, if not aquatic.
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Protists
• About 100,000 species are known to exist.
• The small portion of these that are pathogenic
include Plasmodium falciparum, the cause of
malaria, and the intestinal parasite Giardia,
which contaminates water.
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21.8 Protists and Sexual Reproduction
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Protists and Sexual Reproduction
• For nearly the first 2 billion years after life
appeared, it consisted solely of bacteria and
archaea.
• Protists were the first life-form to evolve other
than bacteria or archaea.
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Protists and Sexual Reproduction
• Protists were the organisms that made
transitions to many of the capabilities and
forms seen in larger organisms today.
• Among these transitions was the change to
sexual reproduction, which protists were the
first to practice.
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Protists and Sexual Reproduction
+ mating type
(haploid)
pairing
– mating type
(haploid)
diploid
cell
fusion
zygote
(diploid)
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Figure 21.14
21.9 Photosynthesizing Protists
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Photosynthesizing Protists: Algae
• Protists that get their nutrition by performing
photosynthesis are known as algae.
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Photosynthesizing Protists: Algae
• Some algal species provide examples of
colonial multicellularity, defined as a form of
life in which individual cells form stable
associations with one another but do not take on
specialized roles.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Photosynthesizing Protists: Algae
• Other algal protists provide examples of true
multicellularity: a form of life in which
individual cells exist in stable groups, with
different cells specializing in different
functions.
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Photosynthesizing Protists: Algae
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Figure 21.15
Photosynthesizing Protists: Algae
• Microscopic algae are important members of
the group of organisms known as
phytoplankton: small photosynthesizing
organisms that float near the surface of water.
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Photosynthesizing Protists: Algae
• Phytoplankton are very important to life in
general because they produce most of Earth’s
oxygen, and because they form the base of so
many aquatic food chains.
• All phytoplankton are either algae or bacteria.
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21.10 Heterotrophic Protists
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Heterotrophic Protists
• Heterotrophic protists do not get their nutrients
by performing photosynthesis but instead get
them from consuming either other organisms or
bits of organic matter.
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Heterotrophic Protists
• Some heterotropic protists have evolved tiny
slender extensions, cilia and flagella, with
which they move toward prey or away from
danger.
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Heterotrophic Protists
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Figure 21.16
Heterotrophic Protists
• The protists called amoeba move through use of
pseudopods or “false feet”—slender extensions
of the amoeba into which the rest of the body
flows.
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Heterotrophic Protists
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Figure 21.17
Heterotrophic Protists
• Likewise, the protists called plasmodial slime
molds and cellular slime molds move by means
of this “cytoplasmic streaming.”
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Heterotrophic Protists
• The cellular slime mold called Dictyostelium
discoideum exists as a collection of individual
amoeboid cells that come together to form a
tiny “slug” during times of little food.
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Heterotrophic Protists
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Figure 21.18
Heterotrophic Protists
• Subsequently, these cells form a tower-like
reproductive structure possessing cells at the
top that will disperse to begin life as individual
cells on the forest floor.
• “Dicty” raises questions about cell signaling
and the origins of multicellular life.
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