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KH4119_Unit 01 ES_E121-E145 03/16/05 3:44 PM Page 127
lightbulb
a
c
b
radiation counter
gas detector
nutrient
solution
soil sample
1. Soil sample is suspended in a porous cup.
2. Nutrient solution is added to the soil sample.
3. Changes in gas content are measured by a
gas detector.
nutrient solution with
radioactive carbon atoms
soil sample
1. Soil sample is sprayed with radioactively
labeled nutrient solution.
2. Any radioactive carbon dioxide that is produced
by the soil and released into the air above the
sample is detected and counted.
gas processing
tube
gases with
radioactive
carbon atoms
radiation counter
soil sample
1. Radioactive gases are introduced into the chamber
containing the soil.
2. The light is turned on as a source of energy.
3. The chamber is heated to release newly made
substances into the air.
4. The air is processed to separate complex substances
from the simple gases that had been introduced earlier.
5. Any radioactive carbon that is contained in these
complex molecules is detected and counted.
Figure E3.8 Three experiments to test Martian soil. (a) A gas exchange experiment tested the Martian
soil for evidence of organisms that took in gases from the Martian atmosphere and nutrients from the soil and
gave off gases as wastes. This experiment is then performed using earth’s soil. The experiment indicates the
presence of microscopic organisms that take in oxygen and nutrients and give off carbon dioxide. (b) Scientists
next search for the release of carbon dioxide. The experiment tests the Martian soil for evidence of organisms
that could use simple nutrients and give off waste gases (CO2). This experiment was similar to the gas exchange
experiment. It served as an important check on its results. Again, this experiment gives strong, positive results
when earth’s soil is tested. (c) A third experiment tested the Martian soil for evidence of organisms that might
build large, complex substances out of simple gases in the Martian atmosphere. This experiment is then
performed on earth’s soil. The experiment indicates the presence of microscopic organisms that use the energy
of sunlight to help them build sugars and other large, complex molecules.
caution in interpreting even the changes that
the distant instruments did detect. In fact, by
1979, most scientists involved with the project
had agreed that although they could not rule
out the possibility that life exists on Mars, all
the data that they collected in the original
experiments could be explained as resulting
from purely chemical (not biological) causes.
The rovers that landed on Mars in 2004 did
not find life. But they did find evidence
indicating that water, a necessity for life as we
know it, existed on Mars in the past.
Describing life . . . a difficult, but not an
impossible challenge. Looking for life, using
earth’s criteria, in a very different environment
more than 40 million miles away . . . more
difficult to be sure, but impossible?
What do you think?
Five Kingdoms
In which of these pairs of illustrations are
the organisms most closely related? Figure E3.9
shows two animals that bear little resemblance
to each other. In contrast, Figure E3.10 shows
two types of cells, each an individual organism
and each looking quite like the other.
Surprisingly, from an evolutionary point
of view, the two animals are much more
closely related than are the two single-celled
organisms. The animals are an African
elephant and a close relative, a small mammal
known as a hyrax. What you cannot see in
Figure E3.9 is all of the ways in which these
organisms are similar, from the basic
structures of their cells to the structures of
their feet and teeth.
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a
b
Figure E3.9 (a) African elephant
(Loxodonta africana). The average male
African elephant is 350 cm high and weighs
5,000 kg. (b) Rock hyrax (Procavia
capensis). A rock hyrax may be 30 cm high
and weigh 4 kg.
On the other hand, the organisms in
Figure E3.10 are very distant in their
evolutionary connection, despite the fact that
each is a single cell. If you look closely, you
can find one of the characteristics that marks
these two organisms as being very different.
Notice that Peranema has an interior
compartment that is missing in the other cell.
That compartment is a nucleus, a membraneenclosed structure in the cell that houses its
DNA. The second cell is a bacterium called
Escherichia coli. Like other types of bacteria, its
DNA is not separated from the rest of the cell
contents by a surrounding membrane. It lacks
a nucleus.
These two cells illustrate the single largest
dividing point that biologists recognize among
all of the species on earth. The bacterial cell is
a
a very simple type of cell known as a
prokaryote. It has no nucleus, and the genetic
material that it contains is a huge molecule of
DNA, without any fancy packaging. In great
contrast, the Peranema is a more complex type
of cell called a eukaryote. Eukaryotes have
cells with nuclei and DNA that is packaged
with proteins to form structures known as
chromosomes. Eukaryotic cells also may have
other specialized, membrane-enclosed
compartments that perform a variety of
functions, such as energy transformation and
protein storage and packaging. Although
many similar processes go on in prokaryotic
cells, these cells do not contain such
compartments.
The structural differences and the
evolutionary distance between prokaryotes
b
Figure E3.10 (a) This Peranema is about 40 µm. (b) This
Escherichia coli is 3.5 µm in length (photographed at 35,000⫻).
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and eukaryotes are so great that biologists
categorize all organisms on earth on the basis
of this distinction. Figure E3.11 illustrates the
five major types of organisms recognized by
most biologists today. Note that one of the
kingdoms includes all of the prokaryotic
organisms. In contrast, the organisms in each
of the other four kingdoms are eukaryotes.
It would not be surprising if the
classification scheme shown in the figure and
used in this course seems a bit foreign to you.
After all, most of us grow up thinking that the
world contains only two basic categories of
organisms, plants and animals.
We are not alone in this. From the days of
Aristotle to the mid-1800s, almost everyone
was content with this simple subdivision. We
generally have little reason to question it,
because we rarely encounter living systems
that are so different in external appearance
that they don’t seem to fit.
By the middle of the 19th century,
however, some scientists had started to
question whether organisms such as fungi and
bacteria really fit well into either the plant
kingdom or the animal kingdom. Despite
these questions, suggestions to increase the
number of kingdoms were largely ignored. It
was not until the 1960s that the prevailing
attitude in the scientific community began to
change. Scientists were discovering new forms
of life and were using new microscopic and
Animalia
Plantae
Craniata
(vertebrates)
Anthophyta
(flowering plants)
Coniferophyta
(conifers)
Ginkgophyta
(ginkgo)
Fungi
Mollusca
(molluscs)
Filicinophyta
(ferns)
Hermichordata
(acorn worms)
Basidiomycota
(club fungi)
Sphenophyta
(horsetails)
Lycophyta
(club mosses)
Crustacea
Echinodermata
(crustaceans) Mandibulata
(sea stars)
(insects)
Ascomycota
(sac fungi)
Bryophyta
(mosses)
Cycadophyta
(cycads)
Hepatophyta
(liverworts)
Zygomycota
(conjugating fungi)
Cnidaria
(jellies)
Anthocerophyta
(hornworts)
Chelicerata
(spiders)
Rhodophyta
(multicellular
red algae)
Platyhelminthes
(flatworms)
Actinopoda
(heliozoans)
Oomycota
(water molds)
Phaeophyta
(multicellular
brown algae)
Diatoms
Apicomplexa
(sporozoans)
Discomitochondria
(flagellates)
Myxomycota
(slime molds)
Ciliophora
(ciliates)
Dinomastigota
(dinoflagllates)
Rhizopoda
(sarcodines)
green
nonsulfur
bacteria
Cyanobacteria
Gram-positive
Bacteria
Actinobacteria
Endospora
Eukarya
(Eukaryotes)
Prokarya
(Prokaryotes)
Gram-negative Bacteria
Bacteria
(Monera,
Prokaryotae)
Rotifera
(rotifers)
Porifera
(sponges)
Chlorophyta
(uni– and multicellular
green algae)
Protoctista
(Protista)
Annelida
(segmented
worms)
purple
bacteria
methanogens
halophiles
Wall-less
Bacteria
thermoacidophiles
Deinococci
mycoplasmas
Eubacteria
Archaea
Figure E3.11 A five-kingdom scheme. The Bacteria kingdom includes the
organisms that do not have membrane-bound organelles. Plants, animals, fungi,
and protoctists are all eukaryotes. What differences do you think separate the
organisms in each kingdom?
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biochemical techniques to examine cell
structure and function in even well-known
organisms. This led to an increasing amount
of evidence that supported proposals to
increase the number of basic categories that
biologists recognize. Figure E3.12 illustrates
some of these multikingdom schemes. These
ideas will help you trace the changes that have
occurred in scientists’ thinking to bring us to
the five-kingdom system that is most often
used today.
As you read the following brief
descriptions of the five kingdoms, look for
patterns in the criteria that determine each
group. Look as well for differences that
distinguish one basic type of organism from
the next. Do you see some of the reasons that
biologists can no longer accept a twokingdom view?
Kingdom Bacteria (Prokaryotae,
Monera). The main criterion (or
qualification) for membership in this
kingdom is the presence of the prokaryotic
type of cell (a cell that lacks membraneenclosed compartments). The Bacteria
kingdom includes the bacteria (or eubacteria)
Animalia
Plantae
Figure E3.12 Scientific ideas change
across time. (a) The first attempts to
categorize life resulted in this twokingdom division between plants and
animals. (b) This model shows three
kingdoms: plants, animals, and protists.
(c) Scientists developed this four-kingdom
scheme when they realized the great
differences between eukaryotes and
prokaryotes.
chordates
vascular plants
arthropods
mosses and
liverworts
segmented
worms
echinoderms
mollusks
algae
coelenterates
flat worms
sponges
flagellates
sporozoans
?
ciliates
vascular plants
Plantae
mosses and
liverworts
Animalia
Plantae
blue-green
bacteria
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ba
cte
ria
tes
lla
ge
fla
sporo
zoans
flat
worms
slime
molds
e
din
rco
sa iates
cil
Monera
s
Protista
slime
molds
blue-green algae
ba
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sp
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fla rozo
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lusk
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art
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fungi
algae
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segm
mosses and
liverworts
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flat
worms
b
algae
vascular plants
c
fungi
segm
ente
blue-green algae
rms
a
Animalia
sarcodines
d wo
bacteria
ria
s
ine
cod
sar
ciliates
co
lusk
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art
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p
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fungi
Protista
KH4119_Unit 01 ES_E121-E145 03/16/05 3:44 PM Page 131
and the archaea. Bacteria usually are single
cells, but they may occur in groups of cells.
Bacteria come in a variety of shapes, as
depicted in figure E3.13. Some swim by
means of long, whiplike tails. Bacteria live in
almost every environment, from the soil to
inside the human mouth. Archaea often live
in extreme environmental conditions. Some
live at high temperatures or in highly acidic
conditions; others live in high-salt conditions.
Some archaea live in environments where
there is very little oxygen, and they produce
methane gas.
Bacteria show a great diversity in the
processes that they use to obtain energy.
Many bacteria can use the sun’s energy
directly to power the reactions required for
making their own food through
photosynthesis. Others use energy derived
from the matter (food molecules) that they
acquire from their environments. As a group,
bacteria can digest almost anything—even
petroleum. This ability is fortunate for us.
Bacteria that can recycle matter through
decomposition increasingly are being used to
help with environmental cleanup efforts. All
bacteria reproduce by dividing into two. But
a
b
some also exchange small amounts of
DNA—a form of sexual reproduction.
Kingdom Animalia. Among the four
eukaryotic kingdoms is the kingdom in which
humans are found, the kingdom Animalia.
Animals are multicellular—they have a
complex organization of many specialized
cells. Animals also are characterized by their
ability to bring food into their bodies and
digest it. In addition, most animals reproduce
sexually and have senses and nervous systems
that enhance their ability to move.
Animals live in marine and freshwater
environments, inhabit the soil, or live on land.
In addition, animals come in a range of sizes,
from microscopic worms that live in human
blood to whales that can reach lengths of
27 meters (89 feet). Figure E3.14 shows a
diversity of animals.
Kingdom Plantae. Another eukaryotic
kingdom, the kingdom Plantae, includes
organisms that acquire their energy not from
eating, but from the sun. Plants carry out
photosynthesis, a process by which cells use
energy from sunlight to produce their own
food. Photosynthesis takes place in
membrane-enclosed structures within plant
c
Figure E3.13 Examples of
prokaryotes. (a) These Streptococcus
bacteria (photographed at 40,000⫻) can
cause strep throat. (b) Nostoc
(photographed at 400⫻), a cyanobacterium,
is common in freshwater lakes. (c) Spirella
voluntans (photographed at 400⫻) is part
of a group of bacteria named for its
characteristic spiral shape.
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a
b
c
Figure E3.14 Examples of animals. (a) Tube sponges from the Red Sea (b) A click beetle in
Arizona (c) A male hooded oriole from the southwestern region of the United States
cells called chloroplasts. Chloroplasts contain
chlorophyll, the light-absorbing pigment that
gives plants their characteristic green color.
Plants are multicellular, and their cell
membranes are surrounded by a rigid cell
wall that provides support. Most of them
reproduce sexually. Plant forms are diverse
and include mosses, liverworts, club mosses,
ferns, conifers, and flowering plants, as
shown in Figure E3.15. The bulk of the
world’s food and much of its oxygen are
produced by plants.
Kingdom Fungi. Kingdom Fungi, also a
eukaryotic kingdom, includes organisms that
a
b
grow directly from reproductive cells called
spores. Fungi, like plants, have cell walls, but
they do not carry out photosynthesis. You
probably are more familiar with the members
of this kingdom than you realize. Fungi such
as mushrooms become large, multicellular
organisms, with tissues made of slender tubes
of cells (hyphae) that may contain more than
one nucleus. Other fungi, such as yeasts, live
as single cells during their entire life cycle.
Still others, such as molds and rusts, live as
tiny multicellular structures on the surface of
bread that has been sitting around too long or
lettuce that is going bad.
c
Figure E3.15 Examples of plants. (a) This moss, Lycopodium, grows in moist areas.
(b) A sword fern, Polystichum munitium, in Olympic National Park, Washington (c) An apple tree,
Malus spp., in full bloom
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Fungi do not digest food inside their
bodies as humans do. Instead, they release
molecules called enzymes into their
surroundings. These enzymes break down
(digest) biological material that other living
systems have produced. The smaller food
molecules then are absorbed into the cells.
Thus fungi, along with many bacteria, play an
important role as decomposers in many
communities of organisms. The diversity
of fungi includes yeasts, molds, morels,
mushrooms, shelf fungi, puffballs, and
plant diseases such as rusts and smuts (see
Figure E3.16). Some fungi also interact
closely with green algae or cyanobacteria
to form the organisms known as lichens.
a
a
b
b
c
c
Figure E3.16 Examples of fungi.
(a) The mycelium of a wood-rotting
fungus, Stereum complicatum
(b) A mushroom fungus, Mycema lejiana
(c) Microstoma floccosa, a small,
cup-shaped fungus
Figure E3.17 Examples of
protoctists. (a) Trichonympha
(photographed at 135⫻), a protoctist that
lives in the gut of termites (b) Micrasteria
(photographed at 100⫻), a type of green
algae (c) Fuligo septica, a slime mold
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Kingdom Protoctista
(Protista). Finally, the
kingdom Protoctista is a grab
bag of all the remaining
eukaryotes that do not belong
to the animal, plant, or fungi
kingdoms. Protoctists live in
water and in moist habitats,
such as in the soil, on trees,
and in the bodies of other
organisms.
Protoctists show a
remarkable range of diversity
in their methods of obtaining
Figure E3.18 Minerva Terrace, Mammoth Hot
food, their methods of
Springs, Yellowstone National Park. Archaea live in
reproduction, their life cycles,
environments like these hot springs.
and their lifestyles. Most
protoctists are microscopic
more about the organisms that inhabit the
single cells and many grow as colonies—
earth. For example, evidence obtained during
clusters of individual cells. Others, such as
the last two decades suggests that the archaea,
brown algae living in the ocean, may form
which are currently in the kingdom Bacteria,
multicellular structures up to 100 meters
differ from other bacteria in that kingdom.
(328 feet) long. Some protoctists are brightly
The archaea include organisms that live in
colored algae that produce their food through
environments similar to those that probably
photosynthesis. Others are slime molds that
existed early in earth’s history, such as hot
obtain their food by decomposing the dead
springs (like those in Figure E3.18), sulfurtissues of other organisms. Still other
containing muds at the bottom of ponds,
protoctists are parasites of animals, plants, or
salt ponds, and salt lakes. For that reason,
fungi. A single droplet of pond water viewed
biologists think that the archaea are among
under the microscope reveals a world of
the very oldest organisms on earth. Because of
protoctists in their myriad of shapes. Figure
their age and their differences from other
E3.17 depicts several protoctists.
bacteria, they perhaps merit a kingdom of
Scientists may rethink the classification
their own.
system once again as they continue to learn
From Cell to Seed
Have you thanked a green plant today? Plants
play a critical role in our existence on earth.
They produce the oxygen that we breathe, the
food that we eat, and the multitude of
materials that we use, from rubber, to lumber,
to medicines, to coffee. Perhaps we ought to
ask, Have you thanked a 3.5 billion-year-old
single-celled organism today? Such an
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organism likely was the ancestor of all
modern plants.
To understand how that could be, we need
to trace the history of plant evolution. One of
the ways we can begin to understand this
history is to recognize that each of
the major events of plant evolution that
scientists think took place involved the