The Linnaean classification system recognized only two kingdoms of living things—Animals and Plants. When biologists began to use microscopes, they discovered many new micro-organisms and gained a better understanding of the cellular structure of living things. Biologists discovered prokaryotes—unicellular organisms, such as bacteria, that do not have membrane-bound organelles. They also discovered many new eukaryotes—unicellular and small multicellular organisms with organelles. These organisms were called protists—their cells had a nucleus and other organelles but showed an extraordinary diversity of shapes, sizes, and life cycles. Biologists also began to see evidence of significant differences between plants and fungi.
prokaryote: a single-celled organism that does not contain membrane-bound organelles
eukaryote: any organism whose cells contain organelles; some eukaryotic organisms are single-celled, while others are multicellular
Today, most biologists classify the more than 1.7 million known living species into six kingdoms. As you will learn, these six kingdoms are grouped within a higher taxonomic rank.
Kingdoms of Life
Figure 1 shows a typical representation of the six kingdoms of life. The most easily recognizable kingdoms are the Animals, Plants, and Fungi. These kingdoms include many large multicellular organisms. The other kingdoms include mostly or entirely microscopic single-celled, or unicellular, organisms. The eukaryotic Protista is by far the most diverse group of organisms. Many of these are tiny, single-celled organisms such as amoeba and paramecium, while others, such as the kelps, are among the largest living organisms. The Eubacteria, which are commonly called "bacteria: and Archaea are prokaryotic and are often extremely difficult to distinguish due to their extremely small size and lack of internal structure. Most prokaryotes consist of individual cells, but some others consist of many cells that have joined to form long strands or clustered arrangements.
LEARNING TIP
Name Change Archaea used to be called Archaebacteria, but we now know that they are distinct from Eubacteria, the "true bacteria."
You should note that the relative size of each kingdom in most diagrams, including Figure 1, does not represent the diversity, sheer numbers, or evolutionary relationships of the various kingdoms. The Animal, Plant, and Fungi kingdoms are usually greatly exaggerated in size. This reflects our own interest in these groups as well as our familiarity with them in our everyday life.
UNIT TASK Which kingdom or kingdoms do the organisms in your chosen group belong to?
Table 1 gives an overview of key characteristics of the six kingdoms. Keep in mind that there are exceptions to these general descriptions and that the number of different species of micro-organisms is likely grossly underestimated.
Table 1 Common Characteristics of the Six Kingdoms of Life
Kingdom: Eubacteria (greater than) 10 000 described species Cell characteristics: - Prokaryotic - Cell walls contain a unique compound. - Cells are variable in shape and size. Other characteristics: - Diverse means of obtaining energy and nutrients—photosynthetic, chemotropic, or heterotrophic - All can reproduce asexually Examples (Organisms shown in the photos are bolded.): - Escherichia coli, Salmonella, nitrogen-fixing bacteria Image: Escherichia coli
Kingdom: Archaea (greater than) 400 described species Cell characteristics: - Prokaryotic - Cell walls and cell membranes have a unique structure. - Most are extremely small. Other characteristics: - Some colonize extreme environments. - Live in the digestive tracts of mammals and marine environments - All can reproduce asexually. Examples (Organisms shown in the photos are bolded.): - extreme thermophiles (organisms that thrive at high temperatures) - methanogens (micro- organisms that produce methane as a byproduct) Image: extreme thermophiles.
Kingdom: Protista (greater than) 100 000 described species Cell characteristics: - Eukaryotic - Cells have extreme diversity of cellular structure. - Some have chloroplasts and cell walls. Other characteristics: - May be heterotrophic or photosynthetic, or both - Have variable forms of movement - Usually live in aquatic or other moist environments - Reproduce sexually and asexually Examples (Organisms shown in the photos are bolded.): Amoeba, kelps, green algae Image: Amoeba
Kingdom: Fungi (greater than) 100 000 described species Cell characteristics: - Eukaryotic - The cell wall is composed of chitin. - Most are multicellular. - Cells have no chloroplasts. Other characteristics: - All are heterotrophic - Most are terrestrial - Reproduce sexually and asexually Examples (Organisms shown in the photos are bolded.): - mushrooms, yeasts, moulds Image: Mushrooms
Kingdom: Plants (greater than) 250 000 described species Cell characteristics: - Eukaryotic - All are multicellular - Cell walls are composed of cellulose. - Possess chloroplasts Other characteristics: - Autotrophic and photosynthetic - Most are terrestrial - Reproduce sexually and asexually Examples (Organisms shown in the photos are bolded.): - mosses, ferns, conifers, flowering plants Image: conifers
Kingdom: Animals (greater than) 1.2 million described species Cell characteristics: - Eukaryotic - All are multicellular. - Cells have no cell walls or chloroplasts. Other characteristics: - All are heterotrophic - Most reproduce sexually. - Live in terrestrial and aquatic environments Examples (Organisms shown in the photos are bolded.): - elephants, sponges, corals, insects, snails, birds, humans Image: elephants
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A simplified phylogenetic tree (Figure 2) shows key evolutionary relationships among the six kingdoms. Note that the Protist Kingdom does not represent a single Glade. The brown, red, and green algae are more closely related to plants, while the amoeboid protists are more closely related to fungi and animals.
Figure 2 A simplified phylogenetic tree of the six kingdoms
Original cell
Archaea - thermophiles - halophytes - methanogens
Eubacteria - purple bacteria - photosynthetic bacteria - other bacteria
- photosynthetic eukaryotes Protists - brown algae - red algae - green algae Plants
Domains of Life
In 1996, Carl Woese conducted a detailed analysis of living organisms. He revealed that all organisms could be classified into three distinct groups. These groups, called domains, showed significant differences in their genetic makeup, which was very compelling evidence of their distinct evolutionary kinships (Figure 3).
domain: the highest taxonomic level; there are three domains of life
Figure 3 The three domains of life based on the work of Carl Woese. This detailed arrangement of the branches is continually being refined as new data become available.
The Domain Eubacteria contains only the Kingdom Eubacteria. Similarly, the Domain Archaea contains only the kingdom Archaea. The third domain, Eukaryotes, contains the other four kingdoms: Protista, Animals, Plants, and Fungi. The branches of the tree and the distances between the branches show an estimate of how closely related the groups are. You may be surprised to see that, while there are very dramatic differences in the physical appearances of animals, plants, and fungi, they were all placed very close together on this tree. The proximity is determined largely by comparing the genetic makeup of different species and groups.
Investigation 1.4.1
Sampling the Diversity of Life (page 32)
After reading about Kingdoms and Domains, you can complete Investigation 1.4.1.
In this field study, you will sample and then classify organisms from a chosen ecosystem.
Key
K/U: Knowledge and Understanding T/I: Thinking and Investigation C: Communication A: Application
Research This
Who Is in My Clade?
Skills: Researching, Analyzing, Communicating
SKILLS HANDBOOK A 2.1
Taxonomists have studied the phylogenetic relationships of all well-known groups of organisms. Their findings are often presented in the form of phylogenetic trees. In this activity you will examine the phylogenetic relationships of a species of your choosing. You should choose a relatively well known mammal or bird, since you are more likely to be familiar with its related groups.
1. Choose a species and describe its distinguishing characteristics. Using the Internet and other resources, find out its genus name.
2. Research the names and characteristics of all other species that are in the same genus.
3. Find out which taxonomic family and order your species belongs to. List the names of several other species that are in the same order.
4. Find two or three simple phylogenetic trees that illustrate the relationships among these species and clades that your chosen species belongs to. Draw your own version of the tree based on the information you have found. Be creative, but present your findings accurately.
A. How similar were members of the same genus? What features do they share? What features can be used to tell them apart? [K/U T/I]
B. List and describe a number of other species to highlight the diversity within the same order. [K/U T/I]
C. Based on your research, which groups within the order are most closely related? [K/U]
UNIT TASK BOOKMARK
To which domains do the organisms in your chosen group belong?
1.4 Summary
- Most biologists classify living things using a system of six kingdoms and three domains. - The most easily recognized kingdoms are Animals, Plants, and Fungi. - Protista is the only kingdom that does not represent a Glade. - Eubacteria and Archaea are difficult to distinguish because of their extremely small size and lack of internal structure. - The six-kingdom, three-domain system is based on phylogenetic relationships revealed by genetic evidence.
1.4 Questions
1. List the six kingdoms and three characteristics of each. [K/U]
2. Why do you think biologists originally placed fungi within the plant kingdom? [K/U T/I]
*3. Use the simplified phylogenetic tree in Figure 2 (page 28) to answer the following questions: [K/U T/I C] (a) State the four groups that gave rise to the protists. (b) To which other group(s) are the Amoebozoa most closely related? (c) Suggest some possible reasons to account for why biologists have grouped the Amoebozoa with brown, red, and green algae in the kingdom Protista. (d) Are animals more closely related to the Archaea or to the Eubacteria? Explain your reasoning.
5. Describe the most significant differences between the three domains of life. Why are the Eubacteria and Archaea difficult to distinguish? [K/U C]
The Linnaean classification system recognized only two kingdoms of living things—Animals and Plants. When biologists began to use microscopes, they discovered many new micro-organisms and gained a better understanding of the cellular structure of living things. Biologists discovered prokaryotes—unicellular organisms, such as bacteria, that do not have membrane-bound organelles. They also discovered many new eukaryotes—unicellular and small multicellular organisms with organelles. These organisms were called protists—their cells had a nucleus and other organelles but showed an extraordinary diversity of shapes, sizes, and life cycles. Biologists also began to see evidence of significant differences between plants and fungi.
prokaryote: a single-celled organism that does not contain membrane-bound organelles
eukaryote: any organism whose cells contain organelles; some eukaryotic organisms are single-celled, while others are multicellular
Today, most biologists classify the more than 1.7 million known living species into six kingdoms. As you will learn, these six kingdoms are grouped within a higher taxonomic rank.
Kingdoms of Life
Figure 1 shows a typical representation of the six kingdoms of life. The most easily recognizable kingdoms are the Animals, Plants, and Fungi. These kingdoms include many large multicellular organisms. The other kingdoms include mostly or entirely microscopic single-celled, or unicellular, organisms. The eukaryotic Protista is by far the most diverse group of organisms. Many of these are tiny, single-celled organisms such as amoeba and paramecium, while others, such as the kelps, are among the largest living organisms. The Eubacteria, which are commonly called "bacteria: and Archaea are prokaryotic and are often extremely difficult to distinguish due to their extremely small size and lack of internal structure. Most prokaryotes consist of individual cells, but some others consist of many cells that have joined to form long strands or clustered arrangements.
LEARNING TIP
Name Change
Archaea used to be called Archaebacteria, but we now know that they are distinct from Eubacteria, the "true bacteria."
Kingdom Plants:
- angiosperms
- pteridophyta
- gymnosperms
- bryophyte
Kingdom Fungi:
- basidiomycetes
- ascomycetes
- zygomycetes
Kingdom Protista:
- sporozoa
- rhodophyta
- chlorophyta
- phaeophyta
- chrysophyta
- pyrrophyta
- mastigophora
Kingdom Eubacteria:
- proteobacteria
- spirochetes
- cyanobacteria
- other bacteria
Kingdom Archaea:
- extreme thermophiles
- methanogens
- extreme halophiles
- sarcodina
- ciliophora
Kingdom Animals:
- porifera
- platyhelminthes
- rotifer
- annelida
- mollusca
- cnidaria
- chordate
- echinodermata
- arthropoda
- nematoda
Print Page 27
You should note that the relative size of each kingdom in most diagrams, including Figure 1, does not represent the diversity, sheer numbers, or evolutionary relationships of the various kingdoms. The Animal, Plant, and Fungi kingdoms are usually greatly exaggerated in size. This reflects our own interest in these groups as well as our familiarity with them in our everyday life.
UNIT TASK
Which kingdom or kingdoms do the organisms in your chosen group belong to?
Table 1 gives an overview of key characteristics of the six kingdoms. Keep in mind that there are exceptions to these general descriptions and that the number of different species of micro-organisms is likely grossly underestimated.
Table 1 Common Characteristics of the Six Kingdoms of Life
Kingdom: Eubacteria (greater than) 10 000 described species
Cell characteristics:
- Prokaryotic
- Cell walls contain a unique compound.
- Cells are variable in shape and size.
Other characteristics:
- Diverse means of obtaining energy and nutrients—photosynthetic, chemotropic, or heterotrophic
- All can reproduce asexually
Examples (Organisms shown in the photos are bolded.):
- Escherichia coli, Salmonella, nitrogen-fixing bacteria
Image: Escherichia coli
Kingdom: Archaea (greater than) 400 described species
Cell characteristics:
- Prokaryotic
- Cell walls and cell membranes have a unique structure.
- Most are extremely small.
Other characteristics:
- Some colonize extreme environments.
- Live in the digestive tracts of mammals and marine environments
- All can reproduce asexually.
Examples (Organisms shown in the photos are bolded.):
- extreme thermophiles (organisms that thrive at high temperatures)
- methanogens (micro- organisms that produce methane as a byproduct)
Image: extreme thermophiles.
Kingdom: Protista (greater than) 100 000 described species
Cell characteristics:
- Eukaryotic
- Cells have extreme diversity of cellular structure.
- Some have chloroplasts and cell walls.
Other characteristics:
- May be heterotrophic or photosynthetic, or both
- Have variable forms of movement
- Usually live in aquatic or other moist environments
- Reproduce sexually and asexually
Examples (Organisms shown in the photos are bolded.):
Amoeba, kelps, green algae
Image: Amoeba
Kingdom: Fungi (greater than) 100 000 described species
Cell characteristics:
- Eukaryotic
- The cell wall is composed of chitin.
- Most are multicellular.
- Cells have no chloroplasts.
Other characteristics:
- All are heterotrophic
- Most are terrestrial
- Reproduce sexually and asexually
Examples (Organisms shown in the photos are bolded.):
- mushrooms, yeasts, moulds
Image: Mushrooms
Kingdom: Plants (greater than) 250 000 described species
Cell characteristics:
- Eukaryotic
- All are multicellular
- Cell walls are composed of cellulose.
- Possess chloroplasts
Other characteristics:
- Autotrophic and photosynthetic
- Most are terrestrial
- Reproduce sexually and asexually
Examples (Organisms shown in the photos are bolded.):
- mosses, ferns, conifers, flowering plants
Image: conifers
Kingdom: Animals (greater than) 1.2 million described species
Cell characteristics:
- Eukaryotic
- All are multicellular.
- Cells have no cell walls or chloroplasts.
Other characteristics:
- All are heterotrophic
- Most reproduce sexually.
- Live in terrestrial and aquatic environments
Examples (Organisms shown in the photos are bolded.):
- elephants, sponges, corals, insects, snails, birds, humans
Image: elephants
Print Page 28
A simplified phylogenetic tree (Figure 2) shows key evolutionary relationships among the six kingdoms. Note that the Protist Kingdom does not represent a single Glade. The brown, red, and green algae are more closely related to plants, while the amoeboid protists are more closely related to fungi and animals.
Figure 2 A simplified phylogenetic tree of the six kingdoms
Original cell
Archaea
- thermophiles
- halophytes
- methanogens
Eubacteria
- purple bacteria
- photosynthetic bacteria
- other bacteria
Ancestral eukaryotic cell
- nonphotosynthetic eukaryotes
Animals
Fungi
- amoeboza
- photosynthetic eukaryotes
Protists
- brown algae
- red algae
- green algae
Plants
Domains of Life
In 1996, Carl Woese conducted a detailed analysis of living organisms. He revealed that all organisms could be classified into three distinct groups. These groups, called domains, showed significant differences in their genetic makeup, which was very compelling evidence of their distinct evolutionary kinships (Figure 3).
domain: the highest taxonomic level; there are three domains of life
Figure 3 The three domains of life based on the work of Carl Woese. This detailed arrangement of the branches is continually being refined as new data become available.
Eubacteria:
- thermotogales
- flavobacteria
- cyanobacteria
- purple bacteria
- gram positives
- green nonsulfur bacteria
Archaea:
- methanosarcina
- methanobacterium
- methanococcus
- thermoproteus
- pryodictium
- haloarchaea
Eukaeyotes:
- microsporidia
- diplomonads
- trichomonads
- flagellates
- ciliates
- plants
- fungi
- animals
- slime moulds
- entamoebae.
Print Page 29
The Domain Eubacteria contains only the Kingdom Eubacteria. Similarly, the Domain Archaea contains only the kingdom Archaea. The third domain, Eukaryotes, contains the other four kingdoms: Protista, Animals, Plants, and Fungi. The branches of the tree and the distances between the branches show an estimate of how closely related the groups are. You may be surprised to see that, while there are very dramatic differences in the physical appearances of animals, plants, and fungi, they were all placed very close together on this tree. The proximity is determined largely by comparing the genetic makeup of different species and groups.
Investigation 1.4.1
Sampling the Diversity of Life (page 32)
After reading about Kingdoms and Domains, you can complete Investigation 1.4.1.
In this field study, you will sample and then classify organisms from a chosen ecosystem.
Key
K/U: Knowledge and Understanding
T/I: Thinking and Investigation
C: Communication
A: Application
Research This
Who Is in My Clade?
Skills: Researching, Analyzing, Communicating
SKILLS HANDBOOK A 2.1
Taxonomists have studied the phylogenetic relationships of all well-known groups of organisms. Their findings are often presented in the form of phylogenetic trees. In this activity you will examine the phylogenetic relationships of a species of your choosing. You should choose a relatively well known mammal or bird, since you are more likely to be familiar with its related groups.
1. Choose a species and describe its distinguishing characteristics. Using the Internet and other resources, find out its genus name.
2. Research the names and characteristics of all other species that are in the same genus.
3. Find out which taxonomic family and order your species belongs to. List the names of several other species that are in the same order.
4. Find two or three simple phylogenetic trees that illustrate the relationships among these species and clades that your chosen species belongs to. Draw your own version of the tree based on the information you have found. Be creative, but present your findings accurately.
A. How similar were members of the same genus? What features do they share? What features can be used to tell them apart? [K/U T/I]
B. List and describe a number of other species to highlight the diversity within the same order. [K/U T/I]
C. Based on your research, which groups within the order are most closely related? [K/U]
UNIT TASK BOOKMARK
To which domains do the organisms in your chosen group belong?
1.4 Summary
- Most biologists classify living things using a system of six kingdoms and three domains.
- The most easily recognized kingdoms are Animals, Plants, and Fungi.
- Protista is the only kingdom that does not represent a Glade.
- Eubacteria and Archaea are difficult to distinguish because of their extremely small size and lack of internal structure.
- The six-kingdom, three-domain system is based on phylogenetic relationships revealed by genetic evidence.
1.4 Questions
1. List the six kingdoms and three characteristics of each. [K/U]
2. Why do you think biologists originally placed fungi within the plant kingdom? [K/U T/I]
*3. Use the simplified phylogenetic tree in Figure 2 (page 28) to answer the following questions: [K/U T/I C]
(a) State the four groups that gave rise to the protists.
(b) To which other group(s) are the Amoebozoa most closely related?
(c) Suggest some possible reasons to account for why biologists have grouped the Amoebozoa with brown, red, and green algae in the kingdom Protista.
(d) Are animals more closely related to the Archaea or to the Eubacteria? Explain your reasoning.
5. Describe the most significant differences between the three domains of life. Why are the Eubacteria and Archaea difficult to distinguish? [K/U C]
*Remind Mrs. Tien to go over Figure 2 in class!