Six Kingdoms

figure 17-9

The six-kingdom system of classification can be aligned with the newer system of three domains. However, biologists have proposed adding, subdividing, or replacing some kingdoms. Biologists have also proposed other levels of taxa.

The system of three domains generally aligns with the more traditional system of six kingdoms, as represented in Figure 17-9 and summarized in Table 17-3. In the six-kingdom system, the first kingdom aligns with the domain Bacteria, the second kingdom aligns with the domain Archaea, and the remaining four kingdoms align with the domain Eukarya.

Kingdom Eubacteria

The kingdom Eubacteria (YOO-bak-TIR-ee-uh) aligns with the domain Bacteria. The name Eubacteria means "true bacteria," distinguishing this group from the archaea, which are no longer considered to be bacteria.

Domain Bacteria

Domain Archaea

Domain Eukarya

Domain Bacteria

Kingdom Eubacteria

Domain Archaea

Kingdom Archaebacteria
Kingdom Protista

Kingdom Fungi

Kingdom Plantae

Kingdom Animalia

Kingdom Fungi

Kingdom Animalia

Kingdom Archaebacteria

The kingdom Archaebacteria (AHR-kee-bak-TIR-ee-uh) aligns with the domain Archaea. The name Archaebacteria means "ancient bacteria," but archaea are so different from bacteria that many biologists now prefer to use only the name archaea for this group.

Kingdom Protista

One of the four kingdoms of eukaryotes is Protista (proh-TIST-uh), whose members are called protists. This kingdom has been defined as those eukaryotes that are not plants, animals, or fungi. This definition is troublesome because it defines organisms based on what they are not rather than on what they are. Most protists are unicellular organisms, but it is difficult to make other generalizations about them. Molecular analyses indicate that many protists are less related to each other than plants are to animals. For all of these reasons, many biologists think that the taxon Protista is no longer useful, and several new taxa are being considered.

Common examples of unicellular protists are amoebas and para-mecia. Examples of multicellular protists include some kinds of seaweeds and molds. Each of these protists is similar to animals, plants, or fungi in some ways yet different in other ways.

TABLE 17-3 Kingdom and Domain Characteristics

Taxon

Cell type

Cell surfaces

Body plan

Nutrition

Domain Bacteria (aligns with Kingdom Eubacteria)

prokaryotic; lack nucleus and other organelles

cell wall:contains peptidoglycans; cell membrane: contains fatty acids

unicellular

heterotrophic and autotrophic by chemosynthesis or photosynthesis

Domain Archaea (aligns with Kingdom Archaebacteria)

prokaryotic; lack nucleus and other organelles

cell wall: lacks peptidoglycan; cell membrane: contains hydrocarbons other than fatty acids

unicellular

heterotrophic and autotrophic by chemosynthesis

Domain Eukarya Kingdom Protista

eukaryotic; have nucleus and complex organelles

cell wall: made of cellulose or other materials; cell membrane: contains fatty acids

mostly unicellular; multicellular forms: lack tissue organization

autotrophic by photosynthesis, some heterotrophic by phagocytosis, or both

Domain Eukarya Kingdom Fungi

eukaryotic; have nucleus and complex organelles

cell wall: made of chitin; cell membrane: contains fatty acids

unicellular and multicellular

heterotrophic by secreting digestive enzymes into environment

Domain Eukarya Kingdom Plantae

eukaryotic; have nucleus and complex organelles

cell wall: made of cellulose; cell membrane: contains fatty acids

multicellular; develop from embryos

autotrophic by photosynthesis

Domain Eukarya Kingdom Animalia

eukaryotic; have nucleus and complex organelles

cell wall: none cell membrane: contains fatty acids

multicellular; develop from embryos

heterotrophic by phagocytosis

Kingdom Fungi

The second kingdom of eukaryotes is Fungi (FUHN-jie). The kingdom Fungi consists of eukaryotic, heterotrophic organisms that are unicellular or multicellular and that gain nutrients in a unique way. Unlike animal cells and some protists, fungi absorb rather than ingest nutrients. The about 70,000 species of fungi include mushrooms, puffballs, rusts, smuts, mildews, and molds.

Kingdom Plantae

The third kingdom of eukaryotes is Plantae (PLAN-tee), which consists of eukaryotic, multicellular plants. Except for a few parasitic species, most plants are autotrophic, use photosynthesis as a source of energy, and develop from embryos. Most plants live on land and include mosses, ferns, conifers, and flowering plants.

Kingdom Animalia

The fourth kingdom of eukaryotes is Animalia (AN-uh-MAH-lee-uh). Animals are eukaryotic, multicellular, and heterotrophic organisms that develop from embryos. Most animals have symmetrical body organization and move around their environment to find and capture food.

Future Taxonomic Systems

Because taxonomic systems are scientific models, they are subject to change. Since the time of Linnaeus, biologists have proposed, used, evaluated, and modified many classification systems. Modern biologists have proposed alternatives to the six-kingdom and three-domain systems. For example, some biologists have proposed three or more new kingdoms to replace Kingdom Protista. Some use taxonomic categories such as "subkingdom" and "superorder" to adapt the more traditional Linnaean categories to newer systematic models. Some favor pure cladistics over Linnaean categories. Surely, new systems will result from future investigations into the millions of known and unknown species on Earth.

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