Through illustrations and interactives you will explore the major divisions (domains, kingdoms) of life. Before you get started, don’t forget to print out your OnTRACK Biology Journal. Show
TEKS Standards and Student Expectations B(8) The student knows that taxonomy is a branching classification based on the shared characteristics of organisms and can change as new discoveries are made. The student is expected to: B(8)(C) compare characteristics of taxonomic groups, including archaea, bacteria, protists, fungi, plants, and animals Learning Objectives Describe how taxonomists classify living organisms. Describe the three-domain classification system and the six-kingdom classification system. Describe common characteristics of organisms grouped into each of the six kingdoms. Essential Questions How do the three domains of life differ from each other? How are organisms classified into kingdoms? What is the difference between a prokaryote and a eukaryote? What is the difference between an autotroph and a heterotroph? Vocabulary
 The three-domain system is a biological classification introduced by Carl Woese, Otto Kandler, and Mark Wheelis in 1990[2][1] that divides cellular life forms into three domains, namely Archaea, Bacteria, and Eukaryote or Eukarya. The key difference from earlier classifications such as the two-empire system and the five-kingdom classification is the splitting of archaea from bacteria as completely different organism. It has been challenged by the two-domain system that divides organisms into Bacteria and Archaea only, as eukaryotes are considered as one group of archaea.[3][4][5] Background[edit]Woese argued, on the basis of differences in 16S rRNA genes, that bacteria, archaea, and eukaryotes each arose separately from an ancestor with poorly developed genetic machinery, often called a progenote. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdoms. Originally his split of the prokaryotes was into Eubacteria (now Bacteria) and Archaebacteria (now Archaea). Woese initially used the term "kingdom" to refer to the three primary phylogenic groupings, and this nomenclature was widely used until the term "domain" was adopted in 1990.[1] Acceptance of the validity of Woese's phylogenetically valid classification was a slow process. Prominent biologists including Salvador Luria and Ernst Mayr objected to his division of the prokaryotes.[6][7] Not all criticism of him was restricted to the scientific level. A decade of labor-intensive oligonucleotide cataloging left him with a reputation as "a crank," and Woese would go on to be dubbed "Microbiology's Scarred Revolutionary" by a news article printed in the journal Science in 1997.[8] The growing amount of supporting data led the scientific community to accept the Archaea by the mid-1980s.[9] Today, very few scientists still accept the concept of a unified Prokarya.[10] Classification[edit]The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems. This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to Eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus. The three-domain system sorts the previously known kingdoms into these three domains: Archaea, Bacteria, and Eukarya.[3] Domain Archaea[edit]The Archaea are prokaryotic, with no nuclear membrane, but with biochemistry and RNA markers that are distinct from bacteria. The Archaeans possess unique, ancient evolutionary history for which they are considered some of the oldest species of organisms on Earth, most notably their diverse, exotic metabolisms. Some examples of archaeal organisms are:
Domain Bacteria[edit]The Bacteria are also prokaryotic; their domain consists of cells with bacterial rRNA, no nuclear membrane, and whose membranes possess primarily diacyl glycerol diester lipids. Traditionally classified as bacteria, many thrive in the same environments favored by humans, and were the first prokaryotes discovered; they were briefly called the Eubacteria or "true" bacteria when the Archaea were first recognized as a distinct clade. Most known pathogenic prokaryotic organisms belong to bacteria (see[11] for exceptions). For that reason, and because the Archaea are typically difficult to grow in laboratories, Bacteria are currently studied more extensively than Archaea. Some examples of bacteria include:
Domain Eukarya[edit]Eukarya are organisms whose cells contain a membrane-bound nucleus. They include many large single-celled organisms and all known non-microscopic organisms. A partial list of eukaryotic organisms includes: Kingdom Fungi or fungi
Niches[edit]Each of the three cell types tends to fit into recurring specialities or roles. Bacteria tend to be the most prolific reproducers, at least in moderate environments. Archaeans tend to adapt quickly to extreme environments, such as high temperatures, high acids, high sulfur, etc. This includes adapting to use a wide variety of food sources. Eukaryotes are the most flexible with regard to forming cooperative colonies, such as in multi-cellular organisms, including humans. In fact, the structure of a eukaryote is likely to have derived from a joining of different cell types, forming organelles. Parakaryon myojinensis (incertae sedis) is a single-celled organism known to be a unique example. "This organism appears to be a life form distinct from prokaryotes and eukaryotes",[12] with features of both. Alternatives[edit]Alternative versions of the three domains of life's phylogeny Parts of the three-domain theory have been challenged by scientists including Ernst Mayr, Thomas Cavalier-Smith, and Radhey S. Gupta.[13][14][15] Recent work has proposed that Eukarya may have actually branched off from the domain Archaea. According to Spang et al. Lokiarchaeota forms a monophyletic group with eukaryotes in phylogenomic analyses. The associated genomes also encode an expanded repertoire of eukaryotic signature proteins that are suggestive of sophisticated membrane remodelling capabilities.[16] This work suggests a two-domain system as opposed to the three-domain system.[4][5][3] Exactly how and when archaea, bacteria, and eucarya developed and how they are related continues to be debated.[17][3][18] See also[edit]
References[edit]
How are organisms classified into domains and kingdoms?Organisms are placed into domains and kingdoms based on their cell type, their ability to make food, and the number of cells in their bodies. Scientists classify organisms in the domain Eukarya into one of four kingdoms: Protists, Fungi, Plants, or Animals.
What is the reason behind in the creation of three domains instead of using kingdoms?The three-domain system emphasizes the similarities among eukaryotes and the differences among eukaryotes, bacteria, and archaea. By using domains, Woese was able to show these relationships without replacing the popular six-kingdom system.
What is the difference between kingdoms and domains?A domain is a taxonomic category above the kingdom level. The three domains are: Bacteria, Archaea, and Eukarya, which are the major categories of life. Essentially, domains are superkingdoms. A kingdom is a taxonomic group that contains one or more phyla.
How do cell types prokaryotes and eukaryotes fit in the kingdom domain classification systems?The three domains are the Archaea, the Bacteria, and the Eukarya. Prokaryotic organisms belong either to the domain Archaea or the domain Bacteria; organisms with eukaryotic cells belong to the domain Eukarya.
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How did the classification of a microorganism change from kingdoms to domains of life
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