If you were to name an animal, a common response would be a lion, tiger, horse, monkeys, etc. Although these words are commonly used in our daily lives, they are not the formal names of organisms. The system by which we name and classify all organisms is called taxonomy. It is based on a system developed by Carl Linnaeus in the 18th century. Every organism has a two-part name, which is why taxonomy is also called binomial nomenclature. The levels of organization is, from the most general to the most specific, kingdom, phylum, class, order, family, genus, and species. Today we will be learning about the classification of organisms including the general and more specific classifications.
The classification system went through several changes, especially in the 20th century. In the 1950s and 1960s, all organisms were placed into three kingdoms, which were later expanded into five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. In 1990, a sixth kingdom, the Archaebacteria, was added. This kingdom included microorganisms that live in extreme environments. Nowadays, scientists use a three-domain system based on DNA analysis. This new system more accurately expresses the relationships among organisms. The three domains are Bacteria, Archaea, and Eukarya. The kingdom Monera is not used anymore since prokaryotes are separated into two domains. The most important change to the classification system was that Archaea formed there own group due being so different from bacteria. The name Archaebacteria was changed to Archaea since the Archaea are not bacteria.
Three Domains
Domain Bacteria
- Single-celled prokaryotes with no internal membranes
- Consists of both anaerobes and aerobes
- Bacteria play the role of decomposers that recycle dead matter
- Carry out conjugation, a form of sexual reproduction where genetic material is exchanged
- Thick, rigid cell wass
- No introns
- Autotrophic or heterotrophic
- Pathogens that cause disease
- Play a vital role in genetic engineering
Domain Archaea
- Unicellular prokaryotes
- Includes extremophiles (organisms that live in extreme environments). Methanogens obtain energy by producing methane from hydrogen. Halophiles thrive in environments with high salt concentrations. Thermophiles thrive in high temperatures.
- Introns are present
Domain Eukarya
- Have a nucleus and internal organelles
- Includes four kingdoms: Protista, Fungi, Plantae, and Animalia
Four Kingdoms of Eukarya
Kingdom Protista
- Widest variety of organisms
- All are eukaryotes
- Most are single-celled but some are multicelled
- Includes heterotrophs (amoebas and paramecia) and autotrophs (euglenas)
- Move through pseudopods (amoeba), cilia (paramecium), flagellum (euglena)
- Include organisms that do not fit into other kingdoms
- Some carry out conjugation
- Some cause serious diseases
Kingdom Fungi
- Heterotrophic eukaryotes
- Unicellular or multicellular
- Carry out extracellular digestion by secreting enzymes and absorbing nutrients by diffusion
- Act as decomposers
- Cell walls are composed of chitin
- Combine with algae to form lichens
- Reproduce asexually through budding, spore formation, or fragmentation or sexually
- Example: yeast, mold, mushrooms
Kingdom Plantae
- Multicellular, nonmotile, autotrophic eukaryotes
- Cell walls are made of cellulose
- Carry out photosynthesis
- Store carbohydrate as starch
- Reproduce sexually
- Some have vascular tissue
- Examples: mosses, ferns, cone-bearing and flowering plants
Kingdom Animalia
- Heterotrophic, multicellular eukaryotes
- Most can move on their own (motile)
- Most reproduce sexually
- Classified by anatomical features
There is so much more to kingdom Animalia such as evolutionary trends and the nine common phyla. This kingdom is even more interesting because us humans are a part of it. Next week, I will write an article specifically about the fourth kingdom.
Works Cited
Allott, Andrew. Biology: Course Companion. Oxford University Press, 2014.
IUPUI. “The 3 Domains of Life.” The 3 Domains of Life, 2002, http://www.biology.iupui.edu/biocourses/N100/2k23domain.html.
The Science Herald 
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