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Biology is a rather mysterious thing.

How are we related to fish? How can you say we are related to amoebas? We will explore the idea of taxonomy here, sticking with the most generalized concept, the domain.

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In science, we are always looking for ways to better categorize what we are researching and studying. After all, it makes it easier to look up information and draw comparisons and relationships between things if they are categorized and organized properly. One such place where we take great pride and detail in this is in biological taxonomy.Biological taxonomy is the hierarchical breakdown of the different ways to categorize living things. We break everything in the world into living vs. nonliving things.

The system for categorizing living things was revised around 1990 by Carl Woese, a microbiologist. He suggested adding a more general term above the category kingdom, and he added domain.Our current taxonomic system looks like this:

  • Domain
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

You can remember this by the mnemonic: Did King Phillip Come Over For Great Spaghetti.


Domains are our way of breaking down living things more generally than before when we just went into kingdoms. We have found through research that many of the kingdoms were not exactly aligned as best as they could be. By adding domains, we can now show how some kingdoms are actually closely related under a specific domain.There are three distinct domains in biology.

They do an excellent job of making it easy to understand what goes under that domain. There is the Archaea, the Bacteria, and the Eukarya. We will look at each one individually.


The first and oldest known domain is the Archaea. These are ancient forms of bacteria that were originally grouped under the kingdom Monera (now defunct) as Archaeabacteria.We know them to be prokaryotic (lacking membrane-bound nuclei and organelles) that are found in all habitats on Earth. They are single celled microbes that find their origins as the first organisms of life here on Earth.

Hence, we give them the prefix archaea, which in Greek means ‘ancient things.’Archaea organisms are also different from the other domains in that many are extremophiles, meaning they can live in intense environments with high temperature, high acid, and high salt levels. One type of extremophile is the methanogens, or those organisms that produce methane as a product of their metabolism.


The Bacteria domain includes all other bacteria that are not included in the Archaea domain. They are prokaryotic and again found in all of the habitats on Earth. They are very similar to the Archaea domain, except that bacteria gain energy by being phototrophs (getting energy from light), lithotrophs (getting energy from inorganic non-carbon compounds), or finally organotrophs (getting energy from organic carbon-containing compounds).


The final domain is the Eukarya. This domain contains all the organisms that are eukaryotic, or contain membrane-bound organelles and nuclei. These would be considered fairly ‘modern’ since the other domains existed first. Scientists believe that Eukarya evolved from the symbiotic relationship exhibited by a prokaryotic bacteria that ate other prokaryotic bacteria but did not digest them. We also see here the more complex and larger organisms.Within the Eukarya domain are four kingdoms: Animalia, Plantae, Fungi, and Protista.

Comparing the Domains

The Archaea domain is the oldest, followed by Bacteria, and finally Eukarya. In both Archaea and Bacteria, the chromosomes (genetic material/blue prints for life) are circular, whereas in Eukarya, we have a double helix DNA structure (looks like a twisted ladder, again, blue prints for life). In Eukarya, there are membrane-bound organelles as well as nuclei, whereas the other two lack these.

Archaea and Eukarya both will continue to grow in the presence of antibiotics, whereas organisms in the Bacteria domain do not.Under the domains Archaea and Bacteria, we see many different organisms, but they are all lumped together under their respective domains. Most of these are single-celled organisms.

It is in the Eukarya domain that we see multicellular organisms falling in complex categories.

Lesson Summary

Biological taxonomy is the hierarchical breakdown of the different ways to categorize living things. The taxonomic break down of biology starts with our most general category, domain, which is our way of breaking down living things more generally than before when we just went into kingdoms. There are three domains in biology, and each one has distinct characteristics and organisms under it.

The three domains include:

  • Archaea – oldest known domain, ancient forms of bacteria
  • Bacteria – all other bacteria that are not included in the Archaea domain
  • Eukarya – all the organisms that are eukaryotic or contain membrane-bound organelles and nuclei

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