The Three Domains of Life

The Three Domains of Life
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  • 0:01 Taxonomy & Domains
  • 1:41 Archaea
  • 2:31 Bacteria
  • 3:04 Eukarya
  • 3:41 Comparing the Domains
  • 4:30 Lesson Summary
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Lesson Transcript
Instructor: Jeremy Battista
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.

Taxonomy

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

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.

Archaea

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.

Bacteria

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).

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