Domain Eukarya Overview – History, Characteristics & Kingdoms

Domain Eukarya Kingdoms

Domain Eukarya: Life on Earth is truly very diverse. Hence, in order to easily distinguish living organisms, early scientists classified them into two kingdoms: Animalia (animal) and Plantae (plants/vegetables).

However, during the 19th century, this classification[1] was challenged by numerous evidences that were just too insufficient to explain such diversity. To address this, scientists began to propose other systems having four or more kingdoms.

One of the most widely used is the system which classify organisms into five kingdoms namely:

  • Bacteria

    Monera (Bacteria)

  • Fungi

    Fungi

  • Protista

    Protista

  • Animalia

    Animalia

  • Plantae

    Plantae

This system connectedly classify life into two namely Prokarya (includes bacteria) and Eukarya (includes fungi, animals, plants, chromoalveolates, rhizarians and excavates).

However, recent studies revealed and provided support for the emergence of another domain: Archaea.

The former domain Prokarya, which consists only of bacteria, has been divided into two separate domains: Bacteria (and) Archaea. Archaea[2] are minute organisms which thrive at extreme environmental conditions like high pressure and temperature.

Three Domains of Life

In summary, the three domains of life are:

  • Domain Bacteria

    Prokarya (Bacteria)

  • Domain Archaea

    Archaea

  • Domain Eukaryote

    Eukaryote

In the diagram below, the domains Prokarya (Bacteria), Archaea, and Eukarya are illustrated and are differentiated from each other. Apparently, none of these domains is ancestral to each other and each have unique and distinguishable features as well as shared characteristics. In this article, the domain Eukarya will be on focus.

3 Domains of Life Tree
3 Domains of Life Tree (Source: Wikimedia)

What is Domain Eukarya? (also Eukaryota)

Coming from the Greek words “eu” which means “true“, and “karyon” which means, “nut“, the domain Eukarya is composed of organisms having “true nucleus”.
Eukaryotic cells[3], as their cells are called, are perhaps the most complex in terms of both external and internal structures, and physiological and reproductive processes. Among all domains in the biological world, members of the domain Eukarya definitely have the largest body size and body mass.

Where did they come from?

According to various evidences, eukaryotic cells have started to exist more than 0.6 billion years ago. Apparently up until now, their evolution is viewed by many as one of the most unusual events in biological history. This is because over the same period of time a lot characteristics have been developed, evolved, and changed.

To explain such bizarre event, scientist Lynn Margulis[4] proposed the so-called “Endosymbiotic Theory“.

  • This theory states that the mitochondria (powerhouse of the cell), and the chloroplasts (structure for photosynthesis) were once single-celled organisms that have been engulfed by “proto-eukaryotic” cells. Check out the Cellular Respiration Equation here.
  • Interestingly, the eukaryotic mitochondria and chloroplasts have a different set of genetic material as compared to the cell itself, hence a compelling evidence that they were once bacterial cells.
  • Their continuous and maintained symbiosis required both cells to reproduce at the same rate and not to digest each other.
  • As a result, the resulting cells could now produce its own energy, and fix carbon through the use of light.

Characteristics of Eukaryotes

Domain Eukarya Animal Cell
Diagram of a typical animal cell (Organelles are labelled inside the diagram)

1. Presence of membrane bound organelles

The eukaryotic cell contains various internal membrane-bound structures referred to as the “organelles“.

  • In the cell, the job of the organelles is to carry out the physiological and metabolic processes that are important for the survival of the cell.
  • Other organelles function for support and motility. Such were the intracellular filaments, cilia, and flagella.

2. Presence of a double membrane nucleus

Unlike other organisms of other domain (which have their genetic material suspended in the cytoplasm), the DNA of eukaryotic organisms are stored in the nucleus.

  • Interestingly, the nucleus of eukaryotes is surrounded by the nuclear envelope, a double membrane, which has pores in order to allow the movement of the DNA in and out of it.

3. Cell division is different

Another distinguishing feature of eukaryotes is that they have a different mode of replicating themselves. Instead of merely dividing themselves and copying their genetic material (like what other domains do), cell division in eukaryotes involves two process: mitosis and cytokinesis. Check out the cell biology topics here.

  • During mitosis, the nucleus of the cell divides into two while the genetic material, present as chromosomes are equally divided to each opposite of the cell.
  • Following that step is the cytokinesis which is when the cytoplasm of the cell divides, following it the equal division of the genetic material.

4. Mode of reproduction may vary

Eukaryotic cells can reproduce themselves in two ways: asexual (through mitosis) or sexual reproduction (through meiosis).

  • During asexual reproduction, the cell simply divide through mitosis followed by cytokinesis.
  • On the other hand, they may also reproduce sexually by involving their sex cells called the gametes. In this type of reproduction, the offspring inherits a chromosome from each of its parent.

Kingdoms Under Domain Eukarya

1. Fungi

The Kingdom Fungi consists of heterotrophic organisms, or organisms that cannot make their own food. Instead, they simply acquire all the important nutrients by absorption.

  • The cell wall of the members of the kingdom is made of chitin, a type of carbohydrate, whereas their carbohydrates (energy) is stored in the form of glycogen.
  • The Kingdom fungi consists of organisms such as yeast, mushroom, and mold.
  • Fungi break down the organic materials of dead organisms and as a result, they help continue nutrient cycling[5] in ecosystems.

2. Animalia

Coming from the Latin word “animalis[6] meaning “have breath“, the Kingdom Animalia is comprised of heterotrophic organisms. As alluded to earlier, these organisms obtain their nutrient requirements by ingesting organisms.

  • A distinguishing characteristic of this kingdom include multi-cellularity and the lack of cell walls.
  • Most members of this kingdom are capable of movement (locomotion) and reproduction.
  • Members of this kingdom consist of almost all animals known (e.g. fishes, amphibians, reptiles, birds, mammals, insects, etc).

3. Plantae

Also called as the Kingdom Metaphyta, the Kingdom Plantae consists all multicellular, eukaryotic, and photosynthetic organisms in the planet.

  • Being photosynthetic, these organisms are autotrophs and are able to make their own food using the energy from the sun.
  • However, some members are able to be both producer and consumers as they are able to synthesize food and metabolize it from other sources.
  • Members of this kingdom have made possible the perpetuation of a large number of organisms. Basically, without them, heterotrophic organisms would have never survived.

4. Chromoalveolata

Formerly called as Kingdom Chromista[7], this kingdom is one of the “newly-considered” kingdoms in the biological world (as proposed by Thomas Cavalier in 1981).

  • This kingdom is consist of dinoflagellates, diatoms, and ciliates.
  • It is believed that the members of this kingdom originated from a bikont (a cell with two flagella) and a red alga that became the ancestor of all organisms with plastids having chlorophyll c.
  • However at present, it is still being debated as some evidences show that this kingdom is not monophyletic (coming from a common ancestor) as it was originally observed.

5. Rhizaria

The next kingdom in this list is widely composed of unicellular eukaryotes: Kingdom Rhizaria[3]. Also proposed by Thomas Cavalier-Smith (2002), the members of this kingdom differ in form but most of them are amoeboids with pseudopods (false feet).

  • This kingdom is composed of the foramineferans, and radiolarians.
  • Together with chromoalveolates and excavates, the rhizarians were previously under the Kingdom Protista.
  • However, unlike the Kingdom Chromoalveolata, there are evidences showing that rhizarians are a monophyletic group.

6. Excavata

Last but not the least, the kingdom consisting of a wide variety of organisms (photosynthetic, heterotrophic, and parasitic)– the Kingdom Excavata[8].

  • The name of this kingdom came from the two Latin words, “ex cavatum” meaning “cavity”.
  • This is of course in reference to the characteristics of the members’ cells having a depressed structure. Such is believed to be associated with one of the flagella and is used in feeding.
  • The members of this kingdom includes the euglenozoa, kinetoplastids, and parabasalids.

References

  • [1] – “Classification: The Three Domain System.” Classification: The Three Domain System. Accessed December 08, 2016. Link.
  • [2] – Rampelotto, Pabulo Henrique. “Extremophiles and Extreme Environments.” Life : Open Access Journal. 2013. Accessed December 08, 2016. Link.
  • [3] – “Domain Eukaryota.” Domain Eukaryota. Accessed December 08, 2016. Link.
  • [4] – “The Evolution of the Cell.” The Evolution of the Cell. Accessed December 08, 2016. Link.
  • [5] – “Introduction to the Fungi.” Introduction to the Fungi. Accessed December 09, 2016. Link.
  • [6] – “Kingdom Animalia.” Kingdom Animalia | Characteristics, Kingdom Animalia Classification | [email protected] Accessed December 09, 2016. Link.
  • [7] – “Chromalveolata.” Chromalveolata – Biocyclopedia – All about Biology, Chemistry & More. Accessed December 09, 2016. http://www.eplantscience.com/index/lua/chromalveolata.php.
  • [8] – “EXCAVATA.” EXCAVATA. Accessed December 09, 2016. Link.
Domain Eukarya Overview – History, Characteristics & Kingdoms
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