Domain Eukarya : Life on Earth diverse . Hence, to easily distinguish living organisms, early scientists classified them into two kingdoms: Animalia (animals) and Plantae (plants).
However, during the 19th century, this classification was challenged by numerous pieces of evidence that were just too insufficient to explain such diversity. To address this, scientists proposed other systems with four or more kingdoms.
One of the most widely used in the system which classifies organisms into five kingdoms , namely:
Monera (Bacteria)
Fungi
Protista
Animalia
Plantae This system connectedly classifies life into two, namely Prokarya (includes bacteria Eukarya (includes fungi animals plants
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 sub-domains: Bacteria and Archaea . Archaea are minute organisms that thrive in extreme environmental conditions like high pressure and temperature.
Three Domains of Life In summary, the three domains of life are:
Prokarya (Bacteria)
Archaea
Eukaryote The diagram below illustrates and differentiates the domains Prokarya (Bacteria), Archaea, and Eukarya. None of these domains is ancestral, each with unique and distinguishable features and shared characteristics. On this page, the domain Eukarya will be on focus.
3 Domains of Life Tree (Source: Wikimedia) What is Domain Eukarya? (aka 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, as their cells are called, are perhaps the most complex in terms of both external and internal structures and physiological and reproductive processes
Where Did Eukaryotes Come From? According to various archeological evidence, eukaryotic cells started to exist more than 0.6 billion years ago . Up until now, their evolution is viewed by many as one of the most unusual events in natural history
To explain such a bizarre event, scientist Lynn Margulis 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. The eukaryotic mitochondria and chloroplasts have different genetic materials than the cell itself. Hence it proves 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 their energy and fix carbon through light. Characteristics of Eukaryotes Diagram of a typical animal cell (Organelles are labeled inside the diagram)
1. Presence of membrane-bound organelles The eukaryotic cell contains various internal membrane-bound structures referred to as the “organelles .”
In cells, the job of organelles is to carry out physiological and metabolic processes that are important for the cell’s survival. 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 domains (which have their genetic material suspended in the cytoplasm
The nucleus of eukaryotes is surrounded by the nuclear envelope, a double membrane with pores to allow the movement of the DNA in and out.
3. Cell division is different Another distinguishing feature of eukaryotes is their different replicating modes. Instead of merely dividing themselves and copying their genetic materials (like what other domains do), cell division in eukaryotes involves two processes: mitosis cytokinesis .
During mitosis, the cell’s nucleus divides into two while the genetic material, present as chromosomes, are equally distributed to each opposite of the cell. Following that step is cytokinesis, when the cell’s cytoplasm divides, developing 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 divides through mitosis followed by cytokinesis. Conversely, 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 parents.
Kingdoms Under Domain Eukarya
1. Fungi heterotrophic organisms or organisms that cannot make their food. Instead, they acquire all the essential nutrients by absorption.
The cell wall chitin , a type of carbohydrate glycogen . The Kingdom fungi consist of organisms such as yeast Fungi break down the organic materials of dead organisms, and as a result, they help continue nutrient cycling in ecosystems
2. Animalia animalis “, 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 includes 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
3. Plantae Metaphyta , the Kingdom Plantae consists of all multicellular , eukaryotic , and photosynthetic organisms on the planet.
Being photosynthetic, these organisms are autotrophs and can make their food using the energy from the sun. However, some members can be producers and consumers as they can synthesize food and metabolize it from other sources. See carnivorous plants Members of this kingdom have made the perpetuation of many organisms possible. Basically, without them, heterotrophic organisms would have never survived.
4. Chromoalveolata Chromista , 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, it is still being debated as some evidence shows that this kingdom is not monophyletic (coming from a common ancestor) as it was initially observed.
5. Rhizaria Rhizaria is widely composed of unicellular eukaryotes. Also proposed by Thomas Cavalier-Smith (2002), the members of this kingdom differ in form. Still, most of them are amoeboids with pseudopods (false feet).
This kingdom is composed of the foraminiferans and radiolarians . Together with chromalveolates and excavates, the rhizarians were previously under the Kingdom Protista . However, unlike the Kingdom Chromoalveolata, there are pieces of evidence showing that rhizarians are a monophyletic group.
6. Excavata
The name of this kingdom came from the two Latin words, “ex cavatum “, meaning “cavity .” The members of this kingdom include the euglenozoa , kinetoplastids , and parabasalids .
Classification: The Three Domain System. – Accessed December 08, 2016. Link .Extremophiles and Extreme Environments. – Life : Open Access Journal. 2013. Accessed December 08, 2016. Link .Domain Eukaryota . Accessed December 08, 2016. Link .The Evolution of the Cell – Accessed December 08, 2016. Link .Introduction to the Fungi – Accessed December 09, 2016. Link .Chromalveolata – Biocyclopedia. Accessed December 09, 2016. http://www.eplantscience.com/index/lua/chromalveolata.php.EXCAVATA – Accessed December 09, 2016. Link .
