What is a domain?

We explain what a domain is and its relations with the biological kingdoms. In addition, some of its main features.

1. What is a domain?

In biology , domain is understood, sometimes also called empire or superreino, to the broadest taxonomic category in which known living things are classified . That is, it is the widest category in which the different realms of life can be organized , according to the most recent classification models and of greater acceptance in the specialized scientific community.

The current system in this area is the one proposed by the American microbiologist Carl Richard Woese in 1990, and is known as the system of the three domains , since it organizes the different kingdoms of life (which are usually animalia, plantae, fungi, proctist, bacteria and archaea) in three large sets or domains, based on their fundamental cellular characteristics: bacterial domain, archaea domain and eukarya domain.

The first two domains, bacteria and archaea, cover the world of prokaryotic organisms , that is, those that lack a cell nucleus and are much simpler and smaller than those belonging to the remaining domain, eukaryotes . The latter have larger, complex and endowed cells with a cell nucleus where their DNA is found , and therefore they can be single-celled or multicellular organisms.

Thus, all known life fits into one of these domains , perhaps with the exception of viruses , whose parasitic and acellular existence remains so mysterious that it is not yet possible to determine if they are really living beings.

2. Domain and kingdom

Domains are the widest category of life , in which the various known kingdoms are organized. These, on the other hand, are the immediately inferior categories (although in some systems superreines are also understood as an intermediate category between domain and kingdom, or even as an alternative: two superreines, eukaryota and prokaryota, instead of three domains), among which living beings are distributed based on their evolutionary, metabolic, cellular and behavioral similarities.

There are various life classification systems that propose 3, 4, 5, 6 and up to 7 different kingdoms. The most usual includes the following:

• Kingdom bacteria. Where are the simplest and most primitive prokaryotic organisms of all, the most predominant on the planet, dedicated to all kinds of nutritional functions: photosynthesis , chemosynthesis, parasitism , predation, etc.
• Archaea kingdom. Initially considered part of the bacterial kingdom (and called archeobacteria) it was later found that they have substantial evolutionary differences that allow them to be a kingdom (and a domain) apart from bacteria , with whom they share their prokaryotic existence, but with different behaviors ( extreme habitats , chemisynthetic nutrition) and cellular characteristics that resemble eukaryotes.
• Proctist kingdom . Also called protist and formerly moneras , it is the kingdom where all eukaryotic unicellular organisms are contemplated, a sort of step between prokaryotic life and multicellular realms. Here enter the protozoans , unicellular algae and other eukaryotic microorganisms of various eating habits.
• Kingdom plant . The plant kingdom, that is, that of plants, those immobile multicellular eukaryotic organisms, which are nourished from photosynthesis: the biochemical composition of sugars from water , carbon dioxide and sunlight , thanks to a pigment specialized they own, called chlorophyll. Its cells house it in their dishes, and also have a rigid cellulose cell wall.
• Fungi kingdom . The kingdom of fungi, intermediate between plant and animal, since they are not autotrophic like plants, but immobile. They feed on the decomposition of organic matter , either saprophytic or parasitic, and reproduce by spores. Its eukaryotic cells have a cell wall, but chitin.
• Animal kingdom . The animal kingdom, with its enormous variety of genera and species of multicellular , heterotrophic , eukaryotic organisms, endowed with mobility, sexual reproduction and a metabolism based on respiration , that is, the oxidation of glucose obtained from matter organic consumed from other living things. Your cells lack a cell wall.

3. Bacteria domain

The bacterial domain coincides with the kingdom of the same name, within which are exclusively prokaryotic organisms , of simple and primitive cellular structure, which are considered the most abundant forms of life on the planet, and surely the first to emerge in the evolutionary broth of the primitive Earth.

They can be obtained in practically all habitats, even within (in symbiotic or parasitic relationship) of some multicellular organisms, and dedicated to various types of metabolic activity: photosynthesis, such as cyanobacteria (blue-green algae), the decomposition of matter organic, etc.

4. Archaea domain

Together with the bacterial domain, the archaea or archaea domain covers the entire prokaryotic world. It also coincides with the kingdom of the same name, which includes archeobacteria or archaea , prokaryotic organisms that exhibit certain similarities with eukaryotic life, despite existing in very specific and generally hostile habitats (leading an extrememophilic life) such as water underground boiling, although they have also been found among the microorganisms that make up the marine plankton.

5. Eukarya domain

The eukarya or eukaryotic domain is the largest of the three, in the sense that it groups a diverse set of kingdoms: animals, plants, fungi and all protists , that is, all forms of eukaryotic life, possessing cells with a specific cell nucleus (where DNA is housed) and other complex cellular organelles.

The evolutionary step from prokaryotes to eukaryotes is still difficult to understand, but it is also key in the formation of more complex organisms , such as multicellular, in which cells sacrifice their independence to form a more complex and interconnected organized whole. The creatures of this domain are called eukaryotes.