THE KINGDOMS OF NATURE AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The five kingdoms of nature correspond to a classification that groups all organisms into five groups called kingdoms. These are the Protista kingdom, the Monera kingdom, the Fungi kingdom, the Plantae kingdom, and the Animalia kingdom.

This classification was proposed by Robert Whittaker in 1968 and groups living organisms into five kingdoms, according to their ecological and trophic characteristics, common to the members of each kingdom.

Protista, Plantae, Animal, Fungi and Monera

Whittaker was an ecologist recognized for his work in taxonomy. Since 1957, he was already making direct criticisms of the classification of living organisms through a dichotomy that only distinguished between animals and plants.

In 1959 Whittaker published one of his first proposals for a model for the classification of living organisms. This model included four kingdoms that grouped plants, animals, fungi and a new kingdom that Whittaker dubbed "the protists."

Finally, in 1968 Robert Harding Whittaker created a system of five kingdoms based on their trophic and structural characteristics, these were:

- Kingdom M onera: prokaryotic organisms (bacteria, archaea)

- P rotist Kingdom: unicellular eukaryotic organisms (protozoa)

- Kingdom F ungi: eukaryotic saprophytic organisms (fungi)

- Kingdom P lantae: photosynthetic eukaryotic organisms (plants)

- Kingdom A nimalia: eukaryotic organisms, both invertebrates and vertebrates (animals, fish, etc.)

Representation of the tree of life with the five kingdoms: Animalia, Plantae, Protista and Fungi (eukaryotes) and Monera (bacteria and prokaryotic archaea) (Source: Maulucioni and Doridí via Wikimedia Commons)

Whittaker's proposed five kingdom classification system was reported in all biology, ecology, and biodiversity textbooks in the 20th century; some books even include it for simplicity to understand the groupings.

The 5 kingdoms of nature

Monera Kingdom

In this kingdom the most "simple and primitive" organisms were grouped: eubacteria and archaebacteria. This group included unicellular, sessile and mobile organisms, with a cell wall, without chloroplasts or any other organelle and without a nucleus.

The taxonomists of the time cataloged within this kingdom all organisms that measured between 3 and 5 micrometers, and that had a free circular DNA (not enclosed in a membrane).

The predominant reproduction of these organisms is asexual and occurs by binary fission and budding. Within the monera kingdom, eubacteria (true bacteria), archaebacteria, and green algae or cyanobacteria were included.

The name "Monera" comes from the Greek word "moneres" which means simple or solitary. This was used for the first time by Ernst Haeckel in 1866 when he raised the organization of life with evolutionary bases.

Ernst Haeckel cataloged all the microscopic organisms within the Monera kingdom and placed them at the beginning of his tree of life scheme, referring to these as the least evolved organisms of all.

The main characteristic of the Monera kingdom was assigned by Edouard Chatton in 1929 who, when observing some of these organisms under a microscope, realized that they had no nucleus. Fred Barkley, in 1939, used the term "Monera" to refer to prokaryotes.

protist kingdom

Composition of photographs of organisms belonging to the Prostate kingdom (Source: Respectively: Claire Fackler, CINMS, NOAA, User: Wiedehopf20, Frank Fox, Patrick De Wever, CDC / Dr. Stan Erlandsen, Jacob Lorenzo-Morales, Naveed A. Khan and Julia Walochnik, Koeh-034.jpg: Franz Eugen Köhler, Köhler's Medizinal-Pflanzen, Urmas Tartes via Wikimedia Commons)

The Protista kingdom, also called Protoctista, included unicellular eukaryotic organisms, characterized by the use of cilia or flagella for their locomotion (although some move amoeboid). These organisms generally do not have a cell wall, but do have nuclei and other eukaryotic organelles. They are organisms that cannot be included in any of the other kingdoms.

This kingdom was proposed for the first time by Whittaker in 1959 and its name derives from the Greek "Protoctista" which means "primordial", "first of the first" or "first creatures". Most of the species in this kingdom are between 2 and 200 μm in size.

A great variety of feeding methods can be found in this group, the organisms can be heterotrophic, autotrophic, saprophytic, phagocytic, holozoic or parasitic.

The kingdom includes a wide variety of different organisms. It has been estimated that between 100 and 200 million different species belong to this kingdom, of which only 30 million have been described.

Many taxonomists define as a general rule that organisms belonging to the Protista group lack a level of tissue organization, despite the fact that they can be found in colonies.

The first classification of the Protista kingdom included the Archaeplastida or Primoplantae, Stramenopiles or Heterokonta, Alveolata, Rizharia, Excavata, Amoebozoa, Opisthokonta, among others.

fungi kingdom

Photograph of organisms belonging to the Fungi kingdom (Source: Ryan Hodnett via Wikimedia Commons)

In this kingdom all eukaryotic organisms characterized by a chitin cell wall were grouped. They are heterotrophic organisms and they absorb their food by secreting enzymes that break it down. These organisms are not photosynthetic and have filamentous bodies made up of hyphae.

The word "fungi" comes from the plural of the Latin word "fungus" which means mushroom. Today this kingdom is also recognized as the Mycota kingdom.

Fungi, in general, can vary in size from several microns to several meters in length. They can have sexual and asexual reproduction. Some fungi are found forming symbiotic associations with algae, trees, and other organisms.

The Fungi kingdom was also proposed by Whittaker in 1959, in order to permanently separate them from the plants. More than 150 thousand different species have been grouped in this kingdom, including all fungi, mushrooms, molds and yeasts.

Most organisms in the Fungi kingdom are decomposers, some are highly pathogenic parasites, but others are an important part of the diet of animals. For example, yeasts are used to make bread and beer are very important to man.

The antibiotics we use to fight bacterial infections come from fungi, an example is penicillin, which was discovered by Alexander Fleming in 1928 in fungi of the genus Penicillium.

Currently, within the Fungi kingdom the Basidiomycota, Ascomycota, Glomeromycota, Zygomycota and Chytridiomycota are cataloged.

Kingdom Plantae

This is, along with the Animalia kingdom, one of the longest proposed kingdoms. It is made up of autotrophic eukaryotic organisms, which obtain their energy from photosynthesis (from sunlight); with cellulose cell wall, chloroplasts and the majority of sessile character.

This group of organisms is present in almost all ecosystems on the planet, with the exception of the north and south poles.

The first to classify plants as a separate kingdom was, in 1735, Carolus Linnaeus in his famous publication Systema naturae. There he made the taxonomic classification of three kingdoms: the animal, the vegetable and the mineral.

Linnaeus published his most impactful work "Botanical Philosophy" in 1751, in which he described the sexual reproduction of plants and gave names to the parts that make up the flowers.

Formerly the kingdom Plantae brought together two groups: the Biliphytas, which included the unicellular algae and the red algae, and the Chlorobiotes, represented by the Chlorophytas and the Streptophytas.

Currently plants are divided into two large groups: Gymnosperms (plants without flowers) and Angiosperms (plants with flowers). Between the two groups they bring together about 50 different orders that include approximately 460 different families of plants.

Animalia Kingdom

All organisms classified within this group are heterotrophic (they acquire their food by ingestion), they lack a cell wall and move through cilia, flagella or tissues with specialized contractile proteins for this purpose.

It is estimated that within the Animalia kingdom about 2 million different species are classified, classified according to the presence or absence of bones, such as vertebrates and invertebrates.

Unicellular protozoa are perhaps the simplest animal-like organisms. These perform all the basic life functions of an animal, but at the unicellular level.

Some of the most prominent phyla within the Animalia kingdom are Porifera, Cnidaria, Acanthocephala, Anelida, Arthropoda, Brachiopoda, Bryozoa, Cordata, Echinodermata, Mollusca, Nematoda, among many others.

Today zoologists recognize 32 different multicellular animal phyla, with their own archetypes and organization models, even with a set of biological properties that distinguish them from each other.

The members of the Animalia kingdom are distributed in all the ecosystems of the biosphere and can have surprisingly variable sizes, with great morphological diversity.

Current ranking

At present all living organisms are grouped into three domains known as Eukarya, Archaea, and Bacteria. This classification was proposed by Woese and Fox in 1977, when comparing the coding genes for 16S ribosomal RNA with the use of molecular biology tools.

The study by Woese and Fox divided what was formerly known as Monera into two separate domains, that of eubacteria and that of archaebacteria; but it brought together all eukaryotic organisms in the Eukaryotic kingdom.

In the Eubacteria domain, cyanobacteria and heterotrophic bacteria are grouped. The eukaryotic domain (currently called Eukarya) groups fungi, animals, plants, chromists, alveolates, and rhinophytes, among others.

Extremophilic organisms are generally found in the Archaeobacteria domain (currently called Archaea).

Studies after that of Woese and Fox delved into the classification of the domains of nature, determining that these were separated into three different groups but coming from the same common ancestor (of monophyletic origin).

References

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