Five-Kingdom classification – Erakina

In 1959, Robert Whittaker proposed a five-kingdom system for the organism classification. These five kingdoms were classified based on modes of nutrition.

Biological classification

Biological classification involves the classification of organisms into categories based on nutrition, habitat, reproductive capabilities etc. Even after the popularisation of Whittaker’s five-kingdom classification, the following systems for biological classification were proposed-

• H.Copeland proposed the four-kingdom system which created space for the kingdom Monera. This preceded Whittaker’s five-kingdom classification.
• Carl Woese proposed the six-kingdom classification based on differences in RNA ribosomal structures. His classification created space for the separation of bacteria into two separate kingdoms- archaebacteria and eubacteria.
• T. Cavalier-Smith proposed the division of eubacteria into two kingdoms- negibacteria and posibacteria based on the Gram nature of the bacteria. This led to the creation of eight kingdoms.

Ranking system

The following taxonomic terms are employed in the ranking and classification of organisms. The ranks are as follows, with the domain being the highest and species being the lowest-

• Domain
• Kingdom 
• Division/Phylum
• Class 
• Order 
• Family 
• Genus 
• Species 

For example- Mangifera indica belongs to the domain Eukaryota, kingdom Plantae, phylum Spermatophyta, and subphylum Angiospermae, class Dicotyledonae, order Sapindales, family Anacardiaceae and finally genus Mangifera followed by species Mangifera indica.

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The five-kingdom classification: An overview

The classification of living organisms into five separate kingdoms was done by the following parameters-

• Mode of nutrition; whether the organism is autotrophic or heterotrophic.
• Cellular organisation; multicellular or unicellular.
• Cell type; organisms can have either prokaryotic or eukaryotic cells.
• Respiration can be either aerobic or anaerobic.
• Modes of reproduction can be sexual, asexual and through spore formation and distribution.
• Based on movement, organisms can be divided into two categories, motile and non-motile.

The five kingdoms of the biological classification system proposed by Robert Whittaker are-

• Kingdom Monera
• Kingdom Protista
• Kingdom Fungi
• Kingdom Plantae
• Kingdom Animalia

Kingdom Monera

This kingdom is composed of all unicellular, heterotrophic and autotrophic organisms. The cellular organisation of organisms in this kingdom is at the prokaryotic level. This kingdom can be further divided into-

• Eubacteria are also known as true bacteria.
• Archaebacteria are ancient organisms that can thrive even in extreme environmental conditions. 

Kingdom Protista

This kingdom is composed of the most primitive eukaryotic organisms. These organisms are heterotrophic eukaryotes that are the common ancestors of all other eukaryotic organisms. Protozoans are a common example of an organism belonging to this kingdom.

Kingdom Fungi

These organisms are known for their characteristic saprophytic nature. This kingdom is composed of organisms that are heterotrophic and eukaryotic. Fungal cell walls are made up of chitin. This makes them distinct from plants. Fungi reproduce through spores and respire aerobically. Common examples of fungal organisms include- yeasts, mushrooms and moulds.

Kingdom Plantae

Plants are essential for life as their metabolic processes involve the production of oxygen which is the most basic life requirement. Plants are characterised by their immobility. They are autotrophic, eukaryotic and aerobic. Reproduction in plants can be either sexual or asexual. The most distinct features of plants are the presence of chlorophyll in the chloroplasts in their cells and the presence of cellulose in their cell walls.

Kingdom Animalia

This is the kingdom we, homo sapiens, belong to. This kingdom is composed of eukaryotic, heterotrophic organisms that possess a multicellular organisation of cells and are characterised by mobility. Animals are found in a variety of habitats. Reproduction is sexual. This kingdom can be further divided into-

• Vertebrates 
• Invertebrates 

This kingdom is incredibly diverse and is considered to be the most evolved.

Conclusion

The five-kingdom classification proposed by Robert Whittaker is based on a set of parameters. This classification is widely accepted and followed all over the world. Biological classification enables the easy study of organism structures and phylogenetic relationships.