The system of assembling organisms into groups or sets on the basis of likenesses and variances is called classification. It simplifies the study of a wide variety of organisms in a very systematic manner.

R.H. Whittaker proposed the five-kingdom classification in 1969. This classification was based upon certain characters like mode of nutrition, thallus organization, cell structure, phylogenetic relationships and reproduction.  This form of kingdom classification includes five kingdoms Monera, Protista, Fungi, Plantae and Animalia.

Also Read: Taxonomy

Five Kingdom Classification

The five-kingdom classification that we see today was not the initial result of the classification of living organisms. Carolus Linnaeus first came up with a two-kingdom classification, which included only kingdom Plantae and kingdom Animalia.

The two-kingdom classification lasted for a very long time but did not last forever because it did not take into account many major parameters while classifying. There was no differentiation of the eukaryotes and prokaryotes; neither unicellular and multicellular; nor photosynthetic and the non-photosynthetic.

Putting all the organisms in either plant or animal kingdom was insufficient because there were a lot of organisms which could not be classified as either plants or animals.

All this confusion led to a new mode of classification which had to take into account cell structure, the presence of cell wall, mode of reproduction and mode of nutrition. As a result, R H Whittaker came up with the concept of the five-kingdom classification.

The five-kingdom classification of living organisms included the following kingdoms:

Kingdom Monera

Bacteria are categorized underneath the Kingdom Monera.

Features of Monerans

They possess the following important features:

• Bacteria occur everywhere and they are microscopic in nature.
• They possess a cell wall and are prokaryotic.
• The cell wall is formed of amino acids and polysaccharides.
• Bacteria can be heterotrophic and autotrophic.
• The heterotrophic bacteria can be parasitic or saprophytic. The autotrophic bacteria can be chemosynthetic or photosynthetic.

Types of Monerans

Bacteria can be classified into four types based on their shape:

• Coccus (pl.: cocci) – These bacteria are spherical in shape
• Bacillus (pl.: bacilli) – These bacteria are rod-shaped
• Vibrium (pl.: vibrio) – These bacteria are comma-shaped bacteria
• Spirillum (pl.: spirilla) – These bacteria are spiral-shaped bacteria

Monera has since been divided into Archaebacteria and Eubacteria.

Kingdom Protista

Features of Protista

Protista has the following important features:

• They are unicellular and eukaryotic organisms.
• Some of them have cilia or flagella for mobility.
• Sexual reproduction is by a process of cell fusion and zygote formation.

Sub-groups of Protista

Kingdom Protista is categorized into subsequent groups:

• Chrysophytes: The golden algae (desmids) and diatoms fall under this group. They are found in marine and freshwater habitats.
• Dinoflagellates: They are usually photosynthetic and marine. The colour they appear is dependent on the key pigments in their cells; they appear red, blue, brown,  green or yellow.
• Euglenoids: Most of them live in freshwater habitation in motionless water. The cell wall is absent in them, instead, there is a protein-rich layer called a pellicle.
• Slime Moulds: These are saprophytic. The body moves along putrefying leaves and twigs and nourishes itself on organic material. Under favourable surroundings, they form an accumulation and were called Plasmodial slime moulds.
• Protozoans: They are heterotrophs and survive either as parasites or predators.

Kingdom Fungi

The kingdom fungi include moulds, mushroom, yeast etc. They show a variety of applications in domestic as well as commercial purposes.

Features of Kingdom Fungi

• The fungi are filamentous, excluding yeast (single-celled).
• Their figure comprises slender, long thread-like constructions called hyphae. The web of hyphae is called mycelium.
• Some of the hyphae are unbroken tubes which are jam-packed with multinucleated cytoplasm. Such hyphae are labelled Coenocytic hyphae.
• The other type of hyphae has cross-walls or septae.
• The cell wall of fungi is composed of polysaccharides and chitin.
• Most of the fungi are saprophytes and are heterotrophic.
• Some of the fungi also survive as symbionts. Some are parasites. Some of the symbiont fungi live in association with algae, like lichens. Some symbiont fungi live in association with roots of higher plants, as mycorrhiza.

Kingdom Plantae

Features of Kingdom Plantae

• The kingdom Plantae is filled with all eukaryotes which have chloroplast.
• Most of them are autotrophic in nature, but some are heterotrophic as well.
• The Cell wall mainly comprises cellulose.
• Plants have two distinct phases in their lifecycle. These phases alternate with each other. The diploid saprophytic and the haploid gametophytic phase. The lengths of the diploid and haploid phases vary among dissimilar groups of plants. Alternation of Generation is what this phenomenon is called.

Kingdom Animalia

Features of Kingdom Animalia

• All multicellular eukaryotes which are heterotrophs and lack cell wall are set aside under this kingdom.
• The animals are directly or indirectly dependent on plants for food. Their mode of nutrition is holozoic. Holozoic nutrition encompasses ingestion of food and then the use of an internal cavity for digestion of food.
• Many of the animals are adept for locomotion.
• They reproduce by sexual mode of reproduction.

Also Read: Kingdom Monera, Protista and Fungi

The five-kingdom classification of living organisms took a lot into consideration and is till now the most efficient system.

The older system of classification was based only on one single characteristic according to which two highly varied organisms were grouped together. For example, the fungi and plants were placed in the same group based on the presence of the cell wall. In the same way, unicellular and multicellular organisms were also grouped together.

Therefore, all the organisms were classified again into the five kingdoms known as the five-kingdom classification, starting with Monera, where all the prokaryotic unicellular organisms were placed together.

Following that, all the eukaryotic unicellular organisms were placed under the kingdom Protista.

The organisms were then classified based on the presence and absence of a cell wall.  The ones without the cell wall were classified under kingdom Animalia and the ones with cell wall were classified under kingdom Plantae.

The organisms under kingdom Plantae were further classified into photosynthetic and non-photosynthetic, which included Plantae and fungi respectively.

This system of classification of living organisms is better than following the older classification of plants and animals because it eradicated the confusion of putting one species in two different kingdoms.

Also Read: Basis of Biological Classification

For more information on the five-kingdom classification or any other kingdom classification of organisms, keep visiting BYJU’S website or download BYJU’S app, for further reference.