Five-Kingdom Classification

Presently, the planet Earth is home to more than 8 million species, and continuously the new ones keep being discovered. Numerous scientists have tried to classify these living organisms for centuries. In the 19th and 20th centuries, the observation by the Greek philosopher Aristotle was expanded. Around 2400 years ago, Aristotle suggested that the planet’s biodiversity was either of animals or can be described as a classification of kingdom Plantae. Other biologists like Ernst Haeckel, Robert Whittaker, and Carl Woese also tried to classify living organisms into different Kingdoms. New kingdoms were discovered, and eventually, the Five Kingdom Classification was given. 

Who proposed the Five Kingdom Classification of living organisms?

In 1969, Robert Whittaker was the biologist who proposed the Five Kingdom Classification. The Kingdoms in this classification were classified based on cell structure, thallus organisation, mode of nutrition, reproduction, and phylogenetic relationships. 

The Five Kingdom Classification

Earlier, the organisms were classified into just two kingdoms- the classification of kingdom Plantae and animals. The classification of plants kingdom included every living organism that did not eat or move and grow continuously throughout their lives. The animal kingdom during this time included organisms that moved, ate, and stopped growing after a specific size.

• H. Whittaker proposed the Five Kingdom Classification, and this classification made it easier to classify organisms into five different kingdoms-

1. Monera
2. Protista 
3. Fungi
4. Plantae 
5. Animalia

Classification basis for Five Kingdom Classification

The five kingdoms in this widely accepted classification consist of species of similar characteristics concerning their growth and functioning. The organisms are divided into the five kingdoms based on their general features like:

1. Cell type: Organisms can have either prokaryotic cells (cells lacking membrane) and be prokaryotes or have eukaryotic cells (a membrane covers genetic material) and be eukaryotes. The kingdom Monera is the only kingdom that consists of prokaryotes, as the other four kingdoms have eukaryotic organisms. 
2. Cell wall: The kingdoms Monera, Fungi, and Plantae consist of organisms that have cell walls in their cells. Some of the Protists also have cell walls. But, the cells of the organisms of the Animalia kingdom do not have cell walls.
3. Nuclear membrane: The organisms under the Protists kingdom have cells without a nuclear membrane, while the others have a nuclear membrane. 
4. Cell organization: This characteristic divides organisms into unicellular and multicellular. 
5. Nutrition mode: The Plantae kingdom consists of autotrophs, i.e., they make their food. Fungi and Animalia kingdom consist of heterotrophs, i.e., organisms under this kingdom depend on others for food. Monera and Protista consist of both- autotrophs and heterotrophs. 

Monera

The kingdom Monera of the five classification system categorizes bacteria underneath it has the following characteristics:

1. It contains all microscopic living organisms. 
2. Kingdom Monera groups the prokaryotes together. 
3. The monerans can be found in all habitats.
4. They are single-celled organisms with an absence of a well-defined nucleus. 
5. The cells of monerans have cell walls formed from amino acids and polysaccharides. 
6. The mode of nutrition of monerans can be either autotrophic or heterotrophic.

Based on their shape, bacteria are divided into four categories- cocci, bacilli, spirilla, and vibrio. 

Protista

The general characteristics: 

1. The cell type of the Protists is eukaryotic.
2. The organisms under Protista are unicellular. 
3. These organisms usually use cilia, flagella, or amoeboid movement to move. 
4. It has both autotrophs and heterotrophs. 
5. The presence of a nuclear membrane marks the kingdom. 
6. Some Protists have cell walls. 
7. Cell fusion or zygote formation is used for sexual reproduction by Protists.

The kingdom Protista is further divided into Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, and Protozoans. 

Fungi

General characteristics of Fungi are:

1. The kingdom contains eukaryotes. 
2. A non-cellulosic cell wall made of chitin and polysaccharides is present in species of this kingdom. 
3. A nuclear membrane is present in the organism’s cells.
4. Fungi consist of heterotrophs, which can be parasitic or saprophytes. 
5. Few Fungi are classified as symbionts. The symbionts living with algae are called lichens. At the same time, the ones living in association with higher plants are known as mycorrhiza.
6. Fungi species contain slender hyphae with long thread-like constructions. The mycelium is a web of hyphae. 
7. Some hyphae are unbroken tubes with multinucleated cytoplasm called coenocytic hyphae. 

Plantae

The features of the classification of plants kingdom are: 

1. The classification of kingdom Plantae consists of eukaryotes that have chloroplasts. 
2. The kingdom mostly has autotrophs, but there are certain exceptions. 
3. Plants have a cellulosic cell wall with the presence of a nuclear membrane.
4. They are multicellular. 
5. The diploid saprophytic and the haploid gametophytic phases have two different life phases.

Animalia

This kingdom has the following features: 

1. Animals are eukaryotes. 
2. They do not have cell walls.
3. Unlike the classification of kingdom Plantae, there are heterotrophs underneath the Animalia kingdom. 
4. Most species of this kingdom are adept at locomotion. 
5. The reproduction mode for animals is sexual. 

Conclusion

Before R, H, and Whittaker, numerous biologists proposed their classification systems. The Five Kingdom Classification system divides the organisms into five kingdoms- Monera, Protista, Fungi, Plantae, and Animalia. The Two Kingdom Classification, which was the classification of Kingdom Plantae and Animalia, followed before this. But, as soon as the Five Kingdom Classification was proposed, it was widely accepted, and it eradicated the limitations of the Two Kingdom Classification.