Five Kingdom Classification

Introduction

No one knows when life began. For centuries, scientists have been trying to classify living organisms. However, according to the observation by Aristotle, 2400 years ago, the Earth’s biodiversity was that of plant or animal origin. His initial observation was then expanded in the later centuries with the discovery of new kingdoms. At present, there are five recognised kingdoms covering more than 8 million species surviving on the planet. 

Biological Kingdom

The biological kingdom is used in science to classify living organisms based on their ancestry through the years of evolution. It means all species living on this planet specifically come under these five groups. A few recent studies have split this classification into 6-7 groups. These groups have common ancestors and share some of their genes. Thus, it can be said that the living beings under one group belong to the same family tree. 

Five Kingdom Classification

Robert Whittaker, Ernst Haeckel, and Carl Woese are a few biologists who have made the classification broader. The five kingdoms they have classified are now widely used in the scientific field. 

According to Whittaker, organisms need to be divided into kingdoms based on their characteristics, such as mode of nutrition, cell structure, source of their nutrition, body organisation, interrelationship and reproduction. Now, we will take a look at the five kingdoms in detail. 

Kingdom Monera

It is a kingdom of microscopic living organisms. These organisms are unicellular and prokaryotic. They lack most membrane bound cell organelles and don’t come with a well-defined nucleus. A few of them have a cell wall, but others don’t. The cell wall is made of polysaccharides and amino acids. It is a group that is present in all habitats. A majority of the bacteria are heterotrophic and aerobic. Even though the archaea are anaerobic, their metabolism is chemosynthetic. 

The types of monerans are-

• Bacillus: These are rod-shaped bacteria
• Spirillum: These are spiral-shaped bacteria
• Coccus: These are spherical-shaped bacteria
• Vibrium: These are comma-shaped bacteria

Kingdom Fungi

Kingdom Fungi has multicellular, heterotrophic, and eukaryotic organisms. It includes moulds, yeast, all species of toadstools, and mushrooms. Their nutrition mode is saprophytic, since they have decaying matter as food. The cell walls of the Fungi Kingdom are made of Chitin. They reproduce through their spores. The Fungi Kingdom has multiple applications in the commercial and domestic fields. 

Fungi are filamentous, other than yeast. They have a structure which has a long and slender thread-like construction known as hyphae. A few hyphae are broken tubes that are packed with multinucleated cytoplasm. These are known as Coenocytic hyphae. A majority of the fungi are heterotrophic or saprophytes. Some fungi survive as symbionts but a few are parasites. A few fungi are known to survive in symbiotic association with algae, such as lichens. In contrast, others are associated with higher plant roots, such as mycorrhiza. 

Kingdom Protista

Kingdom Protista comprises unicellular organisms. They are eukaryotic organisms. They are the simplest forms of eukaryotes. Kingdom Protista has either a heterotrophic or autotrophic mode of nutrition. A few of them have flagella or cilia or pseudopodia for mobility. They reproduce through a process of zygote formation or cell fusion. This group is the most primitive among eukaryotes, and others are just its descendants. It includes eukaryotic organisms that are deemed to be precursors of plants, fungi, or animals. It is so heterogeneous that it becomes difficult to categorise them as the members of the group have very little in common. 

Yet, experts have categorised it into five main categories. 

• Dinoflagellates: These are generally marine and photosynthetic. Their colour depends on the primary pigments of their appearance. So, they might appear to be brown, blue, yellow, red, or green. They are considered to have given rise to plants
• Chrysophytes: It is a group of diatoms and golden algae. These are found in freshwater and marine habitats
• Slime Moulds: Slime moulds are saprophytic and are known to live on twigs and putrefying leaves. It gets its food from organic material. They can accumulate to form plasmodial slime moulds. They are believed to have given rise to Fungi
• Euglenoids: These live in fresh and motionless water. They don’t have cell walls. Rather there is a protein-rich layer called the pellicle
• Protozoans: Protozoans are heterotrophs that live in water as predators or parasites

Kingdom Animalia

Kingdom Animalia has evolved the most and has been divided into two more categories-

• Vertebrates
• Invertebrates

These animals are heterotrophic eukaryotes and multicellular, with aerobic respiration. They do not have cell walls. Kingdom Animalia gets their nutrition heterotrophically. They are indirectly or directly dependent on the plant for their food. Holozoic is their mode of nutrition. It encompasses food ingestion and the use of the internal cavity to digest food. 

Animals reproduce through sexual mode. They are also adept at locomotion. It is one of the most diverse kingdoms and consists of mammals, reptiles, fish, insects, birds, amphibians, annelids, and many more. 

Kingdom Plantae

The name makes it evident as to what the group comprises. Plants, trees, and other vegetarian species make up this kingdom. It is one of the oldest kingdoms. They are characterised by their multicellular, immobile, and eukaryotic nature. The Kingdom Plantae have chloroplast. 

A majority of these are autotrophic. However, some are also heterotrophic. They have cell walls that are made of cellulose. They are an essential part of life on Earth as they release oxygen through photosynthesis. Their reproduction can be asexual or sexual. 

Plants have two phases in their life, and these phases get altered with each other. These two phases are the haploid gametophytic phase and diploid saprophytic. However, the length of haploid and diploid phases might vary with plant groups. The phenomenon is known as alternation of generation. 

Conclusion

The five-kingdom classification that we have discussed here is the most efficient system. Previously, the classification was based on external characteristics. So, just two varying organisms had been grouped. But the present-day theory classified organisms into five kingdoms, called the five-kingdom classification. This remains the most accepted classification. But latest advances in the field of genetic research have led to new revisions, which have opened the gate for debate among science experts.