Five Kingdom Classification

The five kingdom classification is a system that  classifies living organisms into five groups based on their cell structure, body organization, mode of nutrition, mode of reproduction and phylogenetic relationships. The five kingdom classification system helps identify and understand organisms' characteristics, including their origins, growth patterns, reproduction, and structure.

1.0Introduction

Before the emergence of the five kingdoms classification system, organisms were traditionally categorised into just two kingdoms: plants and animals, a system proposed by Linnaeus. However, this simplistic approach couldn't adequately capture the diversity of life on Earth.

With the discovery of single-celled organisms, they were split between plants and animals: mobile forms grouped with animals as Protozoa, and algae and bacteria placed in the plant division Thallophyta or Protophyta. In response, Ernst Haeckel proposed a third kingdom, Protista, in 1866, acknowledging the need for a more comprehensive classification system.

In 1937, Chatton introduced a groundbreaking division into two empires, Eukaryota and Prokaryota, based on the presence or absence of a nucleus. Prokaryotes lack a nucleus and membrane-bound organelles, while eukaryotes possess these features, allowing for specialised cellular functions. Though initially met with scepticism, Chatton's concept gained traction over time, especially after Stanier and van Niel's advocacy in 1962.

Meanwhile, Herbert Copeland's 1938 classification suggested a separate kingdom for prokaryotes, eventually evolving into a four-kingdom system. However, the two-empire framework gradually gained acceptance, providing a more robust understanding of the fundamental divisions in the living world.

2.0Whittaker Five Kingdom Classification

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

3.0Basis of Five Kingdom Classification

Main characteristics used for five-kingdom system classifications are -

• Cellular Structure: Organisms are categorised based on the structure and complexity of their cells. This includes whether they are prokaryotic (lacking a true nucleus and membrane-bound organelles) or eukaryotic (possessing a true nucleus and membrane-bound organelles).
• Body Organisation: The overall body structure and organisation of organisms play a significant role in classification. This includes whether an organism is unicellular (composed of a single cell) or multicellular (consisting of multiple cells organised into tissues, organs, and systems).
• Mode of Nutrition: Organisms are classified based on how they obtain their nutrition. This can include autotrophic (producing their own food through processes like photosynthesis) or heterotrophic (relying on other organisms for food).
• Mode of Reproductive: Reproduction methods are considered in classification. This includes whether an organism reproduces sexually or asexually, as well as the complexity of reproductive structures and processes.
• Phylogenetic Relationships: Phylogenetic relationships refer to the evolutionary relationships between organisms, typically depicted in the form of a phylogenetic tree. These relationships are inferred based on shared characteristics, such as genetic sequences, morphological features, or biochemical traits.

4.0Merits And Demerits of Five Kingdom Classification

Merits of Five Kingdom Classification: 

The five-kingdom classification system has following demerits-

• Based on cell structure and organization-The five-kingdom classification system is based on the complexity of cell structure and organization of the body. 
• Separates unicellular and multicellular organisms-The five-kingdom classification system keeps unicellular and multicellular organisms separate. 
• Separates fungi from plants-The five-kingdom classification system separates fungi from plants based on their different modes of nutrition. 
• Shows phylogeny-The five-kingdom classification system shows the phylogeny of organisms. 

Demerits of five kingdom classification: 

The five kingdom classification system has following demerits- 

• Viruses are not included- Viruses are obligate parasites that are considered non-living and are not included in the five kingdom classification.
• Organisms that are similar are placed apart- For example, unicellular algae are placed in the kingdom Protista, while multicellular algae are placed in the kingdom Plantae.
• Organisms with different characteristics are grouped together- For example, Archaebacteria and Mycoplasma are both placed in the kingdom Monera, even though they differ in structure, composition, and physiology
• Symbiotic associations are not considered- For example, lichens are organisms that are formed by the symbiotic association between fungi and algae. 
• Slime molds are different from the rest of Protista- Slime molds located under Protista differ significantly from the rest of Protista. 

5.0Characteristics of Five Kingdoms

Kingdom Monera 

• Bacteria are the sole members of the Kingdom Monera.They are the most abundant organisms. Bacteria occurs almost everywhere. They also live in extreme environments such as hot springs, deserts, snow  and deep oceans where very few other organisms can survive. Many of them live in or on other organisms as parasites.
• Bacteria are grouped under four categories based on their shape: the spherical Coccus, the rod shaped Bacillus,the comma shaped Vibrio and the spiral shaped Spirillum. 

Features of kingdom monera:

• Organisms in Kingdom Monera are composed of prokaryotic cells, lacking a nucleus and membrane-bound organelles.
• Monerans inhabit a wide range of environments, including soil, water bodies (fresh and marine), extreme environments like hot springs, and even the bodies of other organisms as parasites or symbionts.
• Monerans can be autotrophic, obtaining energy through photosynthesis, or heterotrophic, obtaining energy from organic compounds.
• Reproduction in Monerans primarily occurs through asexual means such as binary fission, though some may engage in forms of genetic exchange like conjugation, transformation, and transduction.
• Kingdom Monera is divided into two main groups: Bacteria and Archaea. Bacteria are the more familiar group and can be further classified into various phyla based on differences in cell structure, metabolism, and habitat.

Kingdom Protista

The kingdom Protista is a group of organisms that are unicellular eukaryotes i.e. all unicellular eukaryotes are placed in kingdom protista, but the boundaries of this kingdom are not well defined.

Features of Kingdom Protista

• All members of Kingdom Protista are eukaryotic, meaning their cells have a nucleus and other membrane-bound organelles.
• Protists can be found in various aquatic environments, including freshwater, marine habitats, and moist terrestrial environments. Some protists are symbiotic, living in close association with other organisms.
• Protists exhibit diverse nutritional strategies. Some are photosynthetic autotrophs, capable of producing their own food through photosynthesis. Others are heterotrophic, feeding on organic matter or other organisms.
• Protists reproduce both sexually and asexually. Asexual reproduction can occur through binary fission, budding, or multiple fission. Sexual reproduction involves the fusion of gametes to form zygotes.
• Some protists are pathogens that cause human, animal, and plant diseases. For example, Plasmodium spp. Causes malaria, etc. 

Basis of protista classification:

This kingdom includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and protozoans. The Kingdom Protista is classified into three categories namely Plant-like Protists, Animal-like Protists and Fungi-like protists.

Kingdom Fungi

A fungus is a member of the group of eukaryotic organisms that includes microorganisms such as yeasts and moulds, as well as the more familiar mushrooms. This is a unique kingdom of heterotopic organisms. Kingdom fungi show great diversity in morphology and habitats.

Features of kingdom Fungi

• Fungi are composed of eukaryotic cells, meaning their cells have a true nucleus enclosed within a membrane, along with other membrane-bound organelles. Fungi have cell walls composed primarily of chitin, a tough, flexible polysaccharide. This sets them apart from plants, which have cell walls mainly made of cellulose.
• Fungi are cosmopolitan and occur in air, water, soil, and on animals and plants. They are primarily terrestrial. They prefer to grow in warm and humid places. 
• The body of a fungus is typically made up of a network of thread-like structures called hyphae. Hyphae collectively form a structure called mycelium, which is the main vegetative body of the fungus. The mycelium is usually hidden within its substrate, such as soil, wood, or other organic matter. The hyphae may be aseptate and multinucleate. Such a hypha is termed coenocytic. In most of the fungi, the mycelium is septate.
• Fungi are heterotrophic organisms, meaning they cannot produce their own food through photosynthesis like plants. Instead, they obtain nutrients by absorbing organic material from their environment. Fungi secrete enzymes that break down complex organic molecules into simpler compounds, which can then be absorbed by the fungus.
• Fungi reproduce both sexually and asexually. Asexual reproduction occurs through the production of spores, which are often released into the air. Sexual reproduction involves the fusion of specialised reproductive cells (gametes) from different mating types.

Basis of fungi classification:

The morphology of the mycelium, mode of spore formation and fruiting bodies form the basis for the classification of this kingdom  into various classes.

Kingdom Plantae

• Kingdom Plantae includes all eukaryotic chlorophyll-containing organisms commonly called plants. A few members are partially heterotrophic such as the insectivorous plants or parasites. Bladderwort and Venus fly trap are examples of insectivorous plants and Cuscuta is a parasite. 
• The plant cells have an eukaryotic structure with prominent chloroplasts and cell walls mainly made of cellulose. 
• Plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

Features of kingdom Plantae

• This kingdom consists of eukaryotic autotrophic organisms capable of producing their own food through photosynthesis. They use chlorophyll and other pigments to capture sunlight and convert it into chemical energy, primarily in the form of glucose. Their cell walls are made of cellulose, providing structural support and protection.
• The plant kingdom exhibits immense diversity, with plants found in almost every terrestrial habitat on Earth, ranging from deserts to rainforests and from polar to tropical regions. Kingdom Plantae includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
• Plants typically exhibit a life cycle alternating between haploid (gametophyte) and diploid (sporophyte) generations. This alternation involves the production of gametes (sperm and eggs) by the gametophyte generation and the production of spores by the sporophyte generation.

Basis of plant classification:

Kingdom Animalia

This kingdom is characterised by heterotrophic eukaryotic organisms that are multicellular and their cells lack cell walls. They directly or indirectly depend on plants for food. They digest their food in an internal cavity and store food reserves as glycogen or fat. Their mode of nutrition is holozoic – by ingestion of food.

Features of Kingdom Animalia

• Animals are composed of multiple cells organised into tissues and organs. Unlike plants, animals obtain nutrients by ingesting organic matter, which can produce their food through photosynthesis. Many animals can move at some stage of their life cycle, whether crawling, swimming, flying, or walking.
• Many animals can move at some stage of their life cycle, whether crawling, swimming, flying, or walking. 
• Animals typically have a nervous system that allows them to perceive and respond to their environment. This may range from simple nerve nets in jellyfish to complex brains in mammals.
• Most animals reproduce sexually, although some species can also reproduce asexually.

Basis of animal classification:

Inspite of differences in structure and form of different animals, there are fundamental features common to various individuals in relation to the arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory or reproductive systems. These features are used as the basis of animal classification

6.0Salient Features of Different Phyla in the Animal Kingdom