Five kingdom classification.

In the previous write up we came to  know about how the system of five kingdom classification evolved from two kingdom classification, through three and four kingdom classification  systems  and how the six  kingdom classification has come into being by a little advancement over the five kingdom classification.

 

R.H. Whittaker in 1969 introduced Five Kingdom classification. 

 

This system of classification holds its ground even today as the criteria chosen for the classification are logical and scientific. 

 

The five kingdoms as were proposed by Whittaker include 1. MONERA 2. PROTISTA 3. FUNGI 4. PLANTAE 5. ANIMALIA. This system of classification takes into account five criteria, which are as under: 

 

1. Complexity of the cell: On the basis of complexity of the cell, all living organisms are categorized into two groups which include prokaryotes and eukaryotes.

A. Prokaryotes: Those organisms which lack well developed nucleus i.e., nucleus lacking nuclear membrane and histone proteins and the genetic material is in circular form, such a nucleus is called as nucleoid. Prokaryotes also lack membrane bound cytoplasmic bodies such as mitochondria, endoplasmic reticulum, Golgi complex and chloroplasts etc. Such organisms include bacteria and blue green algae etc. Whittaker placed all such prokaryotic organisms in kingdom MONERA (1). 

 B. Eukaryotes : Those organisms which have well developed nuclei i.e.,  nuclear membranes and histone proteins are present, besides having the presence of membrane bound cytoplasmic organelles such as mitochondria, endoplasmic reticulum, Golgi complex and chloroplasts etc. were placed in eukaryotes.

 

2. Complexity of the organism: On the basis of  complexity of the organisms, the eukaryotes were categorised into two groups which included unicellular eukaryotes and multicellular eukaryotes. All the unicellular Eukaryotes were placed in Kingdom PROTISTA (2) e.g., Amoeba, Euglena and Paramecium.

 

3. Mode of nutrition: On the basis of mode of nutrition the organism are grouped as  autotrophs, which are either photoautotrophs or chemoautotrophs and heterotrophs, which are either saprozoic or holozoic. Autotrophs prepare their own food while photoautotrophs use sunlight as the source of energy for manufacturing food , the chemoautotrophs use energy released from the chemical reactions for synthesizing their own food . Heterotrophs can not synthesize their own food and thus depend upon other organisms for food. Those heterotrophs which take solid food are said to have the holozoic mode of nutrition where as those heterotrophs which absorb liquified form of food prepared by other organisms are said to have the saprozoic mode of nutrition.

 

4. Ecological role of the organism: On the basis of the ecological role played by the organisms in their environment, the  organisms are categorised as producers and consumers. Producers are those organisms which manufacture their own food where as consumers are heterotrophs which derive their food from producers and other consumers. The consumers are further subgrouped as herbivores, carnivores, omnivores and decomposers.

 

5.Evolutionary history i.e. phylogeny of the organism . As we know that the present day organisms are the outcome of the evolutionary process and have been evolved from the organisms of past . Thus they have a common ancestry and  are releated to each other. Organisms which evolved first were simple and primitive and those which evolved later  are complex hence the prokaryotes evolved first followed by unicellular eukaryotes and the multicellular organisms were last to evolve.

 

All multicellular eukaryotes which produce their own food (autotrophs/producers), have cell wall made up of cellulose,  are placed in Kingdom PLANTAE (3) . Plant cells also contain a large central vacoule and plastids which contain chlorophyll the pigment that plays a main role in the photosynthesis. The stored form of food in plants is Starch. They are non-motile.

Rest of multicellular eukaryotes which depend on other organisms for their food are heterotrophs  and are thus consumers . These heterotrophs are grouped into either Kingdom ANIMALIA or Kingdom  FUNGI. All holozoic organism which lack cell wall and chloroplasts are included in Kingdom ANIMALIA (4). Stored form of food in all animals is glycogen. 

 

Whereas the heterotrophic multicellular eukaryotic organisms which have saprozoic mode of nutrition i.e. decomposers are placed in the Kingdom FUNGI (5). The organisms belonging to this kingdom have cell wall made up of chitin and polysaccharides and the body of such organisms is called as thallus which consists of network of thread like structures called as hyphae which are collectively known as mycelium e.g. mushrooms, yeast, and mold.

 

Kingdom	Complexity of  Cell	Complexity of Organism	Mode of nutrition	Ecological role
MONERA	Prokaryotic	Unicellular	Autotrophic   or Heterotrophic	Producers or Consumers
PROTISTA	Eukaryotic	Unicellular	Autotrophic or Heterotrophic	Producers or Consumers
PLANTAE	Eukaryotic	Multicellular	Autotrophic	Producers
FUNGI	Eukaryotic	Multicellular	Heterotrophic  Saprozoic	Decomposers
ANIMALIA	Eukaryotic	Multicellular	Heterotrophic  Holozoic	Consumers