The Five Kingdom System of Classification
In order to suggest a better system of classification of living organisms, R.H. Whittaker (1969) an American Taxonomist divided all the organisms into 5 kingdoms based on their phylogenetic relationships. This classification takes into account the following important criteria.
Complexity of Cell structure - prokaryote to Eukaryote
Mode of nutrition - autotrophs and heterotrophs
Body organization -unicellular or multi-cellular
Phylogenetic or evolutionary relationship
The Five kingdoms are Monera, Protista, Fungi, Plantae and Animalia.
This kingdom includes all prokaryotic organisms i.e. mycoplasma, bacteria, actinomycetes(filamentous bacteria) and cyanobacteria (blue green Algae). They show the following characters.
They are microscopic. They do not possess a true nucleus. They lack membrane bound organelles.
Their mode of nutrition is autotrophic or heterotrophic. Some bacteria are autotrophic and are photosynthetic. i.e. they can synthesize their organic food in the presence of sunlight eg. Spirillum. Some bacteria are chemosynthetic i.e. they can synthesize their organic food by deriving energy from some chemical reactions. eg. Nitrosomonasand Nitrobacter.
Many other bacteria like Rhizobium, Azotobacter and Clostridium can fix atmospheric nitrogen into ammonia. This phenomenon is called Biological Nitrogen Fixation .
Some bacteria are parasites and others live as symbionts.
Some monerans like Archaebacteria can live in extreme environmental conditions like absence of oxygen (anaerobic), high salt condition, high temperature like 800c or above and highly acidic soils.
This kingdom includes eukaryotic unicellular mostly aquatic cells. They show the following characters.
They have a typical Eukaryotic cell organization.
They often bear cilia or flagella for locomotion. Most of them are photosynthetic autotrophs. They form the chief producers of food in oceans and in fresh water. All unicellular plants are collectively called as phytoplanktons and unicellular animals as zooplanktons. Phytoplanktons are photosynthetically active and have cell wall.
Zooplanktons are mostly predatory. They lack cell wall and show holozoic mode of nutrition as in Amoeba.
Some protists are parasitic. Some are symbionts while others are decomposers.
Euglena, a protozoan has two modes of nutrition. In the presence of sunlight it is autotrophic and in the absence of sunlight it is heterotrophic. This mode of nutrition is known as myxotrophic and hence they form a border line between plants and animals and can be classified in both.
This kingdom includes moulds, mushrooms, toad stools, puffballs and bracket fungi. They have eukaryotic cell organization. They show the following characteristics.
They are either unicellular or multi-cellular organisms.
Their mode of nutrition is heterotrophic since they lack the green pigment chlorophyll. Some fungi like Puccinia are parasites while others like Rhizopus are saprotrophic and feed on dead organic matter.
Their body is made up of numerous filamentous structures called hyphae.
Their cell wall is made up of chitin.
It includes all multi-cellular plants of land and water. Major groups of Algae, Bryophytes, Pteridophytes, Gymnosperms andAngiosperms belong to this kingdom. It shows the following characteristics.
The cells have a rigid cell wall made up of cellulose.
They show various modes of nutrition. Most of them are autotrophs since they have chlorophyll. Some plants are heterotrophs. For eg. Cuscuta is a parasite. Nepenthes and Drosera are insectivorous plants.
This kingdom includes all multi-cellular eukaryotic organisms. They are also referred to as metazoans. They show the following characteristic features.
All animals show heterotrophic mode of nutrition. They form the consumers of an ecosystem.
They have contractibility of the muscle cells.
They can transmit impulses due to the presence of nerve cells.
Some groups of animals are parasites eg. tapeworms and roundworms.
It shows the phylogenetic relationships among the organisms.
It is based on the complexity of the cell structure from prokaryotic to eukaryotic cell organization.
It is based on the complexity of body organization from unicellular to multi-cellular.
It is based on the modes of nutrition: autotrophic or heterotrophic mode of nutrition.
Chlamydomonas and Chlorella are included under the kingdom Plantae. They should have been included under kingdom Protista since they are unicellular.
Since living organisms exhibit great variety and diversity and also they have evolved through millions of years and there are many missing links between groups, it is very difficult to have a clear cut and well defined classification. Biological classification reflects the state of our knowledge. It changes as we acquire new information. By the 1970s molecular biologists realized that prokaryotes consist of two different and unrelated groups. To accommodate this new information three microbiologists, C.Woese,O.Kandler, and M.L Wheelis introduced a new classification scheme in 1990. They proposed that all organisms be divided into three major groups called domains: the Eucarya (containing all eukaryotes), the Bacteria (containing most familiar prokaryotes), and theArchaea (originally called archaebacteria and containing prokaryotes that live mostly in extreme environments.) This scheme is currently accepted by most biologists.
Classification will undoubtedly continue to change.