0The first person to scientifically classify plants and animals was Aristotle.
He classified plants into trees, herbs and shrubs based on simple morphological characters.
He classified animals into two groups, red-blooded and non red-blooded.
Two kingdom classification-
The two kingdom system of classification was given by Carolus Linnaeus.
He divided all organisms into two groups- Kingdom Animalia and Kingdom Plantae. This system only relied on gross morphology and did not take into account other characters such as structure, nature of cell wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships etc.
Five kingdom classification
Five kingdom classification was given by R.H. Whittaker. He classified all organisms into 5 groups-
Kingdom Monera
Kingdom Protista
Kingdom Fungi
Kingdom Plantae
Kingdom Animalia
The criteria used to classify organisms into five kingdoms are-
1. Cell structure
Based on cell structure, organisms can be divided into two groups, as prokaryotes (do not contain nucleus) and eukaryotes (contain nucleus).
2. Body organization
Based on body organzation, organisms can be classified as single-celled (unicellular) or Multicellular.
Kingdom Monera contains all prokaryotic unicellular organisms and Kingdom Protista contains Unicellular Eukaryotes.
3. Mode of Nutrition
Kingdom Plantae is the only kingdom that contains Autotrophic organisms i.e. organisms that can synthesize their own food from inorganic nutrients. Plants do this by photosynthesis.
Kingdom Monera contains autotrophic organisms that can be photoautotrophs or chemoautotrophs (use chemical reactions to synthesise organic nutrients from inorganic substrates) aand heterotrophs that are can either be parasitic or derive their nutrients from dead organic matter.
Heterotrophs are organisms that can not synthesise their own food and rely on other sources for their nutritional needs e.g. all animals.
4. Cell wall
Nature of cell wall is one of the defining feature of various kingdoms. Kingdom Monera contains organisms that have polyaminogycans in their cell wall, Fungi have chitin, Plantae contain cellulose-based cell wall while Animal cells do not contain a cell wall.
