Biological Classification

• There have been many attempts to classify organisms, but early attempts were not using criteria that were useful
• Earlier classification was an attempt to classify organisms for our own use.
• Aristotle attempted to classify organisms based on simple morphological characteristics.
• Aristotle divided animals into those that had red blood and those that did not

Two Kingdom Classification

• This classification included plants and animals
• This system did not distinguish between eukaryotic and multicellular organisms or green algae and non-photosynthetic fungi
• It was determined that placing organisms into plants and animals was inadequate as the number of organisms was large

Five Kingdom Classification

• R.H. Whittaker proposed this system.
• The kingdoms defined by him were named Monera, Protista, Fungi, Plantae, and Animalia.
• The classification criteria used by him include cell structure, body organization, mode of nutrition, reproduction, and phylogenetic relationships. Table 2.1 gives a comparative account of different characteristics of the five kingdoms.

Comparing Kingdoms

• Monera are prokaryotic
• Protista, Fungi, Plantae, and Animalia are Eukaryotic
• Monera cell walls are noncellulosic (polysaccharide + amino acid)
• Protista cell wall is present in some
• Fungi cell wall is present with chitin
• Plantae cell wall is present cellulose
• Animalia cell wall is absent
• The nuclear membrane is absent in Monera but present in the other 4 kingdoms
• The body organization for Monera and Protista is cellular
• The body organization for Fungi is Multicullar/ loose tissue
• The body organization for Plantae is Tissue/ organ
• The body organization for Animalia is Tissue/organ/organ system
• The Mode of nutrition for Monera is autotrophic (chemosynthetic and photosynthetic) and Heterotrophic (saprophytic/parasitic)
• The Mode of nutrition for Protista is Autotrophic (photosynthetic) and Heterotrophic
• The Mode of nutrition for Fungi is Heterotrophic (Saprophytic/Parasitic)
• The Mode of nutrition for Plantae is Autotrophic (photosynthetic)
• The Mode of nutrition for Animalia is Heterotrophic (Holozoic/ Saprophytic etc.)

Three-Domain System

• The three-domain system has also been proposed, dividing the Kingdom Monera into two domains.
• The remaining eukaryotic kingdoms are in the third domain giving a six kingdom classification
• Earlier classification systems included bacteria, blue green algae, fungi, mosses, ferns, gymnosperms, and the angiosperms under 'Plants'.
• The character that unified plants was that all the organisms included had a cell wall in their cells.
• This placed together groups which widely differed in other characteristics.
• The earlier system brought together the prokaryotic bacteria and the blue green algae (cyanobacteria) with other groups which were eukaryotic.
• It also grouped together the unicellular organisms and the multicellular ones, say, for example, Chlamydomonas and Spirogyra were placed together under algae
• The classification did not differentiate between the heterotrophic group-fungi, and the autotrophic green plants, though the fungi had chitin in their walls composition