Biological Classification — Class 11 Biology

This chapter traces how classification systems evolved and presents Whittaker's Five Kingdom system. Aristotle gave an early scientific basis using simple morphology, and Linnaeus used a Two Kingdom system of Plantae and Animalia, which failed to separate prokaryotes from eukaryotes or autotrophs from heterotrophs. In 1969 R.H. Whittaker proposed five kingdoms: Monera, Protista, Fungi, Plantae and Animalia, using criteria such as cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships. Kingdom Monera contains the prokaryotic bacteria, including archaebacteria of extreme habitats and eubacteria such as cyanobacteria and mycoplasma; bacteria show the widest metabolic diversity. Kingdom Protista groups single-celled eukaryotes: chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans. Kingdom Fungi covers heterotrophic organisms with chitinous walls and a body of hyphae forming a mycelium, classed into Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes. Kingdom Plantae includes chlorophyll-bearing plants and Kingdom Animalia the multicellular heterotrophs lacking cell walls. The chapter ends with acellular entities outside the five kingdoms: viruses (nucleoprotein particles with RNA or DNA), viroids (free infectious RNA), prions (misfolded proteins) and lichens (symbiotic associations of algae and fungi).