Biological ClassificationMind Map

The need for classification arose because millions of organisms differ widely in structure, nutrition and reproduction. Early two-kingdom classification placed organisms only in Plantae and Animalia, but it could not properly classify bacteria, fungi, euglenoids and slime moulds. R.H. Whittaker proposed the Five Kingdom Classification in 1969: Monera, Protista, Fungi, Plantae and Animalia. His system used five major criteria: cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationships. This system was better because it separated prokaryotes from eukaryotes, unicellular eukaryotes from multicellular forms, and fungi from plants due to their absorptive nutrition and chitinous wall.

Monera includes all prokaryotic organisms, mainly bacteria. They are the most abundant microorganisms and occur in soil, water, air, extreme habitats and inside other organisms. Monerans lack a true nucleus and membrane-bound organelles. Their genetic material is naked circular DNA in the nucleoid region, and many possess plasmids. Bacteria show varied nutrition: photosynthetic autotrophic, chemosynthetic autotrophic, saprophytic, parasitic or symbiotic. Major NCERT groups include archaebacteria, eubacteria, cyanobacteria and mycoplasma. Archaebacteria live in extreme habitats such as hot springs, salty areas and marshes. Eubacteria are true bacteria, cyanobacteria are photosynthetic, and mycoplasma are the smallest living cells without cell walls.

Protista includes mostly unicellular eukaryotic organisms. It acts as a connecting kingdom because protists show plant-like, animal-like and fungus-like features. They possess a true nucleus and membrane-bound organelles, unlike Monera. Protists live mostly in aquatic or moist environments and may be photosynthetic, heterotrophic or mixotrophic. NCERT groups include chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans. Chrysophytes include diatoms and golden algae; dinoflagellates are often marine and may cause red tides; euglenoids show both plant and animal characters; slime moulds form plasmodium and fruiting bodies; protozoans include amoeboid, flagellated, ciliated and sporozoan forms.

Fungi are eukaryotic, heterotrophic organisms that absorb soluble organic food from their surroundings. Most fungi are multicellular and filamentous, forming hyphae that collectively make a mycelium, while yeast is unicellular. Their cell wall contains chitin and polysaccharides. Fungi may be saprophytes, parasites or symbionts, as seen in lichens and mycorrhiza. Reproduction occurs by vegetative, asexual and sexual methods. NCERT classifies fungi into Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes based on mycelium, spores and fruiting structures. Important NEET concepts include aseptate/coenocytic hyphae, conidia, ascospores, basidiospores, dikaryophase, imperfect fungi and economic roles such as antibiotics, fermentation, food and diseases.

Viruses, viroids and prions are infectious agents not included in Whittaker’s five kingdoms because they are acellular. Viruses consist of genetic material, either DNA or RNA, enclosed in a protein coat called capsid. They are inert outside host cells but multiply inside living cells using host machinery. Viroids are smaller than viruses and consist only of infectious naked RNA without a protein coat; the potato spindle tuber disease is a classic example. Prions are infectious protein particles associated with neurodegenerative diseases. Lichens are not acellular; they are symbiotic associations between an algal/cyanobacterial partner and a fungal partner. They are ecologically important as pioneer organisms and indicators of air pollution.