Introduction
Early naturalists classified organisms into just two groups — plants and animals. As microscopes revealed a vast microbial world and biochemistry uncovered fundamental differences at the cellular level, this two-kingdom system proved inadequate. Modern classification systems reflect evolutionary relationships and cellular organisation.
History of Classification Systems
Two-Kingdom System (Linnaeus)
Divided all life into Plantae and Animalia. Problems: fungi, bacteria, and protists did not fit neatly.
Five-Kingdom System (R.H. Whittaker, 1969)
Currently followed in NCERT Class 11. Criteria used: cell structure (prokaryote/eukaryote), body organisation (unicellular/multicellular), nutrition mode (autotrophic/heterotrophic/saprotrophic), and reproduction.
- 1.The five kingdoms are:
- 2.Monera — prokaryotes (bacteria, cyanobacteria)
- 3.Protista — unicellular eukaryotes
- 4.Fungi — absorptive heterotrophs with cell walls of chitin
- 5.Plantae — multicellular autotrophs (cell walls of cellulose)
- 6.Animalia — multicellular heterotrophs without cell walls
Kingdom Monera
- All prokaryotes (no membrane-bound nucleus or organelles)
- Includes Bacteria and Cyanobacteria (blue-green algae)
- Cell wall present in most (peptidoglycan/murein in bacteria)
- Nutritional diversity: autotrophs (photosynthetic, chemosynthetic) and heterotrophs
- Archaebacteria: ancient bacteria in extreme environments (halophiles — high salt; thermoacidophiles — hot acid springs; methanogens — produce methane in gut of ruminants)
- Eubacteria: true bacteria; cell wall of peptidoglycan. Includes · Mycoplasma · (no cell wall — smallest living cell)
- Cyanobacteria: photosynthetic, contain chlorophyll; some fix nitrogen ( · Nostoc · , · Anabaena · )
- Mycoplasma: smallest known cells; no cell wall; obligate parasites
Kingdom Protista
- Unicellular eukaryotes; mostly aquatic
- Chrysophytes: diatoms and golden algae; cell wall of silica (diatomaceous earth); major component of phytoplankton
- Dinoflagellates: mostly marine; two flagella; red tides caused by · Gonyaulax ·
- Euglenoids: · Euglena · ; no cell wall but protein pellicle; mixotrophic (photosynthetic in light, heterotrophic in dark); reservoir of paramylon
- Slime moulds: saprotrophic; form plasmodium; produce spores with true cell walls
- Protozoans: heterotrophic; · Amoeba · (pseudopodia), · Paramecium · (cilia), · Trypanosoma · (flagella — causes sleeping sickness), · Plasmodium · (sporozoans — causes malaria)
Kingdom Fungi
- Eukaryotic, mostly multicellular (except yeast — unicellular)
- Cell wall of chitin; nutrition by absorption (saprotrophic, parasitic, or symbiotic)
- Body = mycelium (network of thread-like hyphae)
- Hyphae may be coenocytic (no cross-walls, multiple nuclei) or septate (with cross-walls)
- Phycomycetes: aquatic/on decaying wood; coenocytic hyphae; · Mucor · , · Rhizopus · (bread mould), · Albugo · (white rust of mustard)
- Ascomycetes (Sac fungi): · Aspergillus · , · Penicillium · , · Neurospora · (used in biochemical genetics), · Saccharomyces · (yeast); sexual spores in asci; conidia for asexual reproduction
- Basidiomycetes (Club fungi): mushrooms, puffballs, · Agaricus · , · Ustilago · (smut), · Puccinia · (rust); club-shaped basidia; no asexual spores
- Deuteromycetes (Imperfect fungi): sexual stage unknown; · Alternaria · , · Colletotrichum · , · Trichoderma ·
- Lichens: symbiosis between algae (phycobiont) and fungi (mycobiont); excellent pollution indicators
Kingdom Plantae
- Multicellular, eukaryotic, autotrophic (photosynthetic)
- Cell wall of cellulose; chlorophyll in chloroplasts
- Show alternation of generations (gametophyte and sporophyte phases)
- Includes all green plants, mosses, ferns, gymnosperms, angiosperms (covered in Ch. 3)
Kingdom Animalia
- Multicellular, eukaryotic, heterotrophic
- No cell wall; cells have glycocalyx
- Holozoic nutrition (ingestion); most show locomotion
Viruses, Viroids, and Prions (Outside the Five Kingdoms)
- Viruses: non-cellular; nucleoprotein particles; discovered by D.J. Ivanowsky (tobacco mosaic virus); term coined by M.W. Beijerinck; genetic material is DNA or RNA (not both); obligate intracellular parasites
- Viroids: infectious RNA without a protein coat; cause potato spindle tuber disease
- Prions: misfolded proteins; cause BSE (mad cow disease) and Creutzfeldt-Jakob disease
Common mistakes
- Fungi are NOT plants — their cell wall is chitin, not cellulose, and they are heterotrophic.
- · Mycoplasma · belongs to Monera, not Protista, despite having no cell wall.
- Viruses are not placed in any kingdom because they are acellular (non-living outside a host).
- Cyanobacteria are prokaryotes (Monera), not algae in the plant kingdom.
Summary
- Whittaker's Five-Kingdom classification (1969) is based on cell type, body organisation, and nutrition.
- Monera: prokaryotes; Protista: unicellular eukaryotes; Fungi: absorptive heterotrophs; Plantae: autotrophs; Animalia: ingestive heterotrophs.
- Lichens are mutualistic associations of fungi and algae.
- Viruses, viroids, and prions are acellular infectious agents outside the five kingdoms.