Biological Classification — Exam-Eve Review (10 Essential Sentences)

1. R.H. Whittaker proposed the Five Kingdom Classification in 1969, dividing all living organisms into Monera, Protista, Fungi, Plantae, and Animalia based on cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships.

2. Kingdom Monera includes all prokaryotes: Archaebacteria (cell walls lack peptidoglycan; live in extreme environments as methanogens, halophiles, or thermoacidophiles) and Eubacteria (peptidoglycan cell walls; four shapes: coccus, bacillus, vibrio, spirillum).

3. Cyanobacteria (Nostoc, Anabaena) are photosynthetic prokaryotes in Kingdom Monera that fix atmospheric nitrogen in specialised thick-walled cells called heterocysts.

4. Mycoplasma is the smallest known living cell, belongs to Kingdom Monera, and uniquely lacks any cell wall — making it resistant to penicillin.

5. Diatoms (Chrysophytes) in Kingdom Protista have siliceous cell walls called frustules ($SiO_{2}$) that accumulate as diatomaceous earth used in filtration and polishing; Euglena is mixotrophic with a proteinaceous pellicle instead of a cell wall.

6. Fungi have chitin cell walls, store energy as glycogen (not starch), and exist as septate (Ascomycetes, Basidiomycetes, Deuteromycetes) or coenocytic (Phycomycetes) mycelium.

7. The four fungal classes differ in spore types: Phycomycetes form zygospores, Ascomycetes form ascospores in asci, Basidiomycetes form basidiospores on basidia, and Deuteromycetes form only conidia with no known sexual stage.

8. Lichens are a mutualistic symbiosis between a phycobiont (algal partner — photosynthesises) and a mycobiont (fungal partner — absorbs water and minerals), and are sensitive bioindicators of $SO_{2}$ air pollution.

9. Viruses contain either DNA or RNA (never both) enclosed in a protein capsid; viroids (discovered by T.O. Diener, 1971) consist of naked circular RNA with no protein coat; prions (Prusiner, 1982) are infectious proteins with no nucleic acid whatsoever.

10. Key NEET traps: viruses never have both DNA and RNA; fungi store glycogen not starch; cyanobacteria are in Monera not Protista; Euglena has a pellicle not a cell wall; Archaebacteria lack peptidoglycan; prions have no nucleic acid.