Biological Classification: Core NEET Facts

Classification Framework

R.H. Whittaker proposed Five Kingdom Classification in 1969 based on 5 criteria: cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships.
Estimated species on Earth: 8.7 million.
The five kingdoms in order of complexity: Monera → Protista → Fungi → Plantae → Animalia.

Kingdom Monera (Prokaryotes)

All prokaryotes — no membrane-bound nucleus.
Archaebacteria lack peptidoglycan cell walls; live in extreme environments (methanogens, halophiles, thermoacidophiles).
Eubacteria have peptidoglycan cell walls; four shapes: coccus, bacillus, vibrio, spirillum.
Gram-positive = thick peptidoglycan, retains crystal violet (purple). Gram-negative = thin peptidoglycan + outer LPS layer, does not retain crystal violet (appears pink).
Cyanobacteria (Nostoc, Anabaena) are photosynthetic prokaryotes that fix $N_{2}$ in heterocysts.
Mycoplasma = smallest living cell; no cell wall; a eubacterium despite being wall-less.

Kingdom Protista (Eukaryotic Unicellular)

Diatoms (Chrysophytes): siliceous frustules ( $SiO_{2}$ ) → diatomaceous earth.
Dinoflagellates: cellulose plates; Gonyaulax causes red tides.
Euglenoids: Euglena — mixotrophic; pellicle (not cell wall); flagellum.
Protozoans: Amoeba (pseudopodia/Sarcodina), Paramoecium (cilia/Ciliata), Plasmodium (non-motile/Sporozoa, malaria), Trypanosoma (flagella/Flagellata, sleeping sickness).

Kingdom Fungi

Cell wall = chitin. Energy storage = glycogen (not starch).
Body plan = mycelium (hyphae: septate or coenocytic).
Phycomycetes: coenocytic, zygospores (Mucor, Rhizopus).
Ascomycetes: septate, ascospores in asci (Saccharomyces, Aspergillus, Neurospora, Claviceps).
Basidiomycetes: septate, basidiospores on basidia (Agaricus, Ustilago, Puccinia).
Deuteromycetes: conidia only, no sexual stage (Alternaria, Colletotrichum, Trichoderma).

Lichens

Phycobiont (alga — photosynthesises) + mycobiont (fungus — absorbs water/minerals).
Bioindicators of $SO_{2}$ air pollution — absent from polluted areas.
Forms: crustose, foliose, fruticose.

Sub-cellular Agents

Virus: DNA OR RNA (never both) + protein capsid; obligate intracellular parasite.
Viroid: naked circular RNA, no protein; T.O. Diener (1971); plant diseases.
Prion: protein only, no nucleic acid; Prusiner (1982); BSE, CJD.