Section 1 — Need and Basis of Classification

With an estimated 8.7 million species on Earth, systematic classification became essential. R.H. Whittaker's Five Kingdom Classification (1969) uses five criteria: cell structure (prokaryotic/eukaryotic), body organisation (unicellular/multicellular), mode of nutrition, mode of reproduction, and phylogenetic relationships. Earlier two-kingdom (Linnaeus: Plantae + Animalia) and three-kingdom (Haeckel: + Protista) systems were inadequate because they could not properly accommodate fungi, prokaryotes, and unicellular eukaryotes.

Section 2 — Kingdom Monera (Prokaryotes)

All prokaryotes — no membrane-bound nucleus. Two major groups:

• Archaebacteria: Cell walls lack peptidoglycan. Three types by habitat: methanogens (marshy areas, cattle rumen — produce $CH_{4}$), halophiles (high salt — salt lakes), thermoacidophiles (hot acidic springs like Sulfolobus). These organisms can survive extreme conditions that would destroy most life.
• Eubacteria: Cell walls have peptidoglycan. Four morphological shapes: coccus, bacillus, vibrio, spirillum. Gram staining separates Gram-positive (thick peptidoglycan, crystal violet retained, purple) from Gram-negative (thin peptidoglycan + outer LPS, pink after safranin). Key members: Cyanobacteria (Nostoc, Anabaena — photosynthetic, fix $N_{2}$ in heterocysts), Mycoplasma (no cell wall, smallest living cell, causes plant and animal diseases).

Section 3 — Kingdom Protista (Eukaryotic Unicellular)

Eukaryotic organisms that are mostly unicellular. Five key groups:

• Chrysophytes/Diatoms: Siliceous frustule cell walls ($SiO_{2}$); diatomaceous earth; passive flagella or absent.
• Dinoflagellates: Cellulose plates; two flagella; Gonyaulax causes red tides (toxic); some bioluminescent.
• Euglenoids: Euglena — mixotrophic; pellicle not cell wall; flagellum; found in freshwater.
• Slime Moulds (Physarum): Saprophytic; form plasmodium → fruiting body → spores (true cell walls on spores).
• Protozoans: Sarcodina (pseudopodia — Amoeba), Ciliata (cilia — Paramoecium), Sporozoa (non-motile — Plasmodium malaria), Flagellata (flagella — Trypanosoma sleeping sickness).

Section 4 — Kingdom Fungi

Heterotrophic eukaryotes with chitin cell walls and glycogen energy storage. Body = mycelium (network of hyphae). Four classes:

Section 5 — Lichens

Mutualistic symbiosis: phycobiont (alga — photosynthesises food) + mycobiont (fungus — absorbs water, minerals, provides structure). Three growth forms: crustose (flat), foliose (leaf-like), fruticose (shrub-like). Sensitive to $SO_{2}$ → excellent bioindicators of air quality.

Section 6 — Viruses, Viroids, and Prions

Sub-cellular infectious agents outside the five kingdoms:

• Viruses: Non-cellular, obligate intracellular parasites. Contain DNA OR RNA (never both). Examples: TMV (RNA, helical capsid — Ivanowsky 1892), Bacteriophage (DNA, icosahedral head + tail), HIV (RNA). Discovered by Pasteur (rabies) and Ivanowsky (TMV).
• Viroids: Naked circular RNA, no protein coat. Discovered by T.O. Diener (1971). Cause plant diseases only — potato spindle tuber disease.
• Prions: Infectious misfolded proteins, absolutely no nucleic acid. Discovered by Stanley Prusiner (1982). Cause BSE (mad cow disease) in cattle and Creutzfeldt-Jakob disease in humans.