Biological Classification

2.1 Core concepts

• Historical progression: Aristotle classified plants as trees/shrubs/herbs and animals by presence/absence of red blood; Linnaeus' Two-Kingdom system (Plantae + Animalia) failed to separate prokaryotes/eukaryotes, unicellular/multicellular, and photosynthetic/non-photosynthetic forms (NCERT Ch 2 opening, p. 10).
• Whittaker (1969) Five Kingdoms: Monera, Protista, Fungi, Plantae, Animalia — based on cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships (NCERT §intro, p. 11).
• Three-domain system: divides Monera into two domains, retaining the four eukaryotic kingdoms in a third domain → effectively a six-kingdom classification (NCERT p. 11).
• Kingdom Monera (bacteria only): prokaryotic; non-cellulosic cell wall (polysaccharide + amino acid); shapes — Coccus, Bacillus, Vibrium, Spirillum; reproduce mainly by fission, sometimes spores, and a primitive form of DNA transfer (NCERT §2.1, pp. 12–14, Fig. 2.1).
• Archaebacteria: halophiles (salty), thermoacidophiles (hot springs), methanogens (marshes, gut of ruminants — produce biogas from dung); distinctive cell wall structure enables extremophile survival (NCERT §2.1.1, p. 13).
• Eubacteria: rigid cell wall + flagellum (if motile); includes cyanobacteria (blue-green algae) — photosynthetic autotrophs with chlorophyll a, may be unicellular/colonial/filamentous, gelatinous-sheathed colonies, form blooms; heterocysts of Nostoc and Anabaena fix atmospheric nitrogen; chemosynthetic autotrophs oxidise nitrates/nitrites/ammonia and recycle N, P, Fe, S; heterotrophic eubacteria are major decomposers — make curd, antibiotics, fix legume nitrogen; pathogens cause cholera, typhoid, tetanus, citrus canker (NCERT §2.1.2, pp. 13–14, Fig. 2.2).
• Mycoplasma: smallest living cells; completely lack a cell wall; survive without oxygen; many pathogenic to plants/animals (NCERT §2.1.2, p. 14).
• Kingdom Protista — Chrysophytes: diatoms + golden algae (desmids); microscopic plankton; silica-impregnated indestructible cell walls forming two overlapping "soap-box" shells; deposits → diatomaceous earth (polishing, filtration); chief producers in oceans (NCERT §2.2.1, p. 14).
• Dinoflagellates: mostly marine, photosynthetic; appear yellow/green/brown/blue/red by pigment; stiff cellulose plates; two flagella (one longitudinal, one transverse in furrow); Gonyaulax causes red tides whose toxins kill fish (NCERT §2.2.2, p. 15, Fig. 2.4a).
• Euglenoids: mostly fresh-water, stagnant pools; lack cell wall — have protein-rich pellicle; two flagella (one short, one long); photosynthetic in light, heterotrophic predators in dark (mixotrophy); pigments identical to higher plants; e.g., Euglena (NCERT §2.2.3, p. 15, Fig. 2.4b).
• Slime moulds: saprophytic protists; form a multinucleate plasmodium that spreads over decaying matter and engulfs it; under stress form fruiting bodies with true-walled, highly resistant, air-dispersed spores (NCERT §2.2.4, p. 15, Fig. 2.4c).
• Protozoans (4 groups): all heterotrophic, predator/parasite; Amoeboid (Amoeba, Entamoeba — pseudopodia, silica shells in marine forms); Flagellated (Trypanosoma — sleeping sickness); Ciliated (Paramoecium — gullet steers food via cilia); Sporozoans (Plasmodium — malaria, infectious spore-like life-cycle stage) (NCERT §2.2.5, p. 16, Fig. 2.4d).
• Kingdom Fungi: heterotrophic eukaryotes; mostly filamentous with hyphae forming a mycelium; coenocytic (multinucleate, no septa) vs septate hyphae; cell wall of chitin + polysaccharides; saprophytes, parasites, or symbionts (lichens with algae, mycorrhiza with plant roots) (NCERT §2.3, pp. 16–17).
• Fungal reproduction: vegetative (fragmentation/fission/budding); asexual by conidia/sporangiospores/zoospores; sexual by oospores/ascospores/basidiospores; three-step sexual cycle = plasmogamy → karyogamy → meiosis; ascomycetes and basidiomycetes show a dikaryotic (n+n) phase / dikaryophase (NCERT §2.3, p. 17).
• Phycomycetes: aquatic / decaying wood / obligate plant parasites; aseptate coenocytic mycelium; asexual = zoospores or aplanospores in sporangium; zygospore by fusion of isogamous/anisogamous/oogamous gametes; e.g., Mucor, Rhizopus, Albugo (NCERT §2.3.1, p. 17, Fig. 2.5a).
• Ascomycetes (sac fungi): mostly multicellular (Penicillium) or unicellular (Saccharomyces); branched, septate mycelium; asexual conidia produced exogenously on conidiophores; sexual ascospores produced endogenously in asci within fruiting bodies called ascocarps; e.g., Aspergillus, Claviceps, Neurospora (used in genetics), morels and truffles (edible) (NCERT §2.3.2, pp. 17–18, Fig. 2.5b).
• Basidiomycetes: mushrooms, bracket fungi, puffballs, rusts, smuts; branched septate mycelium; asexual spores generally absent; sex organs absent — plasmogamy by fusion of vegetative/somatic cells of different strains → dikaryotic mycelium → basidium where karyogamy + meiosis form four exogenous basidiospores on basidia arranged in basidiocarps; e.g., Agaricus, Ustilago (smut), Puccinia (rust) (NCERT §2.3.3, p. 18, Fig. 2.5c).
• Deuteromycetes (Fungi Imperfecti): only asexual/vegetative phases known; reproduce only by conidia; septate, branched mycelium; many are decomposers in mineral cycling; once sexual stage is discovered they are moved to ascomycetes/basidiomycetes; e.g., Alternaria, Colletotrichum, Trichoderma (NCERT §2.3.4, p. 18).
• Kingdom Plantae: eukaryotic, chlorophyll-containing; cellulose cell wall; includes algae, bryophytes, pteridophytes, gymnosperms, angiosperms; partially heterotrophic exceptions — insectivorous Bladderwort and Venus fly trap; parasitic Cuscuta; life cycle shows alternation of generations (diploid sporophyte ↔ haploid gametophyte) (NCERT §2.4, p. 19).
• Kingdom Animalia: heterotrophic multicellular eukaryotes, no cell wall; holozoic nutrition (ingestion); food stored as glycogen/fat; elaborate sensory/neuromotor systems in higher forms; sexual reproduction by copulation + embryological development (NCERT §2.5, p. 19).
• Viruses: non-cellular obligate parasites; inert crystalline outside host; Ivanowsky (1892) linked microbes to tobacco mosaic disease; Beijerinck (1898) showed tobacco extract caused infection — coined "virus" / Contagium vivum fluidum; Stanley (1935) crystallised viruses, showing they consist largely of protein; genetic material is either RNA or DNA — never both; plant viruses usually ssRNA; animal viruses ssRNA / dsRNA / dsDNA; bacteriophages usually dsDNA; protein capsid of capsomeres (helical/polyhedral); cause mumps, smallpox, herpes, influenza, AIDS; in plants — mosaic, leaf curling, yellowing, vein clearing, stunting (NCERT §2.6, pp. 19–21, Fig. 2.6).
• Viroids: T.O. Diener (1971) — agent of potato spindle tuber disease; free, low-molecular-weight RNA with no protein coat — hence smaller than viruses (NCERT §2.6, p. 21).
• Prions: abnormally folded infectious proteins, virus-sized; cause bovine spongiform encephalopathy (BSE, mad cow disease) in cattle and Creutzfeldt–Jacob disease (CJD) in humans (NCERT §2.6, p. 21).
• Lichens: mutually useful symbiotic associations — phycobiont (autotrophic algal partner, prepares food) + mycobiont (heterotrophic fungal partner, gives shelter, absorbs water and minerals); excellent pollution indicators — do not grow in polluted areas (NCERT §2.6, pp. 21–22).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 2.1, p. 12: Bacterial shapes — Cocci, Bacilli (with spore), Spirilla (with flagellum), Vibrio.
• Figure 2.2, p. 13: Filamentous blue-green alga Nostoc showing heterocyst and mucilaginous sheath.
• Figure 2.3, p. 14: A dividing bacterium showing cell wall, cell membrane, and DNA.
• Figure 2.4, pp. 15–16: (a) Dinoflagellate (with cellulose plates), (b) Euglena (two flagella + pellicle), (c) slime mould, (d) Paramoecium (cilia + gullet).
• Figure 2.5, p. 17: Fungi — (a) Mucor (Phycomycetes), (b) Aspergillus (Ascomycetes), (c) Agaricus (Basidiomycetes).
• Figure 2.6, p. 20: (a) TMV — RNA inside helical capsid; (b) Bacteriophage with head, collar, sheath, tail fibres.
• Table 2.1, p. 11: Five-kingdom comparison across cell type, cell wall, nuclear membrane, body organisation, mode of nutrition.
• Fungal sexual cycle: plasmogamy → karyogamy → meiosis (p. 17).

2.4 Common confusions / NTA trap points

• Mycoplasma vs Archaebacteria: Mycoplasma have no cell wall at all; archaebacteria have a different (not absent) cell wall. NTA often swaps these descriptions.
• Diatom wall vs Dinoflagellate wall: diatom walls are silica (overlapping soap-box shells), dinoflagellate walls are stiff cellulose plates — frequently confused.
• Euglena's pellicle is protein, not cellulose — and Euglena has no cell wall, despite being "plant-like" in pigments.
• Coenocytic = aseptate (Phycomycetes only); ascomycetes/basidiomycetes/deuteromycetes have septate mycelium — a classic match-the-class trap.
• Spore types per class: Phycomycetes → zoospores/aplanospores/zygospores; Ascomycetes → conidia + ascospores; Basidiomycetes → basidiospores (asexual spores generally absent); Deuteromycetes → conidia only. Many MCQs test this exact mapping.
• Conidia exogenous, ascospores endogenous — direction trap.
• No virus has both RNA and DNA — a frequent absolute-statement MCQ. Plant viruses are usually ssRNA; bacteriophages dsDNA.
• Scientist trap (often tested): Ivanowsky (1892) → recognised tobacco-mosaic microbe; Beijerinck (1898) → coined "virus" / Contagium vivum fluidum; Stanley (1935) → crystallised viruses; Diener (1971) → viroid in potato spindle tuber disease. Whittaker (1969) → five kingdoms.
• Lichen partners: phycobiont = autotrophic (algal); mycobiont = heterotrophic (fungal). NTA reverses the roles in distractors.
• Bacterial cell wall composition. NCERT says bacteria have a non-cellulosic cell wall of polysaccharide plus amino acid (NCERT §2.1, p. 12). Distractors that say "cellulosic" or "chitinous" are wrong — chitin is fungal, cellulose is plant/dinoflagellate.
• Five-kingdom criteria. Whittaker's 1969 criteria are precisely five: cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships. CUET sometimes drops phylogenetic relationships and replaces it with "habitat" or "pigment type" — both wrong.
• Diatom example trap. Diatoms are chrysophytes, not dinoflagellates; their indestructible silica walls form diatomaceous earth. They are described as the "chief producers in the oceans" (NCERT §2.2.1, p. 14), a phrase NTA likes to quote.
• Slime mould plasmodium ≠ Plasmodium (sporozoan). The slime-mould multinucleate creeping mass is called a plasmodium (lowercase, common noun); Plasmodium (italic, capital) is the malarial sporozoan. Same word, different referents — a textbook trap.
• Conidia are exogenous, ascospores are endogenous. Conidia are produced exogenously on conidiophores; ascospores are produced endogenously inside sac-like asci within ascocarps. Direction-of-production trap.
• Number of nuclei per fungal cell. Phycomycetes: many nuclei per cell (coenocytic). Ascomycetes/Basidiomycetes: pass through a dikaryotic n+n stage. Deuteromycetes: septate, reproduce only by conidia. Mismatching this is a very common MCQ slip.
• Plant kingdom exceptions. Most Plantae are autotrophic, but NCERT lists insectivorous Bladderwort and Venus fly trap (partially heterotrophic) and Cuscuta (parasitic). Distractors that say "all members of Plantae are autotrophic" are false.
• Body organisation across kingdoms. Monera: prokaryotic single cell. Protista: eukaryotic single cell. Fungi: multicellular (mostly), except yeast (Saccharomyces — unicellular ascomycete). Plantae/Animalia: multicellular eukaryotic. Quick-recall trap for kingdom-feature MCQs.