Biological Classification – Class 11 Biology Chapter 2 Notes & NCERT PDF

Class 11BiologyChapter 2NCERT book: Biology - Class 11

Notes and a simple summary of Chapter 2 of the NCERT Class 11 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 2: Biological Classification

Introduction

People have classified organisms since the dawn of civilisation, first for use (food, shelter, clothing). Aristotle made the earliest scientific attempt: plants into trees, shrubs and herbs; animals into those with red blood and without. In Linnaeus' time the Two Kingdom system (Plantae and Animalia) was used, but it did not distinguish eukaryotes from prokaryotes, unicellular from multicellular, or photosynthetic (green algae) from non-photosynthetic (fungi) organisms. So classification systems kept changing.

Key Concepts

Five Kingdom Classification

R.H. Whittaker (1969) proposed five kingdoms — Monera, Protista, Fungi, Plantae and Animalia. Criteria: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships. A three-domain system has also been proposed that splits Monera into two domains (six-kingdom classification).

#### Characteristics of the five kingdoms (Table 2.1, as lists)

Monera: prokaryotic; cell wall non-cellulosic (polysaccharide + amino acid); nuclear membrane absent; cellular; autotrophic (chemosynthetic and photosynthetic) and heterotrophic (saprophytic/parasitic).

Protista: eukaryotic; cell wall present in some; nuclear membrane present; cellular; autotrophic (photosynthetic) and heterotrophic.

Fungi: eukaryotic; cell wall with chitin; multicellular/loose tissue; heterotrophic (saprophytic/parasitic).

Plantae: eukaryotic; cell wall of cellulose; tissue/organ; autotrophic (photosynthetic).

Animalia: eukaryotic; no cell wall; tissue/organ/organ system; heterotrophic (holozoic/saprophytic etc.).

2.1 Kingdom Monera

Bacteria are the sole members; most abundant micro-organisms; found even in hot springs, deserts, snow and deep oceans.

Four shapes: Coccus (spherical), Bacillus (rod), Vibrium (comma), Spirillum (spiral).

Most extensive metabolic diversity: photosynthetic autotrophs, chemosynthetic autotrophs, but the vast majority are heterotrophs.

2.1.1 Archaebacteria — live in harsh habitats: halophiles (salty areas), thermoacidophiles (hot springs), methanogens (marshy areas). A different cell wall structure helps survival. Methanogens in the gut of cows and buffaloes produce methane (biogas) from dung.

2.1.2 Eubacteria ('true bacteria') — rigid cell wall; flagellum if motile.

- Cyanobacteria (blue-green algae): chlorophyll a, photosynthetic autotrophs; unicellular, colonial or filamentous; form blooms in polluted water; some fix nitrogen in heterocysts (Nostoc, Anabaena).

- Chemosynthetic autotrophs oxidise nitrates, nitrites, ammonia; recycle N, P, Fe, S.

- Heterotrophic bacteria — most abundant; decomposers; used for curd, antibiotics, nitrogen fixation in legume roots; some are pathogens — cholera, typhoid, tetanus, citrus canker.

- Reproduce mainly by fission; spores under unfavourable conditions; primitive DNA transfer (a sort of sexual reproduction).

Mycoplasma — completely lack a cell wall; smallest living cells known; can survive without oxygen; many are pathogenic.

2.2 Kingdom Protista

All single-celled eukaryotes; boundaries not well defined; primarily aquatic; well-defined nucleus; flagella or cilia in some; reproduce asexually and sexually (cell fusion and zygote formation). Groups included:

2.2.1 Chrysophytes — diatoms and golden algae (desmids); plankton; mostly photosynthetic. Diatom walls are two overlapping shells (like a soap box) embedded with silica — indestructible; deposits form diatomaceous earth (used in polishing, filtration of oils and syrups). Diatoms are the chief producers in the oceans.

2.2.2 Dinoflagellates — mostly marine, photosynthetic; stiff cellulose plates; two flagella (one longitudinal, one transverse); red dinoflagellates (Gonyaulax) cause red tides; toxins may kill fishes.

2.2.3 Euglenoids — mostly fresh water; no cell wall, but a protein-rich pellicle; two flagella (short and long); photosynthetic in sunlight, heterotrophic without it; pigments identical to higher plants. Example: Euglena.

2.2.4 Slime moulds — saprophytic protists; form plasmodium that may spread over several feet; fruiting bodies with spores that have true walls and survive for years; spores dispersed by air.

2.2.5 Protozoans — all heterotrophs (predators or parasites); primitive relatives of animals. Four groups:

- Amoeboid — pseudopodia (Amoeba); marine forms have silica shells; Entamoeba is parasitic.

- Flagellated — flagella; Trypanosoma causes sleeping sickness.

- Ciliated — thousands of cilia; gullet; Paramoecium.

- Sporozoans — infectious spore-like stage; Plasmodium causes malaria.

2.3 Kingdom Fungi

Heterotrophic; e.g., bread mould, mushroom, toadstool; white spots on mustard leaves; yeast (unicellular) used for bread and beer; Puccinia causes wheat rust; Penicillium gives antibiotics.

Body of hyphae; network = mycelium; coenocytic hyphae are multinucleate continuous tubes; others have septae.

Cell wall: chitin and polysaccharides.

Nutrition: saprophytes (dead matter), parasites, or symbionts — with algae as lichens, with roots of higher plants as mycorrhiza.

Reproduction: vegetative (fragmentation, fission, budding); asexual by conidia, sporangiospores or zoospores; sexual by oospores, ascospores, basidiospores.

Sexual cycle: (i) plasmogamy — fusion of protoplasms; (ii) karyogamy — fusion of nuclei; (iii) meiosis in zygote → haploid spores. In ascomycetes and basidiomycetes a dikaryon (n + n) stage occurs (dikaryophase).

#### Classes of fungi

2.3.1 Phycomycetes — aquatic/damp places or obligate parasites; mycelium aseptate and coenocytic; asexual by zoospores (motile) or aplanospores (non-motile), endogenous in sporangium; zygospore by fusion of gametes (isogamous, anisogamous or oogamous). Examples: Mucor, Rhizopus, Albugo.

2.3.2 Ascomycetes (sac fungi) — mostly multicellular (Penicillium), rarely unicellular (yeast, Saccharomyces); mycelium branched and septate; asexual conidia on conidiophores (exogenous); sexual ascospores in asci, in ascocarps. Examples: Aspergillus, Claviceps, Neurospora (used in biochemical and genetic work); morels and truffles are edible.

2.3.3 Basidiomycetes — mushrooms, bracket fungi, puffballs; rusts and smuts; asexual spores generally absent; sex organs absent; plasmogamy by fusion of somatic cells → dikaryon → basidium; four basidiospores exogenously; fruiting body basidiocarp. Examples: Agaricus, Ustilago, Puccinia.

2.3.4 Deuteromycetes (imperfect fungi) — only asexual/vegetative phases known; reproduce only by conidia; septate branched mycelium; many are decomposers helping mineral cycling. Examples: Alternaria, Colletotrichum, Trichoderma.

2.4 Kingdom Plantae and 2.5 Kingdom Animalia

Plantae: eukaryotic, chlorophyll-containing; a few partially heterotrophic — insectivorous (bladderwort, Venus fly trap) or parasitic (*Cuscuta); cellulose wall; includes algae, bryophytes, pteridophytes, gymnosperms, angiosperms; life cycle shows alternation of generations* (diploid sporophyte and haploid gametophyte).

Animalia: heterotrophic, multicellular eukaryotes without cell walls; holozoic nutrition; store food as glycogen or fat; definite growth pattern; sexual reproduction by copulation followed by embryological development.

2.6 Viruses, Viroids, Prions and Lichens

Viruses: non-cellular; inert crystalline structure outside a living cell; obligate parasites.

- Dmitri Ivanowsky (1892) — causal organism of mosaic disease of tobacco, smaller than bacteria.

- M.W. Beijerinck (1898) — named it "virus"; called the fluid *Contagium vivum fluidum*.

- W.M. Stanley (1935) — viruses can be crystallised; crystals largely protein.

- Genetic material is RNA or DNA, never both; a virus is a nucleoprotein. Plant viruses usually single-stranded RNA; bacteriophages usually double-stranded DNA. Protein coat = capsid of capsomeres (helical or polyhedral).

- Diseases: mumps, small pox, herpes, influenza, AIDS; in plants — mosaic, leaf rolling/curling, yellowing, vein clearing, dwarfing.

Viroids: discovered by T.O. Diener (1971); free RNA without protein coat; cause potato spindle tuber disease.

Prions: abnormally folded protein; cause BSE (mad cow disease) and CJD in humans.

Lichens: symbiotic associations of algae (phycobiont, autotrophic) and fungi (mycobiont, heterotrophic); very good pollution indicators — do not grow in polluted areas.

Diagrams to Know

Figure 2.1 Bacteria of different shapes — cocci, bacilli, spirilla, vibrio (with flagellum and spore labelled).

Figure 2.6 (a) TMV — rod-shaped with RNA inside the capsid; (b) Bacteriophage — head, collar, sheath, tail fibres.

Key-Facts Sheet

Whittaker — 1969 — five kingdoms.

Diatoms: silica walls; diatomaceous earth.

Gonyaulax: red tides. Trypanosoma: sleeping sickness. Plasmodium: malaria.

Fungal wall: chitin. Plant wall: cellulose. Mycoplasma: no wall.

Albugo: parasite on mustard. Puccinia: wheat rust. Neurospora: genetics.

Exam Focus

Q: How are viroids different from viruses? Outline: viroids are free RNA without a protein coat and of low molecular weight (potato spindle tuber disease, Diener 1971); viruses have a protein coat (capsid) around RNA or DNA.

Q: Describe the four major groups of Protozoa. Amoeboid (pseudopodia, Amoeba, Entamoeba), Flagellated (Trypanosoma), Ciliated (Paramoecium, gullet), Sporozoans (Plasmodium).

Q: Comparative account of fungal classes by nutrition and reproduction — use the four class notes above.

Also: phycobiont/mycobiont; nature of diatom walls; algal bloom and red tides; features of euglenoids.

Key Terms

Prokaryote (असीमकेंद्रकी)

Eukaryote (सुकेंद्रकी)

Heterocyst (हेटेरोसिस्ट / विषमपुटी)

Pellicle (पेलिकल / पतली झिल्ली)

Plasmodium (प्लाज्मोडियम)

Hyphae (कवकतंतु)

Mycelium (कवकजाल)

Plasmogamy (जीवद्रव्य संलयन)

Karyogamy (केंद्रक संलयन)

Dikaryon (द्विकेंद्रकी)

Capsid (कैप्सिड / प्रोटीन आवरण)

Viroid (विरोइड)

Prion (प्रायॉन)

Lichen (लाइकेन / शैक)

Mycorrhiza (कवकमूल)

Common Mistakes

Calling cyanobacteria "algae" in the plant kingdom — they are Monera.

Saying viruses contain both RNA and DNA — no virus contains both.

Confusing sporangiospores/zoospores (phycomycetes) with conidia (ascomycetes/deuteromycetes).

Writing that basidiospores form inside the basidium — they are exogenous; ascospores are endogenous in asci.

Placing Euglena with a cell wall — it has a pellicle instead.

💡 Key Learning Points

✓Classification moved from Aristotle and Linnaeus's two kingdoms to Whittaker's (1969) five kingdoms based on cell structure, body organisation, nutrition, reproduction and phylogeny.
✓Monera (bacteria) shows the greatest metabolic diversity — archaebacteria, eubacteria including cyanobacteria, and wall-less Mycoplasma.
✓Protista groups single-celled eukaryotes: chrysophytes, dinoflagellates, euglenoids, slime moulds and four groups of protozoans.
✓Fungi have chitin walls and hyphae; classes Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes differ in mycelium and spores.
✓Viruses, viroids, prions and lichens lie outside the five kingdoms; each has a distinct nature and examples.

👨‍🏫 Teaching Tips

→Show a timeline: Aristotle → Linnaeus (2 kingdoms) → Whittaker 1969 (5 kingdoms) → three-domain idea, and ask why each change happened.
→Convert Table 2.1 into a card-sorting game: students sort character cards (chitin, cellulose, prokaryotic, no wall) under the right kingdom.
→Prepare a temporary slide of curd bacteria or bread mould to show real microbes and hyphae.
→Use a 'disease–organism' matching quiz: malaria–Plasmodium, sleeping sickness–Trypanosoma, wheat rust–Puccinia, mad cow disease–prion.
→Hold the book's suggested class debate: 'Are viruses living or non-living?'
→Draw TMV and bacteriophage side by side and label capsid, nucleic acid, collar, sheath, tail fibres.
→Show lichens on tree bark near school and discuss them as pollution indicators.

📋 Assessment Questions

Can the student list Whittaker's five kingdoms and the criteria used?
Can the student distinguish the four classes of fungi by mycelium and spores with examples?
Can the student differentiate viruses, viroids and prions?
Can the student explain phycobiont and mycobiont and the importance of lichens?
Can the student give two uses each of heterotrophic bacteria and archaebacteria?

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