Biological Classification

This is Chapter 2 of NCERT's Class 11 Biology (pages 10–22 of the 2026-27 reprint), the second of the four chapters in Unit I, Diversity in the Living World. It has six sections — 2.1 Kingdom Monera, 2.2 Protista, 2.3 Fungi, 2.4 Plantae, 2.5 Animalia and 2.6 Viruses, Viroids, Prions and Lichens — with Table 2.1, Figures 2.1–2.6, a summary and 12 exercises. The words taxon, kingdom, binomial name and the ranks from species to kingdom were set up in Chapter 1, The Living World (kebo101) and are not re-taught here. Protista and Fungi each get two sections on this page, so there are nine concept sections.

• Neighbouring chapters: The Living World (Class 11, Chapter 1) gives the vocabulary; Plant Kingdom and Animal Kingdom (Chapters 3 and 4) expand the last two kingdoms; Human Health and Disease (Class 12, Chapter 7) covers the bacterial, viral, protozoan and fungal diseases named here; Microbes in Human Welfare (Class 12, Chapter 8) covers curd, antibiotics, biogas and biofertilisers; Organisms and Populations (Class 12, Chapter 11) treats lichens and mycorrhiza as mutualism; Ecosystem (Class 12, Chapter 12) treats bacteria and fungi as decomposers.
• Prelims — science and environment: one of the most-asked biology chapters. UPSC has asked about lichens (1984, 2014), mycorrhiza (1990, 1999), red tides (1998), blue-green algae as biofertilisers (2000, 2010, 2013), what viruses can and cannot do (1989, 2013, 2016, 2021, 2025), extremophile microbes (2023), ocean primary producers (2021) and the Himalayan morel ‘Gucchi’ (2022).
• GS3 — Science & Technology, Environment, Agriculture: pathogens and new diseases (viral outbreaks, prion diseases), antibiotics and antimicrobial resistance, biofertilisers and biogas, harmful algal blooms, methane from wetlands, paddies and cattle, and lichens as pollution indicators. GS2 — Health: disease control programmes draw on the same pathogen groups.

• Aristotle: plants as trees, shrubs, herbs; animals with or without red blood. Two kingdoms (Plantae, Animalia; Linnaeus' time) failed to separate prokaryotes and eukaryotes, unicellular and multicellular, photosynthetic green algae and non-photosynthetic fungi.
• R.H. Whittaker (1969): Monera, Protista, Fungi, Plantae, Animalia; criteria = cell structure, body organisation, mode of nutrition, reproduction, phylogenetic relationships. A three-domain system splits Monera into two domains, giving six kingdoms.
• Monera = bacteria: cocci, bacilli, vibrio, spirilla; most extensive metabolic diversity. Archaebacteria (halophiles, thermoacidophiles, methanogens — biogas from dung) have a different wall. Cyanobacteria: chlorophyll a, blooms, N₂ fixed in heterocysts (Nostoc, Anabaena). Mycoplasma: no wall, smallest cells.
• Protista (single-celled eukaryotes): diatoms (silica ‘soap box’ walls, diatomaceous earth, chief ocean producers); dinoflagellates (cellulose plates, two flagella, red tides — Gonyaulax); euglenoids (pellicle, two flagella); slime moulds (plasmodium, resistant spores); protozoans — amoeboid, flagellated (Trypanosoma), ciliated (Paramoecium), sporozoans (Plasmodium, malaria).
• Fungi: chitin walls; hyphae → mycelium (septate or coenocytic); saprophytes, parasites, symbionts (lichens, mycorrhiza). Sex = plasmogamy → karyogamy → meiosis, with a dikaryon (n + n) in asco- and basidiomycetes. Classes: Phycomycetes (Mucor, Rhizopus, Albugo), Ascomycetes (Penicillium, yeast, Neurospora), Basidiomycetes (Agaricus, Ustilago, Puccinia), Deuteromycetes (imperfect; Trichoderma).
• Outside the kingdoms: viruses (nucleoprotein; DNA or RNA, never both; obligate parasites; Ivanowsky 1892, Beijerinck 1898, Stanley 1935), viroids (free RNA; Diener 1971; potato spindle tuber), prions (misfolded protein; BSE, CJD), lichens (alga = phycobiont + fungus = mycobiont; pollution indicators).

Early attempts to classify were instinctive and based on use — food, shelter, clothing. Aristotle was the earliest to try a more scientific basis: he divided plants into trees, shrubs and herbs by simple morphological characters, and animals into those with red blood and those without. In Linnaeus' time a Two Kingdom system — Plantae and Animalia — took in all plants and all animals. It was easy to use but did not distinguish eukaryotes from prokaryotes, unicellular from multicellular organisms, or photosynthetic (green algae) from non-photosynthetic (fungi) organisms, and many organisms fitted neither group. Beyond gross morphology, biologists needed cell structure, nature of the wall, mode of nutrition, habitat, methods of reproduction and evolutionary relationships.

R.H. Whittaker (1969) proposed the Five Kingdom Classification: Monera, Protista, Fungi, Plantae and Animalia. His main criteria: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships. The old ‘Plants’ had been held together only by having a cell wall, which lumped prokaryotic bacteria and blue-green algae with eukaryotes, unicellular Chlamydomonas with multicellular Spirogyra, and heterotrophic fungi (walls of chitin) with autotrophic green plants (walls of cellulose). Whittaker moved fungi into their own kingdom, all prokaryotes into Monera, and unicellular eukaryotes into Protista — which therefore joins Chlamydomonas and Chlorella (once algae in Plants, with walls) with Paramoecium and Amoeba (once animals, without walls).

NCERT adds that a three-domain system has also been proposed: it divides Kingdom Monera into two domains and puts the remaining eukaryotic kingdoms in a third, ‘thereby a six kingdom classification’. Classification will keep changing as our understanding of characters and evolutionary relationships improves; the aim is a system that is not only morphological, physiological and reproductive but phylogenetic. This chapter covers Monera, Protista and Fungi; Plantae and Animalia get Chapters 3 and 4.

Bacteria are the sole members of Kingdom Monera and the most abundant micro-organisms. Hundreds live in a handful of soil; they also live in extreme habitats — hot springs, deserts, snow, deep oceans — and many live in or on other organisms as parasites. By shape there are four kinds: spherical coccus (pl. cocci), rod-shaped bacillus (pl. bacilli), comma-shaped vibrio and spiral spirillum (pl. spirilla). Simple in structure, bacteria are complex in behaviour: as a group they show the most extensive metabolic diversity. Some are autotrophs — photosynthetic or chemosynthetic; the vast majority are heterotrophs.

Archaebacteria live in some of the harshest habitats; they differ from other bacteria in having a different cell wall structure, which is responsible for their survival in extreme conditions. Eubacteria (‘true bacteria’), in their thousands, have a rigid cell wall and, if motile, a flagellum. The cyanobacteria (blue-green algae) have chlorophyll a like green plants and are photosynthetic autotrophs: unicellular, colonial or filamentous, freshwater, marine or terrestrial, their colonies usually in a gelatinous sheath; they often form blooms in polluted water bodies, and some fix atmospheric nitrogen in specialised cells called heterocysts — Nostoc, Anabaena. Chemosynthetic autotrophic bacteria oxidise inorganic substances such as nitrates, nitrites and ammonia and use the energy released to make ATP; they recycle nitrogen, phosphorous, iron and sulphur.

Heterotrophic bacteria are the most abundant in nature; most are important decomposers. Many matter to people: they help make curd from milk, produce antibiotics and fix nitrogen in legume roots; others are pathogens of humans, crops, farm animals and pets — cholera, typhoid, tetanus and citrus canker are bacterial diseases. Bacteria reproduce mainly by fission; under unfavourable conditions they form spores; and they show a sort of sexual reproduction through a primitive DNA transfer from one bacterium to another. The mycoplasma completely lack a cell wall, are the smallest living cells known, can survive without oxygen, and many are pathogenic in animals and plants.

All single-celled eukaryotes are placed in Protista, but the kingdom's boundaries are not well defined: what is ‘a photosynthetic protistan’ to one biologist may be ‘a plant’ to another. NCERT includes five groups — chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans. Protists are primarily aquatic, form a link with plants, animals and fungi, have a well-defined nucleus and membrane-bound organelles, some bear flagella or cilia, and they reproduce asexually and sexually — sexually by cell fusion and zygote formation.

Chrysophytes — diatoms and golden algae — live in fresh and marine water; they are microscopic and float passively in water currents (plankton); most are photosynthetic. A diatom's wall is two thin overlapping shells fitting together like a soap box, embedded with silica and hence practically indestructible. Their deposits, accumulated over billions of years, form diatomaceous earth, a gritty soil used in polishing and in filtering oils and syrups. Diatoms are the chief ‘producers’ in the oceans.

Dinoflagellates are mostly marine and photosynthetic; they appear yellow, green, brown, blue or red according to their main pigments. The wall has stiff cellulose plates on its outer surface. Most have two flagella — one longitudinal, one transverse in a furrow between the plates. Red dinoflagellates such as Gonyaulax can multiply so fast that the sea looks red — red tides — and the toxins released by such numbers may kill fishes and other marine animals. Euglenoids are mostly freshwater organisms of stagnant water; instead of a wall they have a protein-rich pellicle that keeps the body flexible; they have two flagella, one short and one long. They photosynthesise in sunlight but, deprived of light, behave as heterotrophs by preying on smaller organisms; their pigments are identical to those of higher plants (Euglena).

Slime moulds are saprophytic protists. The body moves along decaying twigs and leaves, engulfing organic material. Under suitable conditions they form an aggregation called a plasmodium, which may grow and spread over several feet. In unfavourable conditions the plasmodium differentiates into fruiting bodies bearing spores at their tips. The spores have true walls, are extremely resistant, survive for many years and are dispersed by air currents.

Protozoans are all heterotrophs, living as predators or parasites, and are believed to be primitive relatives of animals. NCERT names four major groups:

Fungi are a unique kingdom of heterotrophs, diverse in form and habitat: moulds on moist bread and rotten fruit, mushrooms and toadstools, the white spots on mustard leaves (a parasitic fungus), yeast for bread and beer, the wheat-rust fungus Puccinia, and Penicillium, a source of antibiotics. Fungi are cosmopolitan — in air, water, soil, on plants and animals — and prefer warm, humid places, which is why we refrigerate food. Except the unicellular yeasts, fungi are filamentous: long slender threads, hyphae, form a network, the mycelium. Some hyphae are continuous multinucleate tubes — coenocytic; others have cross walls (septa). The walls are of chitin and polysaccharides.

Most fungi absorb soluble organic matter from dead substrates — saprophytes; those that depend on living plants and animals are parasites; some live as symbionts — with algae as lichens and with roots of higher plants as mycorrhiza. Reproduction is vegetative (fragmentation, fission, budding), asexual by spores — conidia, sporangiospores or zoospores — and sexual by oospores, ascospores and basidiospores, all produced in distinct structures called fruiting bodies. The sexual cycle has three steps: plasmogamy (fusion of protoplasms of two motile or non-motile gametes), karyogamy (fusion of two nuclei) and meiosis in the zygote, giving haploid spores.

When a fungus reproduces sexually, two haploid hyphae of compatible mating types fuse. In some fungi this immediately gives diploid cells; in ascomycetes and basidiomycetes an intervening dikaryotic stage (n + n) — a dikaryon, in the dikaryophase — occurs before the nuclei fuse. The morphology of the mycelium, the mode of spore formation and the fruiting bodies are the basis for dividing the kingdom into classes (next section).

Phycomycetes have aseptate, coenocytic mycelium and spores made endogenously in a sporangium; their sexual spore is a zygospore. Ascomycetes, the sac fungi, make conidia outside (on conidiophores) and ascospores inside sacs called asci, grouped in ascocarps; Neurospora is used extensively in biochemical and genetic work, and morels and truffles are edible delicacies. Basidiomycetes — mushrooms, bracket fungi, puffballs, rusts and smuts — usually lack asexual spores and sex organs: two somatic cells of different strains fuse, the dikaryotic result gives rise to a basidium, where karyogamy and meiosis produce four basidiospores borne outside it, in basidiocarps. Deuteromycetes, the imperfect fungi, are known only by their asexual or vegetative phases and reproduce only by conidia; when the ‘perfect’ (sexual) stage of one is found, it is moved — usually to the ascomycetes or basidiomycetes. Sometimes the asexual and sexual stages of one fungus had carried two names until the link was established.

Kingdom Plantae includes all eukaryotic, chlorophyll-containing organisms commonly called plants. A few are partially heterotrophic: the insectivorous bladderwort and Venus fly trap, and the parasite Cuscuta. Plant cells have prominent chloroplasts and walls mainly of cellulose (cell structure is Chapter 8). Plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms. The life cycle has two phases — the diploid sporophytic and the haploid gametophytic — that alternate: alternation of generations. How long each phase lasts, and whether each is free-living or dependent, varies among plant groups (Chapter 3).

Kingdom Animalia: heterotrophic, eukaryotic, multicellular organisms whose cells lack walls. They depend directly or indirectly on plants for food, digest food in an internal cavity, and store reserves as glycogen or fat. Nutrition is holozoic — by ingestion. They follow a definite growth pattern into adults of definite shape and size; higher forms have elaborate sensory and neuromotor mechanisms; most can move. Sexual reproduction is by copulation of male and female followed by embryological development. The phyla are in Chapter 4.

Whittaker's five kingdoms make no mention of viruses, viroids, prions — acellular agents — or lichens. Viruses found no place because they are not considered truly ‘living’ if living means having a cell structure. They are non-cellular, with an inert crystalline structure outside the living cell; once they infect a cell they take over its machinery to replicate themselves, killing the host. ‘Virus’ means venom or poisonous fluid. Dmitri Ivanowsky (1892) recognised microbes causing the mosaic disease of tobacco, smaller than bacteria because they passed through bacteria-proof filters. M.W. Beijerinck (1898) showed that the extract of infected tobacco infected healthy plants, named the pathogen ‘virus’ and called the fluid Contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed viruses could be crystallised and the crystals are largely protein. Viruses are inert outside their specific host and are obligate parasites.

Besides protein, viruses contain genetic material — RNA or DNA, never both. A virus is a nucleoprotein, and its genetic material is infectious. In general, plant viruses have single-stranded RNA; animal viruses have single- or double-stranded RNA or double-stranded DNA; bacteriophages (viruses of bacteria) are usually double-stranded DNA. The protein coat, the capsid, made of subunits called capsomeres, protects the nucleic acid; capsomeres are arranged in helical or polyhedral forms. Viruses cause mumps, small pox, herpes and influenza, and AIDS; in plants the symptoms are mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

Viroids: in 1971, T.O. Diener discovered an infectious agent smaller than viruses that caused potato spindle tuber disease. It was a free RNA lacking the protein coat of viruses — hence ‘viroid’ — and of low molecular weight. Prions: certain infectious neurological diseases are transmitted by an agent consisting of abnormally folded protein, similar in size to viruses. The best-known prion diseases are bovine spongiform encephalopathy (BSE), ‘mad cow disease’, in cattle, and its analogous variant Creutzfeldt–Jakob disease (CJD) in humans.

Are viruses alive? NCERT asks the question twice (page 20 and exercise 12) and leaves it open. The positions, side by side:

Lichens are symbiotic, i.e. mutually useful, associations between algae and fungi. The algal partner is the phycobiont (autotrophic) and the fungal partner the mycobiont (heterotrophic). The alga prepares food for the fungus; the fungus provides shelter and absorbs mineral nutrients and water for its partner. So close is the association that a lichen in nature looks like a single organism. Lichens are very good pollution indicators — they do not grow in polluted areas.

The chapter asks two questions in its running text.

Numbers

One-liners

• Aristotle classified plants into trees, shrubs and herbs.
• The two-kingdom system did not separate prokaryotes from eukaryotes.
• R.H. Whittaker proposed the five-kingdom classification in 1969.
• All prokaryotes are placed in Kingdom Monera.
• All single-celled eukaryotes are placed in Kingdom Protista.
• Fungal cell walls contain chitin; plant cell walls contain cellulose.
• Bacteria show the most extensive metabolic diversity of any group.
• Methanogens produce methane (biogas) from the dung of cattle.
• Cyanobacteria fix atmospheric nitrogen in heterocysts.
• Mycoplasma has no cell wall and is the smallest living cell known.
• Diatoms are the chief producers in the oceans.
• Red tides are caused by dinoflagellates such as Gonyaulax.
• Euglenoids have a protein-rich pellicle instead of a cell wall.
• Plasmodium, a sporozoan, causes malaria.
• Trypanosoma, a flagellated protozoan, causes sleeping sickness.
• Ascomycetes and basidiomycetes show a dikaryotic stage.
• Deuteromycetes are called imperfect fungi because no sexual stage is known.
• Puccinia causes wheat rust; Ustilago causes smut.
• A virus contains either DNA or RNA, never both.
• A viroid is free RNA without a protein coat.
• Prions are abnormally folded proteins that cause BSE and CJD.
• In a lichen, the alga is the phycobiont and the fungus the mycobiont.
• Lichens do not grow in polluted areas.

Traps to avoid

• Blue-green algae are bacteria, not algae.
• Slime moulds are protists, not fungi.
• A slime mould's plasmodium is not the malaria parasite Plasmodium.
• A dikaryon (n + n) is not diploid until karyogamy.
• Plasmogamy comes before karyogamy.
• Conidia and basidiospores are exogenous; ascospores are endogenous.
• Rusts and smuts are basidiomycetes.
• Diatom walls are silica; dinoflagellate plates are cellulose.
• A viroid lacks protein; a prion lacks nucleic acid.
• Ivanowsky discovered the virus; Beijerinck named it.
• Typhoid and cholera are bacterial, not protozoan.
• Lichens indicate clean air by being present.
• The three-domain system ranks domains above kingdoms.
• NCERT's summary breaks off mid-sentence and skips viruses and lichens.

Prelims — what UPSC asks

• Asked so far: plankton (1982); protozoan diseases (1983, a flawed key) and disease–pathogen matches (1998); lichens (1984, 2014); viruses — why antivirals are fewer (1989), multiplying only in hosts (1989), ‘is HIV living?’ (1990), enzymes, culture and vectors (2013), what they infect (2016), culture in synthetic media (2021), adenovirus vs retrovirus (2021), oceans, bacteria and transcription (2025); mycorrhiza (1990, 1999); red tides (1998); blue-green algae as biofertiliser (2000, 2010, 2013); methane sources (2008); harmful algal blooms (2011); decomposers (2013); ocean primary producers (2021); ‘Gucchi’ morels (2022); extremophile microbes (2023).
• Likely next: Whittaker's criteria and Table 2.1 contrasts (wall material, nuclear membrane, nutrition); archaebacteria habitats and methanogens; heterocysts and nitrogen fixation; diatomaceous earth and its uses; euglenoid nutrition; protozoan groups and their diseases; fungal class examples (rust, smut, yeast, Penicillium, Trichoderma, truffles); virus–viroid–prion contrasts; prion diseases in the news; lichens as bio-indicators; new viruses and fungal infections in the news, classified by kingdom.

Mains — where it appears and how to answer

• GS3 Science & Technology / Environment: Mains 2023 asked how microorganisms can help meet the current fuel shortage — use this chapter's methanogens (biogas from dung), yeasts (fermentation) and photosynthetic microbes, with Class 12 Chapter 8 for the technology. Use the chapter also for answers on emerging infectious diseases (viruses, prions), antimicrobial resistance (bacteria's DNA transfer; India's 2017 national action plan), biofertilisers (blue-green algae, mycorrhiza), harmful algal blooms in coastal waters, methane from paddies and livestock, and lichen biomonitoring of air quality.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

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