Biological Classification (NEET Biology Class 11): Five Kingdoms, Monera to Animalia

Classification through History & the Five Kingdoms

🎯 NEET priority: High-yield. 5–7 questions every year. NCERT is asked almost line-for-line here — every kingdom, every example organism, every special term is fair game. Learn it all.

How do you sort millions of living things into a system that makes sense? That single question drives this chapter — and NEET loves testing every twist in the answer.

From two kingdoms to five

• Aristotle was the earliest to attempt a scientific classification, sorting plants into trees, shrubs and herbs, and animals by whether they had red blood.

• Linnaeus gave the Two Kingdom system — Plantae and Animalia. Its fatal flaw: it lumped prokaryotes with eukaryotes, and photosynthetic with non-photosynthetic organisms, so it was abandoned.

• R.H. Whittaker (1969) proposed the Five Kingdom Classification — Monera, Protista, Fungi, Plantae, Animalia — using cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.

The Five Kingdoms at a glance (memorise this grid)

The three-domain system — know the answer NEET wants

Later, Carl Woese proposed the three-domain system. It splits Kingdom Monera into two domains — Archaea (the archaebacteria) and Bacteria (the eubacteria) — and groups all eukaryotes into a third domain, Eukarya. So the three domains are Archaea, Bacteria and Eukarya. Splitting Monera this way also yields a six-kingdom classification: Archaebacteria, Eubacteria, Protista, Fungi, Plantae and Animalia. NCERT keeps this brief, but remember the proposer (Woese) and the three domain names — that is exactly what gets asked.

Exam trap: the flaw the five-kingdom system fixed was classifying organisms by a single character — "having a cell wall" — which wrongly lumped prokaryotic bacteria and blue-green algae together with eukaryotic plants and fungi.

Kingdom Monera — the Bacteria

Monera contains the bacteria — the most abundant micro-organisms on Earth, found everywhere from hot springs to deep oceans. They are all prokaryotic.

Bacterial shapes & nutrition

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

• Bacteria show the most extensive metabolic diversity of any group — autotrophic (photosynthetic or chemosynthetic) or, in the vast majority, heterotrophic.

Archaebacteria — the extremophiles

• Live in the harshest habitats, thanks to a different cell wall structure: halophiles (extreme salt), thermoacidophiles (hot springs), methanogens (marshy areas).

• Methanogens live in the gut of ruminants (cows, buffaloes) and produce methane (biogas) from dung.

Eubacteria, cyanobacteria & mycoplasma

• Cyanobacteria (blue-green algae) have chlorophyll a and are photosynthetic; some fix atmospheric nitrogen in specialised cells called heterocysts — e.g. Nostoc and Anabaena. They form blooms in polluted water.

• Chemosynthetic bacteria oxidise inorganic substances (nitrates, nitrites, ammonia) and help recycle nitrogen, phosphorus, iron and sulphur.

• Bacteria reproduce mainly by fission; under stress they form spores.

• Mycoplasma completely lack a cell wall, are the smallest living cells known, and can survive without oxygen.

Kingdom Protista — Single-Celled Eukaryotes

Protista holds all single-celled eukaryotes — the link between Monera and the higher kingdoms. Every protist has a true nucleus and membrane-bound organelles.

• Chrysophytes (diatoms & golden algae): microscopic plankton; diatoms have silica cell walls that fit like a soap-box and are indestructible — their deposits form diatomaceous earth. Diatoms are the chief producers in the oceans.

• Dinoflagellates: mostly marine, photosynthetic, with stiff cellulose plates. Red dinoflagellates (e.g. Gonyaulax) multiply to cause red tides; their toxins can kill fish.

• Euglenoids: freshwater; no cell wall but a protein pellicle; mixotrophic (photosynthetic in light, heterotrophic in dark). e.g. Euglena.

• Slime moulds: saprophytic; form an aggregation called a plasmodium; spores have true walls and resist adverse conditions.

• Protozoans are heterotrophic predators/parasites in four groups: amoeboid (pseudopodia; Amoeba, Entamoeba), flagellated (Trypanosoma → sleeping sickness), ciliated (Paramoecium, with a gullet), and sporozoans (Plasmodium → malaria).

Exam trap: diatomaceous earth (polishing/filtration), red tides (Gonyaulax), and "which protist causes which disease" are classic NEET one-liners.

Kingdom Fungi — the Four Classes

Fungi are heterotrophic eukaryotes with chitin cell walls. Except unicellular yeasts, their body is a network of thread-like hyphae called a mycelium. Multinucleate tubes without cross-walls are coenocytic hyphae.

Most are saprophytes (absorb dead organic matter); others are parasites or symbionts (with algae as lichens, with roots as mycorrhiza). The sexual cycle runs plasmogamy → karyogamy → meiosis (giving haploid spores), sometimes via an intervening dikaryon (n+n) stage.

The four classes

• Phycomycetes: aquatic/decaying wood; aseptate, coenocytic mycelium; asexual zoospores/aplanospores; sexual zygospore. e.g. Mucor, Rhizopus (bread mould), Albugo (mustard parasite).

• Ascomycetes (sac-fungi): septate mycelium; asexual conidia; sexual ascospores in sac-like asci. e.g. Penicillium (antibiotics), yeast (Saccharomyces), Aspergillus, Neurospora (used in genetics).

• Basidiomycetes: mushrooms, rusts, smuts; sexual basidiospores on a basidium. e.g. Agaricus (mushroom), Ustilago (smut), Puccinia (rust).

• Deuteromycetes (imperfect fungi): only asexual/vegetative phase known; reproduce by conidia; many are decomposers. e.g. Alternaria, Trichoderma, Colletotrichum.

Kingdoms Plantae & Animalia

These two kingdoms are detailed in later chapters, but NEET tests their defining features here.

• Plantae: all eukaryotic, chlorophyll-containing organisms with cellulose walls — algae, bryophytes, pteridophytes, gymnosperms, angiosperms. A few are heterotrophic: insectivorous Bladderwort and Venus flytrap, and the parasite Cuscuta.

• Plant life cycles show alternation of generations — a diploid sporophyte alternating with a haploid gametophyte.

• Animalia: heterotrophic, multicellular eukaryotes that lack a cell wall, store glycogen/fat, and have a holozoic mode of nutrition (ingestion). Higher forms show sensory/neuromotor systems and locomotion.

Viruses, Viroids, Prions & Lichens

Whittaker's five kingdoms leave out the acellular agents and lichens — NCERT covers them at the end, and NEET asks about them often.

Viruses

• Non-cellular, inert crystalline structures outside the host; obligate parasites that hijack host machinery to replicate.

• Ivanowsky (1892) found microbes causing tobacco mosaic disease; Beijerinck (1898) named the fluid Contagium vivum fluidum; Stanley (1935) showed viruses can be crystallised.

• Genetic material is RNA or DNA, never both. Plant viruses → usually ss-RNA; bacteriophages → usually ds-DNA. The protein coat is the capsid, made of subunits called capsomeres.

Viroids, Prions & Lichens

• Viroids: Diener (1971) found free RNA without a protein coat, causing potato spindle tuber disease.

• Prions: abnormally folded infectious proteins causing mad cow disease (BSE) and CJD.

• Lichens: symbiotic associations of an alga (phycobiont, autotroph) and a fungus (mycobiont, heterotroph); excellent pollution indicators.

Why this matters for NEET

• Pure recall chapter: "which kingdom / which class / which example" questions dominate — the example organisms (Nostoc, Gonyaulax, Paramoecium, Rhizopus, Puccinia) are as testable as the concepts.

• Favourite traps: Mycoplasma (no wall, smallest), diatoms (silica wall), RNA-or-DNA-never-both, viroid = RNA only, and the scientist–discovery matches (Ivanowsky/Stanley/Diener).

Test Yourself: MCQs, PYQs & Active Recall

Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.

Practice Questions

Q1. Who proposed the Five Kingdom Classification?

• (a) Linnaeus

• (b) Ernst Haeckel

• (c) Aristotle

• (d) R.H. Whittaker

Q2. Which kingdom contains only prokaryotic organisms?

• (a) Fungi

• (b) Monera

• (c) Protista

• (d) Plantae

Q3. The smallest living cells, lacking a cell wall, are:

• (a) Archaebacteria

• (b) Diatoms

• (c) Mycoplasma

• (d) Cyanobacteria

Q4. Diatoms are characterised by cell walls made of:

• (a) Silica

• (b) Peptidoglycan

• (c) Chitin

• (d) Cellulose

Q5. Red tides in the sea are caused by:

• (a) Dinoflagellates

• (b) Chrysophytes

• (c) Slime moulds

• (d) Euglenoids

Q6. Which protozoan causes malaria?

• (a) Paramoecium

• (b) Entamoeba

• (c) Plasmodium

• (d) Trypanosoma

Q7. Nitrogen-fixing cyanobacteria carry out fixation in specialised cells called:

• (a) Heterocysts

• (b) Pellicle

• (c) Asci

• (d) Conidia

Q8. The bread mould Rhizopus belongs to which class of fungi?

• (a) Deuteromycetes

• (b) Ascomycetes

• (c) Basidiomycetes

• (d) Phycomycetes

Q9. Which statement about viruses is correct?

• (a) They contain both RNA and DNA

• (b) They are obligate parasites with a protein capsid

• (c) They have a cellulose cell wall

• (d) They reproduce by binary fission

Q10. Viroids differ from viruses in that they:

• (a) Have only DNA

• (b) Are larger than bacteria

• (c) Have a thick protein coat

• (d) Are free RNA without a protein coat

NEET Previous Year Questions (PYQs)

Real NEET-pattern recall questions with explanations in our own words.

Q11. Methanogens belong to: (NEET PYQ)

• (a) Archaebacteria

• (b) Eubacteria

• (c) Slime moulds

• (d) Dinoflagellates

Q12. Which of the following is the chief producer in the oceans? (NEET PYQ)

• (a) Diatoms

• (b) Cyanobacteria

• (c) Euglenoids

• (d) Dinoflagellates

Q13. The smallest living cells, completely lacking a cell wall, are: (NEET PYQ)

• (a) Mycoplasma

• (b) Archaebacteria

• (c) Diatoms

• (d) Cyanobacteria

Q14. Heterocysts, the site of nitrogen fixation, are found in: (NEET PYQ)

• (a) Rhizopus

• (b) Paramoecium

• (c) Nostoc

• (d) Euglena

Q15. The cell wall of diatoms is mainly composed of: (NEET PYQ)

• (a) Peptidoglycan

• (b) Chitin

• (c) Cellulose

• (d) Silica

Q16. Red tides are caused by the rapid multiplication of: (NEET PYQ)

• (a) Dinoflagellates

• (b) Euglenoids

• (c) Slime moulds

• (d) Diatoms

Q17. Sleeping sickness in humans is caused by: (NEET PYQ)

• (a) Paramoecium

• (b) Entamoeba

• (c) Trypanosoma

• (d) Plasmodium

Q18. Penicillium, a source of antibiotics, belongs to the class: (NEET PYQ)

• (a) Phycomycetes

• (b) Deuteromycetes

• (c) Ascomycetes

• (d) Basidiomycetes

Q19. The 'imperfect fungi', in which only the asexual phase is known, are the: (NEET PYQ)

• (a) Phycomycetes

• (b) Ascomycetes

• (c) Deuteromycetes

• (d) Basidiomycetes

Q20. Viroids differ from viruses in: (NEET PYQ)

• (a) Being larger than bacteria

• (b) Lacking a protein coat

• (c) Possessing a cell wall

• (d) Having only DNA

Q21. The genetic material of a bacteriophage is usually: (NEET PYQ)

• (a) Single-stranded DNA

• (b) Double-stranded RNA

• (c) Single-stranded RNA

• (d) Double-stranded DNA

Q22. Lichens are widely used as: (NEET PYQ)

• (a) Sources of biogas

• (b) Producers of antibiotics

• (c) Nitrogen fixers

• (d) Pollution indicators

Active Recall Prompt

Write everything you can recall, naming each group first. Cover: the five-kingdom history (Aristotle, Linnaeus, Whittaker); Monera (bacterial shapes, archaebacteria, cyanobacteria, mycoplasma); Protista (diatoms, dinoflagellates, euglenoids, protozoans); Fungi (the four classes with examples); and Viruses, Viroids, Prions, Lichens. Begin each fact with its organism/group and end it with a full stop.