Class 11 Ch 2 Biological Classification Notes for NEET

Complete NEET revision notes on Class 11 Ch 2 Biological Classification Notes for NEET – Five Kingdom system, Monera, Protista, Fungi, viruses & lichens, with high-yield tables .

1. History of Classification

System	Proposed by	Kingdoms	Key point
Two Kingdom	Linnaeus	Plantae, Animalia	Didn’t distinguish prokaryotes/eukaryotes, unicellular/multicellular, photosynthetic/non-photosynthetic
Five Kingdom	R.H. Whittaker (1969)	Monera, Protista, Fungi, Plantae, Animalia	Criteria: cell structure, body organisation, mode of nutrition, reproduction, phylogenetic relationships
Three Domain	Carl Woese	Archaea, Bacteria, Eukarya	Splits Monera into 2 domains → gives 6-kingdom system

 

• Aristotle = earliest classification (morphology): plants → trees, shrubs, herbs; animals → red blood / no red blood.
• Old “Plant kingdom” included bacteria, BGA, fungi, mosses, ferns, gymnosperms, angiosperms — united only by presence of a cell wall (a weak/artificial criterion).

2. Table 2.1 — Five Kingdom Comparison 

Character	Monera	Protista	Fungi	Plantae	Animalia
Cell type	Prokaryotic	Eukaryotic	Eukaryotic	Eukaryotic	Eukaryotic
Cell wall	Non-cellulosic (polysaccharide + amino acid)	Present in some	Present (chitin)	Present (cellulose)	Absent
Nuclear membrane	Absent	Present	Present	Present	Present
Body organisation	Cellular	Cellular	Multicellular/loose tissue	Tissue/organ	Tissue/ organ/ organ system
Mode of nutrition	Autotrophic (chemo/photosynthetic) + Heterotrophic (saprophytic/parasitic)	Autotrophic (photosynthetic) + Heterotrophic	Heterotrophic (saprophytic/parasitic)	Autotrophic (photosynthetic)	Heterotrophic (holozoic/ saprophytic)

3. Kingdom Monera

• Only prokaryotes; most abundant microorganisms; found everywhere including extreme habitats (hot springs, deserts, snow, deep oceans).
• Shape-based classification (classic diagram-based question):
  • Coccus (sing.) / Cocci (pl.) — spherical
  • Bacillus / Bacilli — rod-shaped
  • Vibrium / Vibrio — comma-shaped
  • Spirillum / Spirilla — spiral

3.1 Archaebacteria

• Live in extreme habitats; different cell wall structure → survive extreme conditions.
• Types (habitat-based):
  • Halophiles — extreme salty areas
  • Thermoacidophiles — hot springs
  • Methanogens — marshy areas; present in gut of ruminants (cow, buffalo); produce methane (biogas) from dung

3.2 Eubacteria (“true bacteria”)

• Rigid cell wall; flagellum if motile.
• Cyanobacteria (blue-green algae):
  • Chlorophyll a → photosynthetic autotrophs
  • Unicellular, colonial, or filamentous; freshwater/marine/terrestrial
  • Colonies surrounded by gelatinous sheath
  • Cause algal blooms in polluted water
  • N₂-fixation in specialised cells → heterocysts (e.g., Nostoc, Anabaena) — frequently asked
• Chemosynthetic autotrophic bacteria: oxidize inorganic substances (nitrates, nitrites, ammonia) → ATP; recycle N, P, Fe, S.
• Heterotrophic bacteria: most abundant type; decomposers; useful (curd from milk, antibiotics, N-fixation in legume roots) or pathogenic (cholera, typhoid, tetanus, citrus canker).
• Reproduction: mainly fission; spores under unfavourable conditions; primitive sexual-type DNA transfer between bacteria.
• Mycoplasma: completely lack cell wall; smallest living cells known; can survive without O₂; pathogenic in plants/animals.

4. Kingdom Protista

• All single-celled eukaryotes; boundaries not well-defined (overlaps with plant/animal/fungi kingdoms).
• Primarily aquatic; has nucleus + membrane-bound organelles; some have flagella/cilia.
• Reproduce asexually and sexually (cell fusion + zygote formation).
• 5 groups in NCERT: Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, Protozoans.

 

Group	Key features	Examples
Chrysophytes	Diatoms + golden algae (desmids); fresh/marine water; planktonic; photosynthetic; cell wall = 2 overlapping shells embedded with silica (indestructible) → diatomaceous earth (used in polishing, filtration); chief producers in oceans	Diatoms
Dinoflagellates	Mostly marine, photosynthetic; colour (yellow/green/brown/blue/red) depends on pigments; cell wall = stiff cellulose plates; 2 flagella (1 longitudinal + 1 transverse, in a furrow); rapid multiplication of red forms → red tides; release toxins that kill fish	Gonyaulax
Euglenoids	Fresh (stagnant) water; no cell wall, instead protein-rich pellicle (flexible body); 2 flagella (1 short, 1 long); mixotrophic — photosynthetic in light, heterotrophic (predatory) in dark; pigments same as higher plants	Euglena
Slime moulds	Saprophytic; body = plasmodium (aggregation, moves over decaying matter engulfing material); unfavourable conditions → fruiting bodies with spores (true walled, resistant, air-dispersed)	—
Protozoans	All heterotrophic; predators/parasites; primitive relatives of animals; 4 groups	—

Protozoan Subgroups (Very Important — Exact Matching Questions Common)

1. Amoeboid protozoans — fresh/sea water/moist soil; move & capture prey via pseudopodia; marine forms have silica shells; e.g. Amoeba; Entamoeba (parasitic)
2. Flagellated protozoans — free-living or parasitic; flagella; cause sleeping sickness; e.g. Trypanosoma
3. Ciliated protozoans — aquatic, actively moving; thousands of cilia; cavity (gullet) opens to outside; coordinated ciliary movement steers food into gullet; e.g. Paramoecium
4. Sporozoans — infectious spore-like stage in life cycle; e.g. Plasmodium (malaria)

5. Kingdom Fungi

• Heterotrophic eukaryotes; huge diversity in morphology/habitat; cosmopolitan (air, water, soil, on organisms); prefer warm & humid conditions.
• Body: except yeast (unicellular), all filamentous — thread-like hyphae → network = mycelium.
  • Coenocytic hyphae = continuous tubes with multinucleate cytoplasm (no septa)
  • Other hyphae have septa (cross walls)
• Cell wall: chitin + polysaccharides
• Nutritional modes:
  • Saprophytes — absorb from dead organic matter
  • Parasites — depend on living plants/animals
  • Symbionts — with algae = lichens; with plant roots = mycorrhiza
• Reproduction:
  • Vegetative: fragmentation, fission, budding
  • Asexual spores: conidia, sporangiospores, zoospores
  • Sexual spores: oospores, ascospores, basidiospores — produced in fruiting bodies

Sexual Cycle — 3 Steps (Exact Order, High Yield)

5. Plasmogamy — fusion of protoplasms (two motile/non-motile gametes)
6. Karyogamy — fusion of two nuclei
7. Meiosis in zygote → haploid spores

Dikaryon / dikaryophase: in Ascomycetes & Basidiomycetes, an intervening stage (n+n, two nuclei/cell) occurs between plasmogamy and karyogamy.

4 Classes of Fungi (Comparative Table — Very High Yield)

 

Class	Common name	Mycelium	Asexual spores	Sexual spores	Examples
Phycomycetes	—	Aseptate & coenocytic	Zoospores (motile)/ aplanospores (non-motile), in sporangium	Zygospores (isogamous /anisogamous /oogamous fusion)	Mucor, Rhizopus (bread mould), Albugo (mustard parasite)
Ascomycetes	Sac fungi	Branched, septate	Conidia on conidiophores (exogenous)	Ascospores in sac-like asci, arranged in ascocarps	Aspergillus, Claviceps, Neurospora (genetics tool), Penicillium, yeast (unicellular), morels & truffles (edible)
Basidiomycetes	Mushrooms, bracket fungi, puffballs	Branched, septate	Generally absent; vegetative reprod. by fragmentation	Basidiospores exogenously on basidium, in basidiocarps; karyogamy + meiosis occur in basidium	Agaricus (mushroom), Ustilago (smut), Puccinia (rust)
Deuteromycetes	Imperfect fungi	Septate, branched	Only conidia (sexual stage unknown)	None known	Alternaria, Colletotrichum, Trichoderma

Note: Once sexual stage of a Deuteromycete is discovered, it gets reclassified into Ascomycetes/Basidiomycetes — hence “imperfect fungi.”

6. Kingdom Plantae

• Eukaryotic, chlorophyll-containing (autotrophic); cell wall = cellulose; prominent chloroplasts.
• Few partially heterotrophic:
  • Insectivorous plants — Bladderwort, Venus flytrap
  • Parasite — Cuscuta
• Includes: algae, bryophytes, pteridophytes, gymnosperms, angiosperms.
• Alternation of generations: life cycle has 2 phases — diploid sporophytic & haploid gametophytic, alternating; lengths/independence of phases vary by group.

7. Kingdom Animalia

• Heterotrophic, eukaryotic, multicellular; no cell wall.
• Depend directly/indirectly on plants; digest food in internal cavity; store food as glycogen or fat.
• Nutrition = holozoic (ingestion).
• Definite growth pattern, shape, size; higher forms show sensory/neuromotor mechanisms; mostly capable of locomotion.
• Sexual reproduction by copulation (male + female) → embryological development.

8. Viruses, Viroids, Prions, Lichens (Not Part of 5-Kingdom System)

Viruses

• Non-cellular, inert crystalline structure outside living host cell; obligate parasites; not considered truly “living” (no cell structure).
• History (frequently asked):
  • Dmitri Ivanowsky (1892) — discovered causal agent of tobacco mosaic disease; passed through bacteria-proof filters (smaller than bacteria)
  • W. Beijerinck (1898) — showed infected extract causes infection in healthy plants; named it “virus”; called fluid Contagium vivum fluidum
  • M. Stanley (1935) — showed viruses can be crystallised; crystals = largely protein
• Composition: nucleoprotein — protein coat (capsid, made of capsomeres, helical or polyhedral) + genetic material (RNA or DNA, never both).
  • Plant viruses → usually single-stranded RNA
  • Animal viruses → single/double-stranded RNA OR double-stranded DNA
  • Bacteriophages → usually double-stranded DNA
• Diseases: mumps, smallpox, herpes, influenza, AIDS (humans); mosaic formation, leaf rolling/curling, yellowing, vein clearing, dwarfing, stunted growth (plants).

Viroids

• O. Diener (1971) — discovered; smaller than viruses; caused potato spindle tuber disease.
• Free RNA, no protein coat (unlike virus) — key difference; low molecular weight RNA.

Prions

• Infectious agent = abnormally folded protein (no nucleic acid); similar in size to viruses.
• Diseases: BSE (mad cow disease) in cattle; CJD (Creutzfeldt–Jakob disease) in humans.

Lichens

• Symbiotic association between algae and fungi (mutually beneficial).
• Phycobiont = algal component (autotrophic); Mycobiont = fungal component (heterotrophic).
• Algae → food for fungi; Fungi → shelter + absorb minerals/water.
• Good pollution indicators — don’t grow in polluted areas.

9. Quick-Fire Facts (Commonly Tested)

• Five Kingdom classification criteria (5): cell structure, body organisation, mode of nutrition, reproduction, phylogenetic relationships.
• Only Monera lacks nuclear membrane among the 5 kingdoms.
• Chlamydomonas & Chlorella — earlier under Plantae (Algae), now Protista.
• Paramoecium & Amoeba — earlier under Animalia, now Protista.
• Diatomaceous earth = soil-like accumulation of diatom cell walls (silica) over billions of years.
• Heterocyst = site of N₂ fixation in cyanobacteria.
• Smallest living cells = Mycoplasma (no cell wall).
• Virus term meaning = “venom/poisonous fluid.”
• No virus has both RNA and DNA together.
• Difference: Virus (nucleic acid + protein coat) vs Viroid (only RNA, no coat) vs Prion (only protein, no nucleic acid).