Vardaan Learning Institute
Class 11 Biology | Chapter 2
BIOLOGICAL CLASSIFICATION
Chapter Overview
Classification is the process of grouping organisms into convenient categories based on easily observable characters. Early scientists like Aristotle used simple two-kingdom systems. The modern Five Kingdom Classification by R.H. Whittaker (1969) is the most widely accepted system used in NCERT. It classifies all living organisms into: Monera → Protista → Fungi → Plantae → Animalia, based on cell structure, body organisation, nutrition, reproduction, and phylogenetic relationships.
Reference: NCERT Class 11 Biology (kebo102.pdf) — Chapter 2: Biological Classification
1. History and Need for Classification
The diversity of life on Earth is enormous — scientists have identified over 1.7 million species, and millions more remain undiscovered. Without systematic classification, studying and communicating about living organisms would be impossible.
Why Do We Classify?
- Easier study: Grouping similar organisms simplifies their study
- Universal communication: Provides a standard language to refer to organisms worldwide
- Understanding relationships: Reveals evolutionary connections between organisms
- Prediction: Knowing one member of a group helps predict properties of others
- Practical applications: Helps in medicine, agriculture, ecology, and conservation
1.1 History of Classification Systems
A. Aristotle (384–322 BC) — Earliest Classifier
- Classified plants into: Trees, Shrubs, and Herbs (based on morphology)
- Classified animals into: those with red blood (Enaima) and without (Anaima)
- Essentially a broad Two-Kingdom system
B. Linnaeus — Two Kingdom System (1758)
- Carl Linnaeus — “Father of Taxonomy” — proposed Systema Naturae
- Two kingdoms: Plantae and Animalia
- Drawbacks: bacteria and fungi lumped with plants; Euglena (photosynthetic + motile) couldn’t be placed; no distinction between prokaryotes and eukaryotes
C. Ernst Haeckel — Three Kingdom System (1866)
- Added Kingdom Protista for unicellular organisms
- Still did not separate prokaryotes from eukaryotes
D. Copeland — Four Kingdom System (1956)
- Added Kingdom Monera for prokaryotes (bacteria + BGA)
- Kingdoms: Monera, Protista, Plantae, Animalia
E. R.H. Whittaker — Five Kingdom System (1969) [NCERT Standard]
- Added Kingdom Fungi as a separate kingdom
- Five Kingdoms: Monera, Protista, Fungi, Plantae, Animalia
Basis of Five Kingdom Classification (Whittaker 1969)
Whittaker used five criteria:
- Cell structure: Prokaryotic vs Eukaryotic
- Body organisation: Unicellular vs Multicellular
- Mode of nutrition: Autotrophic vs Heterotrophic (absorptive/ingestive)
- Mode of reproduction: Asexual vs Sexual; type of life cycle
- Phylogenetic relationships: Evolutionary history and relatedness
“Make Peace For Plant Animals”
→ Monera | Protista | Fungi | Plantae | Animalia
🎯 NEET PYQ Focus
NEET 2022
Five Kingdom classification was proposed by:
Answer: R.H. Whittaker (1969)
NEET 2020
Kingdom Protista was first proposed by:
Answer: Ernst Haeckel (1866)
NEET 2019
Which is NOT a criterion for Five Kingdom classification?
Answer: Habitat — the 5 criteria are cell type, body organisation, nutrition, reproduction, phylogeny
Five Kingdom Classification Tree — Evolutionary tree showing divergence of Monera, Protista, Fungi, Plantae, and Animalia (Whittaker, 1969)
2. Kingdom Monera
Kingdom Monera includes all prokaryotic organisms — the most primitive, ancient, and metabolically diverse group. They are unicellular, have no membrane-bound nucleus, no membrane-bound organelles. They are found in every possible habitat — soil, water, hot springs, deep oceans, ice, extreme acid/alkaline conditions.
General Characteristics of Monerans
- Cell type: Prokaryotic — DNA is naked, circular, in nucleoid region (no nuclear membrane)
- Organelles: No membrane-bound organelles; ribosomes are 70S type (30S + 50S subunits)
- Cell wall: Present in most — made of peptidoglycan (murein) — polymer of sugars and amino acids
- Size: Very small — typically 1–10 μm
- Nutrition: Autotrophic (photosynthetic or chemosynthetic) OR heterotrophic (saprophytic, parasitic, symbiotic)
- Reproduction: Primarily by binary fission; also budding, conidia, endospores. Genetic recombination by conjugation, transformation, transduction
- Habitat: Ubiquitous — soil, water, air, inside organisms, hot springs, deep ocean vents
2.1 Archaebacteria (Ancient Bacteria)
Archaebacteria are the oldest organisms on Earth found in most extreme environments. Their cell wall does NOT contain peptidoglycan — they have unique lipids and proteins.
Three Groups of Archaebacteria
A. Halophiles (Salt-loving)
- Live in extremely saline environments — salt lakes, salt pans, Great Salt Lake, Dead Sea
- Example: Halobacterium — grows in up to 5M NaCl
- Cell membrane contains bacteriorhodopsin — a light-driven proton pump (purple pigment) that generates ATP without chlorophyll
- Cannot survive in normal saltwater; need extreme salinity
B. Thermoacidophiles (Heat + Acid-loving)
- Live in hot, acidic springs — temperatures up to 80°C, pH as low as 2
- Examples: Sulfolobus, Thermoplasma
- Found in volcanic craters, hydrothermal vents, sulfur springs (e.g., Yellowstone)
- Thermostable enzymes used industrially (e.g., Taq polymerase from Thermus aquaticus — used in PCR)
C. Methanogens (Methane-producing)
- Live in anaerobic (oxygen-free) environments — swamps, marshes, gut of ruminants, sewage sludge
- Produce biogas (methane, CH&sub4;) as metabolic waste
- Examples: Methanobacterium, Methanosarcina
- Found in gut of cattle, goats — contribute to greenhouse gas emissions
- Significance: Basis of biogas plants; methanogens from sewage produce biogas for cooking/electricity
“Halo-Thermo-Methano”
Halophiles = High salt | Thermoacidophiles = Hot + acid | Methanogens = Methane in gut/swamp
2.2 Eubacteria (True Bacteria)
Eubacteria are the “true bacteria” — most familiar and diverse group. They have a rigid cell wall made of peptidoglycan.
A. Classification Based on Shape
Fig. 2.1 — Different morphological shapes of bacteria: cocci, bacilli, vibrio, and spirillum
B. Gram Staining
Gram Positive vs Gram Negative
- Gram Positive (+): Thick peptidoglycan layer → retain crystal violet dye → purple/violet colour. More susceptible to penicillin. Examples: Staphylococcus, Streptococcus, Bacillus, Clostridium
- Gram Negative (−): Thin peptidoglycan + outer lipopolysaccharide (LPS) membrane → do NOT retain crystal violet → pink/red after safranin counterstain. More resistant to antibiotics. Examples: E. coli, Salmonella, Vibrio, Pseudomonas
- Gram staining developed by Christian Gram (1884)
C. Based on Oxygen Requirement
- Obligate aerobes: Require O&sub2; — e.g., Micrococcus, Bacillus subtilis
- Obligate anaerobes: Cannot survive in O&sub2; — e.g., Clostridium, Bacteroides
- Facultative anaerobes: Grow with or without O&sub2; — e.g., E. coli, Lactobacillus
- Microaerophiles: Require very low O&sub2; — e.g., Helicobacter pylori
D. Cyanobacteria (Blue-Green Algae / BGA)
Cyanobacteria — Special Points
- Photosynthetic prokaryotes — have chlorophyll a (same as plants) + phycocyanin (blue) + phycoerythrin (red) pigments
- No chloroplasts — photosynthesis occurs on thylakoid membranes
- Can be unicellular, colonial, or filamentous
- Some produce heterocysts — specialised cells for N&sub2; fixation (contain nitrogenase enzyme)
- Akinetes: Thick-walled resting spores formed in adverse conditions
- Hormogonia: Filament fragments for reproduction/dispersal
- Produced first O&sub2; on Earth ~3 billion years ago (Great Oxidation Event)
- Economic importance:
- Nostoc, Anabaena — N&sub2;-fixing biofertilizers in paddy fields
- Anabaena azollae — symbiotic in water fern Azolla
- Spirulina — protein-rich food supplement
- Bloom formation (Microcystis) — causes eutrophication, produces toxins
- Examples: Nostoc, Anabaena, Oscillatoria, Spirulina, Microcystis
Fig. 2.2 — Nostoc filament showing specialised heterocysts for nitrogen fixation
E. Chemosynthetic Autotrophic Bacteria
- Obtain energy by oxidation of inorganic substances (not light)
- Nitrosomonas — oxidises NH&sub3; → NO&sub2;¹&sup7; (nitritation)
- Nitrobacter — oxidises NO&sub2;¹&sup7; → NO&sub3;¹&sup7; (nitratation)
- Thiobacillus — oxidises H&sub2;S → S (sulfur bacteria)
- Iron bacteria — oxidise Fe²+ → Fe³+
- Play crucial role in biogeochemical cycles (N, S, Fe cycling)
F. Heterotrophic Bacteria
- Most abundant — include saprophytes, parasites, symbionts
- Pathogenic bacteria & diseases:
- Mycobacterium tuberculosis — Tuberculosis (TB)
- Mycobacterium leprae — Leprosy (Hansen’s disease)
- Vibrio cholerae — Cholera
- Salmonella typhi — Typhoid fever
- Clostridium tetani — Tetanus
- Streptococcus pneumoniae — Pneumonia
- Xanthomonas citri — Citrus canker (plant disease)
- Symbiotic bacteria:
- Rhizobium — root nodules of legumes; fixes atmospheric N&sub2;
- E. coli — human gut; synthesises Vitamin K and B&sub1;&sub2;
- Lactobacillus — produces lactic acid; used in curd/yoghurt
- Beneficial uses: Curd, nitrogen fixation, antibiotics (Streptomyces), biogas, leather tanning, sewage treatment
2.3 Reproduction in Bacteria
How Bacteria Reproduce
- Binary fission (primary): Cell divides into two equal daughters; E. coli divides every 20 minutes
- Budding: In some bacteria (e.g., Rhodomicrobium)
- Endospore formation: Dormant, heat-resistant spores in adverse conditions (Bacillus, Clostridium); NOT a reproductive structure — for survival only
- Conidia: In some actinomycetes (e.g., Streptomyces)
- Genetic recombination (Parasexual):
- Transformation: Uptake of naked DNA from environment (Griffith’s experiment; Avery, MacLeod, McCarty proof)
- Transduction: Gene transfer via bacteriophage (virus) as vector
- Conjugation: Direct cell-to-cell contact via sex pilus (F factor/plasmid transfer) — closest to true sexual reproduction
Fig. 2.3 — Binary fission: a dividing bacterium undergoing cell wall constriction and separation into two daughter cells
2.4 Mycoplasma
Mycoplasma — Smallest Living Cells
- Also called PPLO (Pleuro Pneumonia Like Organisms)
- No cell wall at all — pleomorphic (no fixed shape); resistant to penicillin
- Smallest living organisms capable of independent existence (~0.1 μm)
- Mostly parasitic — some obligate intracellular parasites
- Require cholesterol in their membrane (unusual for bacteria)
- Diseases in plants: Little leaf disease of brinjal, citrus greening
- Diseases in animals/humans: Atypical pneumonia (Mycoplasma pneumoniae)
🎯 NEET PYQ Focus — Monera
NEET 2023
Mycoplasma is an example of:
Answer: Organism without cell wall — smallest self-replicating organism
NEET 2022
Which bacteria fixes N&sub2; in root nodules of legumes?
Answer: Rhizobium — symbiotic, heterotrophic bacterium
NEET 2021
Methanogens belong to which kingdom and what do they produce?
Answer: Kingdom Monera (Archaebacteria); produce methane (CH&sub4;) — biogas
NEET 2019
Which organisms fix N&sub2; in paddy fields?
Answer: Nostoc and Anabaena (Cyanobacteria / BGA)
NEET 2018
Specialised cells for N&sub2; fixation in cyanobacteria are called:
Answer: Heterocysts
3. Kingdom Protista
Kingdom Protista includes all unicellular eukaryotes — the link between prokaryotes and more complex eukaryotes. Most are aquatic. They show enormous diversity in nutrition, locomotion, and reproduction.
General Characteristics of Protists
- Cell type: Eukaryotic — membrane-bound nucleus + all organelles
- Body: Primarily unicellular (some colonial forms)
- Habitat: Mostly aquatic (freshwater and marine)
- Nutrition: Autotrophic / Heterotrophic / Mixotrophic
- Locomotion: Pseudopodia, cilia, flagella, or non-motile
- Reproduction: Asexual (binary fission, multiple fission) and Sexual (syngamy, conjugation)
- Represent the first eukaryotes — evolutionary bridge between Monera and higher kingdoms
Fig. 2.4 — Overview of diverse protist forms found in Kingdom Protista
3.1 Chrysophytes (Diatoms and Golden Algae)
Diatoms — Key Features
- Most abundant protists in freshwater and marine ecosystems — base of aquatic food chains
- Cell wall made of silica (SiO&sub2;) — called frustules (fit like a soap box: epithecae + hypothecae)
- Frustules are indestructible — form large deposits called diatomaceous earth (kieselguhr)
- Pigments: Chlorophyll a, c + fucoxanthin (golden-brown) + xanthophylls
- Reserve food: Oils and chrysolaminarin (leucosin)
- Motility: Gliding movement (no flagella in vegetative cells)
- Sexual reproduction produces auxospores
- Ecological: Produce ~25% of Earth’s O&sub2;; major carbon sink
- Economic importance of diatomite: Filtration of oils/syrups, insulator, abrasive in toothpaste; Alfred Nobel mixed nitroglycerin with diatomite to make dynamite
- Examples: Cyclotella, Navicula, Pinnularia, Fragilaria
3.2 Dinoflagellates
Dinoflagellates — Key Features
- Mostly marine, photosynthetic — important primary producers in ocean
- Two flagella: One transverse (in groove called cingulum) + one longitudinal (in groove called sulcus) → spinning motion
- Cell wall (theca): Made of cellulose plates (armoured)
- Pigments: Chlorophyll a, c + peridinin (red-orange) + xanthophylls
- Dinokaryon: Unusual nucleus — chromosomes remain condensed throughout cell cycle; no histones
- Red tide: Explosive bloom of Gonyaulax colours sea red/brown; produces powerful toxins (saxitoxin, brevetoxin) that kill fish and poison shellfish
- Bioluminescence: Noctiluca — produces blue-green light at night
- Examples: Gonyaulax, Gymnodinium, Ceratium, Noctiluca
3.3 Euglenoids
Euglenoids — The Mixotrophic Protists
- Mostly freshwater organisms found in stagnant water
- No cell wall — instead have flexible protein-rich layer called pellicle
- Two flagella: One long (locomotion) + one short (within reservoir)
- Eyespot (stigma): Orange-red pigment spot for phototaxis
- Nutrition: Mixotrophic — photosynthetic in light (chlorophyll a, b); heterotrophic in dark (absorbs dissolved organic matter). Regain chlorophyll in light
- Reserve food: Paramylon (unique β-1,3-glucan) — NOT starch
- Some euglenoids purely heterotrophic (e.g., Peranema)
- Reproduction: Only asexual — binary fission (longitudinal)
- Shows features of BOTH plants and animals — classic example of why Two Kingdom classification failed
- Examples: Euglena, Phacus, Peranema, Trachelomonas
Euglena is placed in Protista because:
- Unicellular eukaryote — characteristic of Protista
- Has chlorophyll (plant-like) but no cell wall (non-plant-like)
- Motile with flagella (animal-like)
- Mixotrophic — cannot be strictly classified as plant OR animal
This is precisely why the Two Kingdom Classification failed — Euglena couldn’t fit in either Plants or Animals.
3.4 Slime Moulds
Slime Moulds (Myxomycetes)
- Found on decaying leaf litter, bark of fallen trees, damp soil in forests
- Vegetative phase: Exists as plasmodium — large, acellular (no cell walls), multinucleate, slimy mass of protoplasm; moves by protoplasmic streaming and engulfs food (holozoic nutrition)
- Reproductive phase: Under adverse conditions, plasmodium forms fruiting bodies (sporangia) on stalks → produce spores with true cell walls (cellulose or chitin)
- Spores extremely resistant — dispersed by wind; germinate to form swarm cells (flagellated or amoeboid)
- Examples: Physarum, Dictyostelium (cellular slime mould), Lycogala
3.5 Protozoans
Protozoans are heterotrophic, animal-like protists. All are unicellular eukaryotes. They ingest food by engulfing (holozoic nutrition) or absorb dissolved organic matter.
Four Groups of Protozoans
A. Amoeboid Protozoans (Sarcodina)
- Move and capture prey using pseudopodia (temporary cytoplasmic extensions)
- Freshwater: Amoeba proteus; Marine: Foraminifera (calcium carbonate shells → chalk and limestone deposits)
- Entamoeba histolytica — causes amoebic dysentery (amoebiasis) in humans
- Radiolaria — marine; siliceous skeleton; form radiolarian ooze on ocean floor
B. Flagellated Protozoans (Mastigophora)
- Move using one or more flagella
- Trypanosoma — causes sleeping sickness (African trypanosomiasis); transmitted by Tsetse fly
- Leishmania donovani — causes kala-azar (visceral leishmaniasis); transmitted by Sandfly (Phlebotomus)
- Trichomonas vaginalis — trichomoniasis (STI)
- Giardia lamblia — giardiasis (waterborne diarrhoea)
C. Ciliated Protozoans (Ciliophora)
- Possess cilia for locomotion and food gathering
- Two types of nuclei: Macronucleus (vegetative function/metabolism) + Micronucleus (sexual reproduction/genetic)
- Sexual reproduction by conjugation (exchange of micronuclei); Asexual by transverse binary fission
- Examples: Paramecium (most studied), Vorticella, Stentor, Balantidium coli (causes dysentery)
- Paramecium has oral groove → cytostome (cell mouth) for food ingestion
D. Sporozoans (Sporozoa)
- All are obligate intracellular parasites
- No locomotory organs in adult stage
- Named for production of spore-like stages in life cycle
- Most important: Plasmodium — causes malaria
- P. vivax — benign tertian malaria
- P. falciparum — malignant tertian/cerebral malaria (most dangerous)
- P. malariae — quartan malaria
- P. ovale — oval malaria
- Vector: female Anopheles mosquito
- Other: Monocystis (in earthworm gut), Toxoplasma (toxoplasmosis)
Life Cycle of Plasmodium (Malaria Parasite) — Alternation of generations between human host (schizogony) and female Anopheles vector (sexual cycle)
🎯 NEET PYQ Focus — Protista
NEET 2023
Organism responsible for “Red tide”:
Answer: Gonyaulax (Dinoflagellate) — produces toxins during bloom
NEET 2022
Diatomaceous earth is formed from cell walls of:
Answer: Diatoms — siliceous frustules accumulate over millions of years
NEET 2021
In absence of light, Euglena behaves as:
Answer: Heterotroph — loses chlorophyll, absorbs dissolved organic matter
NEET 2018
Sleeping sickness is caused by:
Answer: Trypanosoma, transmitted by Tsetse fly
4. Kingdom Fungi
Kingdom Fungi includes eukaryotic, non-photosynthetic organisms that obtain nutrition by absorptive heterotrophy (secrete digestive enzymes externally and absorb products). They are the principal decomposers on Earth, recycling nutrients from dead organic matter.
General Characteristics of Fungi
- Cell type: Eukaryotic
- Cell wall: Made of chitin (a tough polysaccharide of N-acetylglucosamine)
- Body: Mostly multicellular filamentous (except unicellular yeast)
- Mycelium: Mass of thread-like filaments called hyphae
- Hyphae types: Septate (cross-walls present) OR Coenocytic/aseptate (no cross-walls, multinucleate)
- Nutrition: Absorptive heterotrophs — saprophytic (on dead matter), parasitic (on living hosts), symbiotic, or predatory
- Reserve food: Glycogen (same as animals, unlike starch in plants)
- No chlorophyll — strictly heterotrophic
- Some are coprophilous (grow on dung) — e.g., Pilobolus
- Dikaryotic phase: many fungi have two distinct haploid nuclei per cell (“n+n” stage)
4.1 Structure of Fungal Body
- Thallus: The entire fungal body; ranges from unicellular to complex mycelium
- Hyphae: Thread-like tubular filaments; 1–10 μm in diameter
- Mycelium: Mass/network of hyphae
- Septate hyphae: Have cross-walls (septa) dividing hyphae into cells; pores allow cytoplasmic flow — found in higher fungi
- Coenocytic (aseptate) hyphae: No septa; multinucleate mass of cytoplasm — found in lower fungi (e.g., Zygomycetes)
- Haustoria: Special absorbing structures in parasitic fungi that penetrate host cells
- Rhizoids: Root-like structures for attachment and absorption (e.g., Rhizopus)
- Stolons: Horizontal hyphae connecting individual erect hyphae clusters (in Rhizopus)
4.2 Classification of Fungi — Four Classes
A. Phycomycetes (Algal Fungi / Lower Fungi)
Phycomycetes
- Hyphae: Coenocytic (aseptate, multinucleate)
- Habitat: Aquatic, on decaying wood, or as obligate parasites on plants
- Asexual reproduction: Zoospores (motile, in water) or aplanospores (non-motile, on land)
- Sexual reproduction: Produces zygospores (thick-walled resting spores from fusion of two gametes/gametangia)
- Examples: Mucor, Rhizopus (bread mould), Albugo (white rust of crucifers)
- Rhizopus stolonifer — common black bread mould; has stolon, rhizoids, sporangiophores bearing sporangia
- Albugo — parasitic; causes white rust/blister in mustard, potato
B. Ascomycetes (Sac Fungi) — Largest Group of Fungi
Ascomycetes
- Named for: Production of sexual spores in a sac-like structure called ascus (pl. asci)
- Hyphae: Septate and branched
- Sexual spores: Ascospores (produced endogenously in ascus, usually 8 per ascus)
- Asexual spores: Conidia (produced on special hyphae called conidiophores)
- Fruiting body: Ascocarp (cleistothecium, perithecium, apothecium)
- Dikaryophase (n+n): After plasmogamy (fusion of cytoplasm), before karyogamy (fusion of nuclei) — unique dikaryotic mycelium phase (called dikaryon)
- Examples:
- Aspergillus — aflatoxin producer, Aspergillosis lung infection, used in soy sauce, citric acid
- Penicillium — blue-green mould; P. notatum produces penicillin antibiotic
- Neurospora crassa — used extensively in genetic research (Beadle & Tatum’s one gene-one enzyme hypothesis)
- Saccharomyces cerevisiae (Baker’s/Brewer’s yeast) — unicellular; fermentation of sugars to alcohol + CO&sub2;; used in bread, beer, wine, biofuel
- Claviceps purpurea — ergot of rye; produces ergot alkaloids (LSD precursors)
- Morels and Truffles — edible delicacies
C. Basidiomycetes (Club Fungi)
Basidiomycetes
- Named for: Exogenous sexual spores (basidiospores) produced on club-shaped structures called basidium (pl. basidia)
- Hyphae: Septate, with clamp connections
- Asexual reproduction: Usually absent; some produce conidia or oidia
- Dikaryotic phase: Very extensive in life cycle; dikaryotic mycelium formed after plasmogamy
- Fruiting body: Basidiocarp (visible mushroom) = pileus (cap) + stipe (stalk) + gills (lamellae bearing basidia)
- Bioluminescence: Some emit faint glow (“foxfire”)
- Examples:
- Agaricus bisporus — common button mushroom (edible)
- Boletus, Cantharellus — edible mushrooms
- Ustilago (smut) — causes smut disease in wheat, maize
- Puccinia (rust) — causes black stem rust of wheat; heteroecious (two hosts: wheat + Berberis)
- Amanita phalloides — Death Cap; produces amatoxins — extremely poisonous
- Ganoderma — bracket fungi on trees
D. Deuteromycetes (Imperfect Fungi)
Deuteromycetes
- Called “Imperfect Fungi” because sexual reproduction unknown — only asexual conidia known; hence “perfect stage” not observed
- Reproduction: Only by conidia (asexual)
- Also called Fungi Imperfecti
- May be placed in Ascomycetes or Basidiomycetes when sexual stage discovered
- Examples:
- Alternaria — causes early blight of potato and tomato; allergenic conidia
- Colletotrichum — causes anthracnose diseases
- Trichoderma — biocontrol agent against plant pathogens
- Fusarium — vascular wilt diseases of many crops
Fig. 2.5 — Three representative fungi: Mucor (Phycomycetes), Aspergillus (Ascomycetes), and Agaricus (Basidiomycetes)
4.3 Reproduction in Fungi
Stages of Sexual Reproduction in Fungi
Sexual reproduction in fungi occurs in three distinct steps:
- Plasmogamy: Fusion of cytoplasm of two gametes/gametangia; nuclei remain separate → gives dikaryotic (n+n) cell
- Karyogamy: Fusion of two haploid nuclei → gives diploid (2n) nucleus; zygote formed
- Meiosis: Reduction division → produces haploid spores for dispersal
Note: In higher fungi (Ascomycetes, Basidiomycetes), plasmogamy and karyogamy are separated by a lengthy dikaryotic phase (n+n). This “n+n” phase is functionally diploid but genetically different from true diploidy.
Fungal Sexual Reproduction Flowchart — Three successive stages: Plasmogamy (cytoplasm fusion) → Karyogamy (nuclear fusion) → Meiosis (spore formation)
4.4 Economic Importance of Fungi
Harmful Activities of Fungi
- Plant diseases: Late blight of potato (Phytophthora infestans), wheat rust (Puccinia), smuts (Ustilago), powdery mildew, downy mildew, Tikka disease of groundnut (Cercospora)
- Animal diseases: Ringworm (Trichophyton, Microsporum), athlete’s foot (Epidermophyton), candidiasis (Candida albicans), aspergillosis (Aspergillus)
- Food spoilage: Bread mould (Rhizopus), blue-green mould (Penicillium) on citrus
- Mycotoxins: Aflatoxins by Aspergillus flavus (carcinogenic, on peanuts/maize); Ergotism by Claviceps purpurea
4.5 Lichens — Symbiotic Associations
Lichens — Key Facts
- Definition: Mutualistic association between a fungus (mycobiont) and algae/cyanobacteria (phycobiont)
- Fungal partner: Usually an Ascomycete (rarely Basidiomycete); provides water, mineral salts, protection, physical structure
- Algal partner: Green algae (e.g., Trebouxia) or Cyanobacteria (e.g., Nostoc); provides carbohydrates through photosynthesis
- Pioneer organisms: First to colonise bare rock surfaces; begin soil formation (ecological succession)
- Pollution indicators: Extremely sensitive to SO&sub2; (sulfur dioxide) air pollution — lichen absence = polluted air; hence called bioindicators
- Growth forms: Crustose (crust on rock), Foliose (leaf-like, lobed), Fruticose (branched, erect)
- Slow growers: Some grow <1 mm per year; can live for thousands of years
- Uses: Litmus paper (from Rocella tinctoria); dyes, perfumes (Evernia, Lobaria); reindeer food (Cladonia rangiferina — reindeer moss)
- Examples: Cladonia, Parmelia, Usnea, Peltigera
🎯 NEET PYQ Focus — Fungi
NEET 2023
Reserve food material in fungi is:
Answer: Glycogen (same as animals, NOT starch)
NEET 2022
Neurospora is used extensively in biochemical and genetic work. It belongs to class:
Answer: Ascomycetes
NEET 2021
Cell wall of fungi is made of:
Answer: Chitin (N-acetylglucosamine polymer)
NEET 2020
Lichens are used as pollution indicators because they are sensitive to:
Answer: SO2 (sulfur dioxide)
NEET 2019
The sexual stage of Alternaria has not been observed so it is placed in:
Answer: Deuteromycetes (Imperfect Fungi)
NEET 2018
Puccinia (wheat rust) belongs to which class?
Answer: Basidiomycetes — produces basidiospores
5. Viruses, Viroids, Prions & Lichens
These entities do not fit into any of the five kingdoms — they are not considered truly living (or are on the borderline of life). NCERT includes them in this chapter as an addendum to the five-kingdom classification.
5.1 Viruses
Viruses — Complete Notes
Discovery and History
- Dmitri Ivanovsky (1892): Showed that the causative agent of tobacco mosaic disease could pass through bacteria-proof porcelain filters — called it a filterable agent
- M.W. Beijerinck (1898): Showed the filtrate could infect healthy plants AND replicate — called it Contagium vivum fluidum (living contagious fluid). This was the first clear evidence of viruses
- W.M. Stanley (1935): Crystallised Tobacco Mosaic Virus (TMV) — showed viruses are mostly proteins (Nobel Prize 1946)
- The term “virus” comes from Latin meaning “poison”
General Characteristics
- Non-cellular entities: No cytoplasm, no organelles, no metabolism outside host
- Obligate intracellular parasites: Can only replicate inside a living host cell
- Structure: Nucleic acid (DNA or RNA, but never both) + Protein coat (capsid). Some also have an outer lipid envelope (enveloped viruses)
- Capsid: Made of protein subunits called capsomeres
- Core = nucleic acid: Either DNA or RNA (ss or ds)
- Viruses show crystalline structure outside host → not considered alive
- Viruses infecting bacteria are called bacteriophages (or simply phages)
Types of Viruses Based on Host
- Plant viruses: Mostly RNA; examples TMV, TYMV, CMV
- Animal viruses: Mostly DNA; but also RNA (influenza, rabies, HIV)
- Bacteriophages: Mostly DNA; T4 phage (T-even phage) most studied
Viral Diseases
Fig. 2.6 — Structure of Tobacco Mosaic Virus (TMV) showing helical RNA + protein capsid, and T4 bacteriophage structure
HIV — Retrovirus (Special)
- HIV is a Retrovirus — has ssRNA + Reverse Transcriptase enzyme
- Uses reverse transcriptase to convert RNA → DNA (reverse of normal central dogma)
- Attacks CD4+ helper T-lymphocytes → destroys immune system
- AIDS = Acquired Immuno Deficiency Syndrome (not a disease itself, but a syndrome of immunodeficiency)
5.2 Viroids
Viroids — Naked RNA Molecules
- Discovered by T.O. Diener (1971) while studying potato spindle tuber disease
- Viroids = Naked, circular, single-stranded RNA molecules WITHOUT a protein coat (no capsid)
- Smaller than the smallest virus
- Replicate using host’s RNA polymerase
- Pathogenic to plants; cause economically important diseases
- Examples:
- Potato Spindle Tuber Viroid (PSTVd) — spindle-shaped, elongated potato tubers
- Chrysanthemum stunt viroid, Citrus exocortis viroid
- Key Difference from Viruses: Viroids have NO protein coat; only RNA. Viruses have RNA/DNA + protein coat
5.3 Prions
Prions — Infectious Proteins
- Concept proposed by Stanley Prusiner (1982) — Nobel Prize 1997
- Prions = Abnormally folded proteins (proteinaceous infectious particles) with NO nucleic acid whatsoever
- Cause neurodegenerative diseases (spongiform encephalopathies) — the brain develops sponge-like holes
- Misfolded prion protein converts normal PrPC to misfolded PrPSc — propagates like an infection
- Diseases:
- BSE (Bovine Spongiform Encephalopathy) = Mad Cow Disease in cattle
- vCJD (variant Creutzfeldt-Jakob Disease) in humans (from eating BSE-infected beef)
- Kuru — in Fore tribe of Papua New Guinea (ritual cannibalism)
- Scrapie — in sheep
- No treatment available; uniformly fatal
🎯 NEET PYQ Focus — Viruses, Viroids, Prions
NEET 2023
Viroids differ from viruses in that they:
Answer: Have no protein coat (naked RNA without capsid)
NEET 2022
The first virus to be crystallised was:
Answer: TMV (Tobacco Mosaic Virus) by W.M. Stanley in 1935
NEET 2021
Prions are composed of:
Answer: Abnormally folded proteins (no nucleic acid)
NEET 2020
HIV is a retrovirus because it:
Answer: Contains RNA and Reverse Transcriptase enzyme (RNA → DNA)
NEET 2019
Viroids were discovered in potato spindle tuber disease by:
Answer: T.O. Diener (1971)
6. Kingdoms Plantae and Animalia — Overview
These two kingdoms are covered in detail in subsequent chapters. Here is a brief overview for comparative purposes.
Kingdom Plantae — Key Features
- Eukaryotic, multicellular, mostly terrestrial (some aquatic)
- Cell wall: Cellulose + pectin + hemicellulose + lignin (in vascular plants)
- Nutrition: Photoautotrophic (have chloroplasts)
- Reserve food: Starch
- Life cycle: Alternation of generations (sporophyte + gametophyte)
- Includes: Algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms
- Exception: Insectivorous plants (Drosera, Nepenthes) are partially heterotrophic; parasites (Cuscuta) are fully heterotrophic; yet classified in Plantae
Kingdom Animalia — Key Features
- Eukaryotic, multicellular
- No cell wall
- Nutrition: Holozoic heterotrophs (ingest and digest food internally)
- Reserve food: Glycogen
- Locomotion: Most are motile (at least in some stage)
- Ranges from Porifera (sponges) to Mammalia
- Sexual reproduction predominates; development often complex (embryonic stages)
7. High-Yield Summary for NEET
💡 EXAM INSIGHTS — Most Tested Facts
Classification Systems
Whittaker (1969) → 5 kingdoms. Haeckel → added Protista. Copeland → added Monera. Aristotle → 2 kingdoms. Two-kingdom failed because of Euglena/fungi/prokaryotes problem.
Archaebacteria Trio
Halophiles (salt) | Thermoacidophiles (heat+acid) | Methanogens (methane). Methanogens in ruminant gut + swamps. Cell wall NOT peptidoglycan.
BGA / Cyanobacteria
Prokaryotic. Heterocysts for N2 fixation. Nostoc + Anabaena in paddy fields. Spirulina = protein supplement. First O2 producers on Earth.
Mycoplasma
Smallest living organisms. NO cell wall. PPLO. Cause Little leaf disease of brinjal. Resistant to penicillin.
Diatoms
Siliceous cell wall (frustules). Diatomite used in filtration, dynamite. 25% O2 production. Marine + freshwater.
Dinoflagellates
Two flagella (transverse + longitudinal). Red tide = Gonyaulax. Bioluminescence = Noctiluca. Dinokaryon.
Euglena
Pellicle (no cell wall). Mixotrophic. Paramylon as reserve food. Two flagella. Eyespot. Freshwater.
Slime Moulds
Plasmodium (vegetative) → Fruiting bodies (adverse conditions). Spores with true cell walls.
Fungi Cell Wall & Reserve Food
Cell wall = Chitin. Reserve food = Glycogen. Unlike plants (cellulose + starch).
Fungi Classes — Spores
Phycomycetes → Zygospores. Ascomycetes → Ascospores (in ascus). Basidiomycetes → Basidiospores (on basidium). Deuteromycetes → No sexual spore (imperfect).
Penicillin + Neurospora
Penicillin from Penicillium notatum (Fleming 1928). Neurospora crassa in Ascomycetes → one gene-one enzyme hypothesis (Beadle + Tatum).
Lichens
Fungus (mycobiont) + Alga/Cyanobacteria (phycobiont). Pioneer species on bare rock. SO2 sensitive → pollution indicators. Litmus from Rocella.
Viruses
Ivanovsky (1892) → filter-passing agent. Beijerinck (1898) → Contagium vivum fluidum. Stanley (1935) → crystallised TMV. No cell structure; obligate intracellular parasites.
Viroid vs Prion
Viroid = naked RNA (Diener, 1971) → plants. Prion = abnormal protein NO nucleic acid (Prusiner, 1982) → Mad Cow, CJD, Kuru.
8. Memory Tricks & Mnemonics
💡 Memory Tricks
1. Five Kingdoms
“Make Peace For Plants & Animals”
M = Monera | P = Protista | F = Fungi | P = Plantae | A = Animalia
2. Archaebacteria — Three Types
“Hot Salty Marsh”
Hot → Thermoacidophiles | Salty → Halophiles | Marsh → Methanogens
3. Fungi Classes — In order
“Please Attend Biology Department”
P = Phycomycetes | A = Ascomycetes | B = Basidiomycetes | D = Deuteromycetes
4. Sexual Stages in Fungi
“Please Keep Meiosis”
P = Plasmogamy | K = Karyogamy | M = Meiosis
5. Protozoan Diseases
“AMal Try Sleep”
A = Amoeba (Dysentery) | Mal = Plasmodium (Malaria) | Try = Trypanosoma (Sleeping sickness) | Sleep = Leishmania (Kala-azar)
6. Spores in Fungi
Zygomycetes → Zygospore | Ascomycetes → Ascospore | Basidiomycetes → Basidiospore | Deutero → No sexual spore
7. Virus Discovery Order
“I Buy Stanley’s TM Virus”
Ivanovsky (1892) → Filter agent | Beijernick (1898) → Living fluid | Stanley (1935) → Crystallised TMV
9. Practice Questions
- Which of the following is NOT a feature of Kingdom Monera?
(a) Prokaryotic cell (b) 70S ribosomes (c) Membrane-bound nucleus (d) Peptidoglycan cell wall
Answer: (c) — Monera are prokaryotic; they lack membrane-bound nucleus
- Methanogens are found in the gut of ruminants. What is the significance?
Answer: They produce methane (biogas) as a metabolic byproduct. These anaerobic archaea digest cellulose with rumen bacteria, releasing CH4. They are also used in biogas plants.
- Arrange in correct order: Karyogamy, Plasmogamy, Meiosis — in sexual reproduction of fungi.
Answer: Plasmogamy → Karyogamy → Meiosis
- Which organism is used to study the “one gene-one enzyme hypothesis”?
(a) Saccharomyces (b) Neurospora crassa (c) Aspergillus (d) Penicillium
Answer: (b) — Neurospora crassa (Ascomycetes) was used by Beadle and Tatum
- Which of the following has cell wall made of silica?
(a) Dinoflagellates (b) Diatoms (c) Euglenoids (d) Slime moulds
Answer: (b) — Diatom cell walls (frustules) are made of SiO2
- Identify the INCORRECT pair:
(a) Halophiles — Dead Sea (b) Methanogens — Ruminant gut (c) Thermoacidophiles — Hot springs (d) Cyanobacteria — Archaebacteria
Answer: (d) — Cyanobacteria are Eubacteria (BGA), NOT Archaebacteria
- The phycobiont in a lichen is:
(a) Ascomycete fungus (b) Green alga or cyanobacteria (c) Basidiomycete (d) Protozoan
Answer: (b) — Phycobiont = photosynthetic partner = green alga or cyanobacteria
- Mad Cow Disease (BSE) is caused by:
(a) Virus (b) Bacterium (c) Viroid (d) Prion
Answer: (d) — Prions are abnormally folded proteins causing BSE, vCJD, Kuru
- Which of the following statement about viruses is CORRECT?
(a) Viruses contain both DNA and RNA (b) Viruses are cells (c) Viruses can replicate only inside a host cell (d) All viruses have protein coat + lipid envelope
Answer: (c) — Viruses are obligate intracellular parasites; replicate only inside living host cells
- Litmus is obtained from which type of organism?
(a) Algae (b) Fungi (c) Lichen (d) Bacteria
Answer: (c) — Litmus is obtained from the lichen Rocella tinctoria
Short Answer Questions — Board & NEET
- Why is Euglena placed in Kingdom Protista and not in Plantae or Animalia?
Answer: Euglena is unicellular and eukaryotic (Protista characteristic). It has chlorophyll (plant-like) but no cell wall (non-plant). It is motile with flagella (animal-like) and mixotrophic (photosynthetic in light, heterotrophic in dark). Since it shows features of BOTH plants and animals and is unicellular eukaryotic, it is correctly placed in Kingdom Protista.
- Give reasons why fungi are placed in a separate kingdom from plants.
Answer: (i) Fungi are heterotrophic (absorptive), plants are autotrophic; (ii) Fungi cell wall = chitin, plants = cellulose; (iii) Reserve food in fungi = glycogen, plants = starch; (iv) Fungi have no chloroplasts; (v) Fungi body is hyphae-based mycelium, plants have true tissues and organs.
- What is diatomaceous earth? Give two industrial uses.
Answer: Diatomite or diatomaceous earth is a fine siliceous powder formed from the silica frustules (cell walls) of dead diatoms accumulated over millions of years. Uses: (i) Used as a filtering agent for syrups, sugar, and petroleum oils (ii) Alfred Nobel used it as the absorbent/carrier for nitroglycerin to make dynamite (iii) Used as an abrasive/polishing agent in toothpaste.
- Differentiate between Viroid and Prion.
Answer: Viroid = naked circular ssRNA molecule, no protein coat, infects plants (e.g., potato spindle tuber). Prion = abnormally folded protein, NO nucleic acid at all, infects mammals causing neurodegenerative diseases (e.g., BSE, CJD).
- Why are deuteromycetes called “imperfect fungi”?
Answer: Deuteromycetes are called imperfect fungi because only their asexual (imperfect) stage — producing conidia — is known. Their sexual (perfect) stage has not been observed. In fungal nomenclature, the sexual stage is called the “perfect stage.” Examples: Alternaria, Fusarium, Colletotrichum.
