Board exam focus — Biological Classification (CBSE & HBSE)

This chapter traces the history of classification systems and details R. H. Whittaker's Five Kingdom Classification — Monera, Protista, Fungi, Plantae and Animalia. It covers the characteristics of each kingdom, the diversity of bacteria and archaebacteria, and the acellular entities: viruses, viroids, prions, and the symbiotic lichens.

Five Kingdom Classification: Monera and Protista

Evolution of Classification Systems

Criteria used by Whittaker: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.

Kingdom Monera (Bacteria)

• Prokaryotic, unicellular, no true nucleus or membrane-bound organelles.
• The most abundant micro-organisms; found in extreme habitats.
• Shapes: Coccus (spherical), Bacillus (rod), Vibrio (comma), Spirillum (spiral).
• Nutrition: Autotrophic (photosynthetic/chemosynthetic) or heterotrophic (saprophytic/parasitic).

Archaebacteria — live in extreme habitats; cell wall structure is responsible for survival:

Eubacteria ('true bacteria') — rigid cell wall; motile forms have flagellum.

• Cyanobacteria (blue-green algae): photosynthetic autotrophs, have chlorophyll a; can fix atmospheric N₂ in specialised cells called heterocysts (e.g., Nostoc, Anabaena).
• Chemosynthetic autotrophic bacteria: oxidise inorganic substances (nitrogen, iron, sulphur); play a role in recycling nutrients.
• Heterotrophic bacteria: decomposers; some are pathogens (cholera, typhoid, tetanus, citrus canker).

NEET Trap: Mycoplasma are the smallest living cells, lack a cell wall, and can survive without oxygen (anaerobic). Bacteria reproduce mainly by fission; under unfavourable conditions they form spores. A primitive type of DNA transfer (sexual reproduction) also occurs.

Kingdom Protista

• All single-celled eukaryotes (boundary with plants, animals and fungi is not well defined).
• Primarily aquatic. Form a link with plants, animals and fungi.
• Cell contains a well-defined nucleus and membrane-bound organelles; some have flagella/cilia.

Kingdom Fungi, Plantae and Animalia

Kingdom Fungi

• Eukaryotic, heterotrophic; cell wall made of chitin.
• Body is made of long, thread-like structures called hyphae; network of hyphae = mycelium.
• Hyphae may be coenocytic (multinucleate, aseptate) or septate (with cross walls).
• Nutrition: saprophytes (dead matter), parasites, or symbionts (lichens, mycorrhiza).
• Reproduction: Vegetative (fragmentation, fission, budding); Asexual (spores — conidia, sporangiospores, zoospores); Sexual (oospores, ascospores, basidiospores).

Sexual cycle (plasmogamy → karyogamy → meiosis): Two haploid hyphae fuse (plasmogamy), nuclei fuse (karyogamy), then meiosis in the zygote restores haploid spores. In some fungi an intervening dikaryotic stage (n + n) occurs.

Trap: Neurospora is used in biochemical and genetic studies. Penicillium gives the antibiotic penicillin. Deuteromycetes are called 'imperfect fungi' because only the asexual (imperfect) stage is known.

Kingdom Plantae

• Eukaryotic, multicellular, with cellulose cell walls and chlorophyll (autotrophic — photosynthesis).
• Includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
• A few are partially heterotrophic (insectivorous — Venus flytrap, Bladderwort; parasites — Cuscuta).
• Life cycle shows alternation of generations (sporophyte ↔ gametophyte).

Kingdom Animalia

• Eukaryotic, multicellular, heterotrophic organisms.
• No cell wall; cells held together by intercellular junctions.
• Holozoic nutrition (ingestion of food); store food as glycogen or fat.
• Show definite growth pattern, sensory and neuromotor coordination; most can move.
• Reproduction is mostly sexual.

Viruses, Viroids, Prions and Lichens

Acellular Entities — Outside the Five Kingdoms

Viruses, viroids and prions are not included in the five kingdoms because they are non-cellular and obligate parasites.

Viruses

• Term coined by Pasteur; virus means 'venom/poisonous fluid'.
• Non-cellular, inert outside the host (can be crystallised) — on the border of living and non-living.
• D. J. Ivanowsky (1892) recognised them from infected tobacco plants (Tobacco Mosaic Disease).
• M. W. Beijerinck demonstrated infectious fluid; called it Contagium vivum fluidum.
• W. M. Stanley showed viruses could be crystallised (made of protein).

Structure & Genetic Material:

NEET Trap: No virus contains both DNA and RNA. Bacteriophages = viruses that infect bacteria. Viruses cause diseases like mumps, smallpox, herpes, influenza, AIDS; in plants — mosaic, leaf rolling/curling, yellowing.

Viroids

• Discovered by T. O. Diener (1971).
• Free RNA only — no protein coat (smaller than viruses).
• RNA has low molecular weight.
• Caused Potato Spindle Tuber Disease (PSTD).

Prions

• Abnormally folded infectious proteins (no nucleic acid).
• Cause neurological diseases: Bovine Spongiform Encephalopathy (BSE / 'mad cow disease') and Cr-Jacob disease (CJD) in humans.

Lichens

• Symbiotic association between algae (phycobiont) and fungi (mycobiont).
• Algae → autotrophic, prepare food (photosynthesis).
• Fungi → provide shelter, absorb mineral nutrients and water.
• This is a mutualistic (mutually beneficial) relationship.
• Excellent pollution indicators — they do not grow in polluted areas (sensitive to SO₂).

Frequently asked questions

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Do these notes follow CBSE and HBSE?

Yes. The Biological Classification notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Biological Classification chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Biological Classification.