Biological Classification
NCERT Line-by-Line Breakdown for NEET 2026
Unit 1: Diversity in Living WorldClassification has evolved from Aristotle’s simple morphological approach to R.H. Whittaker’s complex phylogenetic system. In this chapter, we explore the Five Kingdom Classification and the unique entities like Viruses and Lichens that defy standard classification.
1. Systems of Classification
🏛️ Two Kingdom System
Proponent: Carolus Linnaeus
Kingdoms: Plantae & Animalia
Drawback: Could not distinguish between Eukaryotes/Prokaryotes, Unicellular/Multicellular, or Photosynthetic/Non-photosynthetic.
⭐ Five Kingdom System (1969)
Proponent: R.H. Whittaker
Kingdoms: Monera, Protista, Fungi, Plantae, Animalia
Criteria: Cell structure, Body organization, Mode of nutrition, Reproduction, and Phylogenetic relationships.
2. Kingdom Monera (Bacteria)
Bacteria are the sole members. They are the most abundant micro-organisms. Based on shape, they are grouped into four categories:
🔥 Archaebacteria: The Survivors
They live in harsh habitats due to a different cell wall structure (Branched chain lipids).
- Halophiles: Salty areas.
- Thermoacidophiles: Hot springs.
- Methanogens: Marshy areas & gut of ruminants (produce Biogas/Methane).
Eubacteria (“True Bacteria”): Characterized by a rigid cell wall and, if motile, a flagellum. Example: Cyanobacteria (Blue-green algae) have chlorophyll-a similar to green plants.
3. Kingdom Protista
“The boundaries of this kingdom are not well-defined.” It includes all single-celled Eukaryotes.
| Group | Key Feature / Example | NEET Importance |
|---|---|---|
| Chrysophytes | Diatoms & Golden Algae (Desmids). | Cell walls form two thin overlapping shells (Soap Box). “Diatomaceous Earth”. |
| Dinoflagellates | Mostly marine, photosynthetic. | Red Tides caused by Gonyaulax. |
| Euglenoids | Fresh water. Have a protein-rich layer called Pellicle. | Mixotrophic nutrition (Photosynthetic in light, Heterotrophic in dark). |
| Slime Moulds | Saprophytic protists. | Form aggregation called Plasmodium under suitable conditions. |
| Protozoans | Heterotrophs (Predators/Parasites). | Includes Amoeboid, Flagellated (Trypanosoma), Ciliated (Paramoecium), and Sporozoans (Plasmodium – Malaria). |
4. Kingdom Fungi
Heterotrophic organisms with cell walls made of Chitin. The body consists of long, slender thread-like structures called hyphae. The network of hyphae is known as mycelium.
Asexual reproduction by zoospores (motile) or aplanospores (non-motile).
Example: Mucor, Rhizopus, Albugo.
Branched/septate mycelium. Produce Ascospores endogenously.
Example: Penicillium, Yeast, Neurospora.
Sex organs absent. Plasmogamy occurs. Produce Basidiospores exogenously.
Example: Agaricus, Ustilago, Puccinia.
Only asexual or vegetative phases are known.
Example: Alternaria, Colletotrichum, Trichoderma.
5. Viruses, Viroids & Lichens
These acellular organisms are NOT mentioned in the Five Kingdom classification.
🦠 Virus History (NEET Favorites)
- Pasteur: Gave the name ‘virus’ (venom).
- D.J. Ivanowsky (1892): Recognized microbes in Tobacco Mosaic Disease.
- M.W. Beijerinck (1898): Called the extract “Contagium vivum fluidum” (infectious living fluid).
- W.M. Stanley (1935): Showed viruses could be crystallized.
Viroids: Discovered by T.O. Diener (1971). Smaller than viruses. Free RNA, no protein coat. Cause Potato Spindle Tuber disease.
Lichens: Symbiotic association between Algae (Phycobiont) and Fungi (Mycobiont). “Lichens are very good pollution indicators – they do not grow in polluted areas.”
📝 Rapid Fire MCQs
Q1. Which of the following are found in extreme saline conditions?
- A) Eubacteria
- B) Cyanobacteria
- C) Mycobacteria
- D) Archaebacteria
Click to check Answer
Answer: D) Archaebacteria (Halophiles).
Q2. Viroids differ from viruses in having:
- A) DNA molecules without protein coat
- B) RNA molecules with protein coat
- C) RNA molecules without protein coat
- D) DNA molecules with protein coat
Click to check Answer
Answer: C) RNA molecules without protein coat.