CBSE Class 11 Biology Chapter 2: Biological Classification Revision Notes

History & Criteria of Biological Classification

Since the dawn of civilization, multiple attempts have been made to classify living organisms. Aristotle was the pioneer who used simple morphological characters to classify plants into trees, shrubs, and herbs, and animals into two groups: those with red blood (Enaima) and those without (Anaima).

Later, Carl Linnaeus introduced the Two-Kingdom Classification System dividing organisms into Kingdom Plantae and Kingdom Animalia. However, this system failed to differentiate between eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic (green algae) and non-photosynthetic (fungi) organisms.

To address these limitations, R.H. Whittaker (1969) proposed the Five-Kingdom Classification. The main criteria used by Whittaker for this classification are:

1. Cell structure (prokaryotic vs. eukaryotic)
2. Thallus (body) organisation
3. Mode of nutrition (autotrophic, saprophytic, heterotrophic, holozoic)
4. Reproduction
5. Phylogenetic relationships (evolutionary history)

Comparative Account of Five Kingdoms

Key Scientific Terms & Glossary

Heterocyst

Specialized, thick-walled nitrogen-fixing cells present in certain filamentous cyanobacteria like Nostoc and Anabaena.

Diatomaceous Earth

The gritty, silica-rich cell wall deposits left behind by dead diatoms in their aquatic habitats over millions of years, used in polishing and filtration.

Mycorrhiza

A mutually beneficial symbiotic association between a fungus (such as Glomus) and the roots of higher vascular plants.

Viroid

An infectious agent smaller than a virus, consisting of free low-molecular-weight single-stranded RNA without any protective protein coat, discovered by T.O. Diener.

Prions

Infectious agents consisting of abnormally folded proteins that cause neurological diseases like Bovine Spongiform Encephalopathy (BSE) and Creutzfeldt-Jakob disease.

Plasmogamy

The initial stage of sexual reproduction in fungi involving the fusion of protoplasms between two compatible motile or non-motile gametes.

Must-Remember Key Highlights

• Kingdom Monera exclusively consists of prokaryotes. Archaebacteria are unique as they survive in extreme environments (halophiles in salty areas, thermoacidophiles in hot springs, methanogens in marshy areas and ruminant guts).
• Kingdom Protista brings together all unicellular eukaryotes. Diatoms (Chrysophytes) are known as the 'Chief Producers of the Oceans'.
• Dinoflagellates like Gonyaulax replicate rapidly, causing 'red tides' that release toxins harmful to marine life.
• Euglena acts as a mixotroph; it is photosynthetic in sunlight but behaves as a heterotroph in the dark.
• Under unfavorable conditions, slime moulds form spores with true, highly resistant walls containing cellulose.
• Fungi are classified into four classes (Phycomycetes, Ascomycetes, Basidiomycetes, Deuteromycetes) based on mycelium morphology, spore formation, and fruiting bodies.
• Deuteromycetes are called 'Fungi Imperfecti' because their sexual stage is unknown or yet to be discovered.
• Viruses, viroids, prions, and lichens are not classified under Whittaker's Five-Kingdom classification scheme.

Step-by-Step Sexual Reproduction Cycle in Fungi

1. Plasmogamy — Fusion of protoplasms of two compatible haploid hyphae (or gametes) belonging to different mating types.
2. Karyogamy — Fusion of the two nuclei. In some fungi (e.g., Ascomycetes & Basidiomycetes), this is delayed, leading to a temporary dikaryotic state (n + n) called the dikaryophase.
3. Meiosis — The diploid zygotic nucleus undergoes reduction division (meiosis) within the fruiting body to produce haploid spores (such as ascospores or basidiospores).

Board Exam Trap Alerts & Scoring Cues

1. Spore Location Trap: Keep a close eye on spore-origin questions.

• Phycomycetes form asexual spores (zoospores/aplanospores) endogenously inside the sporangium.
• Ascomycetes form asexual spores (conidia) exogenous on conidiophores, but sexual spores (ascospores) endogenously inside the asci.
• Basidiomycetes form sexual spores (basidiospores) exogenously on the basidium.

2. Virus vs. Viroid Distinction: Often asked in short-answer questions. Viruses contain a capsid (protein coat) surrounding their genetic material (either DNA or RNA). Viroids have no protein coat (free RNA) and their RNA has a much lower molecular weight.

Practice Questions with Solutions

• Q: Why are Deuteromycetes termed 'Fungi Imperfecti'? A: They are called 'imperfect' because only their asexual or vegetative phases are known. When their sexual stages are discovered, they are reclassified into Ascomycetes or Basidiomycetes.
• Q: What is the composition of the cell walls of diatoms, and what is its consequence? A: Diatom cell walls are embedded with silica, making them indestructible. Over millions of years, their accumulation on the ocean floor forms 'diatomaceous earth', which is used in polishing and filtration.
• Q: What is a dikaryon phase, and in which classes of fungi is it prominent? A: A dikaryon phase occurs when karyogamy does not immediately follow plasmogamy, resulting in two nuclei per cell (n + n). This phase is characteristic of Ascomycetes and Basidiomycetes.
• Q: Why are lichens referred to as excellent pollution indicators? A: Lichens are highly sensitive to air pollution, specifically sulfur dioxide (SO2), and will not grow in polluted industrial areas.

Frequently Asked Questions