Biological Classification Class 11 Notes

Earlier Systems of Classification

Before Whittaker's five-kingdom system, biologists used simpler classification schemes. Understanding these earlier systems helps appreciate why the modern system is more comprehensive.

• Three-Kingdom system (Ernst Haeckel, 1866): Added Kingdom Protista to include unicellular eukaryotes that did not fit neatly into Plantae or Animalia.
• Four-Kingdom system (Copeland, 1938): Separated Monera (prokaryotes) from Protista, recognising the fundamental prokaryote–eukaryote divide.
• Five-Kingdom system (Whittaker, 1969): The most widely accepted system — Monera, Protista, Fungi, Plantae and Animalia — based on cell structure, body organisation, mode of nutrition, reproduction and evolutionary relationships.

Kingdom Monera

Kingdom Monera includes all prokaryotic organisms — the simplest and most ancient forms of life. They are unicellular, lack a membrane-bound nucleus, and are found in almost every habitat on Earth.

• Cyanobacteria (blue-green algae) — photosynthetic prokaryotes that form colonies and have gelatinous sheaths. Examples: Nostoc, Anabaena. They are the earliest oxygen-evolving organisms.
• Chemosynthetic bacteria — obtain energy by oxidising inorganic substances (e.g. nitrifying bacteria, sulphur bacteria).
• Mycoplasma — the smallest known prokaryotes. They completely lack a cell wall and can survive without oxygen — they are obligate parasites.
• Bacteria reproduce primarily by binary fission. Under unfavourable conditions, some form endospores (highly resistant structures).

Kingdom Protista

Kingdom Protista includes all eukaryotic, unicellular organisms. They are often referred to as 'the stepping stone between prokaryotes and multicellular eukaryotes'. Their cell structure is eukaryotic — with a well-defined nucleus and membrane-bound organelles.

• Chrysophytes (diatoms and golden algae) — diatoms have silica-containing cell walls that form two halves fitting together like a petri dish. Their cell wall deposits form diatomaceous earth.
• Dinoflagellates — mostly photosynthetic, with two flagella arranged in grooves. Some are red (Gonyaulax) causing red tide.
• Euglenoids — mixotrophic organisms with a protein-rich pellicle instead of a cell wall. They have a photosynthetic pigment (chlorophyll) and can also absorb nutrients.
• Slime moulds — saprophytic organisms that live on decaying plant matter. Under favourable conditions they form a plasmodium (multinucleate mass); under unfavourable conditions they form fruiting bodies (sporangia).
• Protozoans — the animal-like protists. They are heterotrophic and classified by their mode of locomotion.

Kingdom Fungi

Kingdom Fungi is a kingdom of eukaryotic, multicellular (except yeast) organisms with chitinous cell walls. They are heterotrophs with saprophytic nutrition — they secrete digestive enzymes on food and absorb the nutrients (absorptive heterotrophy).

• Fungal body consists of thread-like hyphae (singular: hypha) that collectively form a mycelium.
• Reproduction occurs by spores — asexual (conidia, oidia) and sexual (oospores, ascospores, basidiospores).
• Sexual reproduction involves three stages: plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei) and meiosis.

Lichens and Mycorrhiza

Kingdom Plantae and Animalia — Overview

Viruses, Viroids and Prions

Viruses, viroids and prions are non-cellular entities that challenge our definition of life. They are considered 'organisms at the edge of life' because they cannot reproduce independently.

Solved Examples

Example: Why are viruses not classified in any of the five kingdoms?

Solution: Viruses are acellular — they lack a cellular structure, cannot reproduce independently, and do not exhibit metabolism outside a host cell. Since all five kingdoms are based on cellular life forms, viruses fall outside the system. They are often described as 'organisms at the edge of life'.

Example: A student observes a microorganism with a peptidoglycan cell wall that reproduces by binary fission and lives in hot springs. To which kingdom and domain does it belong?

Solution: The presence of a peptidoglycan cell wall and binary fission indicates it is a bacterium. However, hot-spring-dwelling organisms are typically Archaebacteria — but archaebacteria lack peptidoglycan. The correct answer is: Kingdom Monera (bacteria) and Domain Bacteria. The key discriminator is the peptidoglycan wall, which rules out Archaea.