Key Points

Aristotle used simple morphological characters to classify plants and animals. Linnaeus later developed the Two Kingdom system (Plantae and Animalia), which was found inadequate as it did not distinguish between prokaryotes and eukaryotes.

R.H. Whittaker (1969) proposed a Five Kingdom Classification: Monera, Protista, Fungi, Plantae, and Animalia. The main criteria used were cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships.

This kingdom includes all prokaryotes, like bacteria. They are unicellular, lack a nuclear membrane, have noncellulosic cell walls, and show the most extensive metabolic diversity.

Monera is divided into Archaebacteria, which live in harsh habitats (e.g., halophiles, thermoacidophiles), and Eubacteria, or 'true bacteria', which have rigid cell walls and may possess flagella.

This kingdom includes all single-celled eukaryotes and forms a link with the other kingdoms. Members are primarily aquatic and include Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, and Protozoans.

Diatoms have indestructible silica walls, forming diatomaceous earth. Dinoflagellates can cause 'red tides'. Euglenoids are photosynthetic but can be heterotrophic. Slime moulds are saprophytic.

Fungi are heterotrophic, eukaryotic organisms whose bodies are made of hyphae that form a mycelium. Their cell walls are composed of chitin and polysaccharides.

Fungi reproduce asexually (spores) and sexually. In ascomycetes and basidiomycetes, sexual reproduction involves a dikaryotic stage ((n+n)) where two nuclei exist per cell before fusing to form a diploid ((2n)) cell.

The kingdom is divided into Phycomycetes (e.g., Mucor), Ascomycetes (sac-fungi, e.g., Penicillium), Basidiomycetes (club fungi, e.g., Agaricus), and Deuteromycetes (imperfect fungi).

This kingdom includes all eukaryotic, chlorophyll-containing organisms (autotrophs). They have cell walls made of cellulose and exhibit alternation of generations between diploid sporophytic and haploid gametophytic phases.

This kingdom comprises multicellular, eukaryotic, heterotrophic organisms that lack cell walls. Their mode of nutrition is holozoic, and they store food reserves as glycogen or fat.

Viruses are non-cellular, obligate parasites with an inert crystalline structure outside a host cell. They consist of genetic material (either DNA or RNA) protected by a protein coat called a capsid.

Viroids are infectious agents smaller than viruses, consisting only of free RNA without a protein coat. Prions are abnormally folded infectious proteins that cause neurological diseases.

Lichens are a symbiotic association between algae (phycobiont) and fungi (mycobiont). Algae provide food, and fungi provide shelter and absorb nutrients. They are very good indicators of pollution.