Whittaker's Five Kingdom Classification and Its Basis

In 1969, the American ecologist R.H. Whittaker proposed a five-kingdom classification that remains the most widely taught scheme in Indian school biology. He grouped all organisms into five kingdoms -- Monera, Protista, Fungi, Plantae and Animalia -- using five broad, interconnected criteria rather than a single visible trait. Understanding these five criteria is essential, because the rest of this chapter (and much of what follows on Plantae and Animalia) repeatedly refers back to them.

1. Cell structure. The most fundamental division is between prokaryotic cells, which lack a membrane-bound nucleus and membrane-bound organelles (found only in Kingdom Monera), and eukaryotic cells, which possess a well-defined nucleus enclosed by a nuclear membrane along with membrane-bound organelles such as mitochondria and (in photosynthetic forms) chloroplasts (found in Protista, Fungi, Plantae and Animalia).

2. Body organisation. Within the eukaryotes, organisation ranges from the unicellular level (a single cell performs every life function, as in most Protista) to the multicellular level. Among multicellular eukaryotes, Whittaker further distinguished a simple loose or filamentous organisation (as in many fungi and algae) from a more advanced tissue or tissue/organ level of organisation, in which cells performing the same function are grouped into tissues, and tissues into organs, as seen in complex members of Plantae and Animalia.

3. Mode of nutrition. Organisms differ fundamentally in how they obtain nourishment. Autotrophic nutrition means an organism manufactures its own food, either by photosynthesis (using sunlight, as in most Plantae and many Protista) or by chemosynthesis (oxidising inorganic chemicals, as in certain bacteria). Heterotrophic nutrition means an organism depends on other organisms for food; this is further divided into saprophytic nutrition (absorbing nutrients from dead and decaying organic matter, typical of most fungi) and holozoic/parasitic nutrition (ingesting solid food or living off a host, typical of Animalia and many parasitic protists).

4. Mode of reproduction. Reproduction ranges from simple asexual methods -- such as binary fission in bacteria, budding, or spore formation -- to more elaborate sexual reproduction involving the fusion of gametes and a well-defined, often complex, life cycle, as seen in higher plants and animals. …