Question
Explain R.H. Whittaker’s Five Kingdom Classification system. What criteria did he use, and what are the defining features of each kingdom?
Solution — Step by Step
flowchart TD
A[Whittaker 1969] --> B[Monera - Prokaryotic]
A --> C[Protista - Eukaryotic Unicellular]
A --> D[Fungi - Absorptive Heterotrophs]
A --> E[Plantae - Autotrophs]
A --> F[Animalia - Ingestive Heterotrophs]
B --> B1[Bacteria, Cyanobacteria, Mycoplasma]
C --> C1[Amoeba, Euglena, Paramecium, Diatoms]
D --> D1[Mucor, Agaricus, Penicillium, Yeast]
E --> E1[Algae to Angiosperms]
F --> F1[Sponges to Mammals]Whittaker classified organisms based on: (1) Cell structure — prokaryotic vs eukaryotic, (2) Body organisation — unicellular vs multicellular, (3) Mode of nutrition — autotrophic, heterotrophic (absorptive or ingestive), or both, (4) Reproduction — asexual, sexual, or both, (5) Phylogenetic relationships — evolutionary history.
The only kingdom with prokaryotic cells (no membrane-bound nucleus). Includes bacteria, cyanobacteria, archaebacteria, and mycoplasma. Cell wall present (peptidoglycan in eubacteria). Nutrition can be autotrophic or heterotrophic. Reproduction mainly by binary fission.
Eukaryotic, mostly unicellular. A diverse “catch-all” kingdom. Includes Chrysophytes (diatoms), Dinoflagellates, Euglenoids (mixotrophic), Slime moulds, and Protozoans (Amoeba, Paramecium, Plasmodium). Nutrition varies: autotrophic, heterotrophic, or both.
Eukaryotic, mostly multicellular (except yeast). Cell wall made of chitin. Nutrition by extracellular digestion and absorption (saprophytic or parasitic). Store food as glycogen. Body made of hyphae forming mycelium. Examples: Mucor, Agaricus (mushroom), Penicillium.
Plantae: Eukaryotic, multicellular, autotrophic (photosynthetic). Cell wall of cellulose. Store food as starch. Range from algae to angiosperms. Animalia: Eukaryotic, multicellular, heterotrophic (ingestive nutrition). No cell wall. Most have organ-system level organisation. Range from sponges to mammals.
Why This Works
Whittaker’s system improved upon earlier two-kingdom (plant/animal) and three-kingdom systems by separating prokaryotes (Monera), unicellular eukaryotes (Protista), and fungi (which were wrongly placed in Plantae). Each kingdom is defined by a unique combination of cell type, organisation, and nutritional strategy.
Common Mistake
Students place fungi under Plantae because fungi “look like plants” and are non-motile. Fungi are in a separate kingdom because they have chitin cell walls (not cellulose), are heterotrophic (not autotrophic), and store glycogen (not starch). These are fundamental differences.