Robert Whittaker (1969), proposed the first popular classification system called five-kingdom system, which was accepted widely. The classification system is based on three criteria.
1. Cell types
- Prokaryotie (cells with primitive nucleus and lacking membrane enclosed organelles) and
- Eukaryotic (cells with well-developed nucleus and membrane-enclosed organelles.
2. Level of cellular organization
- Unicellular or
- Multicellular.
3. Mode of nutrition
- Photoautotrophic nutrition, which is concerned with use of sunlight as energy source and CO2 as source of carbon.
- Heterotrophic nutrition, which is concerned with use of organie compounds as source of energy and carbon. It may be absorptive (which means absorption of nutrients by body wall) and
- Ingestive (which means intake of solid food particles ).
This system consists of one prokaryotic kingdom of Monera (which are prokaryotes) and four eukaryotic kingdoms - Protista, Fungi, Plantae and Animalia.
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| The Five Kingdom Classification by Robert Whittaker |
I] Kingdom Monera (Prokaryotes)
- It includes bacteria, cyanobacteria and archaebacteria.
- Unicellular, microscopic, solitary or colonial forms.
- Can respire aerobically or anaerobically and reproduce by asexual, sexual or vegetative methods.
- Act as decomposers and mineralisers and some may be concerned with nitrogen fixation also.
- No membrane bound organelles like mitochondria and Golgi complex. No nuclear membrane.
- Mode of nutrition: Nutritionally. these organisms exhibit great diversity from autotroph to heterotroph, phototroph to chemotroph and organotroph to lithotroph.
II] Kingdom Protista (Unicellular Eukaryotes)
- Usually (phytoplanktons or zooplanktons).
- Organisation ranges from unicellular to multicellular colonial forms. aquatic pue planktonic
- Some are commensals and some are parasites (heterotrophic or absorptive mode of nutrition)
- Some coloured algae contain various types of accessory pigments.
- Reproduction is by asexual as well as sexual modes and both haploid as well as diploid forms exist.
- Locomotion pseudopodial, ciliary or flagellar depending on the type of locomotary appendages present.
- The phytoplanktons are producers whereas the zooplanktons are consumers (holozoic nutrition).
III] Kingdom Fungi (Multicellular Decomposers)
- Include non-green plants that are the important decomposers and mineralizers.
- Hyphae constitute an entangled cottony mass of filaments termed mycelium that may be aseptate coenocytic or septate multicellular.
- Possess a cell wall made up of chitin or fungal cellulose and membrane bound organelles.
- May be solitary unicellular types or multicellular filamentous types called hyphae.
- Plastids are absent.
- Mode of nutrition heterotrophic (absorption and extracellular digestion), saprophytic or parasitic.
- Non-motile in forms but some may produce motile zoospores.
- Embryo formation does not occur but various fruiting bodies are formed.
- Most members pathogenic on plants as well as animals and cause various diseases as rust, smut and mildew (in higher plants).
- Useful fungi include yeast, Penicillium etc. used in bakery, brewery and antibiotic industries.
- Intercellular mycelia give out haustoria that absorb the nutrients from host cells.
- Asexual and sexual reproduction observed: members with no sexual reproduction known fungi imperfecti.
- Edible fungi: mushrooms and morels.
IV] Kingdom Plantae (Multicellular Producers)
- Members consist of muiticellular, green, photosynthetic, primary producers of the biosphere.
- Aquatic as well as terrestrial, large-bodied and non-motile (except bryophytes which produce motile zoospores).
- Cells bound by cellulosic cell wall and contain the photosynthetic pigments or chloroplast.
- Higher plants have complex cellular organization and vascular tissues (tracheophytes).
- Reproduction: by vegetative and sexual methods.
- All plants produce seeds except bryophytes and pteridophytes.
- Nutrition: autotrophic, absorptive (insectivorous plants) or parasitic (totally parasitic e.g. Cuscuta or partial parasite, e.g. Santalum).
- Lower plants possess simple thalloid organization and lack vascular tissue (bryophytes).
V] Kingdom Animalia (Multicellular Consumers)
- Consist of multicellular, eukaryotic organisms, lacking cell wall, plastids and photosynthetic pigments.
- Nutrition: by ingestion and subsequent digestion within the gastric cavity some possess absorption type of nutrition (parasites).
- Reproduction: asexual or sexual in lower forms while predominantly sexual in higher forms.
- Animals: main consumers in the food chain and may be primary consumers (herbivores) or secondary consumers (carnivores).
- Bear locomotory appendages and are, therefore, motile.
- Characterized by well developed muscular and nervous systems.
Limitations of Five Kingdom System
Most microbiologists do not accept the five-kingdom system for following reasons.- It includes prokaryotes in Monera, but it does not distinguish between Bacteria and Archaea.
- There is a great diversity in kingdom Protista. Protozoa are heterotrophs with animal like cell organization. Algae are photoautotrophs with plant like cell organization.
- Boundaries between Protista, Plantae and Fungi are poorly defined. For example, the red algae is not closely related to plants, still it is included in Plantae.
- It does not explain about evolution and phylogenetic relationship.
