The Five Kingdom Classification by Robert Whittaker & It's Limitations

 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

1. Prokaryotie (cells with primitive nucleus and lacking membrane enclosed organelles) and
2. Eukaryotic (cells with well-developed nucleus and membrane-enclosed organelles.

2. Level of cellular organization 

1. Unicellular or
2. Multicellular.

3. Mode of nutrition

1. Photoautotrophic nutrition, which is concerned with use of sunlight as energy source and CO2 as source of carbon.
2. 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
3. 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.

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.

1. It includes prokaryotes in Monera, but it does not distinguish between Bacteria and Archaea.
2. There is a great diversity in kingdom Protista. Protozoa are heterotrophs with animal like cell organization. Algae are photoautotrophs with plant like cell organization.
3. 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.
4. It does not explain about evolution and phylogenetic relationship.