Five Kingdom Classification - Robert Whittaker

  Five Kingdom Classification - Robert Whittaker

  • Robert Harding Whittaker (December 27, 1920 – October 20, 1980) was a distinguished American plant ecologist, active in the 1950s to the 1970s.
  • Whitaker proposed that organisms should be broadly divided into kingdoms, based on certain characters like the structure of the cell, mode of nutrition, the source of nutrition, interrelationship, body organization, and reproduction. According to this system, there are five main kingdoms. They are:
  1. Kingdom Monera
  2. Kingdom Protista
  3. Kingdom Fungi
  4. Kingdom Animalia
  5. Kingdom Plantae

Kingdoms are divided into subgroups at various levels. The following flowchart shows the hierarchy of classification.

Kingdom → Phylum  →  Class  →   Order  →   Family →   Genus  →   Species

Criteria for Delimiting Kingdoms: Whittaker has used five criteria for delimiting the different kingdoms:
(i) Complexity of cell structure, prokaryotic and eukaryotic
(ii) Complexity of body structure or structural organisation, unicellular and multicellular. (iii) Mode of nutrition which is divergent in multicellular kingdoms— photo-autotrophy in plantae, absorptive heterotrophy in fungi and ingestive heterotrophyin animalia. Photoautotrophic nutrition is also called holophytic nutrition while absorptive heterotrophy is known as holozoic nutrition. Absorptive heterotrophy is saprobiotic (= saprophytic) nu­trition.
(iv) Ecological life style like producers (plantae), decomposers (fungi) and consum­ers (animalia).
(v) Phylogenetic relationships. 

1. Monera - Kingdom of Prokaryotes:

  • EXAMPLE: mycoplasma, bacteria, actinomycetes and cyanobacteria or blue green alge. Along with fungi, they are decomposers and mineralizers of the biosphere.
  • NUTRITION: saprobic, parasitic, chemoautotrophic, photoautotrophic and symbiotic. The photoautotrophs include both aerobes and anaerobes.
  • CELL STRUCTURE: Monerans are basically unicellular (monos-single) prokaryotes and contain the most primitive of living forms. 
    • Cell wall is generally present. It contains peptidoglycan and polysaccharides Other than Cellulose.
    • All membrane bound cell organelles are absent, e.g., mitochondria, lysosomes, spherosomes, Golgi bodies, plastids, etc.
    • Genetic material is not organised into a nucleus,
    • The cells are microscopic (0.1 to a few microns in length).
  • MOVEMENT: The flagella, if present, are single stranded instead of being 11 stranded in eukaryotes. They are formed of protein called flagellin.
  • REPRODUCTION: Mitotic spindle is absent, Gametes are absent. Gene recombination has been discovered in certain cases. Otherwise reproduction is by asexual methods.

2. Protista— Kingdom of Unicellular Eukaryotes:

  • EXAMPLE: It in­cludes all unicellular and colonial eukaryotes,Mostly they are aquatic organisms forming plankton
  • NUTRITION: They have diverse modes of nutrition— photosynthetic, saprobic, parasitic, ingestive, or holozoic etc.
    • The photosynthetic plankton are called phytoplankton. They usually possess cell wall and constitute an important group of producers.
    • The non-photosyn­thetic, wall-less and holozoic plankton are called zooplankton.
    • Holozoic nutrition involves ingestion of particulate food. The protistans having holozoic nutrition are collectively called protozoa, though they have been excluded from kingdom animalia.
    • There is a group of Euglena-like organisms which have a dual mode of nutrition, holophytic or photosynthetic in light and holozoic in absence of light or presence of abundant organic matter. 
     
  • CELL STRUCTURE: 

    •  The cellular organisation is of two envelope type, i.e., besides plasma membrane, internal membranes occur around certain organelles.
    • Genetic material is organised in the form of nucleus. DNA is associated with histone proteins,
    • The aerobic forms possess mitochondria. Endoplasmic reticulum, golgi bodies, lysosomes and centrioles occur
    • Tissue system is, absent.
  • MOVEMENT: Flagella, if present, are 11 stranded with 9 + 2 organisation of microtubules that are composed of a protein named tubulin.
  • REPRODUCTION: Both sexual and asexual modes of reproduction are present. However, an embryo stage is absent.

 3.Fungi— Kingdom of Multicellular Decomposers:

  • EXAMPLE: It in­cludes all unicellular and colonial eukaryotes,Mostly they are aquatic organisms forming plankton.It contains achlorophyllous, spore producing, multicellular or multinucleate eukaryotic organisms.
  • NUTRITION: They are heterotrophic with absorptive type of nutrition. It is either saprobic or parasitic. Symbiotic association occurs with some algae and higher plants, e.g., lichens, mycorrhiza.
  • CELL STRUCTURE: 
    • The wall contains chitin and non-cellulosic polysaccharides. Cellulose also occurs in a few cases.
    • The cellular organisation is two envelope type.
    • In most cases, Golgi bodies are unicistemal.
    • The body of fungus is filamentous and is called mycelium. The filaments are known as hyphae.
    • Tissue differentiation is absent,
  • MOVEMENT: Non-motile.
  • REPRODUCTION: Reproduction is both asexual and sexual.

 4.Plantae — Kingdom of Multicellular Producers or Metaphyta:

  • EXAMPLE: The kingdom contains all photosynthetic eukaryotic multicellular plants and their non-photosynthetic relatives. At the lower level it contains multicellular algae— green, brown and red algae.
  • NUTRITION: Mode of nutrition is autotrophic.
  • CELL STRUCTURE: Cellulosic cell wall.
  • MOVEMENT: Non-motile.
  • REPRODUCTION: Reproduction is both asexual and sexual. Acces­sory spores are present in lower plants.

5. Animalia — Kingdom of Multicellular Consumers or Metazoa:

  • EXAMPLE: Groups included are sponges, coelenterates, worms, molluscs, arthropods, star fishes and vertebrates like fishes, amphib­ians, reptiles, birds and mammals.
  • NUTRITION: Mode of nutrition is Heterotrophic.
  • CELL STRUCTURE: cell wall absent.Plastids and photosynthetic pigments are absent,
  • MOVEMENT: Highly motile.Animals are motile or mobile as they have to search for their food. Sponges and corals are an exception
  • REPRODUCTION: Reproduction is both asexual and sexual.

  Advantages of Five Kingdom Classification:

  1. Separation of prokaryotes in a separate kingdom of Monera is a wise step because prokaryotes differ from all other organisms in their genetic, cellular, reproductive and physi­ological organisation.
  2. Many transitional or intermediate forms are present in the unicellular eukaryotes which had been included both amongst plants and animals. Separation of unicellular eukaryotes into kingdom protista has removed this anomaly.
  3. Fungi have never been related to plants. They have their own biochemical, physi­ological and structural organisation. Separation of fungi into a separate kingdom was long overdue.
  4. The five kingdom classification is based on levels of organisation and nutrition which evolved very early and became established in later groups that are existing today.
  5. In this classification, animal and plant kingdoms are more homogeneous than they are in two-kingdom classification.
  6. It has tried to bring out phylogenetic relationships even amongst the primitive forms. 

Disadvantage of Five Kingdom Classification: 

  1. In real terms the phylogenetic system cannot be established till all the distinct evolu­tionary tendencies are separated. This is not possible at the lower level.
  2. For example, certain green algae are known to obtain hydrogen from sources other than water like pho­tosynthetic bacteria, Similarly, Euglena can be photosynthetic as well as saprotrophic. Its relatives can have absorptive as well as ingestive type of heterotrophic nutrition.
  3. A distinction between unicellular and multicellular organisms is not possible in case of algae. It is because of this that unicellular green algae have not been included in kingdom Protista by Whittaker.
  4. Each group has so many diversities that it is difficult to keep them together. For example, monera and protista contain both walled and wall-less organisms, photosynthetic and non-photosynthetic organisms, unicellular and filamentous or mycelial organisms.
  5. Viruses have not been included in this system of classification.
  6. Archaebacteria differ from other bacteria in structure, composition and physiology.
  7. Mycoplasmas are quite different from bacteria where they have been placed along with prokaryotes.

Kingdom protista does not seem to be a natural group due to:

(i) Dinoflagellates are mesokaryotic and not eukaryotic.
(ii) A distinction of unicellular protistan algae and green algae included in volvocales is not valid,
(iii) Slime moulds are quite distinct from rest of the protists.
(iv) There are several evolutionary lines in protista,
(v) Protists of this kingdom have diverse modes of form, structure and life.

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