In this article we will discuss about the Five Kingdom Classification of Organisms (From 1969 to 1990):- 1. Criteria for Delimiting Kingdoms 2. Monera— Kingdom of Prokaryotes 3. Protista— Kingdom of Unicellular Eukaryotes 4. Fungi— Kingdom of Multicellular Decomposers 5. Plantae — Kingdom of Multicellular Producers or Metaphyta 6. Animalia — Kingdom of Multicellular Consumers or Metazoa 7. Advantages of Five Kingdom Classification 8. Drawbacks of Five Kingdom Classification.
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.
Whittaker’s five kingdoms are Monera, Protista, Plantae, Fungi and Animalia.
Monera— Kingdom of Prokaryotes:
The kingdom includes all prokaryotes— mycoplasma, bacteria, actinomycetes and cyanobacteria or blue green alge. Along with fungi, they are decomposers and mineralizers of the biosphere.
(i) Monerans are basically unicellular (monos-single) prokaryotes and contain the most primitive of living forms,
(ii) They are varied in their nutrition— saprobic, parasitic, chemoautotrophic, photoautotrophic and symbiotic. The photoautotrophs include both aerobes and anaerobes,
(iii) The cells are microscopic (0.1 to a few microns in length),
(iv) Cell wall is generally present. It contains peptidoglycan and polysaccharides Other than Cellulose,
(v) Cells have one envelope type of organisation, i.e., the whole protoplast is covered by plasma membrane but internal compartmentalization is absent,
(vi) Genetic material is not organised into a nucleus,
(vii) DNA is naked, i.e., it is not associated with histone proteins. DNA lies coiled inside the cytoplasm. The coiled mass is known as nucleoid. It is equivalent to a single chromosome,
(viii) All membrane bound cell organelles are absent, e.g., mitochondria, lysosomes, spherosomes, Golgi bodies, plastids, etc.
(ix) The flagella, if present, are single stranded instead of being 11 stranded in eukaryotes. They are formed of protein called flagellin.
(x) Mitotic spindle is absent,
(xi) Gametes are absent. Gene recombination has been discovered in certain cases. Otherwise reproduction is by asexual methods,
(xii) Some of the monerans have the ability to convert di-nitrogen into ammonia state.
Protista— Kingdom of Unicellular Eukaryotes:
All prokaryotic organisms were grouped together under Kingdom Monera and the unicellular eukaryotic organisms were placed in Kingdom Protista.
Kingdom Protista has brought together Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both having cell walls) with Paramecium and Amoeba (which were earlier placed in the animal kingdom which lack cell wall. It has put together organisms which, in earlier classifications, were placed in different kingdoms.
This happened because the criteria for classification changed. This kind of changes will take place in future too depending on the improvement in our understanding of characteristics and evolutionary relationships.
Over time, an attempt has been made to evolve a classification system which reflects not only the morphological physiological and reproductive similarities, but is also phylogenetic, i.e., is based on evolutionary relationships. Kingdom protista includes flagellates (euglenophyceae), diatoms, dinoflagellates, slime moulds, sarcodines, ciliates, sporozoans, etc.
The important characteristics are:
(i) It includes all unicellular and colonial eukaryotes,
(ii) Mostly they are aquatic organisms forming plankton,
(iii) They have diverse modes of nutrition— photosynthetic, saprobic, parasitic, ingestive, or holozoic etc.
(iv) 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.
(v) 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.
Slime moulds are a group of protista which are intermediate between wall-less and walled organisms. They are devoid of a wall in vegetative phase. In the vegetative phase, the nutrition is of ingestive type. In the reproductive phase, the slime moulds come to have cell walls,
(vi) The cellular organisation is of two envelope type, i.e., besides plasma membrane, internal membranes occur around certain organelles,
(vii) Genetic material is organised in the form of nucleus. DNA is associated with histone proteins,
(viii) The aerobic forms possess mitochondria. Endoplasmic reticulum, golgi bodies, lysosomes and centrioles occur,
(ix) Flagella, if present, are 11 stranded with 9 + 2 organisation of microtubules that are composed of a protein named tubulin,
(x) Both sexual and asexual modes of reproduction are present. However, an embryo stage is absent,
(xi) Tissue system is, absent.
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.
Fungi— Kingdom of Multicellular Decomposers:
The kingdom includes moulds, mildews, yeasts, rust causing fungi, pencillium, morels, mushrooms, puffballs, bracket fungi, etc., i.e., all the fungi of the two kingdom classification except slime moulds:
(i) It contains achlorophyllous, spore producing, multicellular or multinucleate eukaryotic organisms. Basically unicellular yeasts are also included amongst fungi because their sexual reproduction is similar to that of some fungi,
(ii) The organism: 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. The saprobic fungi excrete hydrolytic or digestive enzymes in the external medium for digesting complex organic compounds. The parasitic fungi absorb nourishment directly from another living organism called host,
(iii) The body of fungus is filamentous and is called mycelium. The filaments are known as hyphae.
(iv) Hyphae are either multicellular or multinucleate. Nuclei are very small and show intra-nuclear spindle,
(v) The wall contains chitin and non-cellulosic polysaccharides. Cellulose also occurs in a few cases,
(vi) The cellular organisation is two envelope type,
(vii) In most cases, Golgi bodies are unicistemal.
(viii) Reproduction is both asexual and sexual,
(ix) Vegetative body or mycelium is not clear externally in most of the cases due to its subterranean nature. Reproductive bodies, are, however, apparent as in mushrooms, toadstools, puff balls, bracket fungi,
(x) Tissue differentiation is absent,
(xi) Food reserve is glycogen and fat.
The kingdom is important in nutrient cycling because along with some protistans and monerans, fungi are decomposers and mineralizers of the biosphere.
Plantae — Kingdom of Multicellular Producers or Metaphyta:
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. Other groups included in the kingdom plantae are bryophytes, pteridophytes and spermatophytes.
Important characters of this kingdom are as follows:
(i) Organisms are multicellular.
(ii) They are eukaryotic.
(iii) Body form is less regular,
(iv) Growth is usually indefinite,
(v) Organs are commonly external,
(vi) Irritability is poor,
(vii) Mode of nutrition is autotrophic.
(viii) The photosynthetic regions contain plastids in their cells. Due to photosynthetic activity, plants are called producers,
(ix) Most of the plants are restricted to land, sea-shores and fresh water reservoirs.
(x) The plants are usually fixed or free floating. Active locomotion is generally absent,
(xi) Structural differentiation into tissues is found except for certain algae,
(xii) Food reserve is usually starch and fat.
(xiii) Some of the plants are heterotrophic. They are mostly parasitic. A few are saprobes. A small group of autotrophic plants catch small animals and insects for obtaining extra nitrogen. They are called carnivorous or insectivorous plants,
(xiv) Reproduction is both asexual and sexual. Accessory spores are present in lower plants. An embryo stage is absent in the algal group but is present in others.
Animalia — Kingdom of Multicellular Consumers or Metazoa:
Members of this kingdom are also known as metazoa or multicellular animals. The kingdom has maximum number and most diverse types of organisms. It includes all the animals of the two kingdom classification except Protozoa.
Groups included are sponges, coelenterates, worms, molluscs, arthropods, star fishes and vertebrates like fishes, amphibians, reptiles, birds and mammals. Insects, a group of arthropods, outnumber all other organisms in variety and number.
The important characteristics of animalia are:
(i) Organisms are multicellular eukaryotes,
(ii) Body form is regular,
(iii) Organs are internal,
(iv) Growth is definite. Well defined growing points are absent,
(v) Cellular, tissue and organ- system levels of organisation occurs in different groups,
(vi) Response to stimuli is quick,
(vii) A cell does not possess central vacuole. Instead small vacuoles may occur,
(viii) Centrioles occur in the ceils,
(ix) A cell wall is absent,
(x) Plastids and photosynthetic pigments are absent,
(xi) The organisms have holozoic or ingestive type of nutrition. A few animals are, however, parasitic. They live on or inside the bodies of other eukaryotes,
(xii) Animals are motile or mobile as they have to search for their food. Sponges and corals are an exception,
(xiii) The organisms possess muscle cells for their mobility and nerve cells for conduction of impulses. They are, however, absent in sponges,
(xiv) Reproduction is mostly sexual. Regeneration of whole organism and formation of spores are found in lower animals,
(xv) Embryo stage is present,
(xvi) Ecologically animals are consumers. These consumers constitute links in the food chains and food webs.
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.
Drawbacks 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.
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.
2. 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.
3. 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.
4. Viruses have not been included in this system of classification.
5. Archaebacteria differ from other bacteria in structure, composition and physiology.
6. Mycoplasmas are quite different from bacteria where they have been placed along with prokaryotes.