In biology, Kingdoms are the highest taxonomic groups of living organisms. Biologists since the time of Aristotle (384-322 BC) have divided the living world into two kingdoms, Plants and animals.
The word “plant” suggests grasses, bushes, shrubs, creepers, climbers, vines and trees and “animal” suggests cats, dogs, lions, tigers, birds, frogs and fish.
Further thought brings to mind such forms as ferns, mosses, mushrooms and pond scrums (algae), quite different but recognizable as “plants” and insects, lobsters, clams, worms and snails that are definitely animals.
But if you have ever had the pleasure of climbing over the rocky shore of the sea coast, looking at the organisms that cling to the rocks or live in a tide pool, you undoubtedly found some things that were difficult to recognize as animals and plants. The one-celled organisms visible under the microscope cannot easily be assigned to the plant or animal kingdom.
The German biologist Earnst Haeckel (1866) in his book Generelle Morphologie der Organismen suggested a three-kingdom system (Protista, Plantae and Animalia). In the third kingdom Protista he grouped all the single-celled organisms that are intermediate in many respects between plants and animals. Herbert Copeland (1956) have suggested establishing a fourth kingdom, originally called Mycota but later referred to as the Monera, to include the prokaryotes like bacteria and blue-green algae, which have many characteristics is common.
They have a single membrane system without a nucleus, and membrane bounded sub-cellular organelles such as mitochondria or chloroplasts. All other organisms are eukaryotes have a more complex structure with a nucleus and other organelles divided by intracellular membranes. R. H. Whittaker (1969) recognized an additional kingdom for the Fungi. The resulting five- kingdom system suggested by him has received wide acceptance. However, this may not be the end of the story. Some scientists have proposed that organisms be divided into even more (may be as many as 8) kingdoms.
Currently most biologists recognize six kingdoms: two prokaryotic kingdoms (Archaebacteria and Bacteria), a large unicellular eukaryotic kingdom (Protista) and three Multicellular eukaryotic kingdoms (Fungi, Plantae and Animalia). Viruses are not included in any of the present 5 kingdoms – mainly due to their many nonliving characteristics (for example, viruses are not cells).
[Note that the equivalences in this table are not perfect. For example, Haeckel placed the red algae (Haeckel’s Florideae; modern Floridiophyceae) and blue-green algae (Haeckel’s Archephyta; modern Cyanobacteria) in his Plantae, but in modern classifications they are considered protists and bacteria respectively. However, despite this and other failures of equivalence, the table gives a useful simplification]
I. Two Kingdoms Classification:
In his Systema Naturae, first published in 1735, Carolus Linnaeus distinguished two kingdoms of living things: Animalia for animals and Plantae (Vegetabilia) for plants. He classified all living organisms into two kingdoms – on the basis of nutrition and locomotion (mobility).
Linnaeus placed unicellular protozoans and multicellular animals (metazoans) under animal kingdom because of their compact body, holozoic nutrition (ingestion of food) and locomotion. All other organisms were grouped under plant kingdom because of their immobility, spread out appearance and autotrophic mode of nutrition. Thus, the traditional plant kingdom comprised bacteria, algae, plants and fungi
Demerits or Limitations:
(a) The two kingdom system of classification did not indicate any evolutionary relationship between plants and animals.
(b) It grouped together the prokaryotes (bacteria, BGA) with other eukaryotes.
(c) It also grouped unicellular and multi-cellular organisms together.
(d) This system did not distinguish the heterotrophic fungi and the autotrophic green plants.
(e) Dual organisms like Euglena and lichens did not fall into either kingdom.
(f) Slime mould, a type of fungi, can neither be grouped in fungi nor plants. This is because they are wall less and holozoic in vegetative stage, but develop cell wall in the reproductive stage.
(g) It did not mention some acellular organisms like viruses and viroids.
II. Five Kingdoms Classification:
R.H. Whittaker (1969), an American Taxonomist, classified all organisms into five kingdoms: Monera, Protista, Fungi, Plantae and Animal.
He used following criteria for classification:
(i) Complexity of cell structure
(ii) Complexity of body organization
(ii) The mode of nutrition
(iv) Life style (ecological role) and
(v) Phylogenetic relationship.
1. Monera (Kingdom of Prokaryotes):
(a) The members of this kingdom are microscopic prokaryotes.
(b) Monerans are mostly unicellular. But some are mycelial, filamentous (e.g. Nostoc) or colonial.
(c) The cells are prokaryotic with one envelope system or organisation.
(d) Cell wall usually present (except Mycoplasma) which composed of peptidoglycan or murein.
(e) True nucleus and other membrane bounded organelles absent.
(f) Genetic material is a circular naked DNA (without histone proteins) lies coiled near the centre of cell called nucleoid.
(g) More than one structural genes (cistrons) arranged together and regulated in units called operons.
(h) Ribosomes 70s type. (30S + 50S type)
(i) Cytoskeleton (microtubules, microfilaments and intermediate filaments) absent.
(j) Flagella if present consists of flagellant proteins.
(k) Nutrition may be autotrophic (photoautotrophic or chemoautotrophic). Saprot-rophic, parasitic or symbiotic.
(l) Reproduction mainly occurs by binary fission. Sexual reproduction (Gamete formation) absent. In some cases genetic recombination occurs.
(m) They are the important decomposers and mineralizes and help in recycling of nutrients in biosphere.
(n) Most are found in deep ocean floor, hot deserts, hot springs and even inside other organisms.
Monera includes archeabacteria, bacteria, cyanobacteria (BGA), and filamentous actinomycetes.
2. Protista (Kingdom of Unicellular eukaryotes):
(a) The members are unicellular and colonial eukaryotes.
(b) Most of them are aquatic and constitute plankton.
(c) Their eukaryotic cell body contains membrane bounded cell organelles like nucleus, mitochondria, endoplasmic reticulum and Golgi complex etc.
(d) They may have cilia or flagella for their movements which show 9 + 2 arrangements of microtubules.
(e) On the basis of nutrition, the protists are grouped as: (a) Photosynthetic protists (protistan algae) like diatoms, dinoflagellates and euglenoids. They are known as phytoplankton’s. (b) Consumer- decomposer protists (slime moulds) and (c) Predator protists (Protozoans).
(f) Both asexual and sexual modes of reproduction are present.
3. Fungi (Kingdom of Multi-cellular decomposers):
(a) The members are achlorophyllus, spore-bearing eukaryotic thallophytes.
(b) It includes unicellular yeasts and multi-cellular mycelial forms but not slime moulds.
(c) Cell wall composed of chitin (fungal cellulose), a nitrogen containing carbohydrate.
(d) Their mode of nutrition is saprobiotic or parasitic. They can also live as symbionts in association with algae as Lichens and with roots of higher plants as mycorrhiza.
(e) They help in decomposition of organic matter and help in recycling of minerals.
(f) Vegetative reproduction takes place by fragmentation, fission and budding.
(g) Asexual reproduction takes place by motile spores (zoospores) or non-motile spores (condia, oidia, aplanospores or chlamydospores).
(h) Sexual reproduction occurs by oospores, ascospores and basidiospores. Sexual reproduction involves three steps: (a) Plasmogamy (fusion of protoplasm between motile or non-motile gametes, (b) karyogamy (fusion of two nuclei) and (c) Meiosis in Zygote producing haploid spores.
Fungi include Phycomycetes (e.g. Mucor,Rhizopus, Albugo etc.), Ascomycetes (e.g. Sacbaromyces, Penicillium, Aspergillus, Claviceps, Neurospora etc.), Basidiomycetes (e.g. Agaricus, Mushrooms; Ustilago, Smuts; and Puccinia, rust fungi), Deuteromycetes.
4. Plantae (Kingdom of Multicellular Producers):
1. Their members are Multicellular, eukaryotic, chlorophyll-containing organisms. A few are parasitic (e.g. Cuscuta) or partially heterotrophic such as insectivorous plants (e.g. bladderwort, Venus fly trap, Sun few, Pitcher Plant etc.)
2. Their cells are eukaryotic with plastids and cell wall composed of cellulose.
3. Life cycle exhibit alternation between diploid sporophyte and the haploid gametophyte. This Phenomenon is called alternation of generation.
Plantae includes Green algae, brown algae, Red algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
5. Animalia (Kingdom of Multicellular consumers):
1. The members are eukaryotic Multicellular heterotrophic consumers.
2. Cells lack cell walls. They contain glycogen or fat as reserve food.
3. The organization may be cellular level (porifera), tissue level (colenterates), organ level (Platyhelminthes and Nemathelminthcs) and Organ system level (Annelids, Arthropods, Molluscs, Echinoderms and Chordates).
4. Symmetry may be radial, biradial, bilateral or asymmetrical.
5. On the basis of number of germ layers in embryonic gastrula, animals are diploblastic and triploblastic.
6. On the basis of presence of absence of coelom (body cavity) the animals are coelornates, pseudocoelomates or acoelomates.
Merits and Demerits of Five Kingdom:
1. Kingdom animalia become more homogenous with the separation of protozoa.
2. Kingdom plantae also become more homogeneous with the exclusion of bacteria, fungi and some unicellular algal forms.
3. Separation of prokaryotes into a separate kingdom – Monera is due for long time.
4. Separation of fungi from plants is a wise step.
5. Separation of intermediate or transitional forms of unicellular eukaryotes into kingdom – Protista is well thought out. So that the plant and animal kingdoms become more systematic.
6. It brings our phylogenetic relationships in the living world.
1. The Monera and Protista kingdoms are still heterogenous because both include autotrophic and heterotrophic forms and some with or without cell wall.
2. Phyolgeny in lower organisms is not fully reflected.
3. Slime moulds don’t fit in kingdom protista.
4. Red and brown algae are not related to other members of kingdom plantae.
5. Viruses have not been included in this system of classification.
III. Six Kingdoms and Three Domains Classification
In the years around 1980 there was an emphasis on phylogeny and redefining the kingdoms to be monophyletic. The Animalia, Plantae, and Fungi were generally reduced to core groups of closely related forms, and the others thrown into the Protista. Based on rRNA studies Carl Woese divided the prokaryotes into two kingdoms, called Eubacteria and Archaebacteria.
Such six-kingdom systems have become standard in many works. In 1990, Carl Woese proposed that the Eubacteria, Archaebacteria, and Eukarvota represent three primary lines of descent and accordingly he promoted them to domains, naming them Bacteria, Archaea, and Eukarya. This three-domain classification has received notable criticism but has generally displaced the older two-empire system as a way of organizing kingdoms together.
Initially bacteria were considered as primitive animals by some, primitive plants by the others and a few considered them as primitive fungi. But, now, bacteria are considered as the simplest prokaryotic organisms which evolved about 3.5 billion years ago and treated solely under Kingdom- Monera. On the basis of molecular homology of 16S RNA, monerans are divided into two major groups: the Archaebacteria and the Eubacteria.
Plant-like characters of bacteria:
(1) Cell wall is rigid.
(2) Some bacterial cells join together to form algae like simple filament.
(3) Bacteria absorb food form the medium in the form of sap (solution) through their general surface.
(4) Some bacteria, like green plants, have the capability of carbon assimilation (photosynthesis) and form organic food.
(5) Bacteria also synthesize some enzymes and vitamins.
Fungi-like characters of bacteria:
(1) Cell wall contains N-acetylglucosamine (NAG).
(2) Chlorophyll is absent
(3) Nutrition is parasitic or saprophytic.
(4) They reproduce by fission. Hence, related to fission fungi.
Animal-like characters of bacteria:
(1) Chlorophyll absent.
(2) Absence of true vacuole.
(3) Nutrition heterotrophic.
(4) Reserve food is glycogen.
(5) Motility through flagella.
Biological Status of Viruses:
The status of viruses is uncertain and highly debatable as they exhibit the characteristics of both non-living and living. As viruses are metabolically inert outside the host cells, they cannot be regarded as organism. They can be crystallized, but they cannot be reduced to the status of chemicals, because they have the capability to multiply and infect the living cells. Therefore, Andre Lwoff, a Nobel laureate and the former Director of Pasture Research Institute once said “A virus is a virus” which means they have both Living and non-living nature instead of being either of the two.
1. They have genetic material carrying heritable characters.
2. They can multiply only inside the living host cell.
3. They can mutate.
4. They respond to external stimuli like heat, chemical, UV radiation etc.
5. They are strictly obligate parasites.
1. They can be crystallized.
2. They lack protoplasm and cellular organization.
3. Respiration and metabolism absent.
4. Energy storing or utilizing device absent.
5. They cannot be cultured in a non-living culture medium.
6. They lack any evolutionary or phylogenetic relationship.
Because of their acellular nature, viruses are not included under any of the five kingdoms of Whittaker. However, in 1962, Lowff, Home and Tourneir proposed LHT system which was adopted by the International Committee on Taxonomy of viruses (ICTV). LHT system grouped all viruses under a separate phylum ‘Vira’ and divided in form of Linnaean hierarchy.