Whitaker’s Five Kingdom system and Carl Woese’s three Domain system

Kingdom (Latin: regs, pl. regia) is the second-highest taxonomic rank in biology, after the domain. Kingdoms are further subdivided into smaller groups known as phyla.

When Carl Linnaeus introduced the rank-based system of nomenclature into biology, the highest rank was named “kingdom,” followed by four other main or principal ranks: class, order, genus, and species. Later, two additional major ranks were added, resulting in the sequence kingdom, phylum or division, class, order, family, genus, and species. In the 1960s, a new rank, domain (or empire), was introduced above kingdom, so the kingdom is no longer the highest rank.

Whitaker’s Five Kingdom system

R.H. Whittaker proposed a Five Kingdom Classification in 1969. He named the kingdoms as Monera, Protista, Fungi, Plantae, and Animalia. He uses cell structure, thallus organization, mode of nutrition, reproduction, and phylogenetic relationships as his primary classification criteria. The table compares and contrasts the various characteristics of the five kingdoms.

1) Kingdom Monera – Characteristics

  • Unicellular or filamentous prokaryotes
  • Omnipresent (air, soil, hot springs, deserts, deepsea, snow and as parasites),
  • The cell wall is composed of polysaccharides and amino acids (peptidoglycans or murein)
  • Autotrophic (photo and chemosynthetic) and heterotrophic (saprophytic and parasitic). Reproduce by vegetative, asexual and sexual methods

Major groups of Monera

a) Archaebacteria: Methanogens, Halophiles, thermoacidophiles

b) Eubacteria: Vibrio, Mycobacteria

c) Cyanobacteria: Nostoc, Anabaena

2) Kingdom Protista – Characteristics

  • Unicellular eukaryotes
  • Freshwater, marine, or parasitic forms.
  • The cell wall is usually absent if present it is impregnated with silica (diatoms).
  • Photosynthetic or nonphotosynthetic.
  • The locomotory structure may be cilia, flagella, pseudopodia, or absent.
  • Reproduce by sexual and asexual methods.

Major groups of kingdom Protista:

Chrysophyta: Diatom, Pinnularia

Dinoflagellates: Gonyaulax

Euglenoids : Euglena, Peranema

Slime Moulds: Physaram, Stemonitis

Protozoa: amoeba

3) Kingdom fungi – Characteristics

  • Unicellular or multicellular eukaryotes
  • Heterotrophs – Parasites (Puccinia), saprophytes (Agaricus), Symbionts (Parmelia in lichens) and some are associated with the roots of higher plants (mycorrhizal)
  • The thalloid plant body is called mycelium
  • Mycelium is made up of tubular thread-like hyphae
  • The cell wall is made up of chitin.
  • Reproduce by

a) Vegetative (fragmentation. fission, budding)

b) Asexual (sporangiospores, conidia )

c) Sexual (Plasmogamy, Karyogamy, and meiosis resulting in the formation of haploid spores like oospore, ascospore, and basidiospore)

Major Group of Kingdom Fungi

a) Phycomycetes: Mucor Rhizopus

b) Ascomycetes (Sac Fungi): Penicillium

c) Basidiomycetes (Club Fungi): Agaricus, Puccinia

d) Deuteromycetes (Imperfect Fungi): Cercospora, Alternaria

4) Kingdom Plantae – Characteristics

  • Plants cells have cellulose walls that uphold their size
  • They contain chlorophyll A and B and carotenoid pigment. to utilize the sun’s energy.
  • Plants store their food in the form of starch
  • Plants are a multicellular organism with cells that are specialized to form tissues and organs
  • They possess reproductive organs that are shielded by non-reproductive cells to avoid gametes or sex cells from drying up.
  • The female reproductive structure of plants supplies nutrients and water to the developing embryo.
  • They have a life cycle that displays an alternation of two-generation:

(a) A diploid sporophyte that makes spores by meiosis

(b) A haploid gametophyte that creates gametes and eventually produces a sporophyte by fertilization.

Major groups of tracheophytes

a) Filicinophyta — ferns (Pterophyta)

b) Cycadophyta — cycads

c) Coniferophyta — pines

d) Angiospermophyta — flowering plants (Anthophyta)

5) Kingdom Animalia – Characteristics

  • Multicellular – This means that they are made up of more than one cell. Some members of other kingdoms are just made up of one cell, like bacteria or amoeba.
  • They are heterotrophic, which means they must obtain their own food. Autotrophic plants produce their own food through photosynthesis. Members of the Animalia Kingdom must ingest, or eat, other organisms because they cannot produce their own food.
  • Eukaryote – This is the type of cells animals have. Eukaryote cells are more complex than the simpler prokaryote cells found in bacteria.
  • No cell wall – plants, fungi, and prokaryote cells have a cell wall, which is a rigid outer layer that gives cells structure. Animal cells do not have this structure.
Major Group Kingdom Animalia
a) Porifera
b) Annelida
c) Mollusks (Mollusca)
d) Arthropods (Arthropoda)
e) Echinoderms (Echinodermata)
f) Chordates (Chordata)

Three-Domain System of Carl Woese

Carl Woese introduced the three-domain system in 1977, which divides cellular life forms into archaea, bacteria, and eukaryote domains. The three-domain system adds a level of classification (the domains) “above” the previously used five-or-six-kingdom systems.
The current system has the following listed kingdoms in the three domains

1) Domain Archaea

Prokaryotic organisms with no nuclear membrane, distinct biochemistry and RNA markers from bacteria, and a unique ancient evolutionary history, making them some of the oldest species of organisms on the planet; traditionally classified as archaebacteria; frequently characterized by living in extreme environments. Methanogens, which produce the gas methane, halophiles, which live in very salty water, and thermoacidophiles, which grow in acidic high-temperature water are all examples of archaeal organisms.

2) Domain Bacteria

Prokaryotic cells have primarily diacyl glycerol diester lipids in their membranes and bacterial rRNA, but no nuclear membrane, and are thus traditionally classified as bacteria. The majority of known pathogenic prokaryotic organisms are bacteria, which are currently being studied more thoroughly than Archaea. Cyanobacteria photosynthesizing bacteria that are related to the chloroplasts of eukaryotic plants and algae, Spirochaetes — Gram-negative bacteria that include those causing syphilis and Lyme disease, and Firmicutes — Gram-positive bacteria that include Bifidobacterium animals, which is present in the human large intestine, are some examples of bacteria.

3) Domain Eukarya

Eukaryotes, organisms that contain a membrane-bound nucleus. An inexhaustive list of eukaryotic organisms includes —
  • Kingdom Fungi or fungi
Examples: Saccharomycotina — includes true yeasts
Basidiomycota — includes blue oyster mushrooms
  • Kingdom Plantae or plants
Examples: Bryophyta — mosses
Magnoliophyta — flowering plants
  • Kingdom Animalia or animals
Examples: Arthropoda — includes insects, arachnids, and crustaceans
Chordata — includes vertebrates and, as such, human beings
  • Kingdom Chromalveolate — a group of eukaryotes that represent a descent from an organism that had an endosymbiosis between a line related to a bikont and a red alga. However, the monophyly of this group is challenged.