Whittaker (1969) five criteria: Cell structure (prokaryote/eukaryote), Body organisation (unicellular/multicellular), Mode of nutrition (autotrophic/heterotrophic/absorptive), Reproduction, Phylogenetic relationships.
A ✅ Cell structure — Yes (prokaryotic/eukaryotic is fundamental)
B ✅ Body organisation — Yes (unicellular, colonial, multicellular)
C ❌ Presence of flagellum — NOT one of Whittaker's main criteria
D ✅ Reproduction — Yes (sexual/asexual, spores)
E ✅ Phylogenetic relationships — Yes (evolutionary relationships)
Before Whittaker, the living world was divided into just two kingdoms: Plantae and Animalia (Linnaeus, 1758). Problems with two-kingdom system: fungi have no chlorophyll but were kept in Plantae (absorptive nutrition). Bacteria are prokaryotes fundamentally different from all eukaryotes. Protozoa are unicellular unlike multicellular animals. Algae — unicellular to multicellular, all photosynthetic but some structurally simple. Ernst Haeckel (1866): proposed a third kingdom Protista for unicellular organisms. Copeland (1956): proposed four kingdoms. R.H. Whittaker (1969): proposed Five Kingdom Classification based on cell structure, organisation, mode of nutrition, reproduction, and phylogenetic relationships. Carl Woese (1977): proposed three domains (Bacteria, Archaea, Eukarya) based on rRNA sequences — now widely accepted but not part of standard NEET curriculum at this level.
Whittaker's five kingdoms: (1) Monera: prokaryotes — bacteria and cyanobacteria (blue-green algae). No nuclear envelope, no membrane-bound organelles. (2) Protista: unicellular eukaryotes — protozoa, unicellular algae (diatoms, dinoflagellates), slime moulds. All are unicellular eukaryotes. (3) Fungi: multicellular (mostly) eukaryotes with absorptive heterotrophic nutrition. Cell wall of chitin. Spore-forming. Examples: Agaricus (mushroom), Puccinia (rust), Aspergillus, Penicillium. (4) Plantae: multicellular, autotrophic (photosynthetic) eukaryotes with cell wall of cellulose. All land plants and some algae. (5) Animalia: multicellular, heterotrophic eukaryotes without cell walls. Ingestion mode of nutrition. All animals. The classification criteria separated these groups based on: cell type (prokaryote/eukaryote), level of organisation (unicellular/multicellular), mode of nutrition (autotrophic/heterotrophic absorptive/heterotrophic ingestive).
Kingdom Monera: all prokaryotic organisms. Characteristics: no nuclear envelope (nucleoid — circular DNA in cytoplasm), no membrane-bound organelles (no mitochondria, chloroplasts, Golgi, ER), ribosomes 70S, cell wall usually present (peptidoglycan in bacteria), reproduction asexual (binary fission), some parasexual processes (conjugation, transformation, transduction). Bacteria: shapes — coccus (sphere), bacillus (rod), spirillum (spiral), vibrio (comma-shaped). Gram positive/negative based on peptidoglycan amount. Examples: Lactobacillus (curd), E. coli (gut), Streptococcus (throat infection), Clostridium (tetanus, botulism), Rhizobium (nitrogen fixation), Azotobacter (free-living N-fixer), Nitrosomonas + Nitrobacter (nitrification). Cyanobacteria: prokaryotic photosynthetic organisms (once called blue-green algae). Have chlorophyll a. Anabaena, Nostoc, Oscillatoria. Archaebacteria (now Archaea): unique cell wall (no peptidoglycan), different membrane lipids, found in extreme environments.
Kingdom Protista: unicellular (or simple colonial) eukaryotes. Very diverse group. Major groups: Chrysophytes: diatoms (beautiful silica shells — diatomaceous earth used as filters, polishers) and desmids. Dinoflagellates: phytoplankton, bioluminescent (Noctiluca), cause red tides (toxic blooms). Euglenoids: Euglena — photosynthetic but can be heterotrophic, has flagellum, no cell wall (pellicle), eyespot (phototaxis). Slime moulds: Physarum — acellular slime mould with plasmodium (multinucleate mass). Spores similar to fungi. Protozoa: Amoeba (pseudopodia), Paramecium (cilia), Trypanosoma (sleeping sickness, flagellum), Plasmodium (malaria, no locomotory organelle — intracellular parasite). Diatoms: major component of phytoplankton → contribute ~20-25% of total photosynthesis on Earth. Fossils: diatomaceous earth deposits.
Kingdom Fungi: eukaryotic, heterotrophic (absorptive — secrete enzymes, absorb digested nutrients), cell wall of chitin, mostly multicellular (except yeasts). Body: mycelium of hyphae (filaments). Hyphae: septate (with cross-walls, Ascomycetes, Basidiomycetes) or aseptate/coenocytic (no cross-walls, Zygomycetes). Nutrition: saprophytic (dead organic matter), parasitic (on living hosts), symbiotic (lichens = fungus + algae/cyanobacteria; mycorrhizae = fungus + plant roots). Reproduction: asexual (spores — conidia, zoospores, sporangiospores) and sexual. Classification: Phycomycetes (Zygomycetes): Rhizopus (bread mould), Mucor. Ascomycetes (sac fungi): Neurospora (genetic studies), Aspergillus, Penicillium, yeast (Saccharomyces), morels (Morchella). Basidiomycetes (club fungi): Agaricus (button mushroom), Ustilago (smut), Puccinia (rust), Ganoderma. Deuteromycetes (Fungi Imperfecti): no sexual stage known, classified here (Alternaria, Colletotrichum, Trichoderma).
Kingdom Plantae: multicellular, eukaryotic, photosynthetic (autotrophic), cell wall of cellulose, chlorophyll in chloroplasts. Includes: Algae (thallophyta), Bryophyta, Pteridophyta, Gymnosperms, Angiosperms. Algae: Chlorophyceae (green algae — Chlamydomonas, Volvox, Ulva, Spirogyra). Phaeophyceae (brown algae — Fucus, Laminaria, Sargassum). Rhodophyceae (red algae — Porphyra, Gracilaria, Polysiphonia). Bryophyta: mosses (Funaria), liverworts (Marchantia), hornworts (Anthoceros). Pteridophyta: ferns (Nephrolepis), horsetails (Equisetum), club mosses (Selaginella). Gymnosperms: Pinus, Cycas, Gnetum. Angiosperms: monocots (grasses, palms, orchids, lilies) and dicots (most flowering plants). Alternation of generations: haploid gametophyte and diploid sporophyte generations alternate. In bryophytes: gametophyte dominant; in pteridophytes and seed plants: sporophyte dominant.
Kingdom Animalia: multicellular, eukaryotic, heterotrophic (ingestive — consume food), no cell wall, no chlorophyll. Major phyla: Porifera: sponges (Sycon, Spongilla) — simplest metazoans, have pores (ostia), choanocytes. Coelenterata: Hydra, Aurelia (jellyfish), coral — radially symmetrical, cnidoblasts (stinging cells). Platyhelminthes: flatworms — Taenia (tapeworm, parasite), Fasciola (liver fluke), Planaria (free-living). Aschelminthes: roundworms — Ascaris (intestinal parasite), Wuchereria (filaria), Ancylostoma (hookworm). Annelida: segmented worms — Earthworm (Pheretima), Leech (Hirudinaria). Arthropoda: largest animal phylum — insects (cockroach, mosquito), crustaceans (Palaemon), arachnids (scorpion, spider). Mollusca: snails (Pila), clams (Unio), squid, octopus. Echinodermata: spiny skin — starfish, sea urchin, sea cucumber. Chordata: vertebrates (fish, amphibians, reptiles, birds, mammals) + non-vertebrate chordates (Balanoglossus, Amphioxus, sea squirt).
Carl Woese and George Fox (1977) proposed the Three Domain system based on 16S/18S rRNA gene sequences. Three domains: Bacteria: most familiar prokaryotes (E. coli, Lactobacillus, Streptomyces). Have ester-linked phospholipids, peptidoglycan cell wall. Archaea: prokaryotes that look like bacteria but are fundamentally different. Ether-linked lipids, no peptidoglycan (instead have pseudopeptidoglycan or S-layer). Found in extreme environments (halophiles, thermoacidophiles like Sulfolobus, methanogens like Methanobacterium). Genetically more closely related to Eukarya than to Bacteria! Eukarya: all eukaryotes (plants, animals, fungi, protists). Significance: Archaea were originally classified in Monera (with bacteria) but rRNA sequencing revealed they are as different from bacteria as they are from eukaryotes — a completely separate lineage. This changed our understanding of the tree of life. The three domains replaced the Five Kingdom system in research but Whittaker's classification is still taught in NEET curriculum.