Classification is the process of grouping organisms into categories based on similarities and differences. It helps in: (1) Study of large variety of organisms becomes easier. (2) Understanding evolutionary relationships. (3) Finding new organisms and placing them correctly. Carolus Linnaeus (Father of taxonomy) gave the binomial nomenclature system – each organism has two names: genus + species (e.g., Homo sapiens). Classification hierarchy: Kingdom → Phylum → Class → Order → Family → Genus → Species.

R.H. Whittaker (1969) proposed five kingdoms: (1) Monera – prokaryotes, unicellular, no nuclear membrane; bacteria, cyanobacteria, mycoplasma. (2) Protista – eukaryotic, unicellular; Amoeba, Paramecium, Euglena, algae. (3) Fungi – eukaryotic, mostly multicellular, heterotrophic, cell wall of chitin; mushrooms, yeast, Aspergillus, Rhizopus; reproduce by spores. (4) Plantae – eukaryotic, multicellular, autotrophic, cell wall of cellulose, chlorophyll. (5) Animalia – eukaryotic, multicellular, heterotrophic, no cell wall.

Plant kingdom divided into: (1) Thallophyta (Algae) – no true roots/stem/leaves, body is thallus; Spirogyra, Chara, Ulva. (2) Bryophyta – no vascular tissue, need water for reproduction, "amphibians of plant kingdom"; mosses (Funaria), liverworts (Marchantia). (3) Pteridophyta – first vascular plants, no seeds; ferns, Marsilea, Equisetum. (4) Gymnosperms – seeds not enclosed in fruit, naked seeds; Pinus, Cycas, deodar. (5) Angiosperms – seeds enclosed in fruit (flowering plants); divided into monocots (one cotyledon, e.g., wheat, maize) and dicots (two cotyledons, e.g., pea, mango).

Major invertebrate phyla: (1) Porifera – pores all over, no true tissues, aquatic; sponges. (2) Coelenterata – two layers, hollow body, cnidoblasts for stinging; Hydra, jellyfish, coral. (3) Platyhelminthes – flatworms, bilaterally symmetrical, no body cavity; Planaria, tapeworm, liver fluke. (4) Nematoda – roundworms, body cavity present; Ascaris, Wuchereria (filaria). (5) Annelida – segmented worms, true coelom; earthworm, leech, Nereis. (6) Arthropoda – largest phylum, jointed legs, exoskeleton of chitin; insects, spiders, crabs, centipede. (7) Mollusca – soft body, mantle, shell; snail, octopus, Unio. (8) Echinodermata – spiny skin, water vascular system; starfish, sea urchin.

Chordates have notochord, dorsal nerve cord, gill slits at some stage. Vertebrates have vertebral column. Classes: (1) Pisces – fish, cold-blooded, gills, scales, two-chambered heart; shark (cartilaginous), rohu (bony). (2) Amphibia – cold-blooded, moist skin, three-chambered heart, aquatic larvae with gills/adult on land; frog, toad, salamander. (3) Reptilia – cold-blooded, dry scaly skin, three-chambered heart (crocodile has four); lizard, snake, turtle, crocodile. (4) Aves – birds, warm-blooded, four-chambered heart, feathers, hollow bones, forelimbs modified to wings; sparrow, pigeon. (5) Mammalia – warm-blooded, four-chambered heart, mammary glands, hair on body; bat, whale, human.

Biodiversity refers to the enormous variety of living organisms on Earth. Scientists estimate there are 8–10 million species, of which about 1.7 million have been identified and named. Classification is the process of grouping organisms based on their similarities and differences. Importance of classification: (1) Makes study of such a huge number of organisms manageable. (2) Helps understand evolutionary relationships. (3) Helps in identifying new organisms. (4) Facilitates communication among scientists worldwide. Taxonomy is the science of naming, describing and classifying organisms. Aristotle was the first to attempt classification (animals with/without blood). Carolus Linnaeus (1758) is called the father of modern taxonomy; he introduced the binomial nomenclature system.

Modern classification is hierarchical — organisms are grouped at different levels called taxa (singular: taxon). From broadest to most specific: Kingdom → Phylum (animals) / Division (plants) → Class → Order → Family → Genus → Species. Mnemonic: "King Philip Came Over For Good Soup". Species is the most basic unit of classification — organisms of the same species can interbreed and produce fertile offspring. Genus groups related species. Classification basis: (1) Presence/absence of nucleus (prokaryote/eukaryote). (2) Single-celled or many-celled. (3) How nutrition is obtained (autotroph/heterotroph). (4) Presence/absence of cell wall. (5) Level of organization of body. (6) Mode of reproduction. (7) Evolutionary history (phylogeny). Modern classification uses DNA, protein sequences and fossil evidence in addition to morphology.

R.H. Whittaker (1969) proposed the Five Kingdom Classification based on cell structure, mode of nutrition, and phylogenetic relationships. The five kingdoms: (1) Monera — prokaryotes, no nuclear membrane, single-celled; e.g., bacteria, cyanobacteria (blue-green algae). (2) Protista — unicellular eukaryotes; e.g., Amoeba, Paramecium, Euglena, diatoms. (3) Fungi — eukaryotic, heterotrophic (saprophytic/parasitic), cell wall of chitin, no chlorophyll; e.g., Rhizopus, Aspergillus, mushrooms, yeast. (4) Plantae — multicellular eukaryotes, autotrophic (photosynthesis), cell wall of cellulose; includes all plants. (5) Animalia — multicellular eukaryotes, heterotrophic, no cell wall; includes all animals. Earlier: Two kingdom system (Linnaeus) — Plantae and Animalia. Three kingdom: Haeckel added Protista. Four kingdom: Copeland added Monera. Ernst Mayr later proposed Bacteria and Archaea distinction.

Kingdom Plantae includes all multicellular autotrophic organisms with cellulose cell walls. Major divisions (from primitive to advanced): (1) Thallophyta (Algae) — no differentiated body (thallus); no vascular tissue; e.g., Spirogyra, Chara, Ulva, Sargassum. (2) Bryophyta — plant body differentiated into stem-like and leaf-like structures; no true roots, stems or leaves; no vascular tissue; called amphibians of plant kingdom; e.g., Funaria (moss), Marchantia (liverwort). (3) Pteridophyta — first vascular plants (xylem and phloem); true roots, stems and leaves; reproduce by spores; no seeds; e.g., ferns (Dryopteris), Equisetum (horsetail), Marsilea. (4) Gymnosperms — bear seeds but seeds are naked (not enclosed in fruit); vascular tissue present; e.g., Pinus, Cycas, Ginkgo. (5) Angiosperms — seeds enclosed in fruits (flowering plants); most evolved; divided into monocots (one cotyledon: wheat, rice, maize) and dicots (two cotyledons: mango, gram, pea).

Kingdom Animalia consists of multicellular heterotrophic eukaryotes without a cell wall. Major non-chordate phyla: (1) Porifera — pore-bearing; simplest animals; body with many pores (ostia) and canals; sessile; e.g., Sycon, Spongilla, Euspongia. (2) Coelenterata (Cnidaria) — radially symmetrical; diploblastic; stinging cells (cnidoblasts/nematocysts) for capturing prey; e.g., Hydra, Obelia, Aurelia (jellyfish), sea anemone, coral. (3) Platyhelminthes — flatworms; bilaterally symmetrical; acoelomate; parasitic (mostly); e.g., Planaria (free-living), Fasciola (liver fluke), Taenia (tapeworm). (4) Nematoda (Aschelminthes) — roundworms; cylindrical body; pseudocoelomate; mostly parasitic; e.g., Ascaris (roundworm), Wuchereria (filarial worm), Ancylostoma (hookworm). (5) Annelida — segmented worms; true coelom; first animals with closed circulatory system; e.g., Earthworm (Lumbricus/Pheretima), Leech (Hirudinaria), Nereis.

Continuing major phyla: (6) Arthropoda — largest animal phylum; jointed legs (arthron = joint); exoskeleton of chitin; open circulatory system; e.g., insects (cockroach, mosquito), crustaceans (prawn, crab), arachnids (spider, scorpion), myriapods (centipede). (7) Mollusca — soft body with hard shell (usually); muscular foot; e.g., snail (Pila), octopus, squid (Loligo), Unio (freshwater mussel). (8) Echinodermata — spiny-skinned; water vascular system; tube feet; exclusively marine; radially symmetrical; e.g., starfish (Asterias), sea urchin, sea cucumber, brittle star. Chordates — have notochord (at some stage), dorsal nerve cord, pharyngeal gill slits; include: Pisces (fish), Amphibia (frogs), Reptilia (lizards), Aves (birds), Mammalia (mammals). Mammals: warm-blooded, hair/fur on body, mammary glands for nursing young; e.g., whale, bat, human.

Binomial nomenclature is the internationally accepted system of giving every organism a two-part scientific name. It was introduced by Carolus Linnaeus (1758) in his book Systema Naturae. The two parts: (1) Generic name (Genus) — first word, always capitalised. (2) Specific epithet (species) — second word, always in small letters. Rules: Both words in Latin or Latinized form. The name is printed in italics (or underlined when handwritten). The generic name starts with capital letter; specific epithet with small letter. Example names: Homo sapiens (human), Panthera tigris (tiger), Panthera leo (lion), Felis domesticus (cat), Mangifera indica (mango), Solanum tuberosum (potato), Rana tigrina (Indian frog), Musca domestica (housefly). Advantages: same name worldwide (avoids confusion of common names), indicates evolutionary relationships through genus.

Vertebrates (subphylum Vertebrata) have a vertebral column (backbone). Five classes: (1) Pisces (Fish) — aquatic; gills for breathing; scales; cold-blooded (ectothermic); two-chambered heart; e.g., Rohu (Labeo), shark (Scoliodon), Exocoetus (flying fish). (2) Amphibia — can live in water and on land; moist, glandular skin; three-chambered heart; cold-blooded; lay eggs in water; e.g., frog (Rana), toad (Bufo), salamander. (3) Reptilia — scaly skin; three-chambered heart (crocodile: four); cold-blooded; lay eggs with leathery shells on land; e.g., lizard, snake, turtle, crocodile. (4) Aves (Birds) — warm-blooded; feathers; hollow bones; four-chambered heart; forelimbs modified into wings; lay eggs; e.g., crow, pigeon, parrot, penguin. (5) Mammalia — warm-blooded; hair/fur; four-chambered heart; mammary glands; most viviparous; e.g., bat, whale, human, dog. Platypus and echidna (egg-laying mammals) are exceptions.

Fungi are eukaryotic organisms that are heterotrophic and reproduce by spores. They are neither plants nor animals. Structure: body made of thread-like hyphae (singular: hypha); network of hyphae = mycelium; cell walls made of chitin (not cellulose). Nutrition: (1) Saprophytic — feed on dead/decaying organic matter; release digestive enzymes outside body, then absorb nutrients (extra-cellular digestion); e.g., Rhizopus on bread, Aspergillus. (2) Parasitic — live on living hosts; e.g., Puccinia (wheat rust), ringworm fungi. (3) Symbiotic — lichens (fungi + algae in mutually beneficial relationship); mycorrhiza (fungi + plant roots). Reproduction: asexual (spores, fragmentation), sexual (varies by group). Importance: decomposers (recycle nutrients), food (mushrooms, yeast in bread/beer), antibiotics (Penicillium → penicillin), plant diseases (rusts, smuts), some cause human infections (athlete's foot, ringworm).