Biological Classification — NEET Notes

From Aristotle to Whittaker — why classification kept changing

Classification began as a working list of what to eat, what to wear and what to fear. Aristotle made it scientific by using morphological criteria — he sorted plants into trees, shrubs and herbs and split animals by whether or not they had red blood. Linnaeus formalised a Two-Kingdom system (Plantae and Animalia), but it could not tell prokaryotes from eukaryotes, unicellular from multicellular, or photosynthetic green algae from non-photosynthetic fungi. Bacteria, blue-green algae, fungi, mosses, ferns and seed plants all sat under "plants" simply because their cells had walls — an arrangement that hid more than it revealed.

The deeper problem was that walls themselves differ: fungi have chitin walls, plants have cellulose walls, and bacteria have peptidoglycan. Once cell structure, body organisation, mode of nutrition, reproduction and evolutionary relationships were taken together, the two-kingdom scheme collapsed under its own weight. R.H. Whittaker proposed his five-kingdom alternative in 1969; an extended three-domain (six-kingdom) system later split Monera into Archaea and Bacteria. NCERT teaches Whittaker as the canonical scheme for NEET.

The Five Kingdoms at a glance

The five kingdoms can be drawn on a single grid using five characters: cell type (prokaryotic vs eukaryotic), cell wall composition, nuclear membrane, body organisation, and mode of nutrition. This is the single most-tested table in NCERT Chapter 2 — every cell of it has appeared in NEET at least once.

Kingdom Monera — bacteria, cyanobacteria, mycoplasma

Monera is the only prokaryotic kingdom. Bacteria are its sole members and the most abundant micro-organisms on Earth. They live everywhere — soil, water, hot springs, deserts, snow, the deep ocean, and in or on other organisms as parasites or symbionts. Despite their structural simplicity, bacteria show the most extensive metabolic diversity of any group: they can be photosynthetic autotrophs, chemosynthetic autotrophs, saprophytes, or parasites. By shape, bacteria are grouped as cocci (spherical), bacilli (rod), vibrio (comma) and spirilla (spiral). Reproduction is mainly by binary fission; under stress they form resistant spores; and a primitive form of DNA transfer (conjugation) provides genetic recombination.

NCERT divides Monera into Archaebacteria and Eubacteria, with Cyanobacteria treated as a photosynthetic group within Eubacteria, and Mycoplasma as a wall-less special case. Each subgroup is a regular NEET hotspot.

Archaebacteria — extremophiles with a distinctive cell wall

Archaebacteria are the oldest known cellular lineage and live in the harshest places on Earth. Halophiles live in extreme salty environments (salt pans, salt lakes). Thermoacidophiles live in hot acidic springs. Methanogens live in marshy and anaerobic environments, including the gut of ruminants such as cows and buffaloes, where they produce methane (biogas) from dung. Their survival in extremes comes down to a different cell-wall structure — the wall lacks peptidoglycan, replaced by pseudo-peptidoglycan or other unusual polymers.

Eubacteria — the "true bacteria"

Eubacteria are by far the largest group. They carry a rigid peptidoglycan cell wall, and motile forms have a single flagellum. The bacterial envelope often carries a glycocalyx — either a loose slime layer or a tight capsule — which gives bacterial cells their sticky character. Eubacteria include both autotrophs and heterotrophs. Chemosynthetic autotrophs oxidise inorganic substrates — nitrates, nitrites, ammonia, sulphur — and use the released energy for ATP synthesis. They drive the recycling of nitrogen, phosphorus, iron and sulphur in nature. Heterotrophic bacteria are the most abundant of all: decomposers that recycle dead matter; industrial allies that make curd, antibiotics, and fixed nitrogen in legume roots; and pathogens behind cholera, typhoid, tetanus and citrus canker.

Cyanobacteria — blue-green algae, but firmly Monera

Cyanobacteria contain chlorophyll a like green plants and are photosynthetic autotrophs. They are unicellular, colonial or filamentous; freshwater, marine or terrestrial. Colonies are typically wrapped in a gelatinous sheath, and many form algal blooms in polluted water. Some cyanobacteria fix atmospheric N₂ in specialised cells called heterocysts — the classic NEET examples are Nostoc and Anabaena. They are firmly prokaryotic: no nucleus, no membrane-bound organelles, peptidoglycan wall.

Mycoplasma — the smallest living cells

Mycoplasma completely lack a cell wall. They are only about 0.3 µm long, the smallest living cells known, and can pass through filters with pore sizes less than 1 µm. They survive without oxygen (facultative anaerobes) and many are pathogens of plants and animals. The wall-less feature is the single most-tested fact about mycoplasma — NEET has asked it directly at least three times since 2016.

Kingdom Protista — single-celled eukaryotes

All single-celled eukaryotes are placed in Protista. NCERT acknowledges that the boundaries of this kingdom are not well defined — what one biologist calls a "photosynthetic protistan" another may call "a plant". For NEET, Protista includes five groups: Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, and Protozoans. Members are primarily aquatic. Being eukaryotes, they carry a true nucleus and membrane-bound organelles; many have flagella or cilia; they reproduce both asexually and sexually (cell fusion + zygote formation).

Chrysophytes — diatoms and golden algae

Chrysophytes include diatoms and golden algae (desmids). They are microscopic, float passively as plankton, and are mostly photosynthetic. Diatom cell walls form two thin overlapping shells fitting together like a soap box; the walls are embedded with silica and are virtually indestructible. Over geological time their walls have accumulated as diatomaceous earth, used for polishing and for filtration of oils and syrups. Diatoms are the chief producers in the oceans — the molecular foundation of marine food chains.

Dinoflagellates — red tides and toxin blooms

Dinoflagellates are mostly marine and photosynthetic. They appear yellow, green, brown, blue or red depending on their dominant pigments. The cell wall carries stiff cellulose plates on the outer surface, and most have two flagella — one longitudinal, one transverse, set in a furrow between the wall plates. Red dinoflagellates like Gonyaulax can multiply so explosively that they turn the sea red — the red tide phenomenon — releasing toxins that kill fish and other marine animals.

Euglenoids — pellicle, mixotrophs, plant-like pigments

Euglenoids (e.g., Euglena) are mostly freshwater organisms of stagnant water. They lack a cell wall; instead they have a flexible protein-rich pellicle. They carry two flagella, one short and one long. In sunlight they are photosynthetic; without sunlight they switch to heterotrophy and predate on smaller organisms. Their pigments are identical to those of higher plants — a recurring NEET fact.

Slime moulds — saprophytic plasmodia

Slime moulds are saprophytic protists. Their body creeps along decaying twigs and leaves, engulfing organic matter. Under favourable conditions they aggregate into a plasmodium that may spread over several feet. When conditions turn unfavourable, the plasmodium differentiates into fruiting bodies bearing spores at their tips. These spores possess true walls, are extremely resistant, and can survive for years — dispersing through air currents.

Protozoans — the four classical groups

All protozoans are heterotrophs living as predators or parasites; they are considered primitive relatives of animals. NCERT divides them into four groups based on locomotion and life cycle.

Kingdom Fungi — heterotrophs with chitin walls

Fungi are eukaryotic, heterotrophic organisms with chitin-rich cell walls. With the exception of yeasts (unicellular), the fungal body is filamentous: long thread-like hyphae form a network called mycelium. Some hyphae are continuous tubes packed with multinucleated cytoplasm — coenocytic hyphae; others bear septae (cross-walls). Most fungi are saprophytes, absorbing dissolved organic matter from dead substrates. Some are parasites; others live as symbionts — with algae as lichens and with plant roots as mycorrhiza.

Reproduction can be vegetative (fragmentation, fission, budding), asexual (conidia, sporangiospores, zoospores) or sexual (oospores, ascospores, basidiospores produced inside fruiting bodies). The fungal sexual cycle proceeds in three steps: plasmogamy (fusion of protoplasms) → karyogamy (fusion of nuclei) → meiosis in the zygote, generating haploid spores. In ascomycetes and basidiomycetes, an intervening dikaryotic phase (n + n) sits between plasmogamy and karyogamy.

Classification within Fungi is based on the morphology of the mycelium, the mode of spore formation and the type of fruiting body. The four classes are Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes.

Kingdom Plantae — eukaryotic chlorophyll-bearers

Plantae includes all eukaryotic, chlorophyll-containing organisms commonly called plants. A few members are partially heterotrophic — insectivorous plants like the bladderwort and Venus fly trap, and the parasite Cuscuta. Plant cells carry an eukaryotic architecture with prominent chloroplasts and a cellulose-based cell wall. The kingdom embraces algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

A defining feature of plant life cycles is alternation of generations: a diploid sporophytic phase alternates with a haploid gametophytic phase. The lengths of these phases — and whether they are free-living or dependent on each other — vary across the plant kingdom, from algae (gametophyte-dominant) up through angiosperms (sporophyte-dominant). Full treatment of these subgroups appears in the next NCERT chapter (Plant Kingdom).

Kingdom Animalia — heterotrophs without cell walls

Animalia comprises heterotrophic, eukaryotic, multicellular organisms whose cells lack cell walls. They depend, directly or indirectly, on plants for food. Their nutrition is holozoic — food is ingested, digested in an internal cavity, and stored as glycogen or fat. Animals follow a definite growth pattern, attain a definite shape and size at maturity, and most are capable of locomotion. Higher animals show elaborate sensory and neuromotor mechanisms. Sexual reproduction is by copulation followed by embryological development. Detailed coverage of animal phyla appears in the next NCERT chapter (Animal Kingdom).

Viruses, Viroids and Prions — outside the five kingdoms

Whittaker's five-kingdom scheme excludes a handful of acellular agents — viruses, viroids and prions — because they are not "truly living" in the cellular sense. Each is treated separately by NCERT.

Viruses — nucleoprotein parasites

The word virus means "venom" or "poisonous fluid". Dmitri Ivanowsky (1892) first recognised these submicroscopic agents as the cause of tobacco mosaic disease — they passed through bacteria-proof filters. M.W. Beijerinck (1898) showed that extracts of infected plants could infect healthy ones; he named the agent Contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) crystallised viruses and showed the crystals are largely protein. Viruses are obligate parasites: inert crystalline particles outside a host cell, but once inside, they commandeer the host's machinery to replicate.

A virus is a nucleoprotein: its genetic material is enclosed in a capsid of protein subunits called capsomeres, arranged in helical or polyhedral geometry. Crucially, no virus contains both DNA and RNA — it carries either one, never both, and the nucleic acid is the infectious component (not the protein coat). In general:

Plant viruses → single-stranded RNA · Animal viruses → ssRNA, dsRNA or dsDNA · Bacteriophages → double-stranded DNA

Genetic material across virus types (NCERT)

Viruses cause mumps, smallpox, herpes, influenza and AIDS in humans; mosaic, leaf-curling, vein-clearing and stunted growth in plants. Bacteriophages are bacterial viruses with a characteristic head-tail-tail-fibre architecture.

Viroids — naked, low-molecular-weight RNA

In 1971, T.O. Diener discovered a new infectious agent, smaller than any known virus, that caused potato spindle tuber disease. The agent was a free RNA molecule of low molecular weight, with no protein coat — hence the name viroid. Viroids are smaller than viruses, lack the capsid altogether, and still cause infection. This single line is the source of multiple NEET PYQs across 2016, 2017, 2019 and 2020.

Prions — infectious misfolded proteins

Certain neurological diseases are transmitted by an agent that contains no nucleic acid at all — only an abnormally folded protein. These agents are prions, similar in size to viruses. The classic prion diseases are bovine spongiform encephalopathy (BSE) — "mad cow disease" in cattle — and its analogous human variant, Creutzfeldt–Jakob disease (CJD).

Lichens — symbiotic association of alga and fungus

Lichens are one of biology's most elegant partnerships: a mutually useful association between an alga and a fungus, so intimate that they appear as a single organism. The algal component is the phycobiont — autotrophic, prepares food by photosynthesis. The fungal component is the mycobiont — heterotrophic, provides shelter and absorbs water and mineral nutrients for its partner. Lichens are highly sensitive to atmospheric pollution and serve as excellent pollution indicators: they do not grow in polluted areas.