Diversity in Living World

Every year NEET asks 4–6 questions from this cluster of chapters — usually direct, fact-based questions on characteristic features, examples and life cycles. Because the marks come from recall rather than reasoning, a well-organised mental map of the kingdoms and plant groups is one of the cheapest scoring investments in botany.

Why We Classify, and the Logic of Five Kingdoms

Living organisms number in the millions, so biologists group them into categories arranged one inside another — the taxonomic hierarchy of species, genus, family, order, class, phylum/division and kingdom. Each grouping is only as good as the criteria behind it. For nearly two centuries after Linnaeus, only two kingdoms were recognised: Plantae and Animalia. That scheme collapsed under its own contradictions — it lumped together prokaryotes with eukaryotes, photosynthetic organisms with heterotrophic ones, and unicellular forms with multicellular ones. Fungi, for instance, were parked among plants merely because they do not move, even though they have chitinous walls and absorb pre-formed food.

R. H. Whittaker (1969) proposed the five-kingdom system: Monera, Protista, Fungi, Plantae and Animalia. The criteria he used were:

1. Cell structure — prokaryotic versus eukaryotic
2. Cell wall composition — present/absent, cellulose versus chitin versus peptidoglycan
3. Body organisation — unicellular, colonial, thalloid or tissue/organ-grade
4. Mode of nutrition — photoautotrophic, chemoautotrophic, saprophytic, parasitic, holozoic
5. Reproduction and phylogenetic (evolutionary) relationships

Even this system is imperfect. Chlamydomonas and Chlorella (unicellular algae) sit in Protista while multicellular Spirogyra goes to Plantae, splitting closely related green algae apart. Slime moulds and unicellular animals share Protista despite being very different. Consequently the kingdom Protista has been called "assembled by convenience". Later work by Carl Woese, based on ribosomal RNA sequences, split prokaryotes into two fundamentally distinct lineages and gave the three-domain system: Archaea, Bacteria and Eukarya — often described as a six-kingdom refinement of Whittaker's plan.

Kingdom Monera — The Prokaryotes

Monera includes all bacteria: the most abundant and metabolically versatile organisms on Earth. They occupy hot springs, deserts, snow, deep oceans, acidic bogs and the guts of animals. Structurally they are simple but they are anything but primitive in their biochemistry.

By shape, bacteria are grouped as coccus (spherical), bacillus (rod-shaped), vibrio (comma-shaped) and spirillum (spiral). Nutritionally they may be:

• Photosynthetic autotrophs — cyanobacteria, purple and green sulphur bacteria
• Chemosynthetic autotrophs — oxidise inorganic substances such as ammonia, nitrite, iron or sulphur and use the released energy to make ATP; nitrifying bacteria belong here and are vital to nutrient cycling
• Heterotrophs — the largest group; decomposers, nitrogen-fixers, curd-formers, antibiotic producers, and also pathogens causing cholera, typhoid, tetanus and citrus canker

Archaebacteria live in the harshest habitats and have a distinctive cell wall chemistry (they lack peptidoglycan) and branched-chain membrane lipids. Three familiar groups: halophiles (salt-saturated water), thermoacidophiles (hot acidic springs) and methanogens (marshes and the rumen of cattle, where they generate biogas/methane).

Cyanobacteria (blue-green algae) are photosynthetic prokaryotes with chlorophyll a, occurring as unicells, colonies or filaments, usually wrapped in a gelatinous sheath. In Nostoc and Anabaena, specialised thick-walled cells called heterocysts carry out nitrogen fixation. Nutrient-rich water bodies allow them to bloom heavily.

Reproduction in bacteria is chiefly asexual binary fission; under stress many form resistant spores. A primitive form of sexual reproduction occurs — transfer of DNA from one cell to another — but there is no gamete fusion or meiosis.

Mycoplasma are the smallest known living cells, completely lack a cell wall, can survive without oxygen and cause diseases in plants and animals.

Kingdom Protista — Eukaryotic Beginnings

Protista contains all single-celled eukaryotes, and functions largely as a bridge group between Monera on one side and the multicellular fungi, plants and animals on the other. Cell structure is unmistakably eukaryotic — a true membrane-bound nucleus and other membranous organelles — but body organisation stays at the unicellular level, sometimes forming loose colonies. Reproduction includes both asexual division and a primitive sexual process involving cell fusion and zygote formation.

Protists are grouped by nutrition into broad clusters:

• Chrysophytes — diatoms and golden algae (desmids), mostly photosynthetic, found in fresh and marine water. Diatom cell walls are impregnated with silica and fit together like a soap box; the indestructible walls accumulate over ages as diatomaceous earth, used in polishing, filtration and as packing material because it is gritty, inert and free-flowing.
• Dinoflagellates — mostly marine and photosynthetic, with two flagella (one longitudinal, one transverse lying in a groove), cellulose plates in the cell wall, and pigments giving yellow, green, brown, blue or red colour depending on species. Rapid multiplication of red-pigmented forms (e.g. Gonyaulax) causes the "red tide", turning the sea red and releasing toxins that can kill marine animals.
• Euglenoids — mostly freshwater, in stagnant water, with a protein-rich pellicle instead of a cell wall (making them flexible) and two flagella, one short and one long. Photosynthetic in light with chlorophyll a and b like higher plants, but they turn heterotrophic in the dark by ingesting organic material — a textbook example blurring the plant/animal boundary that justified giving protists their own kingdom.
• Slime moulds — chemoheterotrophic, saprophytic protists that form an aggregated, moving, multinucleate mass called a plasmodium under unfavourable conditions, which creeps over decaying matter engulfing organic material.
• Protozoans — heterotrophic, predatory or parasitic, considered ancestral to animals; grouped by locomotory structure into amoeboid (Amoeba, using pseudopodia), flagellated (Trypanosoma, causing sleeping sickness), ciliated (Paramecium, actively moving with a distinct gullet) and sporozoans (Plasmodium, the malarial parasite, with an infectious spore-like stage in its life cycle).

Kingdom Fungi, Lichens and Acellular Agents

Fungi are heterotrophic eukaryotes, mostly multicellular (except unicellular yeast), that absorb nutrients from dead organic matter (saprophytes), from living hosts (parasites) or through mutually beneficial associations (symbionts). The plant body is a mass of branched, thread-like hyphae collectively called the mycelium; hyphae may be coenocytic (continuous cytoplasm, no septa, multinucleate) or septate (cross-walls dividing cells). Cell walls are made of chitin, unlike the cellulose walls of plants. Reproduction can be vegetative (fragmentation, budding), asexual (spores such as conidia, sporangiospores, zoospores) or sexual (ascospores, basidiospores) formed after a three-step process — plasmogamy (fusion of protoplasms), karyogamy (fusion of nuclei) and meiosis in the zygote.

The four fungal classes are separated chiefly by their sexual spores and fruiting bodies:

• Phycomycetes — aquatic or on decaying wood, or obligate parasites on plants; coenocytic mycelium; asexual zoospores or aplanospores, sexual zygospores. Examples: Mucor, Rhizopus (bread mould), Albugo.
• Ascomycetes — mostly multicellular (some unicellular, like yeast), septate mycelium; sexual spores are ascospores produced inside sac-like asci. Examples: Aspergillus, Penicillium (source of the antibiotic), Saccharomyces (baker's/brewer's yeast), morels and truffles.
• Basidiomycetes — familiar mushrooms, bracket fungi and puffballs, plus rusts and smuts that are plant pathogens; septate mycelium; sexual spores are exogenous basidiospores borne on a club-shaped basidium. Examples: Agaricus, Ustilago (smut), Puccinia (rust).
• Deuteromycetes ("fungi imperfecti") — only asexual/vegetative stages are known; when a sexual stage is later discovered, the species is reclassified into Ascomycetes or Basidiomycetes. Examples: Alternaria, Trichoderma, Colletotrichum.

Lichens are not a taxonomic group but a stable, mutually beneficial symbiosis between a fungus (mycobiont) and a photosynthetic partner, alga or cyanobacterium (phycobiont): the fungus provides shelter and absorbs water/minerals, the phycobiont supplies food. Lichens are extremely sensitive to pollution, especially sulphur dioxide, and are therefore used as pollution indicators.

Viruses are acellular, obligate intracellular parasites — outside a host cell they are inert particles with no metabolism of their own, which is why their status as "living" is debated. A virion consists of genetic material (DNA or RNA, never both) enclosed in a protein coat, the capsid, built of subunits called capsomeres. Plant viruses mostly carry single-stranded RNA; animal viruses may carry single- or double-stranded RNA or DNA; bacteriophages are typically double-stranded DNA viruses. Viroids, discovered by T. O. Diener, are smaller than viruses and consist of free RNA with no protein coat at all — the causative agents of some plant diseases. Prions, abnormally folded infectious proteins, cause diseases such as scrapie in sheep and Creutzfeldt–Jakob disease in humans.

Plant Kingdom I: Algae and Bryophytes

Plants are multicellular, eukaryotic, chlorophyll-bearing autotrophs with a cellulose cell wall, showing an alternation of generations between a haploid gametophyte and a diploid sporophyte in their life cycle.

Algae are chiefly aquatic, photosynthetic, and range from unicellular (Chlamydomonas) to colonial or filamentous forms; the largest, kelps, can exceed 100 metres. Reproduction is vegetative (fragmentation), asexual (zoospores) or sexual, ranging from isogamous (fusing gametes look alike) through anisogamous (gametes differ in size) to oogamous (a large, non-motile female gamete fuses with a small, motile male gamete). Algae are grouped into three classes based on pigments and stored/wall material:

• Chlorophyceae (green algae) — grass-green, chlorophyll a and b dominant, cellulose walls, food stored as starch; Chlamydomonas, Spirogyra, Ulva, Chara.
• Phaeophyceae (brown algae) — brown due to fucoxanthin, walls with cellulose and algin, food stored as laminarin and mannitol; Ectocarpus, Fucus, Laminaria, Sargassum.
• Rhodophyceae (red algae) — red due to r-phycoerythrin, food stored as floridean starch; Polysiphonia, Porphyra, Gracilaria, Gelidium. Agar, extracted from Gelidium and Gracilaria, is a staple of microbiology culture media.

Algae are ecologically and economically vital: roughly half of global carbon fixation is by algae (chiefly marine phytoplankton), agar and algin are extracted commercially, and forms like Chlorella and Spirulina are used as protein-rich food.

Bryophytes, called the amphibians of the plant kingdom, need water for fertilisation and stay restricted to moist, shaded habitats. The dominant, photosynthetic phase is the gametophyte, bearing male antheridia (producing biflagellate antherozoids) and female archegonia (each producing a single egg). Fertilisation gives a zygote that develops into a dependent, short-lived sporophyte, which remains attached to and nutritionally dependent on the gametophyte, producing haploid spores by meiosis inside a capsule. Bryophytes are grouped into liverworts (thalloid, e.g. Marchantia), mosses (leafy gametophyte arising from a protonema stage, e.g. Funaria, Sphagnum) and hornworts. Mosses form mats that reduce soil erosion, and Sphagnum (peat moss) is used as fuel and as packing material for perishables.

Plant Kingdom II: Pteridophytes and Gymnosperms

Pteridophytes were the first plants to possess true vascular tissue and are accordingly called the first true land plants, found in cool, damp, shaded places though some are aquatic. Here the sporophyte is the dominant, independent, differentiated plant body with true root, stem and leaves; sporangia borne on leaves called sporophylls produce haploid spores by meiosis (in ferns these may cluster into visible sori). A spore germinates into a small, free-living, photosynthetic gametophyte called a prothallus, which bears antheridia and archegonia and needs water for motile antherozoids to swim to the egg; the resulting zygote grows into the next sporophyte while still attached to the prothallus. Pteridophytes are classified into Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (ferns). Where all spores produced are similar, the plant is homosporous; where two kinds — smaller microspores and larger megaspores — are produced, it is heterosporous (Selaginella, Salvinia), a condition that anticipates the seed habit of gymnosperms and angiosperms.

Gymnosperms ("naked-seed plants") bear ovules not enclosed by any ovary wall, so after fertilisation the seeds remain exposed, unlike angiosperm seeds enclosed within a fruit. Gymnosperms are typically evergreen, perennial and woody, ranging from tall conifers to the slow-growing, long-lived cycads. Leaves may be simple (conifers) or pinnately compound (cycads), often needle-like and adapted to withstand temperature and water extremes. Roots are typically tap roots; some (Pinus) form fungal mycorrhizal associations, others (Cycas) develop small, specialised, nitrogen-fixing coralloid roots. Gymnosperms are heterosporous: haploid microspores form in male cones, while megaspores are retained within the megasporangium (nucellus), itself enclosed by integuments forming the ovule. Pollen is carried to the ovule by wind, and the pollen tube delivers non-motile male gametes to the egg; the zygote develops into an embryo while the ovule matures into a seed. Common NEET examples: Cycas, Pinus, Ginkgo (the only living gymnosperm with fan-shaped leaves), Gnetum.

Common Mistakes and Exam Traps

1. Confusing gametophyte and sporophyte dominance. In bryophytes the gametophyte is dominant and the sporophyte is dependent; in pteridophytes, gymnosperms and angiosperms the sporophyte is dominant and independent — this reversal is one of the most repeated NEET traps.
2. Treating water-dependent fertilisation as identical across groups. Algae and bryophytes both need water for motile male gametes, but only bryophytes have a dependent sporophyte generation — the two groups should not be equated on life-cycle grounds.
3. Mixing up heterospory with the seed habit. Heterospory occurs in some pteridophytes (Selaginella, Salvinia) that do not produce seeds; the seed habit itself is unique to gymnosperms and angiosperms, where the megaspore is retained and nourished within the megasporangium.
4. Placing viruses inside the five/six-kingdom system. Viruses are acellular and fall entirely outside Whittaker's classification, which requires cellular organisation — they are discussed separately, never as a "sixth kingdom".