Biological Classification

Early classification was simple but inadequate. Aristotle made the first scientific attempt, and for a long time Linnaeus's two-kingdom system (Plantae and Animalia) was used. It failed badly because it lumped together organisms with utterly different cell types and nutrition — bacteria, fungi, photosynthetic and non-photosynthetic forms were forced into 'plants'. The need to separate prokaryotes from eukaryotes, unicellular from multicellular, and to recognise different modes of nutrition drove the search for a better scheme.

The system now followed is the five-kingdom classification proposed by R.H. Whittaker in 1969: Monera, Protista, Fungi, Plantae and Animalia. Whittaker used five criteria — cell structure (prokaryotic vs eukaryotic), body organisation (unicellular vs multicellular), mode of nutrition (autotrophic vs heterotrophic, and within heterotrophs, absorptive vs ingestive), mode of reproduction, and phylogenetic relationships. These five criteria, and the year and author, are very frequent NEET one-markers.

Kingdom Monera comprises the bacteria — the sole prokaryotes, and the most abundant micro-organisms on Earth. By shape they are coccus (spherical), bacillus (rod), vibrio (comma) and spirillum (spiral). Despite their simple structure, bacteria show the greatest metabolic diversity of any group: nutrition may be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic or parasitic), the latter being the majority and including the most important decomposers.

Monera is split into archaebacteria and eubacteria. Archaebacteria are special because they live in the most extreme habitats — methanogens (in marshes and the guts of ruminants, producing methane/biogas), halophiles (extreme salt), and thermoacidophiles (hot springs) — owing to a different cell wall structure. Eubacteria or 'true bacteria' include the photosynthetic cyanobacteria (blue-green algae) with chlorophyll like green plants; many fix atmospheric nitrogen in specialised cells called heterocysts (e.g. Nostoc, Anabaena). A special mention is mycoplasma: the smallest living cells, the only bacteria without a cell wall, able to survive without oxygen, and pathogenic. Bacteria reproduce mainly asexually by binary fission and can also transfer DNA (a primitive sexual mechanism).

Kingdom Protista contains all single-celled eukaryotes and forms a link between the simple Monera and the complex plants, animals and fungi. Because its members are so varied, the boundaries of Protista are not sharp, but NCERT groups them into chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans. All have a well-defined nucleus and membrane-bound organelles, and many bear flagella or cilia; reproduction is asexual and also sexual, by a process involving cell fusion and zygote formation.

Chrysophytes include the diatoms and golden algae (desmids). They are mostly photosynthetic and found in fresh and marine water, floating passively as the chief 'producers' of the oceans (plankton). Their most examinable feature is the cell wall: diatoms have indestructible silica walls made of two overlapping halves that fit like a soapbox, and the accumulated remains over billions of years form 'diatomaceous earth', used in polishing and filtration. Dinoflagellates are mostly marine and photosynthetic; appearing yellow, green, brown, blue or red by their pigments. They have two flagella (one longitudinal, one transverse). Red dinoflagellates such as Gonyaulax can multiply so rapidly that the sea looks red — a 'red tide' — releasing toxins that can kill fish and other marine life.

Euglenoids, typified by Euglena, are mostly fresh-water flagellates lacking a cell wall; instead they have a flexible protein-rich pellicle. They are mixotrophic: photosynthetic in sunlight but heterotrophic (predatory) in the dark, a point NEET loves to test as a 'plant-and-animal-like' organism. Slime moulds are saprophytic protists. Under favourable conditions their body forms an aggregation called a plasmodium that may grow and spread over several feet; under unfavourable conditions it differentiates and forms fruiting bodies bearing spores with true walls that are extremely resistant and dispersed by air.

Protozoans are heterotrophic, living as predators or parasites, and are believed to be primitive relatives of animals. There are four major groups, each with a defining locomotory feature: amoeboid protozoans move and capture prey using pseudopodia (Amoeba; the parasite Entamoeba); flagellated protozoans bear flagella, some being parasites such as Trypanosoma (sleeping sickness); ciliated protozoans are aquatic, with thousands of cilia and a gullet, e.g. Paramoecium; and sporozoans have an infectious spore-like stage, the most notorious being Plasmodium, the malarial parasite. Memorising the locomotory organelle and one representative for each group is a reliable mark.

Kingdom Fungi is a group of heterotrophic eukaryotes of great ecological and economic importance — from bread mould and mushrooms to yeasts, antibiotics and crop diseases. Except for unicellular yeasts, fungi are filamentous: their body is made of thread-like hyphae, and a network of hyphae is a mycelium. Hyphae may be coenocytic (continuous, multinucleate tubes without cross walls) or septate (with cross walls). A defining chemical feature, frequently tested, is that the fungal cell wall is made of chitin and polysaccharides, not cellulose.

Fungi are heterotrophic and absorb nutrition (absorptive nutrition). They may be saprophytes (on dead organic matter), parasites (on living hosts), or live in symbiotic associations — with algae as lichens and with plant roots as mycorrhiza. Reproduction is varied: vegetative by fragmentation, fission and budding; asexual by spores such as conidia, sporangiospores and zoospores; and sexual by oospores, ascospores and basidiospores. The sexual cycle proceeds through three steps — plasmogamy (fusion of protoplasms), karyogamy (fusion of nuclei) and meiosis in the zygote — and in many fungi an intervening dikaryotic (n+n) stage occurs where two nuclei coexist before fusing.

Fungi are divided into four classes on the basis of mycelium and spores. Phycomycetes (e.g. Mucor, Rhizopus the bread mould, Albugo the parasite on mustard) have aseptate, coenocytic mycelium; asexual reproduction is by zoospores or aplanospores, and sexual spores are oospores/zygospores. Ascomycetes or 'sac fungi' (e.g. Aspergillus, Penicillium, Claviceps, Neurospora, and the unicellular yeast Saccharomyces) have septate, branched mycelium; asexual spores are conidia produced on conidiophores, and sexual spores are ascospores produced endogenously in sac-like asci. Neurospora is famous as a tool in genetics.

Basidiomycetes include the familiar mushrooms, bracket fungi and puffballs (e.g. Agaricus), as well as the crop pathogens Ustilago (smut) and Puccinia (rust). Their mycelium is septate; they have no asexual spores, vegetative reproduction is by fragmentation, and sex organs are absent — plasmogamy occurs by fusion of two vegetative/somatic cells, after which basidiospores are produced exogenously on a basidium. Finally, Deuteromycetes are the 'imperfect fungi' (e.g. Alternaria, Colletotrichum, Trichoderma): only the asexual or vegetative phases are known, so when their sexual stage is discovered they are moved to Ascomycetes or Basidiomycetes. Matching each class to its hyphae type, characteristic spore and a representative is the single most rewarding NEET exercise in this chapter.

Kingdom Plantae includes all eukaryotic, chlorophyll-containing organisms — the algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Plant cells have a cellulose cell wall, and most are autotrophic, though a few are heterotrophic: insectivorous plants like the Venus flytrap and Nepenthes trap insects, and parasites like Cuscuta draw nutrition from a host. A characteristic feature of plant life cycles is alternation of generations between a diploid sporophyte and a haploid gametophyte. Kingdom Animalia contains heterotrophic, multicellular eukaryotes without a cell wall; they show holozoic nutrition (ingestion of food), store reserve food as glycogen or fat, and higher forms have elaborate sensory and neuromotor systems; reproduction is mostly sexual.

Some entities do not fit any kingdom because they are acellular — chiefly the viruses. A virus is a non-cellular, inert nucleoprotein particle that becomes active only inside a living host, where it is an obligate parasite. The name (Latin for poison) and key discoveries are examinable: D.J. Ivanowsky first recognised viruses from infected tobacco; M.W. Beijerinck called the extract contagium vivum fluidum (infectious living fluid); and W.M. Stanley showed viruses could be crystallised. A virus consists of genetic material enclosed in a protein coat, the capsid, made of subunits called capsomeres.

The nature of the genetic material is a high-yield NEET fact: viruses infecting plants usually have single-stranded RNA, those infecting animals have single- or double-stranded RNA or double-stranded DNA, and bacteriophages (viruses that infect bacteria) are usually double-stranded DNA viruses. In general, a virus is a nucleoprotein and its infectivity resides in the nucleic acid. Viruses cause many diseases — mumps, smallpox, herpes and influenza in animals, AIDS in humans, and mosaic, leaf-rolling, yellowing and dwarfing in plants.

Two even simpler infectious agents and one symbiotic partnership complete the picture. Viroids, discovered by T.O. Diener, are smaller than viruses: they are free, low-molecular-weight RNA without any protein coat, and cause potato spindle tuber disease. Prions are infectious agents made only of abnormally folded protein; they cause neurological diseases such as bovine spongiform encephalopathy (BSE, mad-cow disease) and its human equivalent Creutzfeldt–Jakob disease (CJD). Finally, lichens are symbiotic associations between an alga (phycobiont) and a fungus (mycobiont): the alga photosynthesises and provides food, while the fungus gives shelter and absorbs water and minerals. Because lichens are very sensitive to pollutants such as SO₂, they do not grow in polluted areas and serve as pollution indicators — a fact NEET asks almost every year.