Biological Classification

Biological classification organises living organisms into groups using shared characteristics and evolutionary relationships. Systems have evolved from simple to complex:

• Aristotle — first to classify animals into those with red blood and those without. Plants into herbs, shrubs, and trees.
• Two-kingdom system — Plantae and Animalia (Linnaeus). Did not account for fungi, bacteria, or unicellular organisms satisfactorily.
• Five-kingdom system — Whittaker (1969). Based on cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships.

Monera includes all prokaryotic organisms. They lack a membrane-bound nucleus and organelles, and are the most abundant organisms on Earth.

Archaebacteria — ancient bacteria living in extreme habitats:

• Halophiles — high salt environments
• Thermoacidophiles — hot and acidic environments
• Methanogens — marshy areas; produce methane; found in gut of ruminants

Eubacteria — true bacteria with rigid cell walls:

• Cyanobacteria — photosynthetic prokaryotes (blue-green algae); some fix atmospheric nitrogen (e.g., Nostoc, Anabaena).
• Chemosynthetic autotrophs — obtain energy by oxidising inorganic compounds.
• Heterotrophic bacteria — most common; decomposers, nitrogen fixers, pathogens.
• Mycoplasma — smallest living cells; lack cell wall; can survive without oxygen.

Protista includes all unicellular eukaryotes. They form a connecting link between prokaryotes and complex eukaryotes.

• Chrysophytes — diatoms and golden algae; cell walls have silica; found in fresh and marine water; main component of diatomaceous earth.
• Dinoflagellates — mostly marine and photosynthetic; responsible for red tides; e.g., Gonyaulax causes bioluminescence.
• Euglenoids — no cell wall; protein-rich pellicle instead; photosynthetic in light, heterotrophic in dark (mixotrophic); e.g., Euglena.
• Slime moulds — saprophytic protists; during unfavourable conditions, form plasmodium.
• Protozoans — heterotrophic; Amoeba, Paramecium, Plasmodium (malarial parasite).

Fungi are eukaryotic, heterotrophic organisms that absorb nutrients from their surroundings (saprophytic or parasitic). Their cell walls are made of chitin (not cellulose).

Structure: Body is made of thread-like hyphae. A mass of hyphae is the mycelium. Hyphae may be septate or aseptate (coenocytic).

Major classes:

• Phycomycetes — aseptate hyphae; e.g., Mucor, Rhizopus (bread mould), Albugo (downy mildew of mustard).
• Ascomycetes — septate hyphae; ascospores in asci; e.g., Neurospora, Saccharomyces (yeast), Aspergillus, Claviceps.
• Basidiomycetes — includes mushrooms, bracket fungi, puffballs; basidiospores on basidia; e.g., Agaricus, Ustilago, Puccinia.
• Deuteromycetes (imperfect fungi) — only asexual reproduction known; e.g., Alternaria, Colletotrichum, Trichoderma.

Viruses are acellular, obligate intracellular parasites. They are not placed in any kingdom as they are non-cellular. A virus particle (virion) consists of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid).

• Plant viruses usually have single-stranded RNA.
• Animal viruses may have double-stranded DNA or RNA.
• Bacteriophages have double-stranded DNA.
• HIV (AIDS virus) and tobacco mosaic virus (TMV) are common NEET examples.

Viroids — discovered by T.O. Diener; consist of free, small single-stranded circular RNA with no protein coat. They infect plants (e.g., potato spindle tuber disease).

Lichens — mutualistic symbiosis between algae (phycobiont) and fungi (mycobiont). The fungus provides water and minerals; the alga performs photosynthesis. Lichens are sensitive indicators of air pollution and can grow on bare rocks.