Biological Classification – Comprehensive Study Notes

Evolution of Classification Systems

• Earliest human attempts were instinctive, aimed at utilitarian needs (food, shelter, clothing).
• Aristotle: first to apply a more systematic approach.
  • Plants → trees, shrubs, herbs (simple morphology).
  • Animals → those with red blood vs. without.
• Linnaeus (18th C): Two–Kingdom system $\text{Plantae + Animalia}$.
  • Limitations:
  • No distinction between eukaryotes/prokaryotes.
  • Unicellular vs. multicellular not separated.
  • Photosynthetic green algae & non-photosynthetic fungi lumped together.
  • Many organisms could not fit neatly.
• Increased knowledge introduced criteria such as:
  • Cell structure (prokaryotic/eukaryotic, cell wall chemistry).
  • Body organisation (unicellular, tissue, organ, organ-system levels).
  • Mode of nutrition (autotrophic, heterotrophic — saprophytic, parasitic, holozoic, etc.).
  • Habitat, reproduction, phylogeny.
• R.H. Whittaker (1969): Five-Kingdom system $\text{Monera, Protista, Fungi, Plantae, Animalia}$.
  • Criteria used: cell type, nuclear membrane, cell wall nature, body organisation, nutrition, reproduction, phylogenetic relationships.
• Three-Domain concept (Carl Woese) later split Monera into $\text{Archaea + Eubacteria}$ and kept remaining eukaryotes in third domain $\text{Eukarya}$ (studied in higher classes).

Comparative Features of Whittaker’s Five Kingdoms

(Condensed from Table 2.1)

• Cell Type:
  • Monera → prokaryotic; others → eukaryotic.
• Cell Wall:
  • Monera → non-cellulosic (polysaccharide + amino acid) or absent (Mycoplasma).
  • Protista → present in some.
  • Fungi → chitin.
  • Plantae → cellulose.
  • Animalia → absent.
• Nuclear membrane: absent only in Monera.
• Organisation gradient: cellular → tissue → organ → organ system (highest in Animalia).
• Nutrition:
  • Monera & Protista → auto + heterotrophic (multiple sub-types).
  • Fungi → heterotrophic (saprophytes/parasites).
  • Plantae → autotrophic (photosynthetic) predominantly.
  • Animalia → heterotrophic (holozoic, etc.).

Kingdom Monera

• Sole members: Bacteria (inc. Archaebacteria, Eubacteria, Mycoplasma).
• Habitat: ubiquitous — soil, hot springs, deserts, snow, deep oceans; many symbiotic/parasites.
• Shapes (Figure 2.1):
  • Coccus $\text{(spherical)}$, Bacillus $\text{(rod)}$, Spirillum $\text{(spiral)}$, Vibrio $\text{(comma)}$.
• Metabolism: greatest diversity among all organisms.
  • Autotrophic → photosynthetic or chemosynthetic.
  • Majority heterotrophic.

Archaebacteria

• Extreme environments:
  • Halophiles (high salt), Thermoacidophiles (hot springs), Methanogens (marshes, ruminant gut).
• Unique cell-wall composition allows survival in extremes.
• Methanogens generate biogas (methane) from dung in cow/buffalo rumen.

Eubacteria

• ‘True’ bacteria with rigid cell wall; motile types possess flagellum.
• Cyanobacteria (blue-green algae):
  • Chlorophyll-$a$ like plants; unicellular/colonial/filamentous.
  • Surrounded by gelatinous sheath; form water blooms.
  • Nitrogen fixation in heterocysts (e.g., $\text{Nostoc, Anabaena}$).
• Chemosynthetic autotrophs: oxidise $\text{NH}<em>3$, nitrites, $\text{NO}</em>3^-$, $\text{Fe}^{2+}$, $\text{S}^{2-}$ for ATP; recycle N, P, Fe, S.
• Heterotrophic roles:
  • Decomposers, curd formation, antibiotic production, nitrogen fixation in legumes.
  • Pathogens: cholera, typhoid, tetanus, citrus canker.
• Reproduction: binary fission; spores under stress; primitive genetic exchange (conjugation, etc.).
• Mycoplasma: wall-less, smallest cells, anaerobic tolerance, many pathogens.

Kingdom Protista (all single-celled eukaryotes)

• Aquatic primary habitat; link between plants, animals, fungi.
• Cell biology: nucleus + membrane organelles; some with cilia/flagella.
• Reproduction: asexual and sexual (cell fusion → zygote).

Chrysophytes (Diatoms & Desmids)

• Microscopic plankton (fresh/marine); photosynthetic.
• Diatom cell wall: two overlapping silica-impregnated shells → indestructible.
• Ancient deposits form diatomaceous earth (gritty; used for polishing, filtration of oils/syrups).
• Chief oceanic producers.

Dinoflagellates

• Mostly marine, photosynthetic; colours: yellow, green, brown, blue, red (pigments).
• Wall: stiff cellulose plates.
• Two flagella (longitudinal & transverse).
• Rapid blooms of red species (e.g., $\text{Gonyaulax}$) → “red tides”; toxins may kill fish.

Euglenoids

• Freshwater stagnants.
• No wall; protein pellicle provides flexibility.
• Two flagella (1 long, 1 short).
• Photoautotrophic in light; heterotrophic predator in dark.
• Pigment set identical to higher plants. Example: $\text{Euglena}$.

Slime Moulds

• Saprophytic; creeping plasmodium on decaying matter.
• Unfavourable conditions → fruiting bodies with resistant spores (air-dispersed).

Protozoans (animal-like; all heterotrophic)

1. Amoeboids – pseudopodia; free-living or parasitic (e.g., $\text{Entamoeba}$); marine forms have silica shells.
2. Flagellates – free/parasitic; flagella; disease agents (sleeping sickness by $\text{Trypanosoma}$).
3. Ciliates – aquatic, fast due to cilia; gullet for food (e.g., $\text{Paramoecium}$).
4. Sporozoans – spore-like infectious stage; $\text{Plasmodium}$ causes malaria.

Kingdom Fungi (heterotrophic eukaryotes)

• Habitats: cosmopolitan (air, water, soil, on/inside organisms); thrive in warm humid zones.
• Forms: majority filamentous (hyphae → mycelium); yeast unicellular.
  • Hyphae types: coenocytic (aseptate, multinucleate) vs. septate.
• Wall: chitin + polysaccharides.
• Nutrition: saprophytes, parasites, symbionts (lichens with algae; mycorrhiza with roots).
• Reproduction:
  • Vegetative: fragmentation, fission, budding.
  • Asexual spores: conidia, sporangiospores, zoospores.
  • Sexual spores: oospores, ascospores, basidiospores.
  • Sexual cycle stages: plasmogamy → dikaryon ($n+n$) or immediate diploid → karyogamy → meiosis.
• Classification based on mycelium, sporulation, fruiting bodies.

Phycomycetes (Algal Fungi)

• Aquatic, moist decaying wood, obligate plant parasites.
• Mycelium aseptate, coenocytic.
• Asexual: zoospores (motile) or aplanospores (non-motile) in sporangia.
• Sexual: isogamous/anisogamous/oogamous → zygospore.
• Examples: $\text{Mucor, Rhizopus}$ (bread mould), $\text{Albugo}$ (mustard parasite).

Ascomycetes (Sac Fungi)

• Mostly multicellular (e.g., $\text{Penicillium}$), some unicellular (yeast).
• Saprophytes, decomposers, parasites, coprophilous.
• Mycelium septate, branched.
• Asexual: exogenous conidia on conidiophores.
• Sexual: endogenous ascospores inside ascus; asci aggregated into ascocarp.
• Examples: $\text{Aspergillus, Claviceps, Neurospora}$ (model organism); edible morels, truffles.

Basidiomycetes (Club Fungi)

• Mushrooms, puffballs, bracket fungi; plant parasites (rusts, smuts).
• Mycelium septate, branched.
• Usually no asexual spores; vegetative fragmentation common.
• Sex organs absent; plasmogamy via somatic cell fusion → prolonged dikaryotic phase → basidium.
• Basidium: site of karyogamy + meiosis → 4 exogenous basidiospores; basidia in basidiocarp.
• Examples: $\text{Agaricus}$ (edible mushroom), $\text{Ustilago, Puccinia}$.

Deuteromycetes (Imperfect Fungi)

• Only asexual/vegetative stages known (conidia).
• Once sexual stage found, re-classified into Asco/Basidio.
• Mycelium septate, branched; many saprophytes, parasites, litter decomposers (mineral cycling).
• Examples: $\text{Alternaria, Colletotrichum, Trichoderma}$.

Kingdom Plantae

• All eukaryotic, chlorophyllous organisms (algae → angiosperms).
• Some partial heterotrophs: insectivorous $\text{Bladderwort, Venus fly-trap}$; parasitic $\text{Cuscuta}$.
• Cell wall: cellulose; organelles incl. prominent chloroplasts.
• Life cycle exhibits alternation of generations:
  • Sporophyte (diploid, $2n$) ↔ Gametophyte (haploid, $n$). Length & dependence vary across groups.

Kingdom Animalia

• Multicellular, heterotrophic eukaryotes; no cell wall.
• Nutrition: holozoic (ingestion) → internal digestion; reserve food as glycogen/fat.
• Definite growth pattern → adult form; advanced neuromotor & sensory systems; locomotion prevalent.
• Reproduction: sexual (copulation → embryological development).

Viruses, Viroids, Prions & Lichens (Not in Five-Kingdom system)

Viruses

• Non-cellular, inert crystals outside host; obligate intracellular parasites.
• Discovery milestones:
  • Ivanowsky (1892) – TMV passed through bacteria-proof filters.
  • Beijerinck (1898) – coined “virus”; contagium vivum fluidum.
  • Stanley (1935) – crystallised TMV (protein).
• Composition: nucleic acid (DNA or RNA, never both) + protein capsid (capsomeres). Shapes: helical, polyhedral, complex (bacteriophage: head, sheath, tail fibres).
• Genome trends:
  • Plant viruses → ssRNA.
  • Animal viruses → ssRNA, dsRNA or dsDNA.
  • Bacteriophages → dsDNA.
• Diseases: mumps, smallpox, herpes, influenza, AIDS, plant mosaics, leaf curl, yellowing, dwarfing.

Viroids

• Discovered by T.O. Diener (1971).
• Smaller than viruses; naked circular low-MW RNA, no protein coat.
• Cause potato spindle tuber disease.

Prions

• Infectious agents composed solely of abnormally folded protein (no nucleic acid).
• Cause neurodegenerative disorders: bovine spongiform encephalopathy (mad cow), Creutzfeldt-Jakob disease.

Lichens

• Symbiotic association of alga (phycobiont, autotrophic) + fungus (mycobiont, heterotrophic).
• Mutualism: alga photosynthesises; fungus provides water, minerals, shelter.
• Excellent pollution indicators — absent in polluted locales.

Numerical / Statistical References & Misc.

• $\text{Five Kingdoms} = 5$; $\text{Shapes of bacteria} = 4$ basic types.
• $\text{Binary fission}$ = primary bacterial reproduction.
• Dikaryophase described by $n + n$ condition.

Connections & Significance

• Shift from morphology-based to phylogeny-based classification reflects advancing microscopy, biochemistry, genetics.
• Knowledge of bacterial metabolism underpins biotechnology (e.g., biogas via methanogens, antibiotics via fungi/bacteria, nutrient cycling).
• Understanding protist & fungal diversity crucial for ecology (primary productivity, decomposition) and medicine (pathogens, drug sources).
• Virus, viroid, prion studies blur lines of ‘life’, advance molecular biology, epidemiology.
• Lichens demonstrate symbiosis, act as bio-indicators, contribute to primary succession on rocks.

End of Detailed Notes