Biological Classification – Comprehensive Bullet-Point Notes

Historical Evolution of Classification

  • Earliest human attempts
    • Based on immediate utility (food, shelter, clothing); non-scientific.
  • Aristotle (≈ 300 BCE)
    • Plants ➔ trees, shrubs, herbs (morphology based).
    • Animals ➔ with red blood vs. without red blood.
  • Linnaeus (1758)
    • Two-Kingdom system: Plantae & Animalia.
    • Limitations
    • No distinction between prokaryotes vs. eukaryotes, unicellular vs. multicellular, photosynthetic vs. non-photosynthetic.
    • Many organisms did not fit neatly.
  • Need for revision
    • New characters: cell structure, cell-wall chemistry, nutrition, reproduction, habitat, phylogeny.

Whittaker’s Five-Kingdom System (1969)

  • Kingdoms: Monera, Protista, Fungi, Plantae, Animalia.
  • Core criteria
    • Cell type (prokaryotic/eukaryotic & presence of nuclear membrane).
    • Cell wall composition.
    • Body organisation (cellular ⟶ tissue ⟶ organ ⟶ organ system).
    • Mode of nutrition (autotrophic/heterotrophic; photosynthetic, chemosynthetic, holozoic, saprophytic, parasitic).
    • Reproduction & phylogenetic relationships.
  • Comparative summary
    • Monera: prokaryotic, \text{wall} = \text{non-cellulosic (polysaccharide + amino acid)}, no nuclear membrane, cellular level, autotrophic (photo/chemo) & heterotrophic.
    • Protista: eukaryotic, wall in some, nucleus present, unicellular, photo- & heterotrophic.
    • Fungi: eukaryotic, chitinous wall, multicellular/loose tissue, heterotrophic (saprophyte/parasite).
    • Plantae: eukaryotic, \text{wall} = \text{cellulose}, tissue/organ level, autotrophic photosynthesis.
    • Animalia: eukaryotic, no wall, tissue/organ/organ-system, heterotrophic holozoic.
  • Three-Domain proposal (later): splits Monera into Bacteria & Archaea; Eukarya forms third domain (total six kingdoms).

Kingdom Monera

General Features

  • Sole members = Bacteria.
  • Ubiquitous; thrive even in extreme habitats (hot springs, deserts, polar snow, deep ocean).
  • Shapes (Fig 2.1):
    • Coccus (spherical), Bacillus (rod), Vibrio (comma), Spirillum (spiral).
  • Metabolic diversity largest among organisms
    • Autotrophic: photosynthetic or chemosynthetic.
    • Heterotrophic: saprophytic, parasitic, etc.

Archaebacteria (ancient bacteria)

  • Extreme halophiles, thermoacidophiles, methanogens.
  • Unique cell-wall chemistry ↔ extreme survival.
  • Methanogens occur in ruminant guts; generate methane (biogas).

Eubacteria (true bacteria)

  • Rigid cell wall; motile forms possess flagellum.
  • Cyanobacteria (blue-green algae)
    • Chlorophyll-a; unicellular/colonial/filamentous.
    • Gelatinous sheath; water-bloom formation in polluted water.
    • Heterocysts fix \mathrm{N_2} e.g.
    • \textit{Nostoc},\ \textit{Anabaena}.
  • Chemosynthetic autotrophs
    • Oxidise \text{NH3, NO2^-, NO_3^-} etc. ➔ energy for \text{ATP}.
    • Crucial in cycling N, P, Fe, S.
  • Heterotrophs
    • Decomposers; curd formation, antibiotic production, nitrogen fixation in legumes.
    • Pathogens: cholera, typhoid, tetanus, citrus canker.
  • Reproduction
    • Binary fission (Fig 2.3).
    • Spores during stress.
    • Primitive genetic exchange (conjugation–like DNA transfer).
  • Mycoplasma
    • No cell wall; smallest known living cells; anaerobic tolerance; many pathogens.

Kingdom Protista

  • All single-celled eukaryotes; primarily aquatic; form evolutionary link.
  • Cell features: membrane-bound nucleus & organelles; some with cilia/flagella.
  • Reproduction: asexual & sexual (cell fusion ⟶ zygote).

Chrysophytes

  • Diatoms & golden algae (desmids); planktonic.
  • Diatom wall = two silica-impregnated shells ➔ indestructible.
  • Fossilised deposits ⇒ diatomaceous earth (polishing, oil/syrup filtration).
  • Major oceanic primary producers.

Dinoflagellates

  • Mostly marine, photosynthetic; coloured (yellow/green/brown/blue/red).
  • Cell wall = stiff cellulose plates.
  • Two flagella (longitudinal & transverse).
  • Red dinoflagellate \textit{Gonyaulax} blooms ➔ red tides; toxins can kill fish.

Euglenoids

  • Fresh-water, stagnant; pellicle (proteinous) replaces cell wall ➔ flexibility.
  • Two flagella (short & long).
  • Mixotrophic: photosynthetic with light; predatory in dark.
  • Pigments identical to higher plants (\textit{Euglena}).

Slime Moulds

  • Saprophytic; creeping plasmodium on decaying vegetation.
  • In adversity, forms fruiting bodies with resistant spores; air-borne dispersal.

Protozoans (animal-like protists)

  1. Amoeboid – pseudopodia movement; some with silica shells; parasite \textit{Entamoeba}.
  2. Flagellated – free-living/parasitic; flagella; \textit{Trypanosoma} causes sleeping sickness.
  3. Ciliated – abundant cilia; gullet for feeding; \textit{Paramecium}.
  4. Sporozoans – spore-like infectious stage; \textit{Plasmodium} (malaria).

Kingdom Fungi

General Attributes

  • Heterotrophic eukaryotes; prefer warm, humid habitats; cosmopolitan (air, water, soil, organisms).
  • Forms: moulds on bread/fruit, mushrooms, toadstools; pathogens (mustard white spots); yeast (fermentation); antibiotic source \textit{Penicillium}.
  • Body = mycelium (hyphal network).
    • Coenocytic hyphae: aseptate, multinucleate.
    • Septate hyphae: cross-walls present.
  • Cell wall: chitin + polysaccharide.
  • Nutrition
    • Saprophytes, parasites, or symbionts (lichens – with algae; mycorrhiza – with roots).

Reproduction

  • Vegetative: fragmentation, fission, budding.
  • Asexual spores: conidia, sporangiospores, zoospores.
  • Sexual spores: oospores, ascospores, basidiospores.
  • Sexual cycle
    1. Plasmogamy (cytoplasm fusion).
    2. Karyogamy (nuclear fusion) ➔ 2n.
    3. Meiosis in zygote ➔ n spores.
  • Dikaryon stage in Ascomycetes & Basidiomycetes: cells contain n + n nuclei.

Classes

1. Phycomycetes

  • Aquatic / damp decaying wood / obligate plant parasites.
  • Aseptate coenocytic mycelium.
  • Asexual: motile zoospores / non-motile aplanospores (endogenous in sporangia).
  • Sexual: zygospore via isogamy/anisogamy/oogamy.
  • Examples: \textit{Mucor}, \textit{Rhizopus} (bread mould), \textit{Albugo} (mustard parasite).

2. Ascomycetes (sac fungi)

  • Mostly multicellular (\textit{Penicillium}), some unicellular yeast (\textit{Saccharomyces}).
  • Habitat: saprophyte, decomposer, parasite, coprophilous.
  • Septate branched mycelium.
  • Asexual: conidia on conidiophores.
  • Sexual: ascospores inside asci; asci arranged in ascocarps.
  • Examples: \textit{Aspergillus}, \textit{Claviceps}, \textit{Neurospora} (genetics model), morels & truffles (edible delicacies).

3. Basidiomycetes

  • Mushrooms, bracket fungi, puffballs; also rusts & smuts (plant parasites).
  • Septate branched mycelium.
  • Asexual spores generally absent; vegetative fragmentation common.
  • No discrete sex organs; plasmogamy via somatic cell fusion ➔ dikaryotic mycelium.
  • Basidium develops; karyogamy + meiosis ➔ four exogenous basidiospores.
  • Basidia aggregated into basidiocarps.
  • Examples: \textit{Agaricus} (mushroom), \textit{Ustilago} (smut), \textit{Puccinia} (rust).

4. Deuteromycetes (imperfect fungi)

  • Only asexual/vegetative stages known; sexual stage unknown.
  • Discovery of sexual form leads to re-assignment to Asco- or Basidiomycetes.
  • Mycelium septate; asexual conidia.
  • Ecological roles: saprophytes, parasites, litter decomposers (nutrient cycling).
  • Examples: \textit{Alternaria}, \textit{Colletotrichum}, \textit{Trichoderma}.

Kingdom Plantae

  • Eukaryotic, chlorophyllous organisms (autotrophic).
  • Partial heterotrophs exist: insectivorous (Bladderwort, Venus fly trap) & parasites (Cuscuta).
  • Cell wall = cellulose; chloroplasts prominent.
  • Major groups: Algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms.
  • Alternation of generations
    • Sporophytic (2n) & Gametophytic (n) phases alternate.
    • Length & independence of each phase vary across groups.

Kingdom Animalia

  • Multicellular, eukaryotic, no cell wall.
  • Heterotrophic holozoic nutrition; internal digestion.
  • Reserve food: glycogen/fat.
  • Fixed growth pattern → definite adult form.
  • Well-developed sensory & neuromotor systems; locomotion common.
  • Sexual reproduction: copulation → embryonic development.

Viruses, Viroids, Prions & Lichens

Viruses

  • Acellular, crystalline outside host; obligate intracellular parasites.
  • History
    • 1892: Ivanowsky → tobacco mosaic filtrate.
    • 1898: Beijerinck coined term “virus”, called agent Contagium vivum fluidum.
    • 1935: W.M. Stanley crystallised TMV (mainly protein).
  • Structure
    • Capsid (protein), subunits = capsomeres (helical or polyhedral).
    • Genetic material: DNA or RNA, never both.
    • Plant viruses: usually single-stranded RNA.
    • Animal viruses: ssRNA, dsRNA, or dsDNA.
    • Bacteriophages: typically dsDNA (head, sheath, tail fibres).
  • Diseases: mumps, smallpox, herpes, influenza, AIDS (HIV).
  • Plant symptoms: mosaic, leaf curl, vein clearing, dwarfing.

Viroids (T.O. Diener, 1971)

  • Smaller than viruses; free circular RNA of low MW; no protein coat.
  • Cause: potato spindle-tuber disease.

Prions

  • Infectious agents = mis-folded proteins (virus-sized).
  • Diseases: Bovine Spongiform Encephalopathy (mad-cow), Creutzfeldt–Jakob Disease (CJD) in humans.

Lichens

  • Symbiotic association between alga (phycobiont) & fungus (mycobiont).
    • Alga ⇒ photosynthetic food.
    • Fungus ⇒ shelter, water, minerals.
  • Excellent pollution indicators (absent in polluted zones).

Concept Connections & Implications

  • Continuous refinement of classification mirrors advances in microscopy, biochemistry, genetics & evolutionary biology.
  • Cell-wall chemistry (cellulose vs. chitin) influences ecological roles & drug targets.
  • Extremophilic archaea inform biotechnology (e.g., thermostable enzymes for PCR).
  • Diatomaceous earth serves in industrial filtration & abrasives.
  • Red-tide toxins have fisheries & public-health impacts.
  • Fungal pathogens & symbionts directly affect agriculture (rusts vs. mycorrhiza).
  • Viral, viroid & prion studies underpin quarantine, vaccine design & neuro-degenerative research.

Key Numerical / Statistical / Formulaic References

  • Binary fission ⟶ population doubles each generation (exponential 2^n growth under ideal conditions).
  • Dikaryon condition represented as n + n; post-karyogamy diploid stage = 2n.

Figures & Exemplars Mentioned

  • Fig 2.1: bacterial shapes – Cocci, Bacilli, Spirilla, Vibrio.
  • Fig 2.2: filamentous cyanobacterium Nostoc.
  • Fig 2.3: dividing bacterium (binary fission).
  • Fig 2.4: Dinoflagellate, Euglena, Slime mould, Paramecium.
  • Fig 2.5: Fungi – Mucor, Aspergillus, Agaricus.
  • Fig 2.6: Tobacco Mosaic Virus & Bacteriophage diagrams.

Ethical & Practical Notes

  • Refrigerator use retards microbial spoilage (temperature effect on fungal/bacterial metabolism).
  • Red-tide monitoring crucial for coastal resource management.
  • Antibiotic production by fungi highlights need for responsible antimicrobial stewardship.
  • Lichen sensitivity guides urban air-quality assessments.

Linkages to Subsequent Study

  • Chapter 3 & 4 will detail Kingdoms Plantae & Animalia, including alternation of generations & animal phyla traits.
  • Higher-level courses: three-domain system, molecular phylogenetics, microbial genetics, immunology (virus–host interactions).