Biological Classification

Early Attempts: Aristotle used simple morphological characters to classify plants into trees, shrubs, and herbs, and animals based on the presence or absence of red blood.
Two Kingdom System: Developed by Linnaeus, consisting of Kingdom Plantae and Kingdom Animalia. It failed to distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, or photosynthetic and non-photosynthetic organisms.
Inadequacies: Gross morphology alone was insufficient; criteria evolved to include cell structure, wall nature, nutrition mode, habitat, reproduction, and evolutionary relationships.

Criteria for Classification: Cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic (evolutionary) relationships.
The Five Kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.
Six Kingdom System (Three-Domain System): A variation that divides Kingdom Monera into two domains, with the remaining eukaryotic kingdoms in a third domain.

Characteristics: Solely comprised of bacteria; most abundant microorganisms, found in extreme habitats; simple structure but complex behavior with extensive metabolic diversity.
Bacterial Shapes:
- Spherical: Coccus (pl.: cocci)
- Rod-shaped: Bacillus (pl.: bacilli)
- Comma-shaped: Vibrium (pl.: vibrio)
- Spiral: Spirillum (pl.: spirilla)
Archaebacteria: Specialized for harsh environments (halophiles in salt, thermoacidophiles in hot springs, methanogens in marshy areas).
Eubacteria ('True Bacteria'): Characterized by a rigid cell wall and often a flagellum.
- Cyanobacteria (Blue-green algae): Photosynthetic autotrophs; some fix atmospheric $N_2$ in specialized cells called heterocysts (e.g., $Nostoc$ and $Anabaena$ ).
- Chemosynthetic Autotrophs: Oxidize inorganic substances to produce $ATP$ .
- Heterotrophic Bacteria: Most abundant; decomposers; include pathogens causing diseases like cholera, typhoid, and tetanus.
Mycoplasma: Organisms lacking a cell wall; the smallest known living cells; can survive without oxygen.

General Features: All single-celled eukaryotes; primarily aquatic; link between plants, animals, and fungi.
Chrysophytes: Includes diatoms and desmids (golden algae); walls embedded with silica forming indestructible deposits known as diatomaceous earth.
Dinoflagellates: Mostly marine and photosynthetic; two flagella; $Gonyaulax$ causes 'red tides'.
Euglenoids: Fresh water; lack a cell wall, having a protein-rich pellicle instead; mixotrophic.
Slime Moulds: Saprophytic protists; form aggregations called plasmodium and produce resistant spores.
Protozoans: Heterotrophs (predators or parasites); four groups: Amoeboid ( $Amoeba$ with pseudopodia), Flagellated ( $Trypanosoma$ ), Ciliated ( $Paramoecium$ ), and Sporozoans ( $Plasmodium$ , the malarial parasite).

Structure and Nutrition: Heterotrophic; cell walls composed of chitin and polysaccharides; body made of thread-like hyphae (network is mycelium).
Sexual Cycle: Involves Plasmogamy (protoplasm fusion), Karyogamy (nuclear fusion), and Meiosis. Some show a dikaryon stage ( $n + n$ ).
Phycomycetes: Aseptate, coenocytic mycelium; includes $Mucor$ , $Rhizopus$ , and $Albugo$ .
Ascomycetes (Sac-fungi): Septate/branched; asexual spores (conidia) and sexual ascospores; includes $Penicillium$ , $Yeast$ , and $Neurospora$ .
Basidiomycetes (Club fungi): Mushrooms, bracket fungi, and puffballs; produce basidiospores exogenously; includes $Agaricus$ , $Ustilago$ , and $Puccinia$ .
Deuteromycetes (Imperfect fungi): Lack known sexual stages; includes $Alternaria$ and $Trichoderma$ .

Kingdom Plantae: Eukaryotic chlorophyll-containing organisms; includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms; features alternation of generations (sporophytic $2n$ and gametophytic $n$ ).
Kingdom Animalia: Multicellular heterotrophs lacking cell walls; holozoic nutrition (ingestion); store food as glycogen or fat.

Viruses: Non-cellular nucleoproteins; inert outside hosts; genetic material is either RNA or DNA; Dmitri Ivanowsky recognized the TMV pathogen; M.W. Beijerinck coined the term "virus" and called the fluid Contagium vivum fluidum; W.M. Stanley crystallized them.
Viroids: Discovered by T.O. Diener (1971); infectious free RNA of low molecular weight, lacking a protein coat.
Prions: Abnormally folded proteins; cause neurological diseases like BSE (mad cow disease) and CJD.
Lichens: Symbiotic association between phycobiont (algae) and mycobiont (fungi); excellent pollution indicators.