Biological Classification

Since the dawn of civilization, humans have categorized living beings primarily for practical utility—food, shelter, and clothing. The earliest scientific classification was initiated by the Greek philosopher Aristotle, who used simple morphological traits to classify plants into trees, shrubs, and herbs, and divided animals into two groups: those with red blood (Enaima) and those without red blood (Anaima).

In the 18th century, Carolus Linnaeus established the Two-Kingdom Classification consisting of:

1. Kingdom Plantae: Organisms possessing a cell wall, including bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms.
2. Kingdom Animalia: Multicellular organisms lacking a cell wall, possessing locomotion and holozoic nutrition.

Fatal Limitations of the Two-Kingdom Scheme:

• Failure to Distinguish Cellular Architecture: It lumped prokaryotes (bacteria, cyanobacteria) lacking a nuclear membrane with complex eukaryotes.
• Unicellular vs Multicellular Conflation: Unicellular organisms like Chlamydomonas and Amoeba were grouped alongside complex multicellular trees and metazoans.
• Photosynthetic vs Heterotrophic Inconsistency: Photosynthetic green algae and non-photosynthetic heterotrophic fungi (with chitinous walls) were both forced into Plantae.

To overcome these contradictions, American ecologist Robert H. Whittaker (1969) formulated the Five-Kingdom Classification, dividing all living organisms into:

1. Complexity of Cell Structure: Differentiation between prokaryotic and eukaryotic cellular organization.
2. Complexity of Body / Thallus Organization: Transition from unicellular to multicellular and tissue/organ grades.
3. Mode of Nutrition: Absorptive saprotrophic (Fungi), ingestive holozoic (Animals), or synthetic phototrophic (Plants).
4. Reproduction: Modes of asexual, vegetative, and sexual cycles.
5. Phylogenetic Relationships: Evolutionary lineages tracing ancestry from primitive to derived forms.

Kingdom Monera includes all prokaryotic organisms. Bacteria are the sole and most abundant micro-organisms on Earth, inhabiting deep oceans, hot sulfur springs, sub-zero snow, and deep soil. They occur in four basic morphological shapes: spherical Coccus (pl. Cocci), rod-shaped Bacillus (pl. Bacilli), comma-shaped Vibrio (pl. Vibrios), and spiral Spirillum (pl. Spirilla).

Archaebacteria are primitive prokaryotes that survive under Earth's most hostile ecological extremes. Their extraordinary resilience is due to a chemically unique cell wall lacking true peptidoglycan (composed of pseudomurein) and plasma membranes containing branched-chain aliphatic lipids linked by ether bonds instead of ester bonds. These ether-linked branched lipid monolayers prevent membrane fluidity collapse under boiling temperatures and osmotic lysis in hypersaline pools.

• Methanogens: Strict obligate anaerobes living in marshy swamps and the rumen of cud-chewing herbivorous animals (cows, buffaloes). They reduce carbon dioxide into methane ($CH_4$) gas (biogas). Examples: Methanobacterium, Methanococcus.
• Halophiles: Live in saturated hypersaline environments (Great Salt Lake, Dead Sea). Possess bacteriorhodopsin pigment in cell membranes to synthesize ATP under sunlight without chlorophyll. Example: Halobacterium.
• Thermoacidophiles: Inhabit hot acidic sulfur springs (temperature $80^\circ ext{C}-100^\circ ext{C}$, pH 2). Under aerobic conditions, they oxidize elemental sulfur to sulfuric acid ($H_2SO_4$). Example: Thermoplasma, Sulfolobus.

Characterized by a rigid cell wall composed of peptidoglycan (murein)—a polymer of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short tetrapeptide chains.

Cyanobacteria are photosynthetic prokaryotes bearing chlorophyll-a (identical to higher plants) and accessory phycobiliproteins (phycocyanin and phycoerythrin). They occur as unicellular, colonial, or filamentous freshwater/marine or terrestrial forms enclosed in a gelatinous mucilaginous sheath.

• Heterocysts (হেটারোসিস্ট): Specialized, pale, thick-walled cells found in filamentous cyanobacteria (e.g., Nostoc, Anabaena). They lack Photosystem II (PS-II) and possess an enzyme called nitrogenase, creating a strictly anaerobic micro-environment required to reduce atmospheric nitrogen ($N_2$) into ammonia ($NH_3$).
• Algal Blooms: In nutrient-rich polluted water bodies, massive cyanobacterial blooms deplete dissolved oxygen and produce hepatotoxins (microcystins), killing aquatic fauna.

• Chemosynthetic Autotrophs: Oxidize inorganic substrates like nitrates, nitrites, and ammonia to synthesize ATP (e.g. Nitrosomonas, Nitrobacter). They play a crucial role in biogeochemical recycling of nitrogen, phosphorus, iron, and sulfur.
• Heterotrophic Bacteria: The most abundant group in nature, primarily decomposers. Crucial for converting milk to curd (Lactobacillus), synthesizing streptomycin and tetracycline antibiotics (Streptomyces), and fixing nitrogen symbiotically in legume root nodules (Rhizobium). Many are major pathogens causing cholera (Vibrio cholerae), typhoid (Salmonella typhi), tetanus (Clostridium tetani), and citrus canker (Xanthomonas axonopodis).

Bacteria reproduce predominantly by binary fission under favorable conditions. Under harsh starvation or desiccation, they produce highly resistant, thick-walled endospores (containing calcium dipicolinate). Primitive sexual recombination occurs via parasexual mechanisms: Conjugation (plasmid transfer via sex pilus), Transformation (uptake of naked environmental DNA), and Transduction (bacteriophage-mediated gene transfer).

Mycoplasma are completely devoid of a cell wall! They are the smallest known living cellular organisms, with dimensions ranging from $0.1$ to $0.3\,\mu ext{m}$.

• They can survive indefinitely in the complete absence of oxygen (facultative anaerobes).
• Medical Consequence: Because penicillin specifically inhibits bacterial cell wall peptidoglycan transpeptidation, Mycoplasma are completely insensitive to penicillin! They must be treated with protein synthesis-inhibiting antibiotics like tetracycline or erythromycin.
• They cause pleuropneumonia in cattle and "witches' broom" diseases in plants.

Kingdom Protista encompasses all single-celled eukaryotic organisms. Being eukaryotes, their cellular architecture exhibits a distinct membrane-bound nucleus and specialized membrane-bound organelles (mitochondria, chloroplasts, Golgi apparatus). They are primarily aquatic and represent an evolutionary link connecting primitive prokaryotes with multicellular fungi, plants, and animals.

Found in freshwater and marine ecosystems. Microscopic plankton drifting passively with water currents. Most are photosynthetic.

• Diatom Frustule: The cell wall consists of two thin overlapping shells fitting together seamlessly like a soap box (epitheca fitting over hypotheca). The wall is heavily impregnated with amorphous silica ($SiO_2$), making it virtually indestructible!
• Diatomaceous Earth (ডায়াটোমাইট): When diatoms die, their indestructible silica walls accumulate at the ocean bed over billions of years, forming colossal deposits known as diatomaceous earth.
• Commercial Uses: Being gritty and chemically inert, diatomaceous earth is used commercially for polishing metals, manufacturing heat-resistant bricks, and industrial filtration of oils, syrups, and antibiotic broths.
• Ecological Title: Diatoms are celebrated as the "Chief Producers in the Oceans".

Mostly marine and photosynthetic, displaying golden-brown, yellow, green, or red pigmentation based on xanthophylls and peridinin.

• Their cell surface is armored with stiff, articulated cellulose plates.
• Possess two flagella: one lies longitudinally while the other lies transversely in a spiral furrow (sulcus and cingulum), conferring a unique spinning motility ("whirling whips").
• Red Tides (লাল জোয়ার): Rapid exponential population explosions of red dinoflagellates like Gonyaulax impart a fiery red tint to ocean waters. They release lethal neurotoxins (such as saxitoxin) that cause massive fish kills and bioaccumulate in shellfish, causing paralytic shellfish poisoning in humans.

Majority inhabit stagnant freshwater bodies (e.g., Euglena).

• Pellicle: They completely lack a cellulosic cell wall. Instead, their plasma membrane is supported by a flexible, elastic, protein-rich helical layer called the pellicle, allowing graceful metabolic contractions.
• Possess two unequal flagella: a long whip-like anterior flagellum and a short rudimentary one.
• Mixotrophic Nutrition: In the presence of sunlight, they perform autotrophic photosynthesis using pigments identical to higher plants (Chlorophyll-a and b). When deprived of sunlight in dark habitats, they switch reversibly to heterotrophic predation on smaller micro-organisms!

Slime moulds are saprophytic protists that glide along decaying twigs and leaves, engulfing organic debris and bacteria.

• Plasmodium: Under favorable moist conditions, individual amoeboid cells aggregate into a free-flowing, multinucleated, wall-less mass of protoplasm called a Plasmodium, which can spread over several feet.
• Fruiting Bodies: Under harsh desiccation or stress, the plasmodium differentiates into upright stalked fruiting bodies bearing spores at their tips.
• Spore Resilience: Unlike the vegetative plasmodium, the spores possess true cellulosic cell walls. They are exceptionally resistant to extremes of temperature and drought, surviving dormant for years before dispersing via air currents.

Heterotrophic single-celled organisms that live as predators or parasites, considered primitive evolutionary predecessors of metazoans:

1. Amoeboid Protozoans: Move and capture prey using blunt pseudopodia (false feet). E.g. Amoeba, and the parasitic Entamoeba histolytica causing amoebic dysentery. Marine forms possess ornamental silica shells.
2. Flagellated Protozoans: Possess flagella; either free-living or parasitic. Trypanosoma gambiense causes African sleeping sickness, transmitted by the bite of the tsetse fly (Glossina).
3. Ciliated Protozoans: Aquatic, highly active organisms possessing thousands of coordinated cilia and a gullet opening to the cell exterior (e.g., Paramecium caudatum, with nuclear dimorphism: macronucleus and micronucleus).
4. Sporozoans: Endoparasites characterized by an infectious spore-like lifecycle stage. The most infamous is Plasmodium vivax and P. falciparum, the causative agent of malaria.

Kingdom Fungi comprises achlorophyllous, heterotrophic eukaryotic organisms exhibiting immense morphological and ecological diversity. Except for unicellular yeasts (Saccharomyces), fungi are filamentous. Their body consists of slender, thread-like filaments called hyphae, which intertwine to form an extensive vegetative network called the mycelium.

• Coenocytic Hyphae: Continuous, uninterrupted tubular hyphae lacking transverse partitions (aseptate) containing multiple nuclei suspended in cytoplasm (e.g., Phycomycetes).
• Septate Hyphae: Partitioned by internal cross-walls (septa) possessing pores (simple pore or dolipore septum in Basidiomycetes) allowing cytoplasmic continuity.
• Cell Wall: Unlike plant cellulose, fungal cell walls are composed of Chitin (a tough polymer of N-acetylglucosamine) and complex $eta$-glucans.
• Nutrition: Absorptive heterotrophs. They secrete extracellular digestive hydrolases to digest polymers into monomers, acting as saprophytes, parasites, or mutualistic symbionts (Lichens with algae, and Mycorrhizae with plant roots like Pinus).

Sexual reproduction in fungi proceeds through three sequential cellular milestones:

1. Plasmogamy: Fusion of protoplasms between two compatible naked or gametic cells without immediate nuclear fusion.
2. Karyogamy: Fusion of the two haploid nuclei ($n + n ightarrow 2n$) to form a diploid zygotic nucleus.
3. Meiosis: Reduction division of the diploid nucleus occurring in specialized sporangial structures, yielding haploid sexual spores.

The Dikaryophase ($n + n$): In lower fungi (Phycomycetes), plasmogamy is followed almost immediately by karyogamy. However, in higher fungi (Ascomycetes and Basidiomycetes), karyogamy is delayed! A prolonged intervening condition occurs where each compartment contains two distinct unfused haploid nuclei per cell ($n + n$). This distinct condition is called a dikaryon, and the developmental phase is called the dikaryophase.

Whittaker retained Kingdom Plantae and Kingdom Animalia for multicellular eukaryotic phototrophs and heterotrophs respectively, which are explored in exhaustive detail in Chapters 3 and 4 of the WBCHSE syllabus.

• Cellular Characteristics: Multicellular eukaryotes containing photosynthetic plastids bearing chlorophyll-a and b, carotenoids, and a rigid cell wall composed of cellulose, hemicellulose, and pectin.
• Heterotrophic Exceptions: Partial heterotrophic insectivorous plants (e.g., Nepenthes pitcher plant, Drosera sundew, Utricularia bladderwort) and complete parasitic plants like Cuscuta (dodder).
• Alternation of Generations (জনুক্রম): The life cycle of plants exhibits an alternation between a diploid spore-producing Sporophyte ($2n$) generation and a haploid gamete-producing Gametophyte ($n$) generation. The relative dominance and independence of these phases vary across Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.

• Cellular Characteristics: Multicellular, heterotrophic eukaryotes completely devoid of cell walls and photosynthetic plastids. Cells exhibit advanced junctional complexes (tight junctions, desmosomes).
• Nutrition & Storage: Ingestive holozoic nutrition involving internal digestive cavities. Reserve food material is stored primarily as glycogen or neutral fats (lipids).
• Neuromotor Coordination: Possess specialized sensory receptors, neural networks, and muscular contractile systems enabling active locomotion and rapid responses to environmental stimuli.
• Developmental Cycle: Reproduction is predominantly sexual, where motile male gametes fertilize stationary female ova to form a diploid zygote that undergoes cleavage and embryological tissue differentiation.

In Whittaker's Five Kingdom Classification, there is no mention of non-cellular entities such as Viruses, Viroids, and Prions, nor of composite organisms like Lichens. Because these infectious agents lack cellular protoplasmic organization and do not conform to Schleiden and Schwann's Cell Theory, they are classified separately.

Viruses are obligate intracellular parasites that are inert crystalline particles outside host cells but commandeer the host's cellular machinery to replicate when inside.

Historical Milestones in Virology:

• D.J. Ivanowsky (1892): Recognized microbes causing mosaic disease of tobacco; demonstrated they passed through bacterial filters, establishing they were smaller than bacteria.
• M.W. Beijerinck (1898): Demonstrated filtered sap from infected tobacco plants could infect healthy plants; termed the infectious liquid "Contagium vivum fluidum" (living infectious fluid).
• W.M. Stanley (1935): First crystallized Tobacco Mosaic Virus (TMV), showing crystals consist largely of proteins.

Structural Organization:

• Nucleoprotein Nature: A virus consists of an inner genetic core surrounded by an outer protective protein coat called a Capsid.
• Genetic Material Rule: A virus contains EITHER DNA OR RNA, NEVER BOTH!
  • Plant Viruses: Mostly single-stranded RNA (ssRNA), e.g. TMV.
  • Animal Viruses: ssRNA (influenza, rabies, retroviruses like HIV), double-stranded RNA (dsRNA in reoviruses), or double-stranded DNA (dsDNA in poxviruses, herpesviruses, hepatitis B).
  • Bacteriophages (viruses infecting bacteria): Usually double-stranded DNA (dsDNA) with a complex polyhedral head and helical tail.
• Capsomeres: The protein capsid consists of repeating structural subunits called capsomeres, arranged symmetrically in helical or polyhedral geometric lattices.

Discovered in 1971 by American plant pathologist T.O. Diener as a novel infectious agent smaller than viruses.

• Complete Lack of Protein Coat: Unlike viruses, viroids are free, naked RNA molecules devoid of any protective capsid!
• Low Molecular Weight: Consist of short, circular, single-stranded RNA of exceptionally low molecular weight ($100,000-120,000\, ext{Da}$, ~250-400 nucleotides).
• Diseases: Cause Potato Spindle Tuber Disease (PSTD), chrysanthemum stunt, and citrus exocortis.

Prions are pure protein molecules that have folded into an abnormal, protease-resistant, amyloid-forming conformation ($PrP^{Sc}$).

• They contain no detectable nucleic acid (neither DNA nor RNA)!
• They catalyze the conformational conversion of normal cellular prion proteins ($PrP^C$) into the infectious pathological isoform.
• Fatal Neurodegenerative Diseases: Cause Bovine Spongiform Encephalopathy (BSE) commonly known as Mad Cow Disease in cattle, and Creutzfeldt-Jakob Disease (CJD) as well as Kuru in humans, characterized by sponge-like vacuolation of brain tissue.

Lichens are composite associations between an autotrophic alga or cyanobacterium (Phycobiont / Photobiont) and a heterotrophic fungus (Mycobiont, usually an Ascomycete or Basidiomycete).

• Mutualistic Division of Labor: The phycobiont synthesizes carbohydrates via photosynthesis and fixes nitrogen (if cyanobacterial); the mycobiont provides structural shelter, anchorage, and absorbs water and mineral nutrients from the substratum.
• Ecological Pioneers: Lichens secrete organic acids (e.g. usnic acid) that weather bare rocks into soil, initiating primary ecological succession (lithosere).
• Bio-indicators of Air Pollution: Lichens are exceptionally sensitive to Sulfur Dioxide ($SO_2$) air pollution. Because they lack cuticles and stomata, they absorb atmospheric pollutants directly, which destroys algal chlorophyll. Therefore, lichens do not grow in industrial cities or heavily polluted traffic corridors!