Biological Classification Comprehensive Notes

Historical Systems of Classification

• Early Classification Efforts:

  • Since the beginning of human civilisation, attempts to classify living organisms were made instinctively.

  • Criteria were not scientific, but derived from practical human needs to utilise organisms for food, shelter, and clothing.

• Aristotle's Classification:

  • Earliest attempt to provide a scientific basis for biological classification.

  • Used simple morphological characters to divide plants into three groups: trees, shrubs, and herbs.

  • Divided animals into two distinct groups based on red blood presence: those with red blood and those without red blood.

• Linnaeus' Two Kingdom Classification System:

  • Developed during the time of Linnaeus, establishing Kingdom Plantae and Kingdom Animalia to encompass all plants and animals, respectively.

  • Inadequacies and Limitations:

    • Failed to distinguish between eukaryotic and prokaryotic organisms.

    • Failed to differentiate between unicellular and multicellular organisms (e.g., placing unicellular Chlamydomonas and multicellular Spirogyra together under algae).

    • Failed to separate heterotrophic organisms from autotrophic organisms (e.g., grouping non-photosynthetic fungi with cellulosic green plants).

    • A large number of organisms did not fit into either the plant or animal category.

    • Gross morphology alone proved insufficient, highlighting the need to incorporate cell structure, cell wall composition, mode of nutrition, habitat, reproduction methods, and evolutionary/phylogenetic relationships.

• R.H. Whittaker's Five Kingdom Classification (1969):

  • Proposed five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.

  • Main Criteria Used:

    • Cell structure

    • Body organisation

    • Mode of nutrition

    • Reproduction

    • Phylogenetic (evolutionary) relationships

• Three-Domain System:

  • Divides Kingdom Monera into two separate domains.

  • Places the remaining eukaryotic kingdoms into a third domain, resulting in a six-kingdom classification system.

Characteristics of the Five Kingdoms

• Kingdom Monera:

  • Cell Type: Prokaryotic

  • Cell Wall: Noncellulosic (composed of polysaccharides and amino acids)

  • Nuclear Membrane: Absent

  • Body Organisation: Cellular

  • Mode of Nutrition: Autotrophic (chemosynthetic and photosynthetic) and Heterotrophic (saprophytic and parasitic)

• Kingdom Protista:

  • Cell Type: Eukaryotic

  • Cell Wall: Present in some members

  • Nuclear Membrane: Present

  • Body Organisation: Cellular

  • Mode of Nutrition: Autotrophic (photosynthetic) and Heterotrophic

• Kingdom Fungi:

  • Cell Type: Eukaryotic

  • Cell Wall: Present, composed of chitin

  • Nuclear Membrane: Present

  • Body Organisation: Multicellular / loose tissue

  • Mode of Nutrition: Heterotrophic (saprophytic and parasitic)

• Kingdom Plantae:

  • Cell Type: Eukaryotic

  • Cell Wall: Present, composed of cellulose

  • Nuclear Membrane: Present

  • Body Organisation: Tissue / organ

  • Mode of Nutrition: Autotrophic (photosynthetic)

• Kingdom Animalia:

  • Cell Type: Eukaryotic

  • Cell Wall: Absent

  • Nuclear Membrane: Present

  • Body Organisation: Tissue / organ / organ system

  • Mode of Nutrition: Heterotrophic (holozoic, saprophytic, etc.)

Reclassification and Taxonomic Realignments

• Evolution of Classification Criteria:

  • Earlier systems placed bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms together under 'Plants' solely because all possessed cell walls.

  • Reclassification separated prokaryotic bacteria and blue-green algae (cyanobacteria) from eukaryotic groups.

  • Unicellular eukaryotic organisms were transferred to Kingdom Protista.

  • Unicellular algae like Chlamydomonas and Chlorella (previously classed under Algae in Plantae) were grouped alongside Paramoecium and Amoeba (previously in Animalia) under Kingdom Protista.

  • Fungi were assigned their own kingdom (Kingdom Fungi) due to differences in wall composition (chitin in fungi vs. cellulose in green plants).

  • Modern systems aim to be phylogenetic, directly reflecting evolutionary lineage alongside morphological, physiological, and reproductive traits.

Kingdom Monera

• General Features:

  • Bacteria are the sole members of Kingdom Monera and the most abundant micro-organisms in nature.

  • Cosmopolitan distribution: found in soil (hundreds per handful), water, air, and extreme habitats (hot springs, deserts, snow, deep oceans) where few other life forms survive.

  • Many exist as obligate or facultative parasites in or on other organisms.

• Morphological Categories (Bacterial Shapes):

  • Coccus (pl.: cocci): Spherical shape.

  • Bacillus (pl.: bacilli): Rod-shaped, often spore-bearing.

  • Vibrium (pl.: vibrio): Comma-shaped.

  • Spirillum (pl.: spirilla): Spiral shape, typically possessing flagella.

• Metabolic Diversity and Reproduction:

  • Structurally simple, but metabolically complex and diverse.

  • Autotrophic bacteria synthesise food from inorganic substrates via photosynthesis or chemosynthesis.

  • The vast majority are heterotrophs, relying on dead organic matter or living host organisms.

  • Reproduction occurs primarily by binary fission.

  • Under unfavourable environmental conditions, bacteria produce resistant spores.

  • Reproduce sexually through a primitive form of DNA transfer from one cell to another.

• Archaebacteria:

  • Specialised bacteria inhabit extreme environmental niches due to a unique cell wall structure.

  • Halophiles: Inhabit extremely salty environments.

  • Thermoacidophiles: Inhabit acidic hot springs.

  • Methanogens: Inhabit marshy conditions and the gut alimentary canals of ruminant animals (cows, buffaloes).

  • Methanogens produce methane (biogas) from ruminant animal dung.

• Eubacteria ("True Bacteria"):

  • Characterised by a rigid cell wall and, if motile, a flagellum.

  • Cyanobacteria (Blue-Green Algae):

    • Photosynthetic autotrophs possessing chlorophyll a identical to green plants.

    • Unicellular, colonial, or filamentous structures inhabit freshwater, marine, or terrestrial habitats.

    • Colonies are enclosed within gelatinous/mucilaginous sheaths.

    • Frequently form expansive algal blooms in nutrient-polluted water bodies.

    • Certain cyanobacteria fix atmospheric nitrogen in specialized cells termed heterocysts (e.g., Nostoc and Anabaena).

*   **Chemosynthetic Autotrophic Bacteria:**
    *   Oxidise inorganic substrates such as nitrates, nitrites, and ammonia.
    *   Utilise released energy for ATP synthesis.
    *   Crucial for recycling nutrients including nitrogen, phosphorus, iron, and sulphur.
*   **Heterotrophic Bacteria:**
    *   Most abundant in nature; function predominantly as decomposers.
    *   Beneficial human uses: converting milk to curd, antibiotic synthesis, fixing nitrogen in legume root nodules.
    *   Pathogenic strains cause diseases such as cholera, typhoid, tetanus, and citrus canker.

• Mycoplasma:

  • Organisms completely lacking a cell wall.

  • Smallest known living cells.

  • Capable of surviving anaerobically (without oxygen).

  • Pathogenic in various plants and animals.

Kingdom Protista

• General Characteristics:

  • Comprises all single-celled (unicellular) eukaryotic organisms.

  • Primarily aquatic in nature; forms an evolutionary link between Monera and the multicellular kingdoms (Plantae, Animalia, Fungi).

  • Contains a well-defined cell nucleus and membrane-bound intracellular organelles.

  • Locomotion mediated by flagella or cilia in certain groups.

  • Reproduces asexually and sexually (involving cell fusion and zygote formation).

• Chrysophytes:

  • Includes diatoms and golden algae (desmids).

  • Inhabit both freshwater and marine environments.

  • Microscopic organisms floating passively in water currents (plankton).

  • Predominantly photosynthetic.

  • Diatom Cell Wall Structure:

    • Forms two thin overlapping shells fitting together like a soap box.

    • Embedded with silica, making walls indestructible.

    • Accumulated cell wall deposits over billions of years form 'diatomaceous earth'.

    • Gritty soil used in industrial polishing and filtration of oils and syrups.

    • Diatoms serve as the chief producers in oceanic ecosystems.

• Dinoflagellates:

  • Mostly marine and photosynthetic organisms.

  • Exhibit yellow, green, brown, blue, or red coloration based on predominant intracellular pigments.

  • Cell wall features stiff cellulose plates on its outer surface.

  • Possess two flagella: one arranged longitudinally, the other transversely in a furrow between wall plates.

  • Red dinoflagellates (e.g., Gonyaulax) undergo rapid population blooms causing 'red tides'.

  • Toxins released during red tides can cause mortality in marine animals and fishes.

• Euglenoids:

  • Predominantly freshwater organisms inhabiting stagnant water bodies.

  • Lack a cell wall; enclosed by a protein-rich layer called a pellicle that provides structural flexibility.

  • Possess two flagella: one short and one long.

  • Mixotrophic nutrition: photosynthetic in the presence of sunlight, but behave as heterotrophic predators on smaller organisms when light is absent.

  • Photosynthetic pigments are identical to those of higher plants (e.g., Euglena).

• Slime Moulds:

  • Saprophytic protists moving along decaying twigs and leaves to engulf organic material.

  • Form an aggregated mass called plasmodium under favorable conditions, which can spread over several feet.

  • Under unfavorable conditions, plasmodium differentiates into fruiting bodies bearing spores at their tips.

  • Spores possess true, highly resistant cell walls, surviving for years under adverse conditions.

  • Spores are dispersed by air currents.

• Protozoans:

  • Heterotrophic organisms living as predators or parasites; considered primitive animal relatives.

  • Amoeboid Protozoans:

    • Inhabit fresh water, sea water, or moist soil.

    • Develop pseudopodia (false feet) for movement and prey capture (e.g., Amoeba).

    • Marine forms possess silica shells on their surface.

    • Includes parasitic forms such as Entamoeba.

  • Flagellated Protozoans:

    • Free-living or parasitic; possess flagella.

    • Parasitic species cause diseases like sleeping sickness (e.g., Trypanosoma).

  • Ciliated Protozoans:

    • Aquatic, highly motile organisms with thousands of cilia.

    • Possess a cell cavity (gullet) opening to the exterior cell surface.

    • Coordinated beating of cilia rows steers food-laden water into the gullet (e.g., Paramoecium).

  • Sporozoans:

    • Diverse organisms characterized by an infectious spore-like stage in their life cycle.

    • Includes Plasmodium (malarial parasite), responsible for malaria in human populations.

Kingdom Fungi

• General Characteristics and Morphology:

  • Unique kingdom of heterotrophic organisms exhibiting high structural and habitat diversity.

  • Cosmopolitan: present in air, water, soil, and on plants and animals.

  • Thrive in warm, humid environments (refrigeration prevents bacterial/fungal decay).

  • Includes edible mushrooms, toadstools, yeast (unicellular fungal source for bread/beer), and parasitic species like wheat rust-causing Puccinia or antibiotic-producing Penicillium.

  • Except for unicellular yeasts, fungi are filamentous.

  • Fungal bodies consist of long thread-like structures called hyphae; an interconnected network of hyphae is termed mycelium.

  • Coenocytic Hyphae: Continuous tubes filled with multinucleated cytoplasm (aseptate).

  • Septate Hyphae: Hyphae divided by septae or cross-walls.

  • Cell walls are composed of chitin and complex polysaccharides.

• Nutrition and Modes of Life:

  • Saprophytes: Absorb soluble organic matter from dead substrates.

  • Parasites: Depend directly on living plants or animals.

  • Symbionts: Live in mutualistic associations, such as lichens (with algae) or mycorrhiza (with roots of higher plants).

• Reproduction:

  • Vegetative: Fragmentation, fission, and budding.

  • Asexual Spores: Produced in distinct fruiting bodies as conidia, sporangiospores, or zoospores.

  • Sexual Spores: Produced as oospores, ascospores, or basidiospores.

  • Steps of Sexual Reproduction:

    1. Plasmogamy: Fusion of protoplasms between two motile or non-motile gametes.

    2. Karyogamy: Fusion of two haploid nuclei.

    3. Meiosis: Reduction division in the zygote resulting in haploid spores.

  • Dikaryon and Dikaryophase:

    • In Ascomycetes and Basidiomycetes, fusion of two haploid cells produces an intervening dikaryotic stage ($n + n$, two nuclei per cell).

    • This phase is called the dikaryophase, and the cell is termed a dikaryon.

    • Parental nuclei eventually fuse to form diploid ($2n$) cells, followed by reduction division in fruiting bodies.

• Phycomycetes:

  • Found in aquatic habitats, decaying wood in moist/damp environments, or as obligate plant parasites.

  • Mycelium is aseptate and coenocytic.

  • Asexual reproduction via motile zoospores or non-motile aplanospores, produced endogenously within a sporangium.

  • Zygospores form via fusion of isogamous (similar) or anisogamous/oogamous (dissimilar) gametes.

  • Examples: Mucor, Rhizopus (bread mould), and Albugo (parasite on mustard).

• Ascomycetes (Sac-fungi):

  • Mostly multicellular (e.g., Penicillium), rarely unicellular (e.g., yeast, Saccharomyces).

  • Saprophytic, decomposers, parasitic, or coprophilous (growing on animal dung).

  • Mycelium is branched and septate.

  • Asexual conidia are produced exogenously on specialised hyphae called conidiophores.

  • Sexual ascospores are produced endogenously in sac-like asci (sing.: ascus), which are packaged into fruiting bodies called ascocarps.

  • Examples: Aspergillus, Claviceps, Neurospora (used extensively in biochemical and genetic research), and edible delicacies like morels and truffles.

• Basidiomycetes:

  • Includes mushrooms, bracket fungi, and puffballs.

  • Grow in soil, on logs/stumps, and inside living plant bodies as parasites (rusts and smuts).

  • Mycelium is branched and septate.

  • Asexual spores are generally absent; vegetative propagation occurs via fragmentation.

  • Sex organs are absent; plasmogamy occurs via fusion of two vegetative/somatic cells of different strains/genotypes.

  • Produces a dikaryotic basidium wherein karyogamy and meiosis occur, generating four basidiospores exogenously.

  • Basidia are arranged in fruiting bodies termed basidiocarps.

  • Examples: Agaricus (mushroom), Ustilago (smut), and Puccinia (rust fungus).

• Deuteromycetes (Imperfect Fungi):

  • Termed 'imperfect' because only their asexual or vegetative phases are known.

  • When sexual (perfect) stages are discovered, members are reclassified into Ascomycetes or Basidiomycetes.

  • Reproduce exclusively via asexual spores called conidia.

  • Mycelium is septate and branched.

  • Members function as saprophytes, parasites, or major litter decomposers aiding mineral cycling.

  • Examples: Alternaria, Colletotrichum, and Trichoderma.

Kingdom Plantae

• General Features:

  • Comprises eukaryotic, chlorophyll-containing organisms commonly called plants.

  • Cell walls are prominently composed of cellulose.

  • Includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.

• Heterotrophic Exceptions:

  • Partially heterotrophic insectivorous plants: Bladderwort and Venus fly trap.

  • Parasitic plant: Cuscuta.

• Alternation of Generations:

  • Life cycle alternates between two distinct phases: the diploid sporophytic ($2n$) phase and the haploid gametophytic ($n$) phase.

  • The duration and degree of independence (free-living vs. dependent) of these phases vary among plant groups.

Kingdom Animalia

• General Features:

  • Multicellular, eukaryotic, heterotrophic organisms lacking cell walls.

  • Depend directly or indirectly on plants for nutrition.

  • Digest food inside an internal digestive cavity; store energy reserves as glycogen or fat.

  • Nutrition mode is holozoic (ingestion of food).

  • Follow a fixed growth pattern, developing into adults with defined shapes and sizes.

  • Higher forms possess complex sensory and neuromotor mechanisms.

  • Most members are capable of locomotion.

  • Sexual reproduction proceeds via male and female copulation, followed by embryological development.

Acellular Organisms and Symbiotic Associations

• Viruses:

  • Omitted from Whittaker's five-kingdom system because they are non-cellular and not strictly considered 'living'.

  • Characterised by an inert crystalline structure outside living host cells.

  • Obligate parasites that take over host cell machinery upon infection to replicate, ultimately killing the host.

  • The name 'virus' translates to venom or poisonous fluid.

• Historical Milestones in Virology:

  • Dmitri Ivanowsky (1892): Recognized microbes smaller than bacteria (passing through bacteria-proof filters) as the causal agent of tobacco mosaic disease.

  • M.W. Beijerinck (1898): Demonstrated that extract from infected tobacco plants transmits disease to healthy plants; termed the fluid Contagium vivum fluidum (infectious living fluid) and named the agent "virus".

  • W.M. Stanley (1935): Showed viruses could be crystallised, with crystals consisting largely of proteins.

• Viral Structure and Pathogenicity:

  • Nucleoprotein structure containing genetic material enclosed in a protein coat.

  • Contain either RNA or DNA; no virus contains both.

  • Plant-infecting viruses typically possess single-stranded RNA.

  • Animal-infecting viruses possess single- or double-stranded RNA, or double-stranded DNA.

  • Bacteriophages (bacterial viruses) usually contain double-stranded DNA.

*   **Capsid:** Protein coat made of small protein subunits called capsomeres arranged in helical or polyhedral geometric forms to protect nucleic acid.
*   **Human/Animal Diseases:** Mumps, smallpox, herpes, influenza, and AIDS.
*   **Plant Symptoms:** Mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing, and stunted growth.

• Viroids:

  • Discovered by T.O. Diener in 1971 as a new infectious agent smaller than viruses.

  • Causes potato spindle tuber disease.

  • Consists of free, low-molecular-weight RNA lacking a protein coat (hence 'viroid').

• Prions:

  • Infectious agents consisting of abnormally folded proteins, similar in size to viruses.

  • Cause neurological diseases.

  • Notable Diseases: Bovine spongiform encephalopathy (BSE, or 'mad cow disease') in cattle, and Creutzfeldt-Jakob disease (CJD) in humans.

• Lichens:

  • Symbiotic, mutually beneficial associations between algae and fungi.

  • Phycobiont: Algal component; autotrophic; manufactures food via photosynthesis.

  • Mycobiont: Fungal component; heterotrophic; provides shelter, absorbs water and mineral nutrients.

  • Lichens serve as sensitive pollution indicators, as they do not grow in polluted environments.