Biological Classification Comprehensive Study Guide

Early Classification Systems

• Aristotle's Scientific Approach: Aristotle was the earliest to attempt a more scientific basis for the classification of organisms.

  • Classification of Plants: He divided plants into three categories: Trees, Shrubs, and Herbs.

  • Classification of Animals: He divided animals into two groups: those that possessed red blood and those that did not.

• The Two Kingdom System: Proposed by Carl Linnaeus in $1758$.

  • Kingdoms: This system categorized all organisms into two kingdoms: Kingdom Plantae and Kingdom Animalia.

  • Limitations of the Two Kingdom System:

    • It did not distinguish between prokaryotes (such as Bacteria and Cyanobacteria) and eukaryotes (such as fungi, mosses, ferns, gymnosperms, and angiosperms) which were all initially included under "plants."

    • The basis was only the presence of a cell wall, although prokaryotes and eukaryotes differ significantly in other features.

    • It grouped unicellular and multicellular organisms together; for example, Chlamydomonas and Spirogyra were both placed under algae.

    • It failed to differentiate between heterotrophic fungi and autotrophic green plants. Fungi possess a chitinous cell wall, whereas green plants possess a cellulosic cell wall.

The Five Kingdom Classification

• Proposed by: R.H. Whittaker in $1969$.

• Kingdoms Included: Monera, Protista, Fungi, Plantae, and Animalia.

• Basis of Classification: This system is based on cinco key criteria:

  • Cell structure.

  • Thallus organization.

  • Mode of nutrition.

  • Reproduction.

  • Phylogenetic relationships.

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Comparative Characteristics of the Five Kingdoms

• Cell Type:

  • Monera: Prokaryotic.

  • Protista, Fungi, Plantae, Animalia: Eukaryotic.

• Cell Wall:

  • Monera: Non-cellular, composed of polysaccharide and amino acids.

  • Protista: Present in some organisms.

  • Fungi: Present, composed of chitin (without cellulose).

  • Plantae: Present, composed of cellulose.

  • Animalia: Absent.

• Nuclear Membrane:

  • Monera: Absent.

  • Protista, Fungi, Plantae, Animalia: Present.

• Body Organization:

  • Monera: Cellular.

  • Protista: Cellular.

  • Fungi: Multicellular, organized as loose tissue.

  • Plantae: Tissue or organ level.

  • Animalia: Tissue, organ, or organ system level.

• Mode of Nutrition:

  • Monera: Autotrophic (photosynthetic and chemosynthetic) and heterotrophic (saprophyte or parasite).

  • Protista: Autotrophic (photosynthetic) and heterotrophic.

  • Fungi: Heterotrophic (saprophytic or parasitic).

  • Plantae: Autotrophic (photosynthetic).

  • Animalia: Heterotrophic (holozoic, saprophytic, etc.).

Kingdom Monera

• General Characteristics of Bacteria:

  • Bacteria are the most abundant microorganisms.

  • A single handful of soil can contain hundreds of bacteria.

  • They inhabit extreme environments including hot springs, deserts, snow, and deep oceans.

  • Many function as parasites.

  • Structural Simplicity vs. Behavioral Complexity: While biologically simple in structure, they exhibit complex behavior and extensive metabolic diversity.

  • Nutrition: Some are autotrophic (synthesizing food from inorganic substrates), but the majority are heterotrophs (relying on other organisms or dead organic matter).

• Archaebacteria:

  • Habitats: They live in the harshest environments.

    • Halophiles: Extreme salty areas.

    • Thermoacidophiles: Hot springs.

    • Methanogens: Marshy areas.

  • Survival Mechanism: They have a distinct cell wall structure that allows survival in extreme conditions.

  • Methanogens Detail: Found in the guts of ruminant animals (e.g., cows and buffaloes); they produce methane ($CH_4$, biogas) from animal dung.

• Eubacteria ("True Bacteria"):

  • Characteristics: Rigid cell wall and a flagellum (if motile).

  • Cyanobacteria (Blue-green Algae):

    • Contain chlorophyll a, similar to green plants.

    • Forms: Unicellular, colonial, or filamentous.

    • Environments: Marine or terrestrial.

    • Structure: Colonies are typically surrounded by a gelatinous sheath.

    • Ecology: Form blooms in polluted water bodies. Some fix atmospheric $N_2$ using specialized cells called heterocysts (e.g., Nostoc and Anabaena).

  • Chemosynthetic Autotrophs: Oxidize inorganic substances like nitrates, nitrites, and ammonia to release energy for $ATP$ production. They recycle nutrients such as nitrogen, phosphorous, iron, and sulphur.

  • Heterotrophic Bacteria:

    • Most abundant in nature.

    • Important as decomposers.

    • Applications: Curd production from milk, antibiotic production, and nitrogen fixation in legume roots.

    • Pathogens: Cause diseases such as Cholera, typhoid, tetanus, and citrus canker.

• Reproduction in Bacteria:

  • Primarily by fission.

  • Under unfavorable conditions, they produce spores.

  • Primitive sexual reproduction occurs via DNA transfer from one bacterium to another.

• Mycoplasma:

  • Smallest living cells known.

  • They lack a cell wall entirely.

  • Can survive without oxygen and are often pathogenic in animals and plants.

Kingdom Protista

• General Features:

  • Includes all single-celled eukaryotes.

  • Primarily aquatic; forms a link between plants, animals, and fungi.

  • Contains a well-defined nucleus and membrane-bound organelles; some have flagella or cilia.

  • Reproduction: Asexually and sexually (via cell fusion and zygote formation).

• Chrysophytes:

  • Includes diatoms and golden algae (desmids).

  • Found in freshwater and marine environments.

  • Microscopic plankton that float passively.

  • Diatoms: Siliceous cell walls forming two thin overlapping shells (like a soap box). The accumulation of these walls over billions of years creates "diatomaceous earth," used in polishing and the filtration of oils and syrups. Diatoms are the chief "producers" in the oceans.

• Dinoflagellates:

  • Mostly marine and photosynthetic.

  • Colors: Yellow, green, brown, blue, or red, based on pigments.

  • Stiff cellulose plates on the outer cell wall surface.

  • Flagella: Two flagella; one longitudinal and one transverse in a wall plate furrow.

  • Gonyaulax: A red dinoflagellate that multiplies rapidly causing "red tides." Toxins released can kill marine animals like fish.

• Euglenoids:

  • Mainly stagnant freshwater organisms.

  • Body: No cell wall; instead, they have a protein-rich layer called a pellicle, making the body flexible.

  • Flagella: Two (one short, one long).

  • Nutrition: Mixotrophic. Photosynthetic in sunlight; heterotrophic (predatory) when deprived of light.

  • Example: Euglena (pigments identical to higher plants).

• Slime Moulds:

  • Saprophytic protists.

  • Aggregation: Under suitable conditions, they form Plasmodium (can grow several feet).

  • Fruiting: Under unfavorable conditions, Plasmodium forms fruiting bodies with spores at the tips. Spores have true walls and are extremely resistant, dispersed by air.

• Protozoans:

  • Heterotrophs (predators or parasites) and primitive animal relatives.

  • Amoeboid: Move/capture prey via pseudopodia (e.g., Amoeba). Marine forms have silica shells. Entamoeba is a parasite.

  • Flagellated: Free-living or parasitic. Trypanosoma causes sleeping sickness.

  • Ciliated: Aquatic, use cilia for movement. Possess a gullet (cavity) that opens to the outside. Example: Paramecium.

  • Sporozoans: Have an infectious spore-like stage. Example: Plasmodium (malarial parasite).

Kingdom Fungi

• General Characteristics:

  • Unique kingdom of heterotrophic organisms; cosmopolitan (air, water, soil, plants, animals).

  • Prefer warm and humid places.

  • Examples: Bread Mould, Orange Rots, Mushroom, Toadstools.

  • Disease/Parasites: White spots on mustard leaves; Puccinia causes wheat rust.

  • Antibiotics: Penicillium.

  • Unicellular: Yeast (Saccharomyces), used for bread and beer.

• Structure and Nutrition:

  • Body: Filamentous (except yeast). Consists of hyphae (slender threads). A network of hyphae is a mycelium.

  • Hyphae Types: Coenocytic (continuous tubes with multinucleated cytoplasm) or septate (with cross walls).

  • Cell Wall: Composed of chitin and polysaccharides.

  • Nutrition: Saprophytes (dead matter), Parasites, or Symbionts (Lichens with algae; Mycorrhiza with roots of higher plants).

• Sexual Cycle:

  1. Plasmogamy: Fusion of protoplasm between gametes.

  2. Karyogamy: Fusion of two nuclei.

  3. Meiosis: Occurs in the zygote to produce haploid spores.

• Classes of Fungi:

  • Phycomycetes: Found in aquatic habitats and on decaying wood. Asexual reproduction by zoospores (motile) or aplanospores (non-motile). Sexual reproduction forms zygospores (isogamous or anisogamous/oogamous).

  • Ascomycetes (Sac-fungi): Unicellular (yeast) or multicellular (Penicillium). Saprophytic, decomposers, or coprophilous (grow on dung). Mycelium is branched/septate. Asexual spores are Conidia (produced exogenously on conidiophores). Sexual spores are Ascospores (produced endogenously in sac-like asci). Examples: Aspergillus, Claviceps, Neurospora (used in genetic work), Morels, and Truffles.

  • Basidiomycetes: Mushrooms, bracket fungi, or puffballs. Grow in soil, logs, or as parasites (rusts and smuts). Mycelium is branched/septate. Asexual spores absent; vegetative fragmentation is common. Sex organs absent. Plasmogamy involves fusion of two somatic cells to form a dikaryotic structure, eventually leading to a basidium. Karyogamy and meiosis in the basidium produce $4$ exogenous basidiospores. Examples: Agaricus, Ustilago, Puccinia.

  • Deuteromycetes (Imperfect Fungi): Only asexual or vegetative stages are known. Reproduce by conidia. Mycelium is septate and branched. Crucial as litter decomposers and for mineral cycling. Examples: Alternaria, Colletotrichum, Trichoderma.

Kingdom Plantae and Animalia

• Kingdom Plantae:

  • Eukaryotic, chlorophyll-containing organisms with cellulosic cell walls.

  • Partial Heterotrophs: Insectivorous plants (Bladderwort, Venus fly trap) and parasites (Cuscuta).

  • Groups: Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.

  • Life Cycle: Alternation of generation between diploid sporophytic and haploid gametophytic phases.

• Kingdom Animalia:

  • Multicellular, heterotrophic eukaryotes without cell walls.

  • Nutrition: Holozoic (ingestion of food). Store reserves as glycogen or fat.

  • Growth: Definite growth pattern, adults have fixed shape and size.

  • Physiology: Higher forms have sensory and neuromotor mechanisms; most are capable of locomotion.

  • Reproduction: Copulation of male and female followed by embryological development.

Viruses, Viroids, and Lichens

• Viruses:

  • Acellular and not truly "living," thus excluded from Whittaker's five kingdoms.

  • Structure: Inert crystalline structure outside a living cell; obligate parasites.

  • History:

    • Louis Pasteur: Coined the name "virus" (meaning venom or poisonous fluid).

    • D.J. Ivanowsky ($1892$): Identified microbes smaller than bacteria causing mosaic disease of tobacco.

    • M.W. Beijernick ($1898$): Coined the term "Contagium vivium fluidum" (infectious living fluid) after infecting healthy plants with extract from diseased tobacco.

    • W.M. Stanley ($1935$): Showed viruses could be crystallized; crystals comprise mostly proteins.

  • Composition: Nucleoprotein (protein coat called a capsid + genetic material). No virus contains both DNA and RNA.

  • Host Specificity: Plant viruses usually have ssRNA. Animal viruses have ssRNA, dsRNA, or dsDNA. Bacteriophages (bacteria-infecting) usually have dsDNA.

  • Capsid: Made of subunits called capsomeres, arranged in helical or polyhedral geometric forms.

• Viroids:

  • Smaller than viruses; discovered by T.O. Diener in $1971$.

  • Consists of free, low molecular weight RNA without a protein coat.

  • Example: Causes Potato spindle tuber disease.

• Lichens:

  • Symbiotic association between algae (phycobiont - autotrophic) and fungi (mycobiont - heterotrophic).

  • Mutualism: Algae provide food; fungi provide shelter and absorb water/minerals.

  • Pollution Indicators: They do not grow in polluted areas.