Quick Revision Notes

Class 11 Biology - Biological Classification (जैव वर्गीकरण)

1. Introduction to Biological Classification

• Biological Classification is the scientific process of arranging organisms into groups and subgroups based on similarities and dissimilarities.
• Need for Classification: To study the vast diversity of life systematically, understand evolutionary relationships (phylogeny), and identify organisms easily.

2. Historical Systems of Classification

• Aristotle’s Classification: Earliest scientific attempt.
  • Plants: Herbs, Shrubs, and Trees.
  • Animals: Red blood cells present (Enaima) and absent (Anaima).
• Two-Kingdom System: Proposed by Carolus Linnaeus (1758).
  • Kingdom Plantae and Kingdom Animalia.
  • Drawback: Did not distinguish between eukaryotes and prokaryotes, unicellular and multicellular, or photosynthetic (green algae) and non-photosynthetic (fungi) organisms.

3. Five-Kingdom Classification

Proposed by R.H. Whittaker (1969). The main criteria used include cell structure, thallus organization, mode of nutrition, reproduction, and phylogenetic relationships.

4. Detailed Breakdown of Kingdoms

A. Kingdom Monera (The Prokaryotes)

• Sole members: Bacteria.
• Shapes of Bacteria:
  • Coccus (Spherical - गोलाकार)
  • Bacillus (Rod-shaped - छड़नुमा)
  • Vibrio (Comma-shaped - कॉमा के आकार के)
  • Spirillum (Spiral - सर्पिलाकार)
• Types based on Nutrition:
  • Autotrophic: Photosynthetic autotrophs (e.g., Cyanobacteria) and Chemosynthetic autotrophs.
  • Heterotrophic: Most abundant; act as decomposers or pathogens.
• Key Groups:
  • Archaebacteria: Live in extreme habitats (Halophiles in salty areas, Thermoacidophiles in hot springs, Methanogens in marshy areas & gut of ruminant animals like cows producing biogas/methane).
  • Eubacteria ('True Bacteria'): Characterized by a rigid cell wall and a flagellum (if motile).
    • Cyanobacteria (Blue-green algae): Have chlorophyll $a$. Often fix atmospheric nitrogen in specialized cells called Heterocysts (e.g., Nostoc, Anabaena).
  • Mycoplasma: Completely lack a cell wall, smallest living cells known, can survive without oxygen, pathogenic in animals and plants.

B. Kingdom Protista (The Single-celled Eukaryotes)

• All single-celled eukaryotes; primarily aquatic.
• Major Groups:
  1. Chrysophytes: Includes diatoms and golden algae (desmids). Found in freshwater and marine environments. Cell walls form two thin overlapping shells (soap box structure) embedded with silica, leaving behind Diatomaceous earth.
  2. Dinoflagellates: Mostly marine and photosynthetic. Appear yellow, green, brown, blue, or red depending on pigments. Example: Gonyaulax (rapid multiplication causes red tides).
  3. Euglenoids: Freshwater organisms found in stagnant water. Instead of a cell wall, they have a protein-rich layer called pellicle making their body flexible. Photosynthetic in sunlight, behave like heterotrophs in the absence of light (Mixotrophic nutrition). Example: Euglena.
  4. Slime Moulds: Saprophytic protists. Under favorable conditions, form an aggregation called plasmodium. Under unfavorable conditions, form fruiting bodies bearing spores.
  5. Protozoans: Heterotrophs (predators or parasites). Four major groups:
     • Amoeboid: Move using pseudopodia (e.g., Amoeba).
     • Flagellated: Have flagella (e.g., Trypanosoma - sleeping sickness).
     • Ciliated: Have cilia for movement and water intake (e.g., Paramecium).
     • Sporozoans: Infectious spore-like stage in life cycle (e.g., Plasmodium - malaria parasite).

C. Kingdom Fungi (The Kingdom of Rotting/Molds)

• Heterotrophic, eukaryotic organisms with a chitinous cell wall.
• Body Structure: Consist of long, slender thread-like structures called hyphae. A network of hyphae is called mycelium.
  • Aseptate and multinucleate hyphae = Coenocytic hyphae.
• Reproduction:
  • Vegetative: Fragmentation, fission, and budding.
  • Asexual: Spores like conidia, sporangiospores, or zoospores.
  • Sexual: Oospores, ascospores, and basidiospores formed in distinct structures. Steps involve Plasmogamy $\rightarrow$ Karyogamy $\rightarrow$ Meiosis.
• Classes of Fungi:
  1. Phycomycetes: Aseptate and coenocytic mycelium. Aquatic or on decaying wood. Examples: Mucor, Rhizopus (bread mold), Albugo.
  2. Ascomycetes ('Sac fungi'): Branched and septate mycelium. Examples: Aspergillus, Claviceps, Neurospora (used extensively in biochemical work), Penicillium, Yeast (unicellular). Edible forms: Morels and truffles.
  3. Basidiomycetes ('Club fungi'): Branched and septate mycelium. Sex organs absent, plasmogamy occurs by fusion of two vegetative cells. Examples: Agaricus (mushroom), Ustilago (smut), Puccinia (rust fungus).
  4. Deuteromycetes ('Imperfect fungi'): Only asexual or vegetative phases are known. Examples: Alternaria, Colletotrichum, Trichoderma.

D. Kingdom Plantae

• Includes all eukaryotic, chlorophyll-containing organisms (plants).
• Characteristics: Presence of cellulose cell wall, autotrophic nutrition (photosynthesis).
• Includes Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.
• Shows Alternation of Generation (haploid gametophytic and diploid sporophytic phases).

E. Kingdom Animalia

• Heterotrophic, eukaryotic, multicellular organisms lacking cell walls.
• Directly or indirectly dependent on plants for food (Holozoic nutrition - ingestion of food).
• Follow a definite growth pattern and grow into adults that have a definite shape and size.

5. Acellular Organisms (Non-Cellular Forms)

• Viruses (W.M. Stanley showed they can be crystallized):
  • Non-cellular organisms characterized by having an inert crystalline structure outside the living cell.
  • Composed of genetic material (DNA or RNA - never both) enclosed in a protein coat called a capsid (made of capsomeres).
  • Plant viruses have single-stranded RNA; Animal viruses have single or double-stranded RNA or double-stranded DNA. Bacteriophages usually have double-stranded DNA.
  • Diseases caused: Mumps, smallpox, herpes, influenza, AIDS.
• Viroids (T.O. Diener, 1971):
  • Infectious agent smaller than viruses.
  • Free RNA molecule; lack the protein coat (capsid).
  • Example: Potato spindle tuber disease.
• Prions:
  • Abnormally folded protein molecules causing neurological diseases.
  • Example: Bovine Spongiform Encephalopathy (BSE) or Mad Cow disease in cattle, and its analogous variant Cr-Jacob disease (CJD) in humans.
• Lichens:
  • Symbiotic associations (mutually useful associations) between algae (Phycobiont) and fungi (Mycobiont).
  • Algal component prepares food; fungal component provides shelter and absorbs mineral nutrients and water.
  • Note: Excellent pollution indicators (do not grow in polluted areas).