Class 11 Biology Notes Chapter 2 (Biological classification) – Biology Book

Alright class, let's delve into Chapter 2: Biological Classification. Understanding how living organisms are grouped is fundamental not just for biology, but also forms a basis for questions in various competitive government exams. Pay close attention to the criteria for classification and the key characteristics of each kingdom.

Biological Classification: Need and History

• Why Classify? To organize the vast diversity of life forms for easier study, understand evolutionary relationships (phylogeny), and assign a universal name to each organism.
• Early Attempts:
  • Aristotle: Earliest scientific basis. Used simple morphological characters. Classified plants into trees, shrubs, herbs. Classified animals based on presence/absence of red blood (Enaima/Anaima). (Simple, but inadequate).
  • Linnaeus (Two Kingdom System): Proposed Kingdom Plantae and Kingdom Animalia. Easy to use but failed to distinguish between eukaryotes/prokaryotes, unicellular/multicellular, photosynthetic/non-photosynthetic organisms. Many organisms didn't fit neatly (e.g., Euglena, Fungi, Bacteria).
• Five Kingdom Classification (R.H. Whittaker, 1969): The most widely accepted system.
  • Criteria Used:
    1. Cell Structure: Prokaryotic vs Eukaryotic.
    2. Thallus Organisation: Unicellular vs Multicellular (or loose tissue/tissue/organ level).
    3. Mode of Nutrition: Autotrophic (Photosynthetic/Chemosynthetic) vs Heterotrophic (Saprotrophic/Parasitic/Holozoic).
    4. Reproduction: Asexual/Sexual modes.
    5. Phylogenetic Relationships: Evolutionary history and connections.
  • The Five Kingdoms: Monera, Protista, Fungi, Plantae, Animalia.

Characteristics of the Five Kingdoms (Whittaker)

(Note: Sapro = Saprophytic, Para = Parasitic, Photo = Photosynthetic, Chemo = Chemosynthetic, Holo = Holozoic)

Detailed Study of Kingdoms

1. Kingdom Monera (The Bacteria)

• Sole members: Bacteria. Most abundant microorganisms.
• Habitat: Cosmopolitan (found everywhere - hot springs, deserts, snow, deep oceans, as parasites).
• Structure: Prokaryotic, unicellular. Simple structure, complex behaviour. Diverse metabolic capabilities.
• Shapes: Coccus (spherical), Bacillus (rod-shaped), Vibrium (comma-shaped), Spirillum (spiral).
• Nutrition:
  • Autotrophic:
    • Photosynthetic: Use light energy (e.g., Cyanobacteria).
    • Chemosynthetic: Oxidize inorganic substances (nitrites, nitrates, ammonia) for ATP production. Play a great role in nutrient cycling (N, P, Fe, S).
  • Heterotrophic: Majority are heterotrophs.
    • Saprophytes: Decomposers, obtain food from dead organic matter.
    • Parasites: Obtain food from living hosts.
• Groups within Monera:
  • Archaebacteria: Live in harsh habitats. Different cell wall structure allows survival.
    • Halophiles: Extreme salty areas.
    • Thermoacidophiles: Hot springs (high temp, low pH).
    • Methanogens: Marshy areas, gut of ruminants (cows, buffaloes). Produce methane (biogas).
  • Eubacteria ('True Bacteria'): Rigid cell wall. If motile, possess flagellum.
    • Cyanobacteria (Blue-Green Algae): Photosynthetic autotrophs (Chlorophyll a similar to green plants). Unicellular, colonial, or filamentous. Freshwater/Marine/Terrestrial. Often form blooms in polluted water. Some fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc, Anabaena). Surrounded by gelatinous sheath.
    • Heterotrophic Bacteria: Most abundant. Important decomposers. Helpful in making curd, producing antibiotics, fixing nitrogen in legume roots (Rhizobium). Some are pathogens causing diseases like cholera, typhoid, tetanus, citrus canker.
• Reproduction: Mainly by fission (binary fission). Under unfavourable conditions, produce spores. Also reproduce by a primitive type of DNA transfer (sexual reproduction sort) - transformation, transduction, conjugation.
• Mycoplasma: Organisms completely lacking a cell wall. Smallest living cells known. Can survive without oxygen. Pathogenic in animals and plants.

2. Kingdom Protista

• General Characteristics: All single-celled eukaryotes. Primarily aquatic. Forms a link with Plantae, Animalia, and Fungi. Boundaries not well-defined.
• Structure: Eukaryotic cell organisation (well-defined nucleus, membrane-bound organelles). Some have cilia or flagella for locomotion.
• Nutrition: Can be photosynthetic, holozoic, saprotrophic, parasitic, or mixotrophic (Euglena).
• Reproduction: Asexually (cell fusion/binary fission) and Sexually (zygote formation).
• Major Groups:
  • Chrysophytes: Includes diatoms and golden algae (desmids). Found in freshwater and marine environments. Microscopic, float passively (plankton).
    • Diatoms: Cell walls form two thin overlapping shells fitting like a soap box. Walls embedded with silica, hence indestructible. Accumulation forms diatomaceous earth (used in polishing, filtration of oils and syrups). Chief producers in the oceans.
  • Dinoflagellates: Marine, photosynthetic. Appear yellow, green, brown, blue, or red (main pigments). Cell wall has stiff cellulose plates. Most have two flagella (one longitudinal, one transverse in a furrow). Rapid multiplication causes red tides (e.g., Gonyaulax), releasing toxins that kill marine animals.
  • Euglenoids: Majority freshwater, stagnant water. Instead of cell wall, have a protein-rich layer called pellicle (flexible). Two flagella (one short, one long). Mixotrophic nutrition: Photosynthetic in sunlight; predatory (heterotrophic) in absence of light. Pigments identical to higher plants (Chlorophyll a and b). Example: Euglena.
  • Slime Moulds: Saprophytic protists. Body moves along decaying twigs/leaves, engulfing organic matter. Under suitable conditions, form an aggregation called Plasmodium (can grow very large). Under unfavourable conditions, Plasmodium forms fruiting bodies bearing spores at tips. Spores have true walls, extremely resistant, dispersed by air.
  • Protozoans: Heterotrophs (predators or parasites). Believed to be primitive relatives of animals. Four major groups based on locomotory structures:
    • Amoeboid: Move and capture prey using pseudopodia (false feet) (e.g., Amoeba). Marine forms have silica shells. Some are parasitic (Entamoeba causes amoebic dysentery).
    • Flagellated: Have flagella. Free-living or parasitic. Parasitic forms cause diseases like sleeping sickness (Trypanosoma).
    • Ciliated: Aquatic, actively moving using thousands of cilia. Have a cavity (gullet) for food intake. Coordinated cilia movement directs food into gullet (e.g., Paramecium).
    • Sporozoans: Diverse organisms with an infectious spore-like stage in their life cycle. Locomotory structures absent in adults. All are parasitic. Example: Plasmodium (malarial parasite).

3. Kingdom Fungi

• General Characteristics: Eukaryotic, heterotrophic organisms. Great diversity in morphology and habitat. Cosmopolitan. Grow in warm, humid places.
• Structure: Most are multicellular, filamentous. Exception: Yeast (Saccharomyces) is unicellular. Body consists of long, slender thread-like structures called hyphae. Network of hyphae is mycelium.
  • Hyphae types:
    • Coenocytic hyphae: Continuous tubes filled with multinucleated cytoplasm (aseptate).
    • Septate hyphae: Have cross-walls (septa).
  • Cell Wall: Composed of chitin and polysaccharides.
• Nutrition:
  • Saprophytes: Absorb soluble organic matter from dead substrates.
  • Parasites: Depend on living plants and animals.
  • Symbionts: Mutually beneficial association.
    • Lichens: With algae.
    • Mycorrhiza: With roots of higher plants (e.g., Pinus).
• Reproduction:
  • Vegetative: Fragmentation, fission, budding.
  • Asexual: By spores like conidia, sporangiospores, zoospores.
  • Sexual: By oospores, ascospores, basidiospores. Produced in distinct structures called fruiting bodies.
  • Sexual Cycle Steps:
    1. Plasmogamy: Fusion of protoplasms between two motile or non-motile gametes.
    2. Karyogamy: Fusion of two nuclei.
    3. Meiosis: In the zygote, resulting in haploid spores.
  • Dikaryon Stage: In Ascomycetes and Basidiomycetes, an intervening stage (n+n, i.e., two nuclei per cell) occurs between plasmogamy and karyogamy. This phase is called dikaryophase.
• Classes of Fungi: Based on mycelium morphology, spore formation, and fruiting body formation.
  • Phycomycetes (Algal Fungi): Found in aquatic habitats, on decaying wood, moist places, or as obligate parasites on plants. Mycelium is aseptate and coenocytic.
    • Asexual reproduction: Zoospores (motile) or Aplanospores (non-motile), produced endogenously in sporangium.
    • Sexual reproduction: Zygospores formed by fusion of gametes (isogamous, anisogamous, or oogamous).
    • Examples: Mucor, Rhizopus (bread mould), Albugo (parasitic fungus on mustard).
  • Ascomycetes (Sac Fungi): Mostly multicellular (e.g., Penicillium) or rarely unicellular (e.g., Yeast). Saprophytic, decomposers, parasitic, or coprophilous (growing on dung). Mycelium is branched and septate.
    • Asexual reproduction: Conidia produced exogenously on special mycelium called conidiophores.
    • Sexual reproduction: Spores called ascospores produced endogenously in sac-like asci (singular: ascus). Asci are arranged in fruiting bodies called ascocarps.
    • Examples: Aspergillus, Claviceps, Neurospora (used extensively in genetic work), Morels and truffles (edible delicacies).
  • Basidiomycetes (Club Fungi): Includes mushrooms, bracket fungi, puffballs. Grow in soil, on logs, tree stumps, and as parasites (rusts and smuts). Mycelium is branched and septate.
    • Asexual spores: Generally not found. Vegetative reproduction by fragmentation is common.
    • Sexual reproduction: Sex organs absent. Plasmogamy occurs by fusion of two vegetative or somatic cells of different strains. Resulting structure is dikaryotic, which forms a basidium. Karyogamy and meiosis occur inside the basidium, producing four basidiospores exogenously. Basidia are arranged in fruiting bodies called basidiocarps.
    • Examples: Agaricus (mushroom), Ustilago (smut), Puccinia (rust fungus).
  • Deuteromycetes (Imperfect Fungi): Fungi where only asexual or vegetative phases are known. Sexual forms, when discovered, are moved to Ascomycetes or Basidiomycetes. Often called "Fungi Imperfecti". Reproduce only by asexual spores (conidia). Mycelium is septate and branched. Mostly decomposers of litter, help in mineral cycling. Some are saprophytes or parasites.
    • Examples: Alternaria, Colletotrichum, Trichoderma.

4. Kingdom Plantae

• General Characteristics: Eukaryotic, chlorophyll-containing organisms (autotrophs). Cell wall mainly made of cellulose.
• Includes: Algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms.
• Nutrition: Primarily photosynthetic. Some are partially heterotrophic:
  • Insectivorous: Bladderwort, Venus fly trap.
  • Parasitic: Cuscuta (Amarbel).
• Life Cycle: Exhibit alternation of generations – diploid sporophytic phase alternates with haploid gametophytic phase. Lengths and dominance of these phases vary between groups. (Details in Chapter 3).

5. Kingdom Animalia

• General Characteristics: Multicellular, eukaryotic, heterotrophic organisms. Lack cell walls.
• Nutrition: Holozoic (ingesting food). Store food reserves as glycogen or fat.
• Organisation: Cells -> Tissues -> Organs -> Organ Systems.
• Other Features: Definite growth pattern. Capable of locomotion. Higher forms show elaborate sensory and neuromotor mechanisms.
• Reproduction: Mostly sexual, involving copulation and embryological development. (Details in Chapter 4).

Viruses, Viroids, Prions, and Lichens

• These are acellular or symbiotic entities not placed in Whittaker's Five Kingdom classification.
• Viruses:
  • Non-cellular, obligate intracellular parasites. Inert crystalline structure outside a living cell.
  • Structure: Protein coat (capsid) surrounding genetic material (either DNA or RNA, never both). Capsid made of subunits called capsomeres.
  • Genetic material is infectious.
  • Cause diseases: Mumps, smallpox, herpes, influenza, AIDS (humans); Mosaic disease, leaf roll/curl, yellowing (plants).
  • Bacteriophages: Viruses that infect bacteria (usually have double-stranded DNA).
  • Named 'virus' (venom/poisonous fluid) by Pasteur. Discovered by D.J. Ivanowsky (tobacco mosaic disease). M.W. Beijerinck called fluid 'Contagium vivum fluidum'. W.M. Stanley crystallized viruses.
• Viroids:
  • Discovered by T.O. Diener (1971).
  • Infectious agents smaller than viruses.
  • Consist of free RNA only; lack protein coat.
  • RNA has low molecular weight.
  • Cause potato spindle tuber disease.
• Prions:
  • Infectious agents consisting of abnormally folded proteins.
  • Similar in size to viruses.
  • Cause fatal neurodegenerative diseases: Bovine Spongiform Encephalopathy (BSE) or 'mad cow disease' in cattle; Cr-Jakob disease (CJD) in humans.
• Lichens:
  • Symbiotic association (mutualism) between algae (phycobiont) and fungi (mycobiont).
  • Alga prepares food (photosynthesis); Fungus provides shelter, absorbs water and nutrients.
  • Very good pollution indicators, especially for sulphur dioxide (SO2) pollution – they do not grow in polluted areas.

Multiple Choice Questions (MCQs)

1. Which of the following criteria was NOT used by R.H. Whittaker for the Five Kingdom Classification?
   (a) Mode of nutrition
   (b) Thallus organisation
   (c) Phylogenetic relationships
   (d) Mode of locomotion

2. Which kingdom contains prokaryotic organisms?
   (a) Protista
   (b) Fungi
   (c) Monera
   (d) Plantae

3. Diatomaceous earth is formed by the indestructible cell walls of:
   (a) Dinoflagellates
   (b) Diatoms
   (c) Euglenoids
   (d) Slime moulds

4. Which group of fungi is commonly known as 'Imperfect Fungi'?
   (a) Phycomycetes
   (b) Ascomycetes
   (c) Basidiomycetes
   (d) Deuteromycetes

5. Methanogens, belonging to Archaebacteria, are found in:
   (a) Hot springs
   (b) Salty areas
   (c) Gut of ruminants
   (d) Polluted streams

6. Viruses possess:
   (a) DNA only
   (b) RNA only
   (c) Protein coat and genetic material (DNA or RNA)
   (d) Both DNA and RNA

7. Which of the following protists exhibits mixotrophic nutrition?
   (a) Amoeba
   (b) Paramecium
   (c) Euglena
   (d) Plasmodium

8. Mycorrhiza is a symbiotic association between:
   (a) Algae and fungi
   (b) Fungi and roots of higher plants
   (c) Bacteria and roots of higher plants
   (d) Algae and roots of higher plants

9. Which of the following statements is TRUE for Viroids?
   (a) They have a protein coat.
   (b) They have RNA of high molecular weight.
   (c) They are larger than viruses.
   (d) They consist of free RNA without a protein coat.

10. Heterocysts, involved in nitrogen fixation, are found in certain:
    (a) Archaebacteria
    (b) Cyanobacteria
    (c) Dinoflagellates
    (d) Protozoans

Answer Key for MCQs:

1. (d)
2. (c)
3. (b)
4. (d)
5. (c)
6. (c)
7. (c)
8. (b)
9. (d)
10. (b)

Revise these notes thoroughly, focusing on the distinguishing features and examples for each group. Good luck with your preparation!