Class 11 Biology Notes Chapter 2 (Chapter 2) – Examplar Problems (English) Book

Alright class, let's delve into Chapter 2: Biological Classification. Understanding how life is organized is fundamental, not just for your Class 11 studies, but also forms a crucial base for many government exams where Biology is a component. We'll break down the key concepts from the NCERT Exemplar perspective.

Chapter 2: Biological Classification - Detailed Notes

1. Introduction & Need for Classification:

• Earth hosts millions of organisms. Classification provides a framework to study this vast diversity systematically.
• Objectives: To organize organisms based on similarities and differences, establish evolutionary relationships (phylogeny), and provide a universal system for identification and naming.

2. History of Classification Systems:

• Aristotle: Earliest scientific attempt. Used simple morphological characters.
  • Plants: Trees, Shrubs, Herbs.
  • Animals: With red blood (Enaima) and without red blood (Anaima).
  • Limitation: Superficial; didn't consider internal structure, cell type, or evolutionary links.
• Linnaeus (Two Kingdom System - Plantae & Animalia): A significant step, but inadequate.
  • Plantae: Included bacteria, fungi, algae, bryophytes, pteridophytes, gymnosperms, angiosperms (based mainly on presence of cell wall).
  • Animalia: Included protozoa to vertebrates (based on lack of cell wall, locomotion, holozoic nutrition).
  • Demerits:
    • Didn't distinguish Prokaryotes (bacteria) from Eukaryotes.
    • Didn't separate Unicellular (Protists, Bacteria) from Multicellular organisms.
    • Didn't separate Photosynthetic (green algae, plants) from Non-photosynthetic (fungi) organisms.
    • Fungi (chitin cell wall, heterotrophic) were wrongly placed in Plantae.
    • Organisms like Euglena (photosynthetic but lack cell wall, motile) didn't fit well.

3. Five Kingdom Classification (R.H. Whittaker, 1969):

• The most widely accepted system, addressing the shortcomings of the two-kingdom system.
• Main Criteria Used:
  1. Cell Structure: Prokaryotic vs. Eukaryotic.
  2. Thallus Organisation: Complexity of the organism's body (Unicellular vs. Multicellular; tissue/organ level).
  3. Mode of Nutrition: Autotrophic (photosynthesis/chemosynthesis) vs. Heterotrophic (saprophytic/parasitic/holozoic).
  4. Reproduction: Asexual/Sexual modes.
  5. Phylogenetic Relationships: Evolutionary history and connections between groups.

Detailed Characteristics of the Five Kingdoms:

(i) Kingdom Monera:

• General Features: Includes all prokaryotes. Unicellular (most common), colonial, or filamentous. Lack true nucleus and membrane-bound organelles (like mitochondria, ER, Golgi). Cell wall typically made of peptidoglycan (murein), absent in Mycoplasma. Exhibit the most extensive metabolic diversity.
• Nutrition: Autotrophic (Photoautotrophs like Cyanobacteria; Chemoautotrophs like nitrifying bacteria) and Heterotrophic (Saprophytes, Parasites).
• Reproduction: Primarily Asexual (Binary Fission). Genetic recombination occurs through Conjugation, Transformation, Transduction.
• Major Groups:
  • Archaebacteria: Primitive bacteria living in extreme environments. Cell wall structure differs (lacks peptidoglycan), enabling survival.
    • Halophiles: Extreme salt concentrations.
    • Thermoacidophiles: High temperature and low pH (hot springs).
    • Methanogens: Marshy areas, gut of ruminants (cows, buffaloes); produce methane (biogas).
  • Eubacteria ('True Bacteria'): Characterized by rigid peptidoglycan cell wall (if present) and flagellum (if motile).
    • Cyanobacteria (Blue-Green Algae): Photosynthetic (Chlorophyll 'a'). Unicellular, colonial, or filamentous. Some fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc, Anabaena). Often form 'blooms' in polluted waters.
    • Chemosynthetic Autotrophs: Oxidize inorganic substances (nitrites, nitrates, ammonia) for energy (ATP). Play a role in nutrient cycling (N, P, Fe, S).
    • Heterotrophic Bacteria: Most abundant. Majority are decomposers. Some are pathogens, some helpful (making curd, antibiotics, fixing nitrogen in legumes).
  • Mycoplasma: Smallest living cells. Lack a cell wall completely. Can survive without oxygen (facultative anaerobes). Many are pathogenic.

(ii) Kingdom Protista:

• General Features: Includes all unicellular eukaryotes. Forms a link between prokaryotic Monera and multicellular eukaryotes (Fungi, Plantae, Animalia). Primarily aquatic. Possess a well-defined nucleus and membrane-bound organelles. May have cilia, flagella, or pseudopodia for locomotion. Boundaries of this kingdom are not well-defined.
• Nutrition: Diverse - Photosynthetic, Holozoic, Saprotrophic, Parasitic, Mixotrophic (Euglena).
• Reproduction: Both Asexual (binary fission, etc.) and Sexual (cell fusion, zygote formation).
• Major Groups:
  • Chrysophytes: (Diatoms and Golden algae/Desmids). Found in freshwater and marine habitats. Microscopic plankton.
    • Diatoms: Have siliceous cell walls forming two overlapping shells ('soap box' structure). Cell wall deposits accumulate as 'Diatomaceous Earth' (useful for polishing, filtration). Chief producers in the oceans.
  • Dinoflagellates: Mostly marine, photosynthetic. Cell wall has stiff cellulose plates. Have two flagella (longitudinal and transverse). Cause 'Red Tides' (e.g., Gonyaulax) which can be toxic.
  • Euglenoids: Mostly freshwater (stagnant water). Lack cell wall, have a flexible protein layer called pellicle. Have two flagella. Mixotrophic nutrition (photosynthetic in light, heterotrophic in dark). Pigments similar to higher plants. Example: Euglena.
  • Slime Moulds: Saprophytic protists. Under favourable conditions, form an aggregate called Plasmodium (multinucleate protoplasmic mass). Under unfavourable conditions, form fruiting bodies with spores having resistant walls.
  • Protozoans: Heterotrophic (predators or parasites). Considered primitive relatives of animals.
    • Amoeboid: Use pseudopodia for movement and capturing prey (e.g., Amoeba, Entamoeba - parasitic).
    • Flagellated: Possess flagella (e.g., Trypanosoma - causes sleeping sickness).
    • Ciliated: Have numerous cilia for locomotion and food intake (e.g., Paramecium).
    • Sporozoans: Have an infectious spore-like stage. All are parasites (e.g., Plasmodium - causes malaria).

(iii) Kingdom Fungi:

• General Features: Eukaryotic, heterotrophic organisms. Cosmopolitan distribution. Mostly multicellular (except unicellular yeasts, e.g., Saccharomyces). Body consists of thread-like hyphae; network of hyphae forms mycelium. Hyphae can be coenocytic (multinucleate, aseptate) or septate (with cross-walls). Cell wall composed of chitin and polysaccharides. Store food as glycogen and oil.
• Nutrition: Saprophytic (absorb nutrients from dead organic matter), Parasitic (live on/in other living organisms), Symbiotic (in association with algae - Lichens; with roots of higher plants - Mycorrhiza).
• Reproduction:
  • Vegetative: Fragmentation, Fission, Budding.
  • Asexual: Spores like conidia, sporangiospores, zoospores.
  • Sexual: Oospores, Ascospores, Basidiospores. Involves Plasmogamy (fusion of protoplasm), Karyogamy (fusion of nuclei), and Meiosis in zygote. A dikaryotic stage (n+n) occurs in Ascomycetes and Basidiomycetes.
• Classes:
  • Phycomycetes: Found in aquatic habitats, decaying wood, or as obligate parasites. Mycelium is aseptate and coenocytic. Asexual spores: zoospores (motile) or aplanospores (non-motile). Examples: Mucor, Rhizopus (bread mould), Albugo (parasite on mustard).
  • Ascomycetes (Sac Fungi): Mostly multicellular (Penicillium) or unicellular (Yeast). Saprophytic, decomposers, parasitic, or coprophilous. Mycelium is septate and branched. Asexual spores: conidia (exogenous). Sexual spores: ascospores (endogenous, in asci). Asci arranged in fruiting bodies (ascocarps). Examples: Aspergillus, Neurospora (used in genetics), Morels, Truffles (edible).
  • Basidiomycetes (Club Fungi): Includes mushrooms, bracket fungi, puffballs, rusts, smuts. Mycelium is septate and branched. Asexual spores generally absent. Sexual spores: basidiospores (exogenous, on basidia). Basidia arranged in fruiting bodies (basidiocarps). Examples: Agaricus (mushroom), Ustilago (smut), Puccinia (rust).
  • Deuteromycetes (Imperfect Fungi): Sexual reproduction is absent or not discovered. Reproduce by asexual conidia. Mycelium is septate and branched. Mostly decomposers, some parasites. Examples: Alternaria, Colletotrichum, Trichoderma. Once sexual stages are found, they are moved to Ascomycetes or Basidiomycetes.

(iv) Kingdom Plantae:

• General Features: Eukaryotic, multicellular, photosynthetic autotrophs. Cell wall mainly made of cellulose. Includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms. Exhibit alternation of generations (life cycle alternates between diploid sporophyte and haploid gametophyte). Some members are partially heterotrophic (insectivorous plants like Venus fly trap; parasites like Cuscuta).

(v) Kingdom Animalia:

• General Features: Eukaryotic, multicellular, heterotrophic organisms. Lack cell walls. Nutrition is holozoic (ingestion). Store food reserves as glycogen or fat. Capable of locomotion (most). Exhibit definite growth patterns. Reproduction is primarily sexual.

4. Viruses, Viroids, Prions, and Lichens:

• These entities are not included in the Five Kingdom system as they are not truly 'living' in the same sense or don't fit the criteria.
• Viruses:
  • Non-cellular, obligate intracellular parasites. Inert outside host cell.
  • Genetic material: Either DNA or RNA (never both), single or double-stranded.
  • Structure: Genetic material enclosed in a protein coat called capsid (made of capsomeres).
  • Cause diseases: Common cold, flu, AIDS, mumps, smallpox, herpes (animals); mosaic disease, leaf curl (plants).
  • Bacteriophages: Viruses infecting bacteria.
  • Key Scientists: Ivanowsky (discovery), Beijerinck (Contagium vivum fluidum), Stanley (crystallization).
• Viroids:
  • Discovered by T.O. Diener. Smaller than viruses.
  • Consist only of free, low molecular weight RNA. Lack a protein coat.
  • Cause diseases like Potato Spindle Tuber Disease.
• Prions:
  • Infectious agents consisting of abnormally folded proteins.
  • Cause fatal neurodegenerative diseases: Bovine Spongiform Encephalopathy (BSE or 'mad cow disease'), Cr-Jakob disease (CJD) in humans.
• Lichens:
  • Symbiotic association between an alga (phycobiont) and a fungus (mycobiont).
  • Alga provides food (photosynthesis); Fungus provides shelter, water, and mineral absorption.
  • Very sensitive to Sulphur Dioxide (SO2) pollution, hence used as pollution indicators.

Multiple Choice Questions (MCQs):

1. R.H. Whittaker's Five Kingdom Classification is NOT based on:
   a) Mode of nutrition
   b) Complexity of body organisation
   c) Presence or absence of locomotion
   d) Phylogenetic relationships

2. Which kingdom is characterized by prokaryotic cell structure and exhibits the most extensive metabolic diversity?
   a) Protista
   b) Fungi
   c) Monera
   d) Plantae

3. Heterocysts, specialized cells involved in nitrogen fixation, are found in certain members of:
   a) Archaebacteria
   b) Chrysophytes
   c) Cyanobacteria
   d) Dinoflagellates

4. The cell walls of diatoms are embedded with silica, making them indestructible. The accumulation of these walls is known as:
   a) Red tide
   b) Diatomaceous earth
   c) Pellicle
   d) Plasmodium

5. Fungi store reserve food material primarily in the form of:
   a) Starch
   b) Cellulose
   c) Proteins
   d) Glycogen and oil

6. Which class of fungi includes mushrooms, puffballs, and rust fungi, characterized by the formation of exogenous basidiospores?
   a) Phycomycetes
   b) Ascomycetes
   c) Basidiomycetes
   d) Deuteromycetes

7. Viruses are considered non-cellular because:
   a) They lack genetic material.
   b) They lack a protein coat.
   c) They possess an inert crystalline structure outside the living cell.
   d) They cannot reproduce independently.

8. Viroids, discovered by T.O. Diener, are infectious agents consisting of:
   a) Protein coat only
   b) Free DNA without a protein coat
   c) Free RNA without a protein coat
   d) Both RNA and DNA with a protein coat

9. Lichens represent a symbiotic relationship where the phycobiont (algal partner) primarily provides:
   a) Shelter and anchorage
   b) Water and mineral absorption
   c) Protection from UV radiation
   d) Synthesized food through photosynthesis

10. In Whittaker's classification, unicellular eukaryotic organisms like Amoeba and Paramecium are placed in Kingdom:
    a) Monera
    b) Protista
    c) Fungi
    d) Animalia

Answer Key:

1. c
2. c
3. c
4. b
5. d
6. c
7. c (While (d) is true, (c) best explains their non-cellular nature based on structure outside a host)
8. c
9. d
10. b

Study these notes thoroughly. Focus on the defining features of each kingdom and the unique characteristics of viruses, viroids, prions, and lichens. Good luck with your preparation!