📝 Chapter Notes & Revision

Cell: The Unit of Life

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Class 11 Biology — Chapter 8: Cell: The Unit of Life (कोशिका: जीवन की इकाई)

1. Fundamental Concepts & Discoveries (मूल अवधारणाएं एवं खोजें)

  • Cell (कोशिका): The fundamental, structural, and functional unit of all living organisms.
  • Anton von Leeuwenhoek: First saw and described a live cell.
  • Robert Hooke: Discovered the cell (dead cork cell).
  • Robert Brown: Discovered the Nucleus (केंद्रक) in 1831.

2. Cell Theory (कोशिका सिद्धांत)

  1. Matthias Schleiden (1838): Observed that all plants are composed of different kinds of cells.
  2. Theodore Schwann (1839): Reported that animal cells have a thin outer layer (now called plasma membrane). He concluded that the presence of a cell wall is a unique character of plant cells.
  3. Rudolf Virchow (1855): Modified the theory and gave the phrase "Omnis cellula-e-cellula" (All cells arise from pre-existing cells).

Modern Cell Theory Postulates:

  • All living organisms are composed of cells and products of cells.
  • All cells arise from pre-existing cells.

3. Prokaryotic vs. Eukaryotic Cells (प्रोकेरियोटिक बनाम यूकेरियोटिक कोशिका)

Feature / CharacterProkaryotic CellEukaryotic Cell
Nuclear MembraneAbsentPresent
Membrane-bound OrganellesAbsentPresent (Mitochondria, ER, Golgi, etc.)
Ribosome Type70S type (50S + 30S)80S type (60S + 40S) [70S in organelle]
Genetic MaterialNaked circular DNA (Nucleoid)Linear DNA wrapped around Histones
Cell WallMade of Peptidoglycan / MureinMade of Cellulose (Plants) / Chitin (Fungi)
ExamplesBacteria, BGA, Mycoplasma, PPLOProtists, Plants, Fungi, Animals

4. Detailed Prokaryotic Cell Structure

  • Cell Envelope (कोशिका आवरण): Consists of 3 tightly bound layers:

    1. Glycocalyx: Outermost layer. Loose sheath = Slime layer; Thick and tough = Capsule.
    2. Cell Wall: Determines shape and provides strong structural support.
    3. Plasma Membrane: Selectively permeable; interacts with the outside world.

  • Special Structures:

    • Mesosomes (मेसोसोम): Infoldings of plasma membrane in bacteria. Help in cell wall formation, DNA replication, and respiration.
    • Chromatophores: Pigment-containing membranous infoldings (e.g., Cyanobacteria).
    • Flagella: Composed of 3 parts — Filament, Hook, and Basal body.
    • Pili & Fimbriae: Surface structures for attachment (Pili are tubular, made of pilin protein).
    • Inclusion Bodies: Reserve material stored in cytoplasm (e.g., Phosphate granules, Cyanophycean granules, Glycogen granules, Gas vacuoles).

5. Cell Membrane & Cell Wall

Fluid Mosaic Model (तरल मोजेक मॉडल)

  • Proposed by Singer and Nicolson (1972).
  • Membrane is composed of a lipid bilayer (mainly phosphoglycerides) with embedded proteins.
  • Lipid arrangement: Polar hydrophilic heads point outward, non-polar hydrophobic tails point inward.
  • Membrane Proteins:
    • Integral Proteins: Partially or totally buried in the membrane.
    • Peripheral Proteins: Lie on the surface of the membrane.
  • Ratio in Human Erythrocyte Membrane: ~52% Protein, 40% Lipid.

Transport Across Membrane

  1. Passive Transport: Movement along concentration gradient without ATP (e.g., Simple Diffusion, Osmosis).
  2. Active Transport: Movement against concentration gradient using energy (ATP) (e.g., $Na^+/K^+$ Pump).

6. The Endomembrane System (अंतः झिल्लीतंत्र)

Includes organelle functions that are coordinated: ER + Golgi Apparatus + Lysosomes + Vacuoles. (Note: Mitochondria, Chloroplasts, and Peroxisomes are NOT part of the endomembrane system).

A. Endoplasmic Reticulum (ER - अंतःद्रव्यी जालिका)

  • Rough ER (RER): Bears ribosomes on its surface. Active in protein synthesis and secretion.
  • Smooth ER (SER): Lacks ribosomes. Main site for lipid/steroid hormone synthesis.

B. Golgi Apparatus (गॉल्जी काय)

  • Discovered by Camillo Golgi (1898).
  • Consists of flat, disc-shaped sacs called cisternae ($0.5,\mu m - 1.0,\mu m$).
  • Convex face (Cis/Forming face): Receives materials from ER.
  • Concave face (Trans/Maturing face): Discharges modified materials.
  • Function: Modification, packaging, and secretion of proteins; site of glycoprotein and glycolipid formation.

C. Lysosomes (लयनकाय)

  • Formed by packaging in Golgi apparatus.
  • Rich in hydrolytic enzymes (hydrolases — lipases, proteases, carbohydrates) active at acidic pH.
  • Known as the "Suicide Bags" of the cell.

D. Vacuoles (रसधानी)

  • Bound by a single membrane called Tonoplast (selectively permeable).
  • Can occupy up to 90% volume of a plant cell.
  • Contains water, sap, excretory products.
  • Amoeba: Contractile vacuole (for excretion/osmoregulation).

7. Semiautonomous Organelles

A. Mitochondria (सूत्रकणिका - Powerhouse of the Cell)

  • Double membrane-bound organelle.
  • Outer membrane: Smooth and continuous.
  • Inner membrane: Forms infoldings called Cristae (increases surface area).
  • Matrix: Inside inner membrane; contains circular DNA, 70S ribosomes, and enzymes for Kreb's cycle.
  • Function: Site of aerobic respiration and production of cellular energy (ATP).

B. Plastids (लवक)

Found in plant cells and Euglenoids. Classified into 3 types based on pigments:

  1. Chloroplasts (हरितलवक): Contain chlorophyll and carotenoids; function in photosynthesis.
  2. Chromoplasts (वर्णीलवक): Contain fat-soluble pigments like carotene and xanthophylls (yellow, orange, red color).
  3. Leucoplasts (अवर्णीलवक): Colorless storage plastids.
    • Amyloplasts: Store Carbohydrates (Starch) — e.g., Potato.
    • Elaioplasts: Store Fats and Oils.
    • Aleuroplasts: Store Proteins.

Internal Structure of Chloroplast:

  • Double membrane-bound.
  • Stroma: Fluid space inside inner membrane; contains 70S ribosomes, circular DNA, and Rubisco enzyme.
  • Thylakoids: Flattened membranous sacs arranged in stacks called Grana.
  • Stroma Lamellae: Flat tubules connecting thylakoids of different grana.

8. Ribosomes, Cytoskeleton, Cilia & Centrosome

A. Ribosomes (राइबोसोम)

  • Discovered by George Palade (1953) (Granular structures, no membrane).
  • Composed of RNA + Proteins.
  • Prokaryotic Ribosome (70S): Subunits $50S + 30S$.
  • Eukaryotic Ribosome (80S): Subunits $60S + 40S$.
  • (S = Svedberg's Unit / Sedimentation Coefficient).

B. Cytoskeleton (कोशिका कंकाल)

  • Network of filamentous proteinaceous structures in cytoplasm.
  • Composed of: Microtubules (Tubulin), Microfilaments (Actin), and Intermediate filaments.
  • Functions: Mechanical support, motility, maintenance of cell shape.

C. Cilia and Flagella

  • Hair-like outgrowths of cell membrane.
  • Core is called Axoneme, containing microtubules running parallel to the long axis.
  • Arrangement: $9 + 2$ pattern (9 peripheral doublets + 2 central singlets).
  • Emerges from a structure called the Basal Body (centriole-like).

D. Centrosome and Centrioles (तारककाय एवं तारककेंद्र)

  • Centrosome contains two cylindrical structures called Centrioles perpendicular to each other.
  • Structure has a Cartwheel pattern with $9 + 0$ arrangement of peripheral triplet microtubules.
  • Forms the basal body of cilia/flagella and spindle apparatus during cell division.

9. Nucleus and Chromosomes

A. Nucleus (केंद्रक)

  • Stained by basic dyes; nuclear material named Chromatin by Flemming.
  • Components:
    1. Nuclear Envelope: Two parallel membranes with perinuclear space ($10-50,nm$). Interrupted by Nuclear Pores.
    2. Nucleoplasm: Nuclear matrix containing nucleolus and chromatin.
    3. Nucleolus (केंद्रिका): Non-membrane bound site for active ribosomal RNA (rRNA) synthesis.

B. Chromosomes (गुणसूत्र)

Chromatin condenses into distinct chromosomes during cell division. Made of DNA, Basic proteins called Histones, non-histone proteins, and RNA.

Classification of Chromosomes based on Centromere Position:

    Metacentric        Sub-metacentric       Acrocentric        Telocentric
        (V)                 (L)                  (J)                (I)
         o                   o                    o                  o
        / \                 / \                  / \                 |
       /   \               /   \                /   \                |
      /     \             /     \              /     \               |
  1. Metacentric (मध्यकेंद्री): Centromere in the middle, forming two equal arms ('V' shaped).
  2. Sub-metacentric (उप-मध्यकेंद्री): Centromere slightly away from middle, one shorter arm and one longer arm ('L' shaped).
  3. Acrocentric (अग्रकेंद्री): Centromere situated close to its end, forming one extremely short and one long arm ('J' shaped).
  4. Telocentric (अंतकेंद्री): Terminal centromere ('I' shaped).
  • Satellite Chromosome: Certain chromosomes have non-staining secondary constrictions at a constant location, giving the appearance of a small fragment called a Satellite.

C. Microbodies (सूक्ष्मकाय)

  • Minute, single-membrane bound vesicles containing various enzymes present in both plant and animal cells (e.g., Peroxisomes).

10. Key Formulas & Numerical Relationships

  1. Sedimentation Coefficients: $$\text{Prokaryotic Ribosome } (70S) = 50S + 30S$$ $$\text{Eukaryotic Ribosome } (80S) = 60S + 40S$$ (Note: Subunits do not add up mathematically due to sedimentation density shape factors).

  2. Microtubule Arrangements: $$\text{Cilia / Flagella Axoneme} = (9 \times 2) + 2 = \text{9 peripheral doublets + 2 central singlets}$$ $$\text{Centriole / Basal Body} = (9 \times 3) + 0 = \text{9 peripheral triplets + 0 central microtubules}$$