Class 11 Biology Notes Chapter 8 (Chapter 8) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 8, 'Cell: The Unit of Life', from your NCERT Exemplar. This chapter is fundamental not just for your Class 11 understanding but also forms the bedrock for many questions in competitive government exams. Pay close attention to the details.

Chapter 8: Cell - The Unit of Life: Detailed Notes for Exam Preparation

1. What is a Cell?

• Definition: The fundamental, structural, and functional unit of all living organisms.
• Anything less than a complete structure of a cell does not ensure independent living.
• Discovery: Robert Hooke (1665) first observed cells (dead cork cells). Antonie van Leeuwenhoek first saw and described live cells. Robert Brown discovered the nucleus.

2. Cell Theory

• Proposed by Matthias Schleiden (German Botanist, 1838 - observed plants are composed of cells) and Theodor Schwann (British Zoologist, 1839 - observed animals are composed of cells, also noted the thin outer layer - plasma membrane, and presence of cell wall is unique to plants).
• Key Postulates (Schleiden & Schwann):
  • All living organisms are composed of cells and products of cells.
• Modification by Rudolf Virchow (1855): Explained cell division.
  • Added the postulate: "Omnis cellula-e cellula" - All cells arise from pre-existing cells.
• Modern Cell Theory:
  • All known living things are made up of one or more cells.
  • All living cells arise from pre-existing cells by division.
  • The cell is the fundamental unit of structure and function in all living organisms.
  • All cells are basically the same in chemical composition and metabolic activities.
  • Energy flow (metabolism and biochemistry) occurs within cells.
  • Hereditary information (DNA) is passed on from cell to cell.

3. An Overview of Cell

• Size & Shape: Cells vary greatly (e.g., Mycoplasma - smallest, 0.3 µm; Ostrich egg - largest isolated single cell; Human RBCs - 7.0 µm diameter; Nerve cells - longest). Shape varies with function (e.g., disc-like, polygonal, columnar, cuboid, thread-like, irregular).
• Basic Components: Cytoplasm (main arena of cellular activities) and Nucleus. Ribosomes (non-membrane bound) are found in both prokaryotes and eukaryotes, and also within chloroplasts, mitochondria, and on rough ER.
• Types of Cells:
  • Prokaryotic Cells: Lack membrane-bound nucleus and organelles (Bacteria, Blue-green algae/Cyanobacteria, Mycoplasma, PPLO - Pleuro Pneumonia Like Organisms). Generally smaller and multiply faster.
  • Eukaryotic Cells: Possess membrane-bound nucleus and organelles (Protists, Fungi, Plants, Animals).

4. Prokaryotic Cells

• Represented by Bacteria, BGA, Mycoplasma, PPLO.
• Cell Envelope: Outermost protective covering, consisting of 3 layers:
  • Glycocalyx: Outermost. Composition and thickness vary. Can be a loose sheath (slime layer) or thick and tough (capsule). Helps in adhesion and protection.
  • Cell Wall: Middle layer. Determines shape, provides structural support, prevents bursting/collapsing. Made of peptidoglycan in bacteria.
  • Plasma Membrane: Inner layer. Selectively permeable. Structurally similar to eukaryotes. Site of respiratory enzymes (analogous to mitochondria).
• Classification based on Gram Staining (developed by Christian Gram):
  • Gram-positive: Take up Gram stain (thick peptidoglycan layer).
  • Gram-negative: Do not take up Gram stain (thin peptidoglycan layer, have an outer membrane).
• Mesosomes: Infoldings of the plasma membrane.
  • Functions: Cell wall formation, DNA replication and distribution, respiration, secretion processes, increase surface area of plasma membrane and enzymatic content. Can be vesicular, tubular, or lamellar.
• Cytoplasm: Semifluid matrix, no membrane-bound organelles. Contains:
  • Nucleoid: Region containing the genetic material (single circular DNA, naked - not associated with histones). Plasmids (small, circular, extrachromosomal DNA) may be present, conferring unique traits like antibiotic resistance.
  • Ribosomes: Site of protein synthesis. 70S type (subunits: 50S and 30S). Several ribosomes may attach to a single mRNA to form a polysome or polyribosome.
  • Inclusion Bodies: Reserve material storage (phosphate granules, cyanophycean granules, glycogen granules). Not membrane-bound. Gas vacuoles (found in blue-green, purple, and green photosynthetic bacteria) help in buoyancy.
• Appendages:
  • Flagella: Thin filamentous extensions from the cell wall, responsible for motility. Composed of filament, hook, and basal body.
  • Pili: Elongated tubular structures made of pilin protein. Involved in conjugation (transfer of genetic material).
  • Fimbriae: Small bristle-like fibres sprouting out of the cell. Help in attachment to rocks in streams and host tissues.

5. Eukaryotic Cells

• Include Protists, Plants, Fungi, and Animals.
• Extensive compartmentalization due to membrane-bound organelles.
• Possess an organized nucleus with a nuclear envelope.
• Variety of complex locomotory and cytoskeletal structures.
• Genetic material organized into chromosomes.

Key Eukaryotic Organelles:

• Cell Membrane (Plasma Membrane):
  • Detailed structure studied after electron microscope invention (1950s).
  • Chemical studies (especially on human RBCs) showed lipid bilayer (mainly phosphoglycerides) with proteins and some carbohydrates.
  • Fluid Mosaic Model (Singer & Nicolson, 1972): Widely accepted. Describes membrane as quasi-fluid lipid bilayer where proteins are embedded or float ('icebergs in a sea of lipids'). Fluidity allows cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.
  • Functions: Transport of molecules (passive - diffusion, osmosis; active - requires ATP), cell recognition, cell adhesion. Selectively permeable.
• Cell Wall:
  • Non-living rigid structure outer to plasma membrane in Fungi and Plants.
  • Functions: Gives shape, protects from mechanical damage and infection, helps in cell-to-cell interaction, barrier to undesirable macromolecules.
  • Composition: Algae (cellulose, galactans, mannans, minerals like CaCO₃), Plants (cellulose, hemicellulose, pectins, proteins). Fungi (Chitin).
  • Plant cell wall layers: Primary wall (capable of growth), Secondary wall (formed inner to primary), Middle lamella (outermost layer, mainly calcium pectate, holds neighbouring cells together). Plasmodesmata connect cytoplasm of neighbouring cells.
• Endomembrane System: Group of organelles whose functions are coordinated. Includes: Endoplasmic Reticulum (ER), Golgi complex, Lysosomes, Vacuoles. (Mitochondria, Chloroplasts, Peroxisomes are NOT part of it).
  • Endoplasmic Reticulum (ER): Network of tiny tubular structures scattered in cytoplasm. Divides intracellular space into luminal (inside ER) and extraluminal (cytoplasm) compartments.
    • Rough ER (RER): Ribosomes attached to outer surface. Actively involved in protein synthesis and secretion. Continuous with the outer membrane of the nucleus.
    • Smooth ER (SER): Lacks ribosomes. Major site for lipid synthesis. In animal cells, steroid hormones are synthesized in SER. Detoxification of drugs (liver cells).
  • Golgi Apparatus (Golgi Complex/Body): Discovered by Camillo Golgi (1898). Consists of flattened sacs (cisternae, 0.5-1.0 µm diameter) stacked parallel. Cisternae concentrated near nucleus with distinct convex cis (forming) face and concave trans (maturing) face.
    • Functions: Packaging materials (from ER) for delivery (intra- or extracellular targets), modification of proteins, formation of glycoproteins and glycolipids. Important site for formation of lysosomes.
  • Lysosomes: Membrane-bound vesicular structures formed by Golgi. Rich in hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) active at acidic pH.
    • Functions: Digestion of carbohydrates, proteins, lipids, nucleic acids. Digest worn-out cells (autophagy). Called "suicidal bags".
  • Vacuoles: Membrane-bound space found in cytoplasm. Membrane called tonoplast.
    • Functions: Contains water, sap, excretory products, other materials not useful for the cell. In plants, can occupy up to 90% volume. Tonoplast facilitates transport against concentration gradient into vacuole (hence higher concentration inside). In Amoeba, contractile vacuole for osmoregulation & excretion. In Protists, food vacuoles formed by engulfing food.
• Mitochondria:
  • "Powerhouse of the cell". Site of aerobic respiration, produce cellular energy (ATP).
  • Typically sausage-shaped or cylindrical. Diameter 0.2-1.0 µm, length 1.0-4.1 µm.
  • Structure: Double membrane-bound. Outer membrane (continuous boundary). Inner membrane forms infoldings called cristae (increase surface area). Inner membrane encloses the matrix.
  • Matrix: Contains single circular DNA molecule, few RNA molecules, ribosomes (70S type), components required for protein synthesis and enzymes for Krebs cycle.
  • Divide by fission.
• Plastids:
  • Found in all plant cells and euglenoids. Easily observable due to large size. Bear specific pigments.
  • Types based on pigment:
    • Chloroplasts: Contain chlorophyll and carotenoid pigments. Responsible for trapping light energy for photosynthesis. Found in mesophyll cells of leaves. Lens-shaped, oval, spherical, discoid, or ribbon-like. Length 5-10 µm, width 2-4 µm.
      • Structure: Double membrane-bound. Inner membrane less permeable. Space limited by inner membrane is the stroma. Stroma contains flattened membranous sacs called thylakoids. Thylakoids arranged in stacks called grana (singular: granum). Flat membranous tubules called stroma lamellae connect thylakoids of different grana. Thylakoid membrane encloses the lumen.
      • Stroma: Contains enzymes for carbohydrate synthesis (dark reaction), small circular DNA, ribosomes (70S type), RNA.
    • Chromoplasts: Contain fat-soluble carotenoid pigments (carotene, xanthophylls). Give yellow, orange, or red colour to parts of the plant (e.g., petals, fruits).
    • Leucoplasts: Colourless plastids of varied shapes/sizes. Store nutrients.
      • Amyloplasts: Store starch (carbohydrates) (e.g., potato).
      • Elaioplasts: Store oils and fats.
      • Aleuroplasts: Store proteins.
• Ribosomes:
  • Granular structures, non-membrane bound. Composed of ribosomal RNA (rRNA) and proteins.
  • First observed by George Palade (1953).
  • Eukaryotic ribosomes: 80S type (subunits: 60S and 40S).
  • Prokaryotic ribosomes: 70S type (subunits: 50S and 30S). ('S' = Svedberg unit, measure of density and size).
  • Function: Site of protein synthesis ("protein factories").
• Cytoskeleton:
  • Elaborate network of filamentous proteinaceous structures in cytoplasm.
  • Functions: Mechanical support, motility, maintenance of cell shape.
  • Components: Microtubules, Microfilaments, Intermediate filaments.
• Cilia and Flagella:
  • Hair-like outgrowths of the cell membrane. Cilia are small, work like oars causing movement of cell or surrounding fluid. Flagella are longer, responsible for cell movement. Prokaryotic flagella are structurally different.
  • Structure (Eukaryotic): Covered with plasma membrane. Core called axoneme, possesses microtubules running parallel to long axis. Axoneme has 9+2 array (9 pairs of doublet peripheral microtubules, and a pair of central microtubules). Central tubules connected by bridge, enclosed by central sheath. Peripheral doublets connected to central sheath by radial spokes. Peripheral doublets are interconnected by linkers. Both emerge from centriole-like structure called the basal body.
• Centrosome and Centrioles:
  • Centrosome: Organelle usually containing two cylindrical structures called centrioles. Found near nucleus in animal cells. Absent in higher plants. Surrounded by amorphous pericentriolar materials.
  • Centrioles: Lie perpendicular to each other. Organisation like a cartwheel. Made of 9 evenly spaced peripheral fibrils of tubulin protein. Each fibril is a triplet. Central part (hub) is proteinaceous. Hub connected to peripheral triplets by radial spokes.
  • Functions: Form basal body of cilia/flagella, form spindle fibres during cell division in animal cells.
• Nucleus:
  • "Director" of the cell. First described by Robert Brown (1831). Material stained by basic dyes named chromatin by Flemming.
  • Nuclear Envelope: Double membrane structure separating nucleus from cytoplasm. Outer membrane usually continuous with ER, bears ribosomes. Inner membrane smooth. Space between membranes (10-50 nm) called perinuclear space. Nuclear envelope interrupted by nuclear pores (formed by fusion of two membranes), regulate passage of RNA and proteins between nucleus and cytoplasm.
  • Nucleoplasm (Nuclear Matrix): Contains nucleolus and chromatin.
  • Nucleolus: Spherical structure(s) in nucleoplasm. Not membrane-bound. Site for active ribosomal RNA (rRNA) synthesis. Larger and more numerous in cells actively carrying out protein synthesis.
  • Chromatin: Network of nucleoprotein fibres. Contains DNA and basic proteins (histones), some non-histone proteins, and RNA. During cell division, chromatin condenses to form chromosomes.
• Chromosomes:
  • Visible only during dividing cells.
  • Structure: Each chromosome has a primary constriction or centromere. Disc-shaped structures called kinetochores are present on the sides of the centromere (site of attachment for spindle fibres).
  • Based on position of centromere:
    • Metacentric: Centromere in middle, two equal arms.
    • Sub-metacentric: Centromere slightly away from middle, one shorter arm, one longer arm.
    • Acrocentric: Centromere close to one end, one very short arm, one very long arm.
    • Telocentric: Centromere at terminal end.
  • Some chromosomes have a non-staining secondary constriction at a constant location, giving appearance of a small fragment called the satellite.

6. Differences between Plant and Animal Cells

7. Differences between Prokaryotic and Eukaryotic Cells

Multiple Choice Questions (MCQs)

1. Which of the following is NOT a part of the endomembrane system in eukaryotic cells?
   a) Endoplasmic Reticulum
   b) Golgi Complex
   c) Peroxisomes
   d) Lysosomes

2. The 'Fluid Mosaic Model' of the plasma membrane was proposed by:
   a) Schleiden and Schwann
   b) Singer and Nicolson
   c) Robert Brown
   d) Rudolf Virchow

3. Mesosomes in prokaryotic cells are infoldings of the plasma membrane primarily involved in:
   a) Protein synthesis
   b) Respiration and DNA replication
   c) Storage of reserve food material
   d) Providing buoyancy

4. Which type of ribosome is found in the cytoplasm of eukaryotic cells and on the Rough Endoplasmic Reticulum?
   a) 70S
   b) 80S
   c) 50S
   d) 30S

5. The main function of the Golgi apparatus is:
   a) Synthesis of ATP
   b) Synthesis of lipids
   c) Processing, packaging, and transport of proteins and lipids
   d) Detoxification of drugs

6. A chromosome with the centromere located close to one end, resulting in one very short and one very long arm, is termed:
   a) Metacentric
   b) Sub-metacentric
   c) Acrocentric
   d) Telocentric

7. Which of the following structures is common to both prokaryotic and eukaryotic cells?
   a) Nucleus
   b) Mitochondria
   c) Ribosomes
   d) Chloroplasts

8. The non-membrane bound organelle responsible for rRNA synthesis within the nucleus is:
   a) Chromatin
   b) Nucleolus
   c) Nuclear pore
   d) Centriole

9. Plant cell walls are primarily composed of __________, while fungal cell walls are primarily composed of __________.
   a) Chitin, Peptidoglycan
   b) Cellulose, Chitin
   c) Peptidoglycan, Cellulose
   d) Chitin, Cellulose

10. The '9+2' microtubule arrangement is characteristic of the axoneme found in:
    a) Prokaryotic flagella
    b) Centrioles
    c) Eukaryotic cilia and flagella
    d) Microvilli

Answer Key for MCQs:

1. c) Peroxisomes
2. b) Singer and Nicolson
3. b) Respiration and DNA replication
4. b) 80S
5. c) Processing, packaging, and transport of proteins and lipids
6. c) Acrocentric
7. c) Ribosomes
8. b) Nucleolus
9. b) Cellulose, Chitin
10. c) Eukaryotic cilia and flagella

Make sure you revise these concepts thoroughly. Understanding the structure and function of each cell component, and the key differences between cell types, is crucial for your exams. Good luck!