Class 11 Biology Notes Chapter 8 (Cell: the unit of life) – Biology Book

Alright class, let's delve into one of the most fundamental chapters in Biology – 'Cell: The Unit of Life'. Understanding the cell is crucial, as it forms the basis of all life and is a frequently tested topic in various government exams. Pay close attention to the structures, their functions, and the key differences between cell types.

Chapter 8: Cell: The Unit of Life - Detailed Notes

1. What is a Cell?

• The cell is the fundamental, structural, and functional unit of all living organisms.
• Anything less than a complete structure of a cell does not ensure independent living.
• Anton Von Leeuwenhoek: First saw and described live cells.
• Robert Hooke: First discovered cells (dead cork cells).
• Robert Brown: Discovered the nucleus.

2. Cell Theory

• Proposed by Matthias Schleiden (German Botanist, 1838) and Theodore Schwann (British Zoologist, 1839).
  • Schleiden observed that all plants are composed of different kinds of cells.
  • Schwann observed that animal cells have a thin outer layer (plasma membrane) and also concluded, based on studies on plant tissues, that the presence of a cell wall is a unique character of plant cells. He proposed the hypothesis that bodies of animals and plants are composed of cells and products of cells.
• Rudolf Virchow (1855) modified the theory by explaining that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula).
• Modern Cell Theory postulates:
  • All living organisms are composed of cells and products of cells.
  • All cells arise from pre-existing cells.
  • All energy flow (metabolism & biochemistry) of life occurs within cells.
  • Cells contain hereditary information (DNA) which is passed from cell to cell during cell division.
  • All cells are basically the same in chemical composition in organisms of similar species.

3. An Overview of Cell

• Cells differ greatly in size, shape, and activities.
  • Smallest cells: Mycoplasmas (0.3 µm in length).
  • Largest isolated single cell: Egg of an ostrich.
  • Longest cells: Nerve cells.
  • Shapes can be disc-like, polygonal, columnar, cuboid, thread-like, or irregular.
• Basic Components: Cytoplasm (main arena of cellular activities), Cell Membrane (outer boundary), Nucleus/Nucleoid (contains genetic material).
• Types of Cells:
  • Prokaryotic Cells: Lack a membrane-bound nucleus and other membrane-bound organelles (e.g., Bacteria, Blue-green algae, Mycoplasma, PPLO - Pleuro Pneumonia Like Organisms).
  • Eukaryotic Cells: Possess an organised, membrane-bound nucleus and other membrane-bound organelles (e.g., Protists, Fungi, Plants, Animals).

4. Prokaryotic Cells

• Represented by Bacteria, Blue-Green Algae (Cyanobacteria), Mycoplasma, and PPLO.
• Generally smaller and multiply more rapidly than eukaryotic cells.
• Vary greatly in shape (Bacillus - rod-like, Coccus - spherical, Vibrio - comma-shaped, Spirillum - spiral).
• Cell Envelope: Outermost protective covering, consisting of three layers:
  • Glycocalyx: Outermost layer. 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, present in most prokaryotes (except Mycoplasma). Determines the shape of the cell and provides structural support, prevents bursting or collapsing. Made of peptidoglycan in bacteria.
  • Plasma Membrane (Cell Membrane): Inner layer, selectively permeable, interacts with the outside world. Structurally similar to eukaryotes (lipid bilayer + proteins).
• Mesosomes: Infoldings of the plasma membrane into the cytoplasm.
  • Forms: Vesicles, tubules, lamellae.
  • Functions: Cell wall formation, DNA replication and distribution to daughter cells, respiration, secretion processes, increase surface area of plasma membrane and enzymatic content.
• Cytoplasm: Semi-fluid matrix filling the cell. No well-defined organelles.
• Nucleoid: Region in the cytoplasm where the genetic material (single circular DNA, not membrane-bound) is located. Many bacteria also have small, circular DNA outside the genomic DNA called Plasmids, which confer unique characteristics like antibiotic resistance.
• Ribosomes: Site of protein synthesis.
  • Size: 70S type (composed of 50S and 30S subunits).
  • Location: Associated with the plasma membrane; several ribosomes may attach to a single mRNA to form a polysome or polyribosome.
• Inclusion Bodies: Reserve material stored in the cytoplasm, not membrane-bound.
  • Examples: Phosphate granules, cyanophycean granules, glycogen granules, gas vacuoles (found in blue-green and purple/green photosynthetic bacteria, provide buoyancy).
• Surface Structures:
  • Flagella (sing. Flagellum): Thin filamentous extensions from the cell wall, responsible for motility in motile bacteria. Composed of three parts: filament, hook, basal body.
  • Pili (sing. Pilus): Elongated tubular structures made of a special protein (pilin). 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 also to host tissues.

5. Eukaryotic Cells

• Include protists, plants, fungi, and animals.
• Extensive compartmentalization of cytoplasm due to the presence of membrane-bound organelles.
• Possess an organized nucleus with a nuclear envelope.
• Have complex locomotory (cilia/flagella) and cytoskeletal structures.
• Genetic material organized into linear chromosomes within the nucleus.

Components of Eukaryotic Cells:

• Cell Membrane (Plasma Membrane):
  • Structure: Explained by the Fluid Mosaic Model (Singer and Nicolson, 1972). Composed of a phospholipid bilayer with proteins embedded or associated. Lipids have a polar (hydrophilic) head and a non-polar (hydrophobic) tail. Proteins can be integral (partially or totally buried) or peripheral (lie on the surface). Quasi-fluid nature enables lateral movement of proteins, crucial for functions like cell growth, secretion, endocytosis, cell division. Cholesterol is also present (especially in animal cells), affecting fluidity.
  • Functions: Transport of molecules (selectively permeable).
    • Passive Transport: Movement across the membrane without energy expenditure, along the concentration gradient (e.g., simple diffusion, osmosis - movement of water).
    • Active Transport: Movement against the concentration gradient, requires energy (ATP) and carrier proteins (pumps), e.g., Na+/K+ pump.
• Cell Wall:
  • Rigid outer covering found in plant cells and fungi (absent in animal cells).
  • Composition: Cellulose, galactans, mannans, calcium carbonate in algae; Cellulose, hemicellulose, pectin, proteins in other plants; Chitin in fungi.
  • Structure in Plants: Primary wall (capable of growth), Secondary wall (formed on the inner side as the cell matures), Middle lamella (layer mainly of calcium pectate, holds neighbouring cells together). Plasmodesmata connect the cytoplasm of neighbouring cells.
  • Functions: Gives shape to the cell, protects from mechanical damage and infection, helps in cell-to-cell interaction, provides a barrier to undesirable macromolecules.
• Endomembrane System: A group of organelles whose functions are coordinated. Includes: Endoplasmic Reticulum (ER), Golgi complex, Lysosomes, Vacuoles. (Mitochondria, Chloroplasts, Peroxisomes are NOT part of this system).
  • Endoplasmic Reticulum (ER): Network of tiny tubular structures scattered in the cytoplasm. Divides the intracellular space into luminal (inside ER) and extra-luminal (cytoplasm) compartments.
    • Rough Endoplasmic Reticulum (RER): Bears ribosomes on its surface. Actively involved in protein synthesis and secretion. Extensive in cells active in protein synthesis.
    • Smooth Endoplasmic Reticulum (SER): Ribosomes are absent. Major site for synthesis of lipids. In animal cells, lipid-like steroidal hormones are synthesized in SER. Also involved in detoxification of drugs.
  • Golgi Apparatus (Golgi Complex/Golgi Body): First observed by Camillo Golgi (1898). Consists of flattened, disc-shaped sacs or cisternae (0.5µm to 1.0µm diameter) stacked parallelly.
    • Structure: Cisternae are concentrically arranged near the nucleus with distinct convex cis (forming face) and concave trans (maturing face). Cis and trans faces are interconnected but functionally distinct.
    • Functions: Packaging materials for delivery (intra- or extra-cellular targets); Modification of proteins synthesized by RER; Formation of glycoproteins and glycolipids; Site of synthesis of polysaccharides (like cell wall materials); Formation of lysosomes.
  • Lysosomes: Membrane-bound vesicular structures formed by the Golgi apparatus.
    • Contain hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at acidic pH.
    • Functions: Digestion of carbohydrates, proteins, lipids, and nucleic acids; Removal of worn-out cellular organelles (autophagy); Sometimes called "suicidal bags" as they can digest the entire cell if it's damaged.
  • Vacuoles: Membrane-bound space found in the cytoplasm.
    • Membrane: Tonoplast (single membrane).
    • Functions: Contains water, sap, excretory products, and other materials not useful for the cell.
    • In Plants: Large central vacuole can occupy up to 90% of the cell volume. Tonoplast facilitates transport against concentration gradient into the vacuole. Maintains turgor pressure.
    • In Protists: Contractile vacuole for osmoregulation and excretion. Food vacuoles formed by engulfing food particles.
• Mitochondria (sing. Mitochondrion): "Powerhouses of the cell".
  • Structure: Typically sausage-shaped or cylindrical. Double membrane-bound:
    • Outer membrane: Smooth, continuous boundary.
    • Inner membrane: Forms infoldings called cristae (sing. crista) towards the matrix. Cristae increase the surface area.
    • Intermembrane space: Space between outer and inner membranes.
    • Matrix: Dense, homogenous substance filling the inner compartment. Contains single circular DNA molecule, a few RNA molecules, 70S ribosomes, and components required for protein synthesis.
  • Function: Sites of aerobic respiration. Produce cellular energy in the form of ATP.
  • Semi-autonomous: Can synthesize some of their own proteins; divide by fission.
• Plastids: Found in all plant cells and euglenoids. Easily observed under the microscope due to their large size. Bear specific pigments, thus imparting specific colours.
  • Based on pigments, classified into:
    • Chloroplasts: Contain chlorophyll and carotenoid pigments. Responsible for trapping light energy for photosynthesis.
      • Structure: Lens-shaped, oval, spherical, discoid, or ribbon-like. Double membrane-bound. Inner membrane less permeable.
      • Stroma: Space limited by the inner membrane. Contains enzymes for carbohydrate synthesis, small double-stranded circular DNA, 70S ribosomes.
      • Thylakoids: Flattened membranous sacs embedded in the stroma. Contain chlorophyll pigments. Arranged in stacks called grana (sing. granum).
      • Stroma lamellae: Flat membranous tubules connecting thylakoids of different grana.
    • Chromoplasts: Contain fat-soluble carotenoid pigments like carotene, xanthophylls. Give yellow, orange, or red colour to parts of the plant (e.g., fruits, flowers).
    • Leucoplasts: Colourless plastids of varied shapes and sizes. Store nutrients.
      • Amyloplasts: Store carbohydrates (starch), e.g., potato.
      • Elaioplasts: Store oils and fats.
      • Aleuroplasts: Store proteins.
  • Like mitochondria, chloroplasts are also semi-autonomous (have own DNA and 70S ribosomes).
• Ribosomes: Granular structures, composed of ribonucleic acid (RNA) and proteins. Not membrane-bound.
  • First observed by George Palade (1953).
  • Eukaryotic ribosomes: 80S (composed of 60S and 40S subunits).
  • Prokaryotic ribosomes (also in mitochondria and chloroplasts): 70S (composed of 50S and 30S subunits).
  • 'S' (Svedberg's Unit) stands for sedimentation coefficient; it is an indirect measure of density and size.
  • Function: Site of protein synthesis ("protein factories").
• Cytoskeleton: An elaborate network of filamentous proteinaceous structures in the cytoplasm.
  • Components: Microtubules, Microfilaments, Intermediate filaments.
  • Functions: Mechanical support, motility, maintenance of the shape of the cell, muscle contraction, formation of spindle fibers during cell division.
• Cilia and Flagella: Hair-like outgrowths of the cell membrane. Cilia are small, work like oars; Flagella are longer, responsible for cell movement. Prokaryotic flagella are structurally different from eukaryotic ones.
  • Structure (Eukaryotic): Covered with plasma membrane. Core (axoneme) possesses microtubules running parallel to the long axis. Axoneme usually has nine pairs (doublets) of radially arranged peripheral microtubules, and a pair of centrally located microtubules (9+2 array). Central tubules are connected by bridges and enclosed by a central sheath. Radial spokes connect peripheral doublets to the central sheath. Peripheral doublets are interconnected by linkers. Both emerge from centriole-like structures called basal bodies.
• Centrosome and Centrioles: Centrosome is an organelle usually containing two cylindrical structures called centrioles. Found mainly in animal cells. Not membrane-bound.
  • Structure: Centrioles lie perpendicular to each other. Organisation like a cartwheel. Made of nine evenly spaced peripheral fibrils of tubulin protein. Each peripheral fibril is a triplet. Central part (hub) is proteinaceous. Hub connected to peripheral triplets by radial spokes. (9+0 array).
  • Function: Form the basal body of cilia and flagella, and spindle fibres (spindle apparatus) during cell division in animal cells.
• Nucleus: "Director" of the cell. Contains the main genetic material. First described by Robert Brown (1831). Material stained by basic dyes was named chromatin by Flemming.
  • Structure: Generally spherical.
    • Nuclear Envelope: Double membrane structure with a space between (perinuclear space, 10-50 nm). Outer membrane usually continuous with ER and bears ribosomes. Inner membrane smooth. Interrupted by minute nuclear pores, formed by fusion of its two membranes. Pores regulate passage of RNA and proteins between nucleus and cytoplasm.
    • Nucleoplasm (Nuclear Matrix): Contains nucleolus and chromatin.
    • Nucleolus (pl. Nucleoli): Spherical structure(s) within the nucleoplasm. Not membrane-bound. Contents continuous with nucleoplasm. 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 also RNA. During cell division, chromatin condenses to form distinct Chromosomes.
• Chromosomes: Visible only during dividing cells.
  • Structure: DNA is packaged into chromosomes. Each chromosome essentially has a primary constriction or the centromere. On the sides of the centromere, disc-shaped structures called kinetochores are present.
  • Based on the position of the centromere, chromosomes are classified as:
    • Metacentric: Centromere in the middle, forming two equal arms.
    • Sub-metacentric: Centromere slightly away from the middle, resulting in one shorter and one longer arm.
    • Acrocentric: Centromere situated close to one end, forming one extremely short and one very long arm.
    • Telocentric: Terminal centromere (at the tip).
  • Some chromosomes have non-staining secondary constrictions at a constant location, giving the appearance of a small fragment called the satellite.
• Microbodies: Many membrane-bound minute vesicles found in both plant and animal cells. Contain various enzymes. E.g., Peroxisomes (involved in peroxide metabolism), Glyoxysomes (in plants, involved in glyoxylate cycle).

Summary Comparison

Multiple Choice Questions (MCQs)

1. Which of the following is NOT a postulate of the modern Cell Theory?
   a) All living organisms are composed of cells and products of cells.
   b) All cells arise from pre-existing cells.
   c) All cells have a membrane-bound nucleus.
   d) All energy flow of life occurs within cells.
   Answer: c) (Prokaryotic cells lack a membrane-bound nucleus)

2. The main arena of various cellular activities in both plant and animal cells is:
   a) Nucleus
   b) Plasma membrane
   c) Mitochondria
   d) Cytoplasm
   Answer: d)

3. Which of the following structures is common to both prokaryotic and eukaryotic cells?
   a) Nucleus
   b) Ribosomes
   c) Endoplasmic Reticulum
   d) Mitochondria
   Answer: b) (Ribosomes are present in both, though of different types - 70S and 80S)

4. 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
   Answer: b)

5. Which cell organelle is involved in the formation of glycoproteins and glycolipids?
   a) Endoplasmic Reticulum
   b) Golgi apparatus
   c) Lysosome
   d) Vacuole
   Answer: b)

6. The 'powerhouse' of the cell, responsible for ATP synthesis through aerobic respiration, is:
   a) Chloroplast
   b) Ribosome
   c) Mitochondrion
   d) Nucleus
   Answer: c)

7. Which type of plastid stores proteins?
   a) Amyloplast
   b) Elaioplast
   c) Chromoplast
   d) Aleuroplast
   Answer: d)

8. The structural arrangement of microtubules in the axoneme of eukaryotic cilia and flagella is:
   a) 9 + 0
   b) 9 + 2
   c) 8 + 2
   d) 7 + 0
   Answer: b)

9. Mesosomes in prokaryotes are infoldings of the cell membrane primarily involved in:
   a) Protein synthesis
   b) Photosynthesis
   c) Respiration and DNA replication
   d) Lipid storage
   Answer: c)

10. A chromosome with the centromere situated close to one end, forming one extremely short and one very long arm, is called:
    a) Metacentric
    b) Sub-metacentric
    c) Acrocentric
    d) Telocentric
    Answer: c)

Remember to revise these notes thoroughly. Focus on the specific functions of each organelle and the differences between prokaryotic and eukaryotic cells, as well as plant and animal cells. These details are often the target of questions in competitive exams. Good luck with your preparation!