Class 9 Science Notes Chapter 5 (Chapter 5) – Examplar Problem (English) Book

Alright class, let's focus on Chapter 5, 'The Fundamental Unit of Life'. This is a crucial chapter, not just for your Class 9 understanding, but also because basic biology concepts frequently appear in various government exams. We'll go through the key points systematically. Pay close attention.

Chapter 5: The Fundamental Unit of Life - Detailed Notes for Exam Preparation

1. Introduction & Discovery

• Cell: The basic structural and functional unit of all living organisms.
• Discovery:
  • Robert Hooke (1665): Observed cork cells (dead cells) using a self-designed microscope. Coined the term "cell" (from Latin cellula meaning 'little room').
  • Anton van Leeuwenhoek (1674): Observed living cells (bacteria, protozoa, sperm, RBCs) for the first time using an improved microscope.
  • Robert Brown (1831): Discovered the nucleus within the cell.
  • Purkinje (1839): Coined the term "protoplasm" for the fluid substance of the cell.

2. The Cell Theory

• Proposed initially by M. Schleiden (1838 - Botanist) and T. Schwann (1839 - Zoologist).
• Key Postulates (Schleiden & Schwann):
  • All plants and animals are composed of cells.
  • The cell is the basic unit of life.
• Modified by Rudolf Virchow (1855): Added the postulate "Omnis cellula-e cellula" meaning "All cells arise from pre-existing cells."
• Modern Cell Theory States:
  • 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.
  • Cells contain hereditary information (DNA) which is passed from cell to cell during cell division.

3. Types of Organisms based on Cell Number

• Unicellular Organisms: Composed of a single cell that performs all life functions (e.g., Amoeba, Paramecium, Bacteria, Yeast).
• Multicellular Organisms: Composed of many cells that are often organized into tissues, organs, and organ systems. Exhibit division of labour (e.g., Fungi, Plants, Animals).

4. Types of Cells

• Prokaryotic Cells (Pro = primitive; karyon = nucleus):
  • Lack a well-defined nucleus (genetic material lies naked in the cytoplasm in a region called the nucleoid).
  • Lack membrane-bound organelles (like mitochondria, ER, Golgi, lysosomes, plastids).
  • Have 70S ribosomes.
  • Typically smaller in size (1-10 µm).
  • Have a cell wall (usually peptidoglycan in bacteria).
  • Example: Bacteria, Blue-Green Algae (Cyanobacteria).
• Eukaryotic Cells (Eu = true; karyon = nucleus):
  • Possess a well-defined nucleus enclosed by a nuclear membrane.
  • Contain membrane-bound organelles.
  • Have 80S ribosomes (also 70S in mitochondria and plastids).
  • Typically larger in size (5-100 µm).
  • Cell wall present in plants (cellulose), fungi (chitin), but absent in animals.
  • Example: Protists, Fungi, Plants, Animals.

5. Cell Structure (Eukaryotic Cell Components)

• A. Plasma Membrane (Cell Membrane):

  • Structure: Thin, delicate, elastic, living boundary. Made primarily of lipids (phospholipids) and proteins (Fluid Mosaic Model - Singer & Nicolson, though the model name might be beyond Class 9 scope, the concept of fluidity and components is relevant).
  • Nature: Selectively permeable - allows entry and exit of only specific substances.
  • Functions:
    • Regulates passage of substances into and out of the cell.
    • Maintains cell shape (in animal cells).
    • Enables cell communication and recognition.
    • Allows processes like endocytosis (engulfing substances, e.g., Amoeba feeding - phagocytosis) and exocytosis (expelling substances).
  • Transport Mechanisms:
    • Diffusion: Movement of substances (solutes or gases like CO₂, O₂) from a region of higher concentration to lower concentration. Requires no energy.
    • Osmosis: Movement of water molecules across a selectively permeable membrane from a region of higher water concentration to lower water concentration. A special case of diffusion. Requires no energy.
      • Hypotonic Solution: Surrounding solution has higher water concentration (lower solute concentration) than the cell. Water enters the cell. Cell swells (may burst in animal cells - haemolysis; causes turgidity in plant cells).
      • Isotonic Solution: Surrounding solution has the same water concentration as the cell. No net movement of water. Cell stays the same size.
      • Hypertonic Solution: Surrounding solution has lower water concentration (higher solute concentration) than the cell. Water leaves the cell. Cell shrinks (crenation in animal cells; plasmolysis in plant cells - shrinkage of protoplast away from the cell wall).

• B. Cell Wall:

  • Presence: Found in plant cells, fungi, bacteria, algae. Absent in animal cells.
  • Structure: Rigid, non-living, fully permeable outer layer located outside the plasma membrane. Primarily cellulose in plants, chitin in fungi, peptidoglycan in bacteria.
  • Functions:
    • Provides definite shape and structural support.
    • Protects the cell from mechanical injury and pathogens.
    • Prevents excessive water uptake, thus preventing bursting in hypotonic solutions.

• C. Nucleus:

  • Structure: Large, usually spherical organelle. Often centrally located (in animal cells) or peripheral (in mature plant cells due to large vacuole).
    • Nuclear Envelope/Membrane: Double membrane with nuclear pores (regulate transport between nucleus and cytoplasm).
    • Nucleoplasm: Fluid inside the nucleus.
    • Nucleolus: Dense, spherical body within the nucleus; site of ribosome synthesis. May be one or more.
    • Chromatin Material: Thread-like network of DNA and proteins (histones). Condenses to form distinct Chromosomes during cell division. Chromosomes contain genes (segments of DNA), which are units of heredity.
  • Functions:
    • "Control center" of the cell - regulates cell growth and metabolism.
    • Stores genetic information (DNA).
    • Essential for cell division.

• D. Cytoplasm:

  • Jelly-like substance filling the cell, enclosed by the plasma membrane.
  • Includes the cytosol (aqueous component) and various cell organelles suspended in it.
  • Site of many metabolic reactions (e.g., glycolysis).

• E. Cell Organelles (Membrane-bound structures within the cytoplasm performing specific functions):

  • i. Endoplasmic Reticulum (ER): Network of membranes (tubules and sacs called cisternae) extending throughout the cytoplasm. Continuous with the outer nuclear membrane.

    • Rough ER (RER): Ribosomes attached to its surface. Function: Protein synthesis and transport.
    • Smooth ER (SER): Lacks ribosomes. Function: Lipid and steroid synthesis, detoxification of drugs and poisons, storage of calcium ions.
    • General Function: Serves as a channel for transport of materials (especially proteins) between cytoplasm and nucleus, and within the cytoplasm. Provides surface area for biochemical activities. Involved in membrane biogenesis (formation of membranes).

  • ii. Golgi Apparatus (Golgi Complex/Golgi Body): Stack of flattened membrane-bound sacs (cisternae), vesicles, and vacuoles. Discovered by Camillo Golgi.

    • Functions: Modification, sorting, packaging, and dispatching of materials (proteins, lipids) synthesized in the ER to various targets inside and outside the cell. Involved in the formation of lysosomes and complex sugars.

  • iii. Lysosomes: Small, spherical sacs containing powerful digestive (hydrolytic) enzymes.

    • Functions: Intracellular digestion (digest food particles, foreign materials, worn-out organelles). Known as "suicidal bags" because they can burst and release enzymes to digest their own cell if the cell is damaged or old (autolysis). Part of the cell's waste disposal system.

  • iv. Mitochondria (Singular: Mitochondrion): Rod-shaped or spherical organelles.

    • Structure: Double membrane-bound. Outer membrane is porous. Inner membrane is folded into cristae (increases surface area for ATP synthesis). Have their own DNA and 70S ribosomes (semi-autonomous organelles).
    • Function: "Powerhouses of the cell". Site of cellular respiration where glucose is oxidized to release energy in the form of ATP (Adenosine Triphosphate - the energy currency of the cell).

  • v. Plastids: Found only in plant cells and some protists (e.g., Euglena).

    • Structure: Double membrane-bound. Have their own DNA and 70S ribosomes (semi-autonomous organelles).
    • Types & Functions:
      • Chloroplasts: Contain chlorophyll (green pigment). Site of photosynthesis (convert light energy into chemical energy - glucose). Contain stacks of thylakoids called grana, embedded in a matrix called stroma.
      • Chromoplasts: Contain coloured pigments (carotenoids - yellow, orange, red). Impart colour to flowers and fruits.
      • Leucoplasts: Colourless plastids. Store food materials like starch (amyloplasts), oils (elaioplasts), and proteins (aleuroplasts).

  • vi. Vacuoles: Membrane-bound sacs containing fluid (cell sap).

    • Plant Cells: Usually one large central vacuole occupying 50-90% of cell volume. Functions: Storage (water, nutrients, waste products), maintaining turgor pressure (rigidity). Membrane is called tonoplast.
    • Animal Cells: Small, temporary, and few (or absent). Function: Storage, waste disposal. In unicellular organisms like Amoeba, food vacuoles (contain ingested food) and contractile vacuoles (osmoregulation - expel excess water) are present.

  • vii. Ribosomes: Non-membrane bound organelles. Composed of RNA and protein. Found free in cytoplasm, attached to RER, and within mitochondria and chloroplasts. Site of protein synthesis.

6. Differences between Plant and Animal Cells

7. Cell Division

• The process by which a parent cell divides into two or more daughter cells.
• Essential for growth, repair of tissues, and reproduction.
• Two Main Types:
  • Mitosis: Produces two genetically identical daughter cells with the same chromosome number as the parent cell (diploid -> diploid). Occurs in somatic (body) cells for growth and repair.
  • Meiosis: Produces four genetically different daughter cells (gametes - sperm/egg) with half the chromosome number of the parent cell (diploid -> haploid). Occurs in reproductive cells for sexual reproduction.

Multiple Choice Questions (MCQs)

1. Which of the following cell organelles is known as the "suicidal bag" of the cell?
   (a) Mitochondria
   (b) Lysosome
   (c) Golgi Apparatus
   (d) Endoplasmic Reticulum

2. The process by which water moves across a selectively permeable membrane from a region of high water concentration to low water concentration is called:
   (a) Diffusion
   (b) Osmosis
   (c) Active Transport
   (d) Endocytosis

3. Which of these is NOT found in an animal cell?
   (a) Nucleus
   (b) Mitochondria
   (c) Cell Wall
   (d) Plasma Membrane

4. The genetic material in a prokaryotic cell is located in the:
   (a) Nucleus
   (b) Nucleolus
   (c) Nucleoid
   (d) Mitochondria

5. Which organelle is primarily involved in synthesizing lipids and detoxifying poisons?
   (a) Rough Endoplasmic Reticulum (RER)
   (b) Smooth Endoplasmic Reticulum (SER)
   (c) Golgi Apparatus
   (d) Lysosome

6. A plant cell placed in a hypertonic solution will undergo:
   (a) Plasmolysis
   (b) Turgidity
   (c) Haemolysis
   (d) No change

7. Which of the following organelles possess their own DNA and ribosomes?
   (a) Endoplasmic Reticulum and Golgi Apparatus
   (b) Lysosomes and Vacuoles
   (c) Mitochondria and Plastids
   (d) Ribosomes and Nucleolus

8. The folding of the inner membrane of mitochondria is called:
   (a) Cisternae
   (b) Thylakoids
   (c) Cristae
   (d) Grana

9. The statement "Omnis cellula-e cellula" was given by:
   (a) Schleiden
   (b) Schwann
   (c) Robert Hooke
   (d) Rudolf Virchow

10. The main function of the Golgi Apparatus is:
    (a) Protein synthesis
    (b) ATP production
    (c) Modification and packaging of materials
    (d) Intracellular digestion

Answer Key for MCQs:

1. (b) Lysosome
2. (b) Osmosis
3. (c) Cell Wall
4. (c) Nucleoid
5. (b) Smooth Endoplasmic Reticulum (SER)
6. (a) Plasmolysis
7. (c) Mitochondria and Plastids
8. (c) Cristae
9. (d) Rudolf Virchow
10. (c) Modification and packaging of materials

Make sure you revise these notes thoroughly. Understanding the structure and function of each cell component, the differences between cell types, and processes like osmosis is key. Good luck with your preparation!