Class 9 Science Notes Chapter 5 (The Fundamental Unit of Life) – Science Book

Alright class, let's focus on Chapter 5: The Fundamental Unit of Life. This chapter is crucial as it lays the foundation for understanding biology. For your government exams, pay close attention to the definitions, functions of organelles, and the differences between various cell types.

Chapter 5: The Fundamental Unit of Life - Detailed Notes

1. Discovery of the Cell

• Robert Hooke (1665): Observed thin slices of cork (dead plant cells) under a self-designed microscope. Coined the term "cell" (from Latin cellula meaning 'little room') because the structures resembled small compartments.
• Anton van Leeuwenhoek (1674): With an improved microscope, observed living cells for the first time (like bacteria, protozoa, sperm cells, RBCs) in pond water.
• 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 Schleiden (1838 - botanist) and Schwann (1839 - zoologist), and later expanded by Virchow (1855). The main postulates are:

• All living organisms (plants and animals) are composed of cells.
• The cell is the basic structural and functional unit of life.
• All cells arise from pre-existing cells (Omnis cellula-e cellula - Virchow).

3. Types of Organisms based on Cell Number

• Unicellular Organisms: Composed of a single cell that performs all life functions. Examples: Amoeba, Paramecium, Chlamydomonas, Bacteria.
• Multicellular Organisms: Composed of many cells that are often grouped into tissues, organs, and organ systems, showing division of labour. Examples: Fungi, Plants, Animals.

4. Shape and Size of Cells

• Cells vary greatly in shape and size depending on their location and function.
• Shapes can be irregular (Amoeba, white blood cells), spindle-shaped (muscle cells), elongated (nerve cells), oval (red blood cells), cuboidal (germ cells), etc.
• Size can range from microscopic (bacteria - micrometers) to quite large (ostrich egg - centimeters).

5. Types of Cells
Based on the complexity of organization, especially the nucleus:

• Prokaryotic Cells (Pro = primitive; karyon = nucleus):
  • Generally smaller in size (1-10 µm).
  • Lack a well-defined nucleus; the genetic material (usually a single circular chromosome) lies naked in the cytoplasm in a region called the nucleoid.
  • Lack membrane-bound organelles (like mitochondria, ER, Golgi, etc.).
  • Ribosomes are present (70S type).
  • Cell division is mainly by binary fission.
  • Examples: Bacteria, Blue-Green Algae (Cyanobacteria).
• Eukaryotic Cells (Eu = true; karyon = nucleus):
  • Generally larger in size (5-100 µm).
  • Possess a well-defined nucleus enclosed by a nuclear membrane.
  • Contain membrane-bound organelles, allowing for compartmentalization of functions.
  • Ribosomes are present (80S type in cytoplasm/RER, 70S in mitochondria/plastids).
  • Cell division occurs by mitosis or meiosis.
  • Examples: Fungi, Protozoa, Plants, Animals.

6. Structural Organisation of a Cell
Every typical eukaryotic cell has three main components: Plasma Membrane, Nucleus, and Cytoplasm.

• (A) Plasma Membrane (or Cell Membrane):

  • Structure: Outermost covering in animal cells; lies beneath the cell wall in plant cells. Made primarily of lipids (phospholipids) and proteins (Fluid Mosaic Model). It is thin, elastic, and living.
  • Function:
    • Regulates the movement of substances into and out of the cell – it is selectively permeable.
    • Maintains the shape of the cell (in animal cells).
    • Protects the cell from injury.
  • Transport across Membrane:
    • Diffusion: Movement of substances (like CO₂, O₂) from a region of higher concentration to a region of lower concentration. Requires no energy.
    • Osmosis: Movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration. It's a special case of diffusion.
      • Hypotonic Solution: Surrounding solution has higher water concentration (lower solute conc.) than the cell. Water enters the cell; cell swells (may burst in animal cells - haemolysis; becomes turgid in plant cells).
      • Isotonic Solution: Surrounding solution has the same water concentration as the cell. No net movement of water; cell size remains the same.
      • Hypertonic Solution: Surrounding solution has lower water concentration (higher solute conc.) than the cell. Water moves out of the cell; cell shrinks (crenation in animal cells; plasmolysis in plant cells - shrinkage of protoplast away from the cell wall).
    • Endocytosis: Process by which the cell engulfs food and other materials from the external environment by infolding the plasma membrane (e.g., Amoeba obtaining food - phagocytosis; ingestion of fluid - pinocytosis). Requires energy.
    • Exocytosis: Process by which the cell expels waste or secretory products by the fusion of a vesicle with the plasma membrane. Requires energy.

• (B) Cell Wall:

  • Structure: Rigid, non-living outer layer found in plant cells, fungi, and bacteria (composition varies - cellulose in plants, chitin in fungi, peptidoglycan in bacteria). It lies outside the plasma membrane. Freely permeable.
  • Function: Provides definite shape, structural strength, and protection to the cell. Prevents excessive water uptake (prevents bursting in hypotonic solutions).

• (C) Nucleus:

  • Structure: Usually spherical or oval, largest organelle. Covered by a double-layered nuclear envelope with nuclear pores (regulate transport between nucleus and cytoplasm). Contains:
    • Nucleoplasm: Fluid inside the nucleus.
    • Nucleolus: Dense, spherical body (one or more); site of ribosome synthesis. Rich in RNA and proteins.
    • Chromatin Material: Tangled mass of thread-like structures composed of DNA (Deoxyribonucleic acid) and proteins (histones). During cell division, chromatin condenses to form visible, rod-shaped chromosomes.
  • Function:
    • "Control center" of the cell – regulates cellular activities.
    • Contains genetic information (DNA) responsible for inheritance of traits from one generation to the next.
    • Essential role in cell division.
  • Chromosomes: Contain genes (segments of DNA) which carry hereditary information. Human cells typically have 46 chromosomes (23 pairs).

• (D) Cytoplasm:

  • Jelly-like substance filling the cell, enclosed by the plasma membrane. It 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.

  • 1. Endoplasmic Reticulum (ER): Network of membranes (tubules and sheets) throughout the cytoplasm.

    • Rough ER (RER): Studded with ribosomes on its surface. Function: Protein synthesis and transport.
    • Smooth ER (SER): Lacks ribosomes. Function: Synthesis of lipids (fats) and steroids, detoxification of drugs and poisons (liver cells), storage of calcium ions (muscle cells).
    • General Function: Serves as a channel for transport of materials (especially proteins) between cytoplasm and nucleus; provides a surface for biochemical activities. Helps in membrane biogenesis (formation of membranes).

  • 2. Golgi Apparatus (Golgi Complex/Body): System of membrane-bound sacs (cisternae), vesicles, and vacuoles, usually arranged parallel to each other. First described by Camillo Golgi.

    • Function: Modifies, sorts, packages, and dispatches materials synthesized in the ER (like proteins, lipids) to various targets inside and outside the cell. Involved in the formation of lysosomes and complex sugars.

  • 3. Lysosomes: Small, spherical sacs containing powerful digestive enzymes (hydrolases). Found mainly in animal cells.

    • Function: "Waste disposal system" of the cell – digest foreign materials (bacteria, food) and worn-out cell organelles. Known as "suicidal bags" because if the cell gets damaged, lysosomes may burst, and their enzymes digest their own cell.

  • 4. Mitochondria: Rod-shaped or spherical organelles with a double membrane. Outer membrane is porous; inner membrane is folded into cristae (increases surface area). Have their own DNA and ribosomes (70S).

    • Function: Sites of cellular respiration – they use oxygen to break down glucose and release energy in the form of ATP (Adenosine Triphosphate). Known as the "powerhouses of the cell". ATP is the energy currency of the cell.

  • 5. Plastids: Double-membraned organelles found only in plant cells. Have their own DNA and ribosomes (70S). Types:

    • Chloroplasts: Contain chlorophyll (green pigment). Sites of photosynthesis (convert light energy into chemical energy - food). Contain stacks of thylakoids called grana, embedded in a fluid called stroma.
    • Chromoplasts: Contain coloured pigments (carotenoids - yellow, orange, red). Impart colour to flowers and fruits.
    • Leucoplasts: Colourless plastids. Store food in the form of starch (amyloplasts), oils (elaioplasts), or proteins (aleuroplasts).

  • 6. Vacuoles: Membrane-bound sacs used for storage or waste disposal.

    • Plant Cells: Usually have a single, large central vacuole that can occupy 50-90% of the cell volume. Stores water, salts, pigments, waste products; provides turgidity and rigidity. The membrane of the vacuole is called the tonoplast.
    • Animal Cells: Have small, temporary vacuoles, or may lack them altogether.
    • Unicellular Organisms: May have food vacuoles (containing ingested food) and contractile vacuoles (osmoregulation - expelling excess water, e.g., Amoeba, Paramecium).

  • 7. Ribosomes: Tiny, non-membrane-bound granules composed of RNA and protein. Found free in the cytoplasm or attached to RER. Also found in mitochondria and chloroplasts (70S type).

    • Function: Sites of protein synthesis.

7. Differences between Plant and Animal Cells

8. Cell Division
The process by which new cells are formed from pre-existing cells. Essential for:

• Growth of organisms
• Replacement of old, dead, or injured cells
• Formation of gametes for reproduction
• Types:
  • Mitosis: Produces two genetically identical daughter cells (diploid). Occurs in somatic (body) cells for growth and repair.
  • Meiosis: Produces four genetically different daughter cells (haploid - half the chromosome number). Occurs in reproductive cells to form gametes (sperm and eggs).

Multiple Choice Questions (MCQs)

1. Who coined the term 'cell' after observing cork slices?
   a) Anton van Leeuwenhoek
   b) Robert Brown
   c) Robert Hooke
   d) Rudolf Virchow

2. Which of the following is NOT a postulate of the Cell Theory?
   a) All cells arise from pre-existing cells.
   b) All living organisms are composed of cells.
   c) The cell is the basic structural unit of life.
   d) All cells have a well-defined nucleus.

3. The undefined nuclear region in prokaryotic cells is known as:
   a) Nucleolus
   b) Nucleoid
   c) Nucleosome
   d) Nuclear pore

4. Which cell organelle is known as the 'powerhouse of the cell'?
   a) Ribosome
   b) Mitochondrion
   c) Lysosome
   d) Endoplasmic Reticulum

5. The phenomenon where the protoplast shrinks away from the cell wall in a hypertonic solution is called:
   a) Osmosis
   b) Diffusion
   c) Plasmolysis
   d) Endocytosis

6. Which of these is primarily responsible for protein synthesis?
   a) Smooth Endoplasmic Reticulum
   b) Lysosome
   c) Golgi Apparatus
   d) Ribosome

7. Which organelle contains chlorophyll and is the site of photosynthesis?
   a) Mitochondrion
   b) Leucoplast
   c) Chromoplast
   d) Chloroplast

8. Which of the following is present in a plant cell but absent in an animal cell?
   a) Plasma Membrane
   b) Cytoplasm
   c) Cell Wall
   d) Nucleus

9. Lysosomes are known as 'suicidal bags' because they contain:
   a) Respiratory enzymes
   b) Photosynthetic pigments
   c) Hydrolytic (digestive) enzymes
   d) Stored food material

10. The process by which Amoeba obtains its food is an example of:
    a) Exocytosis
    b) Plasmolysis
    c) Diffusion
    d) Endocytosis (Phagocytosis)

Answer Key for MCQs:

1. c) Robert Hooke
2. d) All cells have a well-defined nucleus. (Only eukaryotic cells do)
3. b) Nucleoid
4. b) Mitochondrion
5. c) Plasmolysis
6. d) Ribosome
7. d) Chloroplast
8. c) Cell Wall
9. c) Hydrolytic (digestive) enzymes
10. d) Endocytosis (Phagocytosis)

Make sure you revise these points thoroughly. Understanding the function of each part of the cell is key. Good luck with your preparation!