Quick Revision Notes & Formula Sheet

Class 9 Science - Chapter: The Fundamental Unit of Life (Cell)

MP Board Education

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### 1. Introduction to the Cell

• Cell (कोशिका): The fundamental, structural, and functional unit of all living organisms.
• Discovery:
  • Robert Hooke (1665) discovered cells using a primitive microscope in cork slices. He coined the term "cell" (from the Latin cellula, meaning a little room).
  • Anton van Leeuwenhoek (1674) discovered free-living living cells in pond water for the first time.
  • Robert Brown (1831) discovered the Nucleus in the cell.
• Cell Theory (कोशिका सिद्धान्त): Proposed by M. Schleiden (1838) and T. Schwann (1839), and later expanded by Rudolf Virchow (1855).
  • Postulates: All plants and animals are composed of cells, and all cells come from pre-existing cells.

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### 2. Types of Organisms Based on Cell Number

• Unicellular Organisms (एककोशिकीय जीव): Made up of a single cell.
  • Examples: Amoeba, Paramecium, Chlamydomonas, Bacteria.
• Multicellular Organisms (बहुकोशिकीय जीव): Made up of many cells that perform division of labor.
  • Examples: Fungi, Plants, Animals, Human beings.

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### 3. Types of Cells (Based on Organization)

Feature	Prokaryotic Cells (प्रोकैरियोटिक कोशिका)	Eukaryotic Cells (यूकैरियोटिक कोशिका)
Size	Generally small (1–10 µm)	Generally large (5–100 µm)
Nucleus	Undefined, region called Nucleoid (nuclear membrane absent)	Well-defined, surrounded by a nuclear membrane
Membrane-bound Organelles	Absent	Present (e.g., Mitochondria, Plastids)
Chromosome	Single	More than one
Example	Bacteria, Blue-green algae	Plant and Animal cells

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### 4. Structure of a Plant Cell vs. Animal Cell

• Plant Cell (पादप कोशिका):
  • Has a rigid outer Cell Wall made of cellulose.
  • Contains large permanent Vacuoles.
  • Contains Plastids (chloroplasts for photosynthesis).
• Animal Cell (जंतु कोशिका):
  • Cell wall is absent.
  • Vacuoles are small or absent.
  • Contains Centrosomes / Centrioles (absent in most plant cells).

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### 5. Key Cell Organelles and Their Functions

I. Plasma Membrane / Cell Membrane (प्लाज्मा झिल्ली)

• Nature: Living, selective permeable membrane (selective movement of substances).
• Composition: Made of lipids and proteins.
• Key Processes:
  • Diffusion (विसरण): Spontaneous movement of a substance from a region of high concentration to a region of low concentration (e.g., $CO_2$ and $O_2$ exchange).
  • Osmosis (परासरण): Movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
    • Hypotonic Solution: Cell gains water and swells (endosmosis).
    • Isotonic Solution: Cell stays the same size (no net movement).
    • Hypertonic Solution: Cell loses water and shrinks (exosmosis).

II. Cell Wall (कोशिका भित्ति)

• Nature: Rigid, non-living outer covering lying outside the plasma membrane in plant cells.
• Composition: Cellulose.
• Function: Provides structural strength, shape, and protection to plant cells. Allows plant cells to withstand very dilute external media without bursting (Plasmolysis is the shrinking of cytoplasm away from cell wall).

III. Nucleus (केंद्रक)

• Structure: Double-layered covering called nuclear membrane. Contains pores, Nucleolus, and Chromatin material.
• Chromatin: Consists of DNA and proteins. During cell division, chromatin organizes into Chromosomes.
• Function: Controls all cellular activities and plays a central role in cellular reproduction (contains hereditary information/genes).

IV. Cytoplasm (कोशिकाद्रव्य)

• The fluid content inside the plasma membrane containing various specialized living components called Cell Organelles.

V. Endoplasmic Reticulum - ER (अंतर्द्रव्यी जालिका)

• A large network of membrane-bound tubes and sheets.
• Rough ER (RER): Has particles called ribosomes attached to its surface. Synthesizes proteins.
• Smooth ER (SER): Lacks ribosomes. Synthesizes fats/lipids and detoxifies poisons/drugs.

VI. Golgi Apparatus (गोल्जी उपकरण)

• System of membrane-bound vesicles (cisternae) arranged parallel to each other.
• Function: Storage, modification, and packaging of products in vesicles. Also involved in the formation of Lysosomes.

VII. Lysosomes (लाइसोसोम)

• Membrane-bound sacs filled with powerful digestive enzymes (made by RER).
• Key Term: Known as the "Suicide Bags" (आत्मघाती थैली) of the cell because if the cell gets damaged, lysosomes burst and digestive enzymes digest their own cell.

VIII. Mitochondria (माइटोकॉन्ड्रिया)

• Key Term: "Powerhouse of the Cell" (कोशिका का बिजलीघर).
• Structure: Has two membrane coverings (outer is porous, inner is deeply folded into cristae to increase surface area).
• Function: Releases energy required for various chemical activities in the form of ATP (Adenosine Triphosphate). It has its own DNA and ribosomes.

IX. Plastids (लवक)

• Present only in plant cells.
• Types:
  1. Chromoplasts (वर्णलवक): Coloured plastids (provide colors to flowers and fruits).
  2. Leucoplasts (अवर्णीलवक): Colourless plastids (stores starch, oils, and protein granules).
  3. Chloroplasts (हरितलवक): Contain chlorophyll; essential for photosynthesis. Also possess their own DNA and ribosomes.

X. Vacuoles (रसधानियाँ)

• Storage sacs for solid or liquid contents.
• Large and central in plant cells (provides turgidity and rigidity); small in animal cells.

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### 6. Important Cell Division Terms (कोशिका विभाजन)

• Mitosis (समसूत्री विभाजन): Process by which most cells divide for growth and repair. One mother cell divides to form two identical daughter cells having the same number of chromosomes.
• Meiosis (अर्धसूत्री विभाजन): Specific cells of reproductive organs divide to form four daughter cells having half the number of chromosomes (gametes for sexual reproduction).