ICSE 6 Biology Cell Short

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Table of Contents

    Introduction, Discovery, and Cell Theory

    1. What is a Cell?

    Definition: The cell is the basic structural and functional unit of all living organisms.

    Analogy: Just as bricks are the building blocks of a house, cells are the building blocks of the body.

    2. The Discovery of the Cell

    Scientist Year Contribution
    Robert Hooke 1665 Discovered dead cells in cork; coined the term "Cell."
    Schleiden & Schwann 1838-39 Proposed the Cell Theory (Plants/Animals made of cells).
    Rudolf Virchow 1855 Stated that all cells arise from pre-existing cells.

    3. The Cell Theory (3 Main Points)

    • All living organisms are made up of one or more cells.
    • The cell is the basic unit of life (structure and function).
    • New cells are produced from existing cells through division.

    Visualizing Cells (The Microscope)

    Most cells are microscopic (invisible to the naked eye). We use a Microscope to see them.

    1. The Compound Microscope

    Function: Magnifies objects up to 2,000 times using light and glass lenses.

    • Eyepiece: The lens at the top (usually 10X).
    • Objective Lens: The lens near the object (10X, 40X, or 100X).
    • Mirror: Reflects light through the slide.
    • Adjustment Knobs: Coarse (rough focus) and Fine (sharp focus).

    2. Calculating Magnification

    $$\text{Total Magnification} = \text{Power of Eyepiece}$$ $$\times \text{Power of Objective}$$

    Example: 10X (Eyepiece) $\times$ 40X (Objective) = 400X total magnification.

    3. The Electron Microscope

    Uses beams of electrons instead of light. Magnification goes up to 500,000 times to see tiny organelles.

    Variation and Types of Cells

    1. Cell Number: One vs. Many

    • Unicellular: One cell performs all functions (e.g., Amoeba, Bacteria).
    • Multicellular: Millions of cells work together (e.g., Humans, Mango tree).

    2. Cell Shape: Function Determines Form

    • Spherical: RBCs move easily in blood.
    • Spindle-shaped: Muscle cells contract and pull.
    • Long and Branched: Nerve cells carry signals long distances.
    • Irregular: Amoeba changes shape to move/eat.

    3. Cell Size: Smallest to Largest

    • Smallest Cell: Bacteria (0.1 to 0.5 micrometers).
    • Largest Cell: Egg of an Ostrich ($170 \times 130$ mm).

    4. Two Main Types of Cells (Prokaryotic vs. Eukaryotic)

    Feature Prokaryotic Cell Eukaryotic Cell
    Nucleus No well-defined nucleus. Well-defined nucleus present.
    Nuclear Membrane Absent (DNA floats freely). Present (DNA enclosed).
    Examples Bacteria, Blue-green algae. Humans, Plants, Fungi.

    Cell Organelles & Functions

    1. The Outer Boundaries

    • Cell Membrane: Selectively permeable "guard" found in all cells.
    • Cell Wall: Rigid cellulose layer in plants only for shape/protection.
    • Cytoplasm: Jelly-like site of chemical reactions.

    2. The Control Center: Nucleus

    Known as the "Brain" of the cell. Contains the Nucleolus (makes proteins) and Chromatin (carries genes).

    3. The Specialized Workers (Organelles)

    Organelle Nickname Main Function
    Mitochondria "Powerhouse" Produces energy (ATP).
    Ribosomes "Protein Factories" Synthesizes proteins.
    Lysosomes "Suicide Bags" Digests waste/damaged parts.
    Golgi Bodies "Packaging Center" Modifies/sorts proteins.
    ER "Transport System" Moves materials within cell.
    Plastids "Kitchen" Photosynthesis (Chloroplasts).

    🌟 Quick Revision Tip: The "Onlys"

    • Only in Plants: Cell Wall and Plastids.
    • Only in Animals: Centrosomes.
    • No Nucleus: Mature Human RBCs (to carry more oxygen!).
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