ICSE 7 Biology Photosynthesis Short

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Table of Contents

    Photosynthesis & Respiration

    1.0 Photosynthesis

    • Definition: Process by which green plants make food (glucose) using CO₂, water, and sunlight.
    • Site: Occurs in the Chloroplasts (mainly in leaves).
    • Pigment: Chlorophyll (traps solar energy).
    • End Products: Glucose (stored as Starch) and Oxygen (released).
    Word Equation:
    Carbon Dioxide + Water → (Sunlight/Chlorophyll) → Glucose + Oxygen

    2.0 Gaseous Exchange (Stomata)

    Stomata: Tiny pores on the leaf undersurface for gas exchange.

    • Guard Cells: Bean-shaped cells that control the opening/closing of stomata.
    • Mechanism:
      • Turgid (Swollen) = Stoma Opens.
      • Flaccid (Shrunken) = Stoma Closes.

    3.0 Quick Facts

    Raw Materials: CO₂ (from air), H₂O (from soil).
    Key Factors: Light intensity, Temperature, CO₂ levels.
    Storage: Glucose is converted to Starch for storage.
    💡 Revision Tip: The Iodine Test is used to detect starch. A blue-black color indicates a positive result!

    4.0 The Starch Test (Iodine Test)

    Standard Procedure to Test a Leaf:

    1. Boil in Water: To kill the cells and stop chemical reactions.
    2. Boil in Alcohol: To remove Chlorophyll (decolorize the leaf).
    3. Wash in Cold Water: To soften the brittle leaf.
    4. Add Iodine: If it turns Blue-Black, Starch is present.

    5.0 Key Experimental Aims

    To Prove... The Method The Key Result
    Sunlight is needed Cover part of leaf with black paper. Covered part stays brown; rest turns blue-black.
    CO₂ is needed Use KOH (Potassium Hydroxide) to absorb CO₂. Leaf in jar with KOH fails the starch test.
    Oxygen is released Inverted funnel over Hydrilla plant. Gas bubbles collect; supports combustion.

    6.0 Respiration (Releasing Energy)

    • Definition: Breaking down Glucose to release energy in cells.
    • The Two Types:
      • Aerobic: Uses Oxygen. (End products: CO₂, H₂O, 38 ATP).
      • Anaerobic: Without Oxygen. (End products: Alcohol/Lactic Acid, CO₂, 2 ATP).
    Aerobic Equation:
    Glucose + Oxygen → Carbon Dioxide + Water + Energy
    [Image showing a comparison diagram between aerobic and anaerobic respiration processes]

    💡 Exam Note: Photosynthesis stores energy (Anabolic), while Respiration releases energy (Catabolic).

    7.0 Respiratory Structures in Plants

    How different parts breathe:

    • 🍃 Stomata: Primary sites for gas exchange in leaves and green stems.
    • 🌳 Lenticels: Small openings in the bark of woody trees for gas exchange.
    • 🌱 General Surface: Younger roots take in oxygen from soil air spaces via diffusion.
    • 🌊 Pneumatophores: "Breathing roots" in mangroves that grow upward out of salty, waterlogged soil to get air.

    8.0 Anaerobic Respiration in Daily Life

    • 💪 In Humans: During heavy exercise, muscles produce Lactic Acid due to lack of O₂. This causes muscle cramps.
    • 🍞 In Yeast: Anaerobic breakdown of sugar produces Ethanol (Alcohol) and CO₂. This is called Fermentation.
    [Image showing lactic acid fermentation in human muscle cells during exercise]

    9.0 Comparison Table

    Feature Photosynthesis Respiration
    Energy Stores Energy (Solar) Releases Energy (ATP)
    Time Only in Light All the time (24/7)
    Raw Materials CO₂ and Water Glucose and Oxygen
    Organelle Chloroplast Mitochondria

    ⚠️ Death by Overwatering: Too much water fills air spaces in soil → Roots cannot respire → Roots rot → Plant dies.

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