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ICSE Class 10 · Biology · Chapter 7

Photosynthesis and Respiration

13 topics · 25 questions

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Everything you need, in order

Introduction to Photosynthesis

  • Photosynthesis: Process by which green plants synthesize organic food (glucose) from inorganic raw materials (CO₂ and H₂O) using light energy.
  • Site: Chloroplasts in mesophyll cells of leaves.
  • Significance: (1) Primary source of food for all organisms, (2) Releases oxygen for respiration, (3) Maintains atmospheric balance, (4) Stores solar energy as chemical energy.
  • Autotrophic Nutrition: Plants make their own food through photosynthesis. Self-sufficient.
  • Overall Equation: 6CO₂ + 12H₂O + Light energy → C₆H₁₂O₆ + 6O₂ + 6H₂O (in presence of chlorophyll).
  • Simplified Equation: 6CO₂ + 6H₂O + Light energy → C₆H₁₂O₆ + 6O₂.

Structure of Chloroplast

  • Chloroplast: Double membrane-bound organelle; site of photosynthesis. Found in mesophyll cells.
  • Outer Membrane: Smooth, permeable. Encloses the chloroplast.
  • Inner Membrane: Less permeable. Regulates entry of substances.
  • Stroma: Colorless fluid matrix. Contains enzymes for dark reaction, DNA, ribosomes.
  • Thylakoids: Flattened membrane sacs. Contain chlorophyll. Site of light reaction.
  • Grana: Stacks of thylakoids (10-20 thylakoids per granum). Increase surface area for light absorption.
  • Chlorophyll: Green pigment that absorbs light energy. Located in thylakoid membranes.
  • Diagram Importance: Labeled diagram of chloroplast showing outer membrane, inner membrane, stroma, grana, and thylakoids is crucial.

Raw Materials for Photosynthesis

  • Carbon Dioxide (CO₂): Enters through stomata. Dissolved in cell sap. Used in dark reaction to form glucose.
  • Water (H₂O): Absorbed by roots. Transported via xylem. Split in light reaction (photolysis) to release O₂.
  • Light Energy: Absorbed by chlorophyll. Converted to chemical energy (ATP and NADPH).
  • Chlorophyll: Essential pigment. Absorbs red and blue light, reflects green (why plants look green).
  • Optimal Conditions: Light intensity 1000-1500 foot-candles, Temperature 25-35°C, CO₂ concentration 0.03-0.04%.

Mechanism - Light Reaction (Hill Reaction)

  • Location: Thylakoid membranes of grana.
  • Light-dependent: Requires light energy. Occurs during day.
  • Process: (1) Chlorophyll absorbs light energy, (2) Water molecules split (photolysis): 2H₂O → 4H⁺ + O₂, (3) Oxygen released as by-product, (4) Hydrogen used to reduce NADP to NADPH₂, (5) ATP synthesized (photophosphorylation).
  • Products: ATP (energy), NADPH₂ (reducing power), O₂ (released).
  • Photolysis: Light-induced splitting of water. Source of oxygen released during photosynthesis.
  • Energy Conversion: Light energy → Chemical energy (ATP and NADPH₂).

Mechanism - Dark Reaction (Calvin Cycle)

  • Location: Stroma of chloroplast.
  • Light-independent: Does not require light directly. Uses products of light reaction. Can occur day or night.
  • Process: (1) CO₂ fixation - CO₂ combines with 5-carbon compound (RuBP) using enzyme RuBisCO, (2) Forms unstable 6-carbon compound, (3) Splits into two 3-carbon compounds (PGA), (4) Reduction using ATP and NADPH₂ from light reaction, (5) Forms glucose (C₆H₁₂O₆).
  • Products: Glucose (C₆H₁₂O₆), which is converted to starch for storage.
  • Calvin Cycle: Named after Melvin Calvin. Series of reactions fixing CO₂ into organic compounds.
  • RuBisCO: Most abundant enzyme on Earth. Catalyzes CO₂ fixation.

Factors Affecting Photosynthesis

  • Light Intensity: Increases rate up to saturation point. Beyond that, no effect. Low light limits photosynthesis.
  • Carbon Dioxide Concentration: Increases rate up to 1%. Normal atmospheric CO₂ is 0.03-0.04% (limiting factor).
  • Temperature: Optimum 25-35°C. Below 0°C and above 45°C, enzymes inactive. Affects dark reaction.
  • Water: Essential raw material. Deficiency closes stomata, reducing CO₂ entry.
  • Chlorophyll: More chlorophyll increases photosynthesis. Deficiency (yellowing) reduces rate.
  • Limiting Factor: Factor in shortest supply that limits the rate. Liebig's Law of Minimum.
  • Blackman's Law: Rate limited by factor present in minimum quantity. Increasing other factors won't help.

Experiments Demonstrating Photosynthesis

  • Starch Test (Iodine Test): Tests for starch production. Leaf boiled in water → alcohol (removes chlorophyll) → iodine solution. Blue-black color indicates starch.
  • Necessity of Light: Half leaf covered with black paper. Only exposed part shows starch (turns blue-black).
  • Necessity of CO₂: Plant kept in KOH solution (absorbs CO₂). No starch formed. Control plant shows starch.
  • Necessity of Chlorophyll: Variegated leaf (green and white parts). Only green parts show starch.
  • Oxygen Evolution: Hydrilla experiment in inverted funnel. Bubbles of oxygen collected in test tube. Glowing splinter test confirms O₂.
  • Moll's Half-Leaf Experiment: One half in CO₂-free air, other in normal air. Only normal air side produces starch.

Adaptations of Leaf for Photosynthesis

  • Large Surface Area: Maximum light absorption. Flat and broad shape.
  • Thin Structure: Short distance for gas diffusion. Light penetrates easily.
  • Chlorophyll: Abundant in mesophyll cells. Absorbs light energy.
  • Stomata: Allow CO₂ entry and O₂ exit. More on lower surface.
  • Vein Network: Xylem supplies water. Phloem transports glucose.
  • Transparent Epidermis: Allows light to reach mesophyll.
  • Palisade Mesophyll: Columnar cells with many chloroplasts. Main photosynthetic tissue.
  • Spongy Mesophyll: Loosely arranged with air spaces. Facilitates gas exchange.

Importance and Significance

  • Food Production: Primary source of organic food for all living organisms. Base of food chain.
  • Oxygen Release: Maintains atmospheric oxygen (21%). Essential for aerobic respiration.
  • Carbon Dioxide Utilization: Reduces atmospheric CO₂. Helps control global warming.
  • Energy Storage: Converts solar energy to chemical energy (glucose). Stored as starch.
  • Fuel Formation: Ancient plants formed fossil fuels (coal, petroleum) through photosynthesis.
  • Ecological Balance: Maintains balance between O₂ and CO₂ in atmosphere.
  • Economic Importance: Agriculture, forestry, and food industry depend on photosynthesis.

Introduction to Respiration

  • Respiration: Process of breakdown of organic food (glucose) to release energy. Occurs in all living cells.
  • Types: (1) Aerobic respiration (with oxygen) - complete breakdown, (2) Anaerobic respiration (without oxygen) - incomplete breakdown.
  • Site: Mitochondria in eukaryotic cells. Cytoplasm in prokaryotic cells.
  • Significance: (1) Releases energy for cellular activities, (2) ATP production, (3) Maintains life processes.
  • Overall Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP).
  • Connection to Photosynthesis: Photosynthesis produces glucose and O₂, respiration uses glucose and O₂ to produce CO₂ and H₂O.

Aerobic Respiration

  • Definition: Complete oxidation of glucose in presence of oxygen to release maximum energy.
  • Location: Mitochondria (Powerhouse of cell).
  • Stages: (1) Glycolysis (cytoplasm), (2) Krebs cycle (mitochondrial matrix), (3) Electron transport chain (cristae).
  • Glycolysis: Breakdown of glucose (6C) to pyruvic acid (3C). Produces 2 ATP, 2 NADH. Anaerobic process.
  • Krebs Cycle: Pyruvic acid → Acetyl CoA → CO₂ + H₂O. Produces CO₂, NADH, FADH₂, 2 ATP.
  • Electron Transport Chain: NADH and FADH₂ oxidized, electrons passed through carriers. Produces 34 ATP.
  • Total ATP: 36-38 ATP per glucose molecule (2 from glycolysis + 2 from Krebs + 32-34 from ETC).
  • Products: CO₂, H₂O, Energy (ATP).

Anaerobic Respiration

  • Definition: Incomplete breakdown of glucose in absence of oxygen. Releases less energy.
  • In Animals: Lactic acid fermentation. Glucose → Lactic acid + 2 ATP.
  • In Plants and Yeast: Alcoholic fermentation. Glucose → Ethanol + CO₂ + 2 ATP.
  • Lactic Acid Fermentation: Occurs in muscle cells during heavy exercise. Causes muscle fatigue.
  • Alcoholic Fermentation: Used in baking (CO₂ makes bread rise) and brewing (ethanol production).
  • ATP Production: Only 2 ATP per glucose (very less compared to aerobic respiration).
  • Importance: Provides energy when oxygen is insufficient.

Comparison: Photosynthesis vs Respiration

  • Photosynthesis: (1) Occurs in green plants, (2) Uses CO₂ and H₂O, (3) Produces glucose and O₂, (4) Requires light, (5) Anabolic process, (6) Chloroplasts.
  • Respiration: (1) Occurs in all living cells, (2) Uses glucose and O₂, (3) Produces CO₂ and H₂O, (4) Occurs day and night, (5) Catabolic process, (6) Mitochondria.
  • Relationship: Photosynthesis and respiration are complementary processes. Photosynthesis stores energy, respiration releases energy.
  • Balance: Photosynthesis: 6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂, Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy.
  • Importance: Maintains atmospheric balance of O₂ and CO₂.

Formulas

Keep these ready

Formula 01

6CO₂ + 12H₂O + Light energy → C₆H₁₂O₆ + 6O₂ + 6H₂O

Complete photosynthesis equation

Formula 02

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

Aerobic respiration equation

Formula 03

C₆H₁₂O₆ → 2C₃H₆O₃ + 2ATP

Lactic acid fermentation (anaerobic in animals)

Formula 04

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP

Alcoholic fermentation (anaerobic in plants/yeast)

Quiz

Test your knowledge

Multiple choice

  1. Question 1 of 25Knowledge
    Photosynthesis occurs in:
  2. Question 2 of 25Knowledge
    The green pigment in plants that absorbs light is:
  3. Question 3 of 25Knowledge
    The raw materials for photosynthesis are:
  4. Question 4 of 25Comprehension
    The oxygen released during photosynthesis comes from:
  5. Question 5 of 25Knowledge
    Light reaction of photosynthesis occurs in:
  6. Question 6 of 25Knowledge
    Dark reaction (Calvin cycle) occurs in:
  7. Question 7 of 25Knowledge
    The test for starch in leaves uses:
  8. Question 8 of 25Comprehension
    Which factor is usually the limiting factor for photosynthesis?
  9. Question 9 of 25Knowledge
    Photolysis of water means:
  10. Question 10 of 25Assertion-Reasoning
    Assertion (A): Only green parts of variegated leaf show starch. Reason (R): Chlorophyll is necessary for photosynthesis.
  11. Question 11 of 25Knowledge
    The optimum temperature for photosynthesis is:
  12. Question 12 of 25Knowledge
    RuBisCO enzyme is involved in:
  13. Question 13 of 25Knowledge
    Stomata are more abundant on:
  14. Question 14 of 25Knowledge
    The product of photosynthesis that is stored in plants is:
  15. Question 15 of 25Application
    In the Hydrilla experiment, the gas collected is:
  16. Question 16 of 25Comprehension
    Chlorophyll appears green because it:
  17. Question 17 of 25Knowledge
    The main photosynthetic tissue in leaf is:
  18. Question 18 of 25Comprehension
    ATP and NADPH₂ produced in light reaction are used in:
  19. Question 19 of 25Comprehension
    Photosynthesis is important because it:
  20. Question 20 of 25Comprehension
    Blackman's Law of Limiting Factors states that:
  21. Question 21 of 25Knowledge
    Respiration occurs in:
  22. Question 22 of 25Knowledge
    The energy currency of the cell is:
  23. Question 23 of 25Knowledge
    Aerobic respiration produces:
  24. Question 24 of 25Knowledge
    Anaerobic respiration in muscles produces:
  25. Question 25 of 25Knowledge
    The site of aerobic respiration in cells is:

0 of 25 answered