(99) Photosynthesis - Light Dependent Reactions and the Calvin Cycle

Introduction to Photosynthesis

  • Photosynthesis is the process by which plants use light energy to build carbohydrates.

  • The term "photo" means light, and "synthesis" means to build.

Net Equation for Photosynthesis


  • Overall reaction:6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

  • Reactants: Water and Carbon Dioxide

  • Products: Glucose and Oxygen Gas

  • Entry/Exit of Molecules:

    • Water enters through roots.

    • Carbon Dioxide enters through stomata in leaves.

    • Oxygen exits through stomata.

Role of Chloroplasts

  • Chloroplasts are the organelles where photosynthesis occurs.

  • Mitochondria are responsible for cellular respiration.

  • Photosynthesis and cellular respiration are opposites:

    • Photosynthesis: CO₂ + H₂O → C₆H₁₂O₆ + O₂

    • Cellular respiration: C₆H₁₂O₆ + O₂ → CO₂ + H₂O + energy

Chlorophyll and Light Absorption

  • Chlorophyll is the pigment that absorbs light energy, specifically blue and red light, reflecting green light.

  • Thylakoids are structures where chlorophyll is located.

  • Structures Within Chloroplasts:

    • Granum: Stack of thylakoids.

    • Grana: Multiple stacks of thylakoids (plural).

    • Stroma: Fluid outside thylakoids inside chloroplasts.

    • Intermembrane Space: Space between inner and outer membranes of the chloroplast.

Stages of Photosynthesis

  • Two main stages:

    • Light-dependent Reactions (thylakoids)

    • Light-independent Reactions (Calvin Cycle) (stroma)

Light-dependent Reactions

  • Purpose: Convert solar energy to chemical energy (ATP and NADPH).

  • Reactants: Water, NADP⁺, ADP, and inorganic phosphate (P)

  • Products: Oxygen gas, ATP, and NADPH

  1. Photon Absorption: Light excites electrons in chlorophyll in Photosystem II (PQ).

  2. Water Oxidation: Water is oxidized, providing electrons and producing oxygen gas.

  3. Electron Transport Chain (ETC):

    • Electrons are transferred to plastoquinone, cytochrome b6f, and plastocyanin.

    • Protons are pumped into the thylakoid lumen creating a concentration gradient.

  4. Photosystem I: Electrons are re-energized by light and transferred to ferredoxin, then to NADP reductase.

  5. ATP Production: Protons flow through ATP synthase to generate ATP via chemiosmosis.

Light-independent Reactions (Calvin Cycle)

  • Purpose: Convert CO₂ into glucose (G3P).

  • Reactants: CO₂, ATP, and NADPH

  • Products: G3P, NADP⁺, ADP, and P

  1. Carbon Fixation: CO₂ is fixed with ribulose bisphosphate (RuBP) using rubisco to form 3-phosphoglycerate (PGA).

  2. Reduction Phase: ATP phosphorylates PGA to 1,3-bisphosphoglycerate; NADPH reduces it to G3P.

  3. Regeneration: G3P regenerates RuBP to continue the cycle.

    • Note: 1 G3P is produced for every 3 CO₂, needing six CO₂ to make one glucose (6 G3P).

    • Energy Requirements: For glucose, 18 ATP and 12 NADPH are needed.

Summary

  • Photosynthesis comprises light-dependent reactions producing ATP and NADPH, and light-independent reactions (Calvin Cycle) converting CO₂ into glucose.

  • These processes highlight the interdependence of energy transformation and matter cycling in plant biology.