Photosynthesis Lecture Notes

Introduction

• Photosynthesis: Process of using light energy to build carbohydrates.
• Terminology:
  • "Photo" means light.
  • "Synthesis" means to build.

Photosynthesis Equation

• Combines 6 water molecules + 6 carbon dioxide molecules using light to produce:
  • Glucose (C6H12O6)
  • Oxygen gas (O2)
• Reactants: Water, carbon dioxide.
• Products: Glucose, oxygen.

Process Overview

• Water enters through roots; carbon dioxide enters through stomata in leaves.
• Oxygen exits through stomata.

Chloroplast vs. Mitochondria

• Chloroplast: Organelle for photosynthesis.
• Mitochondria: Organelle for cellular respiration.
• Photosynthesis and cellular respiration are opposite processes.

Chlorophyll and Light Absorption

• Chlorophyll: Pigment absorbing light energy, located in thylakoids.
• Thylakoids: Stacks called granum (plural: grana).
• Chloroplast Structure:
  • Inner and outer membranes.
  • Stroma (fluid inside chloroplast).
  • Lumen (fluid inside thylakoid).

Light Absorption

• Chlorophyll absorbs blue and red light; reflects green light.

Photosynthesis Stages

1. Light Dependent Reactions:

   • Location: Thylakoids.
   • Oxidation of water into oxygen gas.
   • Produces ATP and NADPH.
   • Reactants: Water, NADP+, ADP, phosphate.
   • Products: Oxygen, ATP, NADPH.

2. Light Independent Reactions (Calvin Cycle):

   • Location: Stroma.
   • Carbon dioxide is reduced to sugars.
   • ATP and NADPH are used as energy sources.
   • Reactants: Carbon dioxide, ATP, NADPH.
   • Products: Sugars (glucose), NADP+, ADP, phosphate.

Detailed Light Dependent Reactions

• Electron Transport Chain:
  • Photosystem II (P680) starts the process by exciting electrons.
  • Water is oxidized, releasing oxygen and electrons.
  • Electron Flow: Plastoquinone → Cytochrome B6F → Plastocyanin → Photosystem I (P700) → Ferredoxin → NADP+ reductase.
  • ATP Synthesis: Hydrogen ions flow through ATP synthase, combining ADP and phosphate to form ATP.

Calvin Cycle (Light Independent Reactions)

• Phases:
  1. Carbon Fixation: Carbon dioxide reacts with RuBP catalyzed by rubisco, forming 3-phosphoglycerate (PGA).
  2. Reduction: 1,3-biphosphoglycerate is reduced to G3P using ATP and NADPH.
  3. Regeneration of RuBP: G3P used to regenerate RuBP.
• Net Result: 3 CO2 converted to 1 G3P.
• For glucose production, 6 CO2 needed, requiring doubling of ATP and NADPH use.

Summary

• Light dependent reactions and Calvin cycle are essential for photosynthesis.
• Light dependent reactions occur in thylakoid membranes.
• Calvin cycle occurs in stroma.
• Understanding of chlorophyll, chloroplast structure, and electron transport chain is crucial.

• Conclusion: Photosynthesis involves complex processes of light capture and chemical conversions, essential for plant energy production.