Photosynthesis Lecture Notes

Introduction to Photosynthesis

• Definition: Photosynthesis is the process of using light energy to build carbohydrates (e.g., glucose).
• Net Equation:
  • Reactants: 6 Water (H2O) + 6 Carbon Dioxide (CO2) + Light
  • Products: Glucose (C6H12O6) + Oxygen Gas (O2)

Overview of Reactants and Products

• Reactants:
  • Water enters through roots.
  • Carbon dioxide enters through stomata in leaves.
• Products:
  • Oxygen exits through the same stomata.

Organelles Involved

• Chloroplasts:
  • Organelles where photosynthesis occurs.
• Mitochondria:
  • Responsible for cellular respiration (opposite process of photosynthesis).

Chlorophyll and Light Absorption

• Chlorophyll:
  • Pigment responsible for absorbing light energy.
  • Found in thylakoids.
  • Absorbs blue and red light, reflects green light.
• Thylakoids:
  • Individual stacks of thylakoids are called grana.
  • The fluid inside thylakoids is called lumen, and the fluid in chloroplasts is called stroma.

Stages of Photosynthesis

1. Light-Dependent Reactions

   • Occur in thylakoids.
   • Reactants: Water, NADP+, ADP, Phosphate
   • Products: Oxygen, ATP, NADPH
   • Process:
     • Oxidation of water to produce O2.
     • Electrons move through the electron transport chain.
     • ATP is produced via chemiosmosis using ATP synthase.

2. Light-Independent Reactions (Calvin Cycle)

   • Occur in the stroma.
   • Reactants: Carbon Dioxide, ATP, NADPH
   • Products: Sugars (e.g., glucose), NADP+, ADP, Phosphate
   • Process:
     • Fixes CO2 into 3-phosphoglycerate (PGA) with enzyme Rubisco.
     • PGA is phosphorylated to form 1,3-biphosphoglycerate.
     • Reduced to glyceraldehyde 3-phosphate (G3P) using NADPH.

Details of Light-Dependent Reactions

• Photosystem II (P680):
  • Absorbs light, oxidizes water to produce O2.
• Plastoquinone:
  • Transports electrons to cytochrome B6F.
• Cytochrome B6F Complex:
  • Pumps protons into the lumen, creating a gradient.
• Photosystem I (P700):
  • Absorbs light to re-energize electrons.
• NADP Reductase:
  • Reduces NADP+ to NADPH using electrons.
• ATP Synthase:
  • Produces ATP from ADP and phosphate through chemiosmosis.

Details of Calvin Cycle

• Stages:
  1. Carbon Fixation: CO2 + RuBP (catalyzed by Rubisco) → PGA.
  2. Reduction: PGA phosphorylated to 1,3-biphosphoglycerate, then reduced to G3P using NADPH.
  3. Regeneration of RuBP: Five G3P molecules regenerate three RuBP.
• Overall Reaction:
  • Converts 3 CO2 into 1 G3P, requires 9 ATP and 6 NADPH.
• To produce glucose:
  • Requires 6 CO2, producing 2 G3P, needing 18 ATP and 12 NADPH.

Summary

• Photosynthesis involves two main processes:
  • Light-dependent reactions (in thylakoids) produce ATP and NADPH.
  • Light-independent reactions (Calvin Cycle in stroma) convert CO2 into glucose.

---

Thank you for watching! Don't forget to subscribe!