Photosynthesis: An Introduction

What is Photosynthesis?

• Definition: Using light energy to synthesize carbohydrates.
• Components:
  • "Photo" means light.
  • "Synthesis" means to build.
• Net Equation for Photosynthesis:
  • Reactants: 6 Water (H2O) + 6 Carbon Dioxide (CO2) + Light
  • Products: Glucose (C6H12O6) + Oxygen (O2)
• Process:
  • Water enters through roots.
  • CO2 enters through stomata.
  • O2 is released from stomata.

Chloroplasts

• Function: Site of photosynthesis.
• Comparison: Mitochondria is for cellular respiration.
• Processes:
  • Photosynthesis: CO2 + H2O -> Glucose + O2
  • Cellular Respiration: Glucose + O2 -> CO2 + H2O
• Photosynthesis Pigment: Chlorophyll
  • Absorbs blue and red light.
  • Reflects green light.
• Components:
  • Thylakoids: Stacks called grana.
  • Lumen: Fluid in thylakoids.
  • Stroma: Fluid in chloroplasts.
  • Membranes: Inner, outer, and intermembrane space.

Stages of Photosynthesis

Light Dependent Reactions

• Location: Inside thylakoids.
• Process:
  • Oxidizes water into oxygen gas.
  • NADP+ is reduced to NADPH.
  • ATP is synthesized from ADP and phosphate.
• Products: O2, ATP, NADPH
• Reactants: H2O, NADP+, ADP, Phosphate

Light Independent Reactions (Calvin Cycle)

• Location: Stroma of chloroplasts.
• Process:
  • Takes in CO2 and reduces it into glucose.
  • NADPH oxidized back to NADP+.
  • ATP used in the cycle.
• Products: Sugars (e.g., glucose), NADP+, ADP, Phosphate
• Reactants: CO2, ATP, NADPH

Electron Transport Chain in Photosynthesis

• Photosystem II (P680):
  • Excited by light, loses electrons.
  • Electrons from water replenish lost electrons.
• Mobile Electron Carriers:
  • Plastoquinone -> Cytochrome b6f -> Plastocyanin
  • Proton gradient created.
• Photosystem I (P700):
  • Further excites electrons.
  • Electrons go to Ferrodoxin -> NADP reductase -> NADPH
• ATP Synthase:
  • Uses proton gradient for ATP production.

Calvin Cycle Detailed Steps

1. Carbon Fixation:
   • CO2 reacts with RuBP (Ribulose biphosphate).
   • Catalyzed by rubisco.
2. Reduction:
   • 3-Phosphoglycerate -> 1,3-biphosphoglycerate -> G3P (Glyceraldehyde 3-phosphate)
   • ATP and NADPH used.
3. Regeneration of RuBP:
   • G3P used to regenerate RuBP.

• Net Result:
  • 3 CO2 -> 1 G3P
  • Requires 9 ATP and 6 NADPH for 1 G3P.
  • 6 CO2 needed for 1 glucose.

Summary

• Photosynthesis consists of light-dependent reactions and the Calvin Cycle.
• Light-dependent reactions occur in thylakoid membranes producing O2, ATP, NADPH.
• Calvin Cycle occurs in stroma using CO2, ATP, NADPH to produce glucose.

End of Lecture Notes