Lecture Notes: Photosynthesis Part 2 - Electron Transport and Calvin Cycle

Introduction

• Focus on photosynthetic electron transport and Calvin cycle.
• Goal: Produce organic compounds.

Photosystems and Photon Absorption

• Photosystems are located in the thylakoid membrane.
• Photosystem II absorbs light at 680 nm.
• Water oxidation releases electrons, protons, and oxygen.

Electron Transport

• Purpose: Generate NADPH and ATP for the Calvin cycle.
• Key Components:
  • Photosystem II: Absorbs light, excites electrons.
  • Plastoquinone (PQ): Accepts electrons, reduced near Photosystem II, transfers to the cytochrome complex.
  • Cytochrome Complex: Accepts electrons from PQ.
  • Plastocyanin (PC): Transfers electrons to Photosystem I.
  • Photosystem I: Absorbs light at 700 nm, further excites electrons.
  • Ferredoxin: Transfers electrons to NADP+ reductase.
  • NADP+ Reductase: Produces NADPH.

Proton Motive Force

• Proton motive force is built up in the thylakoid lumen.
• Protons are pumped by hydrolysis of water and plastoquinone activity.

ATP Synthesis

• Occurs via ATP synthase due to proton gradient.
• Converts ADP and phosphate into ATP.

Linear and Cyclic Electron Transport

• Linear Electron Transport: Ends with NADPH production.
• Cyclic Electron Transport: Occurs when NADP+ is unavailable, continues ATP production.

Calvin Cycle

• Occurs in the stroma of the chloroplast.
• Three Phases:
  1. Carbon Fixation
     • CO2 fixation onto an organic molecule (3-phosphoglycerate).
     • Catalyzed by Rubisco.
  2. Reduction
     • NADPH reduces 1,3-bisphosphoglycerate to G3P.
     • Produces one excess G3P for glucose.
  3. Regeneration
     • ATP regenerates RuBP from G3P.

Rubisco Enzyme

• Main catalyst for CO2 fixation.
• Importance: Most crucial enzyme in biosphere.
• Drawback: Can bind oxygen (competitive inhibition).

Conclusion

• Photosynthesis involves complex electron transport and Calvin cycle processes.
• Next topic: Cellular respiration, starting with glycolysis.