Photosynthesis and Photosystem II

Importance of Photosynthesis

• Vital for life on Earth
• Occurs in green plants and some algae
• Oxygen production is a byproduct of photosynthesis

Overview of Photosystem II

• Key Components:
  • Photons
  • Light-harvesting chlorophyll-binding proteins
  • P680 chlorophyll molecules
  • Fyen molecules
  • Plastoquinones (QA and QB)
  • Water and Oxygen
• Function: Initiates the electron transport in photosynthesis

Photosystem II Reaction Center

• Composed of multiple proteins and pigment molecules
• P680 Chlorophyll Molecules:
  • Donate electrons to the electron transport system
• Electron Transfer Process:
  1. Electron passed to fyen molecule
  2. Passed to plastoquinone QA
  3. Then to plastoquinone QB
  4. QB acts as a mobile carrier in the photosynthetic electron transport system

Electron Flow and Water Splitting

• Electrons from water replenish P680's lost electrons
• Additional proteins split electrons from water
• Oxygen evolving complex splits water to provide electrons
  • Produces the oxygen we breathe

Light Harvesting

• Chlorophyll-Binding Proteins:
  • Surround reaction center
  • Harness energy from light photons
• Energy Transfer:
  1. Photon hits chlorophyll, creates resonance energy (vibrational energy)
  2. Energy passed through chlorophyll molecules to P680
• Result: Loss of an electron from the P680 molecule

Cyclic Electron Flow

• Requires two photons for full electron transfer to QB
• Steps:
  1. Photon activates chlorophyll, resonance energy transferred to P680
  2. Electron lost, donated to QA then to QB
  3. Second photon required for QB to become mobile
  4. Fully reduced QB transferred to cytochrome b6f complex
• P680 reduced by electrons from oxygen evolving complex
• Summary: Two water molecules split to provide electrons, producing oxygen