How does PS II supply electrons continuously in noncyclic ETS?

Continuous electron supply mechanism:

Problem: PS II loses electrons when P680 is photooxidized (P680 → P680⁺)

Solution: Water photolysis (splitting)

Reaction:

2H₂O → 4H⁺ + O₂ + 4e⁻

Mechanism:

1. Oxygen-Evolving Complex (OEC):

• Contains 4 manganese ions (Mn), 1 calcium ion (Ca), and 5 oxygen atoms
• Located on lumenal side of PS II
• Tyrosine-161 (TyrZ) acts as electron donor to P680⁺

1. S-state cycle (Kok cycle):

• S₀ → S₁ → S₂ → S₃ → S₄ (releases O₂, resets to S₀)
• Requires 4 photons to release 1 O₂ molecule
• Each photon advances one S-state

1. Electron extraction:

• Water donates electrons to Mn₄CaO₅ cluster
• Cluster oxidizes sequentially
• 4 electrons extracted per 2 H₂O molecules

1. Proton and oxygen release:

• 4 H⁺ released into thylakoid lumen (contributes to proton gradient)
• 1 O₂ diffuses out as by-product

Significance: This process is the source of all atmospheric oxygen and the ultimate electron donor for photosynthesis.