IB Biology Topic C1.3.11: Generation of Oxygen by the Photolysis of Water in Photosystem II Notes

PSII Splits Water to Refill P680, Releasing Protons and Oxygen

1. Photolysis is the splitting of water by light energy, and it happens at photosystem II (PSII).

2. It is carried out by a protein complex in PSII called the oxygen-evolving complex (OEC).

3. The overall reaction is:

2H₂O → O₂ + 4H⁺ + 4e⁻

Why P680 Needs Water in the First Place

1. Light excites an electron in the reaction centre chlorophyll P680, which then leaves PSII.

2. This leaves P680 positively charged and short of an electron.

3. The OEC splits water to supply replacement electrons for P680.

4. Restored this way, P680 can absorb light and release an electron again.

The Three Products of Photolysis

1. Electrons

   1. They replace the electrons lost by P680, keeping PSII working.

2. Protons (H⁺)

   1. They are released into the thylakoid lumen, raising the H⁺ concentration there.
   2. This adds to the proton gradient later used to make ATP, covered in C1.3.12.

3. Oxygen (O₂)

   1. Oxygen is a waste product of photolysis and is not used by the plant.
   2. It diffuses out of the leaf into the atmosphere.

Photolysis Is the Source of Earth's Atmospheric Oxygen

1. Nearly all the oxygen in the atmosphere was first released by photolysis in cyanobacteria, algae, and plants.

2. This build-up of oxygen made aerobic respiration and complex life possible, explored further in the evolution topics.