When a chlorophyll molecule within the LHC contacts a photon of light, resonance energy is produced.

This resonance energy is transferred through several more chlorophyll molecules until it reaches the P680 chlorophyll molecules at the heart of the Photosystem II reaction center.

The resonance energy causes the loss of an electron from the P680 molecules. This electron is then transferred to a pheophytin molecule, then to Qa and finally to Qb. P680 is then reduced by the splitting of a water molecule which replaces the electron lost in this process.

In order to fully reduce Qb, a second electron is required. This means a second photon of light is needed to remove an electron from P680.

Once again resonance energy is transferred through the LHC to the P680 molecules, causing the loss of another electron. As before, the electron is transferred to plastoquinone Qb.

Once Qb is fully reduced, it becomes a mobile carrier and leaves Photosystem II on its way to the next component of the photosynthetic electron transport system.