2 Water + 2 Plastoquinone + 4 Proton → 2 Plastoquinol + Oxygen + 4 Proton (thylakoid lumen)

Photosystem II EC 1.10.3.9

Splits water to get electrons. It is the only known process able to tear them away from a molecule so unwilling to let them go, and all the oxygen in the atmosphere comes from here. Light supplies the energy; only matter appears in the equation.

irreversible
Thylakoid membrane

in depth

ΔG°′: not documented
Location: Thylakoid membrane

Fun fact

The center that makes the cut is a cluster of four manganese atoms and one calcium atom. It stores the four positive charges one at a time and only then releases the oxygen all at once, so as not to let intermediate products loose.