Question

How many protons are needed to break two molecules of water during photolysis of water?

Answer 1

In Photosystem II, each photon absorbed will release one hydrogen atom from one water molecule. When two molecules of water (H${}_2$O) split during photolysis, four hydrogen atoms are released. Hence, 4 photons are required to release 4 hydrogen atoms from 2 molecules of water. Hence, four photons are needed to break two molecules of water during photolysis of water.

$2H_{2}O$ → 4H+ + 4e- + $O_2$