Four photons are required to generate one oxygen molecule. When dark-adapted chloroplasts are exposed to a brief flash of light, one electron passes through photosystem II. Monitoring the O2 released after each flash reveals that four flashes are required to generate each O2 molecule. The peaks in O2 release are after the 3rd, 7th, and 11th flashes because the dark-adapted chloroplasts start in the S1 state—that is, the one-electron reduced state.