Joseph Priestley (1733-1804) discovered that plants consume CO . In this process they produce elemental oxygen, which he also discovered. In chloroplasts, the oxygen-evolving complex loses single electrons when exposed to light. After a specific number of electrons are lost, the complex regains electrons from water to produce oxygen, in a cycle. Consecutive pulses of light were flashed at a solution of chloroplasts, and the amount of oxygen produced from each flash was recorded.