Mechanism of Photosynthesis

In 1931, Van Niel studied photosynthetic bacteria that use hydrogen sulphide (H2S) instead of water to make carbohydrate:

6CO₂ + 12H₂S → (Light) C₆H₁₂O₆ + 6H₂O + 12S↓

From this, Van Niel proposed that in green plants the same basic reaction occurs but with water taking the place of H2S, so that oxygen (rather than sulphur) is released as the byproduct. In 1941, Ruben confirmed this using Chlorella and water labelled with the heavy oxygen isotope 18O (written H2^18O); the oxygen evolved during photosynthesis was found to carry that same 18O label, directly proving Van Niel's hypothesis that the O2 released in photosynthesis comes from water, not from CO2. This gives the modern, isotopically-confirmed equation of photosynthesis:

6CO₂ + 12H₂¹⁸O → (Light) C₆H₁₂O₆ + 6H₂O + 6 ¹⁸O₂

Separately, in 1937, R. Hill cultured isolated chloroplasts in a medium that contained water but no dissolved CO2, along with a ferric compound. On illumination, the ferric compound changed colour as it was reduced by hydrogen removed from water through photolysis (light-splitting of water) — since there was no CO2 present at all, the evolved oxygen could only have come from water, not from CO2. Ruben and Kamen later confirmed this same finding using 18O2. (A related demonstration adds haemoglobin to the culture medium: the oxygen evolved from water reacts with the haemoglobin, turning it red as it forms oxyhaemoglobin, again showing that the oxidised source of the O2 is water.) This observation — that light drives the splitting of water to release electrons, protons and oxygen, quite apart from any CO2 reduction — is called the Hill reaction, and it establishes two key facts: (i) in photosynthesis, oxygen is released from water, and (ii) the electrons used to reduce CO2 are also obtained from water. …