Van Neil 1932 Comparing the biochemistry of autotrophs

Photosynthetic sulphur bacteria use H₂S as their source of hydrogen

This suggested that in green plants the oxygen originates from the water molecule

Ruben: 1941: Confirmed this hypothesis using the heavy isotope ¹⁸O and mass spectrometry.

Studying redox reactions in photosynthesis using artificial electron acceptors

....

The light dependent reaction involves the splitting of water. The electrons released are used to reduce electron acceptors. (This became known as the Hill Reaction)

Natural Electron ACCEPTORS

Pyridine nucleotides: ... NAD+ + 2H+ + 2e- NADH + H+ Nicotinamide Adenine Dinucleotide (NAD) used in respiration in mitochondria, Nicotinamide Adenine Dinucleotide Phosphate (NADP) used in photosynthesis in chloroplasts Flavine Adenine Dinucleotide (FAD) also used in respiration Ferredoxin the most difficult to reduce (and most easily oxidised) Cytochromes: Conjugate proteins which contain a haem group. The iron atom undergoes redox reactions  Fe3+ + e-  Fe2+ NB The iron atom in the haem group of haemoglobin does not go through a redox reaction. Haemoglobin is oxygenated or deoxygenated.

Arnon 1954 : The Hill reaction using natural electron acceptors

Arnon had effectively separated the light dependent reaction, which produces ATP, NADPH + H⁺ and oxygen, from the light independent reaction, which produces sugars.

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