What does the evidence in the Hill reaction investigation tell you about the process of photosynthesis?
Robin Hill, who discovered the Hill reaction, demonstrated through his experiment that isolated chloroplasts would generate oxygen molecules (`O_2`) while not fixing carbon dioxide (`CO_2`) (to generate sugars). Also, he showed that this reaction would happen only in the presence of light and an electron acceptor (a substance that receives electrons from another compound - an oxidizing agent).
At that time, it was believed that the oxygen generated by plants came from `H_2O` , but no experiment had been made until then. Hill showed that the oxygen comes from the water molecules. Robin then observed that oxygen is produced in a different reaction from the carbon dioxide one.
The fact that the reaction needed an electron acceptor to occur implies that the reaction is a result of a series of redox reactions! In plants, we have natural electron acceptors, such as NADP. But Hill actually found out that there are many other electron acceptors - we call one of this substances a Hill reagent. In fact, we can use substances that change color when they reduce. This was used to further observe the reactions happening in the chloroplasts.
After Robin Hill, Daniel Arnon made more studies using NADP as an acceptor. He observed the light independent reaction, where `CO_2` is fixed by the chloroplasts to generate sugars. He was the first to separate both reactions.
Basically, most current knowledge of photosynthesis came from these observations made by Robin Hill, by noting that oxygen is released during the light-dependent reactions.