Hill's Reaction

In 1937, Robert Hill isolated intact chloroplasts and illuminated them in the presence of a suitable artificial electron acceptor - such as ferricyanide - but with no carbon dioxide present at all. He found that, in light, the chloroplasts still reduced the added acceptor (ferricyanide being converted to ferrocyanide) while simultaneously evolving oxygen, even though there was no CO2 available for the chloroplasts to fix. This result, now called Hill's reaction, is now considered essentially equivalent to the light reaction of photosynthesis in isolation. In general form, Hill's reaction can be written $2H_2O + 2A \xrightarrow{light} 2AH_2 + O_2$, where A stands for any suitable hydrogen (electron) acceptor - common in vitro examples being ferricyanide, benzoquinone, and DCPIP (dichlorophenol indophenol). Three conclusions followed directly from Hill's experiment: first, the oxygen evolved during photosynthesis comes from water, not from CO2; second, the electrons eventually used to reduce CO2 originate from water; and third, the reduced substance generated by this light-driven process is what later goes on to reduce CO2 …