<P> Cornelis Van Niel made key discoveries explaining the chemistry of photosynthesis . By studying purple sulfur bacteria and green bacteria he was the first to demonstrate that photosynthesis is a light - dependent redox reaction, in which hydrogen reduces carbon dioxide . </P> <P> Robert Emerson discovered two light reactions by testing plant productivity using different wavelengths of light . With the red alone, the light reactions were suppressed . When blue and red were combined, the output was much more substantial . Thus, there were two photosystems, one absorbing up to 600 nm wavelengths, the other up to 700 nm . The former is known as PSII, the latter is PSI . PSI contains only chlorophyll "a", PSII contains primarily chlorophyll "a" with most of the available chlorophyll "b", among other pigment . These include phycobilins, which are the red and blue pigments of red and blue algae respectively, and fucoxanthol for brown algae and diatoms . The process is most productive when the absorption of quanta are equal in both the PSII and PSI, assuring that input energy from the antenna complex is divided between the PSI and PSII system, which in turn powers the photochemistry . </P> <P> Robert Hill thought that a complex of reactions consisting of an intermediate to cytochrome b (now a plastoquinone), another is from cytochrome f to a step in the carbohydrate - generating mechanisms . These are linked by plastoquinone, which does require energy to reduce cytochrome f for it is a sufficient reductant . Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by Hill in 1937 and 1939 . He showed that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light . The Hill reaction is as follows: </P> <Dl> <Dd> 2 H O + 2 A + (light, chloroplasts) → 2 AH + O </Dd> </Dl>

What is the source of oxygen inside the plant during photosynthesis