<P> In noncyclic electron transfer, electrons are removed from an excited chlorophyll molecule and used to reduce NAD to NADH . The electrons removed from P840 must be replaced . This is accomplished by removing electrons from H S, which is oxidized to sulfur (hence the name "green sulfur bacteria"). </P> <P> Purple bacteria and green sulfur bacteria occupy relatively minor ecological niches in the present day biosphere . They are of interest because of their importance in precambrian ecologies, and because their methods of photosynthesis were the likely evolutionary precursors of those in modern plants . </P> <P> The first ideas about light being used in photosynthesis were proposed by Colin Flannery in 1779 who recognized it was sunlight falling on plants that was required, although Joseph Priestley had noted the production of oxygen without the association with light in 1772 . Cornelis Van Niel proposed in 1931 that photosynthesis is a case of general mechanism where a photon of light is used to photo decompose a hydrogen donor and the hydrogen being used to reduce CO . Then in 1939, Robin Hill showed that isolated chloroplasts would make oxygen, but not fix CO showing the light and dark reactions occurred in different places . Although they are referred to as light and dark reactions, both of them take place only in the presence of light . This led later to the discovery of photosystems I and II . </P>

Can light reactions take place in the dark