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Electron transport chain and ATP synthesis (thylakoid membranes)

During the 20th century, comparisons between photosynthetic processes in green plants and in certain photosynthetic sulfur bacteriaprovided important information about the photosynthetic mechanism. Sulfur bacteria use hydrogen sulfide (H2S) as a source of hydrogen atomsand produce sulfur instead of oxygen during photosynthesis. The overall reaction is

Chemical equation.

In the 1930s Dutch biologist Cornelis van Niel recognized that the utilization of carbon dioxide to form organic compounds was similar in the two types of photosynthetic organisms. Suggesting that differences existed in the light-dependent stage and in the nature of the compounds used as a source of hydrogen atoms, he proposed that hydrogen was transferred from hydrogen sulfide (in bacteria) or water (in green plants) to an unknown acceptor (called A), which was reduced to H2A. During the dark reactions, which are similar in both bacteria and green plants, the reduced acceptor (H2A) reacted with carbon dioxide (CO2) to form carbohydrate (CH2O) and to oxidize the unknown acceptor to A. This putative reaction can be represented as:

Chemical equation.

Van Niel’s proposal was important because the popular (but incorrect) theory had been that oxygen was removed from carbon dioxide (rather than hydrogen from water, releasing oxygen) and that carbon then combined with water to form carbohydrate (rather than the hydrogen from water combining with CO2 to form CH2O).