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General Features of Photophosphorylation:- Light Drives Electron Flow in Chloroplasts

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  724

2026-06-22

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General Features of Photophosphorylation:- Light Drives Electron Flow in Chloroplasts

In 1937 Robert Hill found that when leaf extracts containing chloroplasts were illuminated, they (1) evolved O₂ and (2) reduced a nonbiological electron acceptor added to the medium, according to the Hill reaction:

where A is the artificial electron acceptor, or Hill reagent. One Hill reagent, the dye 2,6-dichlorophenol indophenol, is blue when oxidized (A) and colorless when reduced (AH2), making the reaction easy to follow. When a leaf extract supplemented with the dye was illuminated, the blue dye became colorless and O₂ was evolved. In the dark, neither O₂ evolution nor dye re duction took place. This was the first evidence that absorbed light energy causes electrons to flow from H₂O to an electron acceptor. Moreover, Hill found that CO₂ was neither required nor reduced to a stable form under these conditions; O₂ evolution could be dissociated from CO₂ reduction. Several years later Severo Ochoa showed that NADP is the biological electron acceptor in chloroplasts, according to the equation

To understand this photochemical process, we must first consider the more general topic of the effects of light absorption on molecular structure.