Evidence for a respiratory chain in the chloroplast.

Evidence is given for the existence of an electron transport pathway to oxygen in the thylakoid membranes of chloroplasts (chlororespiration). Plastoquinone is shown to be a redox carrier common to both photosynthetic and chlororespiratory pathways. It is shown that, in dark-adapted chloroplasts, an electrochemical gradient is built up across the thylakoid membrane by transfer of electrons through the chlororespiratory chain as well as by reverse functioning of the chloroplast ATPases. It is proposed that these mechanisms ensure recycling of the ATP and NAD(P)H generated by the glycolytic pathway converting starch into triose phosphates. Chlororespiration is thus an O(2)-uptake process distinct from photorespiration and the Mehler reaction. The evolutionary significance of chlororespiration is discussed.