Photosynthesis by isolated chloroplasts. IV. General concept and comparison of three photochemical reactions

1. 1. Procedures are described for the preparation of chloroplasts capable of carrying out three photochemical reactions, each representing an increasingly complex phase of photosynthesis: photolysis of water (Hill reaction), esterification of inorganic phosphate into adenosine triphosphate (photosynthetic phosphorylation) and the reduction of carbon dioxide to the level of carbohydrates with a simultaneous evolution of oxygen. 2. 2. The three photochemical reactions were separable by variations in the technique for preparation of chloroplasts and by differential inhibition by several reagents. Inhibition of a more complex phase of photosynthesis does not affect the similar one which precedes it and, conversely, the inhibition of a simpler phase of photosynthesis is paralled by an inhibition of the more complex phase which follows. 3. 3. Reversible inhibition of CO 2 fixation and photosynthetic phosphorylation, but not of photolysis, by sulfhydryl group inhibitors suggests that sulfhydryl compounds (enzymes, cofactors, or both) are involved in phosphorylation and CO 2 fixation, but not in the primary conversion of light into chemical energy as measured by the Hill reaction. 4. 4. Evidence is presented in support of the conclusion that the synthesis of ATP by green cells occurs at two distinct sites: anaerobically in chloroplasts, by photosynthetic phosphorylation, and aerobically in smaller cytoplasmic particles, presumably mitochondria, by oxidative phosphorylation independent of light. 5. 5. A general scheme of photosynthesis by chloroplasts, consistent with these findings, is presented.