Light‐induced cytochrome redox reactions were investigated with algal chloroplasts capable of high rates of electron transport coupled to phosphorylation. The electron‐donor pool preceding photosystem I consists of membrane‐bound cytochrome f‐554.5 and a soluble cytochrome c‐553; the latter replaces the function of plastocyanin of higher‐plant chloroplasts. Both cytochromes are reduced by photosystem II and oxidized by photosystem I. A site of energy conservation precedes these c‐type cytochromes. The data obtained with respect to the function of b‐type cytochromes are comparable to those obtained with higher‐plant chloroplasts. Cyclic electron transport is mediated by cytochrome b‐563 in a photosystem‐I‐dependent reaction. In addition, cytochrome b‐563 may be reduced by photosystem II, in accordance with recent findings with intact spinach chloroplasts. It therefore appears that cytochrome b‐563 is a member of both cyclic and non‐cyclic electron transport. In contrast to higher‐plant chloroplasts, redox reactions of cytochrome b‐559 are observable without any pretreatments. Cytochrome b‐559, high‐potential, is reduced by photosystem II through plastoquinone. In the presence of car
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