Ferricyanide was used as a Hill oxidant to localize the site of photoreduction in chloroplast lamellae. The ferrocyanide formed on illumination was complexed with copper ions to form insoluble, electron-dense precipitates of Cu ferrocyanide, which can be easily seen in unstained preparations in the electron microscope. Control experiments showed no precipitates in the dark, at zero time, in the absence of ferricyanide, or on addition of sodium ascorbate. Discrete precipitates of Cu ferrocyanide were seen on both stroma and grana lamellae. It is concluded that both stroma and grana lamellae have photosystem II activity necessary for reduction of ferricyanide.

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