Electron transport in photosystem I in spinach chloroplasts.

We have studied the recovery of the photochemical activity of Photosystem I after the charge separation induced by a flash under conditions where the secondary donors are in the reduced form. The rate-limiting steps are on the donor side. The first step is completed within 400 mus. The second step is much slower (half time approximately equal to 1 ms) and corresponds to the transfer of electrons from plastoquinone. Under our conditions, only one intermediate is involved in electron transfer between the centers and the plastoquinone pool. Electron exchange between the Sytem I centers has been demonstrated.