The mechanism of the electron transfer from the soluble protein plastocyanin to the multiprotein complex of photosystem I from spinach has been studied in detail. The two kinetic components of P700+ reduction by plastocyanin after a laser flash, showing a constant half-life of 11 microseconds and a variable half-life of the second-order reaction, respectively, are used to monitor the electron transfer from bound and soluble plastocyanin. The effect of increasing concentration of reduced plastocyanin on both of these kinetic components and the competition by oxidized plastocyanin is used to estimate the individual dissociation constants of the complex between the proteins in each of its oxidized and reduced state. The dissociation constant of oxidized plastocyanin is about six times larger than that of 7 microM found for reduced plastocyanin and purified PSI. Consistent with this result the midpoint redox potential of plastocyanin bound to photosystem I either in equilibrium with soluble plastocyanin or after cross-linking to photosystem I is found to be 50-60 mV higher than that of soluble plastocyanin. It is concluded that the driving force of the intracomplex electron transfer is decreased in favor of an optimized turnover of photosystem I. Double-flash excitation shows that oxidized plastocyanin has to leave the complex after the electron transfer before a new reduced plastocyanin molecule can bind to photosystem I. This release of oxidized plastocyanin with a half-life of about 60 microseconds limits the turnover of photosystem I. All data are consistently described by a model including the formation of a complex at a single binding site of photosystem I. Differences in the rate and binding constants are discussed with respect to the structure and the electrostatic and hydrophobic interactions stabilizing the complex as well as their modification by the membrane environment in situ.