Protein dynamics due to structural fluctuations of small molecular subgroups on the picosecond time scale has been shown to play an important role in physiological processes in nature. Prominent examples for such dynamics-function correlations include proton transfer in bacteriorhodopsin of halobacterium salinarum, ligand binding to myoglobin, and photosynthetic electron transfer in plant photosystem II. In the case of photosystem II membrane fragments of green plants, quasielastic neutron scattering (QENS) has established that the onset of diffusive molecular motions at ~240 K, and at a relative humidity of ~45% is strictly correlated with the temperature- and hydration-dependent electron transfer efficiency from an electron donor referred to as QA to a transiently bound acceptor molecule named QB. Such a characteristic dependence of a functional process on temperature and hydration may indicate a crucial role of molecular dynamics in the underlying molecular mechanism. We propose to investigate the dynamics of cyanobacterial thylakoid membranes by QENS experiments on IN6 in order to search for a correlation between internal dynamics and function.