Membrane proteins are the gatekeepers of cellular membranes where they act as enzymes, transporters, signaling receptors, or in energy conversion. Traditionally seen as a difficult field, the last decade has brought dramatic progress to membrane protein structural biology. Here, I describe recent advances in studying the conformational dynamics of membrane proteins by X-ray free electron lasers. By integrating sample efficient high viscosity injectors into pump probe setups, it has become

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... le to determine whole series of structural snapshots by time-resolved serial femtosecond crystallography and assemble them to molecular movies of proteins in action. According to the current studies on bacteriorhodopsin, photosystem II and nitric oxide reductase, I outline the technical challenges as well as new possibilities to study membrane protein function.