In our lectures we first describe the history and methods of membrane protein crystallization, before we show how the structure of the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis was solved. Then the structure of this membrane protein complex is correlated with its function as a light-driven electron pump across the photosynthetic membrane. Finally we draw conclusions on the structure of the photosystem II reaction centre from plants and discuss the aspects of membrane protein structure. Sections 1 (crystallization), 4 (conclusions on the structure of photosystem II reaction centre and evolutionary aspects) and 5 (aspects of membrane protein structure) were presented and written by H.M., Sections 2 (determination of the structure) and 3 (structure and function) by J.D. We have arranged the paper in this way in order to facilitate continuous reading.