The conversion of light energy into chemical energy is one of the most important processes in nature. By using light energy to oxidise organic or inorganic compounds, photosynthetic organisms obtain the electrons that are necessary for the fixation of carbon dioxide into carbohydrate which is needed for cell growth. The initial photochemistry occurs within membrane-bound pigment/protein complexes known as reaction centre complexes (RCs). The elucidation of the structure of the photosynthetic reaction centre complex from the purple non-sulphur photosynthetic bacterium Rhodopseudomonas viridis has formed a framework with which to understand the conversion of light energy to chemical energy.