When light impinges on photosynthetic material – a plant leaf, an alga or a photosynthetic bacterium – it is absorbed by an array of lightcollecting pigments. Through resonant energy transfer the absorbed quantum of light is transported to a trap, the reaction centre. Within such a trap, a specialized (bacterio)chlorophyll complex is able to eject from its excited state an electron. This electron is ‘captured’ by an adjacent acceptor, which in turn donates the electron to a second acceptor, and so on. Thus, light energy is converted into chemical energy which is ultimately used in the metabolic processes of the cell.