With the aid of light and two coupled photosystems containing chlorophyll, green plants remove electrons from water, releasing O 2 , and convey them to CO 2 , reducing it to simple sugars and thence to carbohydrate. Photosynthetic bacteria operate more simply, employing a single photosystem. They reduce CO 2 but require substrates more easily oxidized than water, e.g. sulphide and succinate. Bacterial reaction centres free of the bulk chlorophyll that performs the light-harvesting function can be isolated. The reaction centre is the site where the electronic energy of the photoexcited molecule is converted to chemical potential. Thus bacterial reaction centres are ideal subjects for studying the details of this process by kinetic spectroscopy. Picosecond laser studies show that an electron is removed from the primary donor (a chlorophyll dimer) in 4ps or less and transferred in several stages to the ubiquinone acceptor in ca . 200 ps. Remarkably, reverse electron transfer is several orders of magnitude slower. The paper discusses how Nature may have accomplished this.