Small vesicles, called chromatophores, can be isolated from the membranes of photosynthetic bacteria, which exhibit two types of electron transfer chains resembling mitochondria and chloroplasts. Chroma-tophores supported on artificial membranes permit the generation of 200 mV on illumination. The salt-bacteria (Halobacterium halobium) contain a simple protein-vitamin A aldehyde, known as bacteriorhodopsin, when supported on artificial membranes that generate 250 mV on illumina­tion. This system is simpler than its counterpart. There is a probabil­ity that the entire system may be successfully synthesized or assembled. Solar photocells made of bacteriorhodopsin show great promise.