<P> In heterogeneous electron transfer, an electron moves between a chemical species and a solid - state electrode . Theories addressing heterogeneous electron transfer have applications in electrochemistry and the design of solar cells . </P> <P> The first generally accepted theory of ET was developed by Rudolph A. Marcus to address outer - sphere electron transfer and was based on a transition - state theory approach . The Marcus theory of electron transfer was then extended to include inner - sphere electron transfer by Noel Hush and Marcus . The resultant theory, called Marcus - Hush theory, has guided most discussions of electron transfer ever since . Both theories are, however, semiclassical in nature, although they have been extended to fully quantum mechanical treatments by Joshua Jortner, Alexender M. Kuznetsov, and others proceeding from Fermi's Golden Rule and following earlier work in non-radiative transitions . Furthermore, theories have been put forward to take into account the effects of vibronic coupling on electron transfer; in particular, the PKS theory of electron transfer . </P> <P> Before 1991, ET in metalloproteins was thought to affect primarily the diffuse, averaged properties of the non-metal atoms forming an insulated barrier between the metals, but Beratan, Betts and Onuchic subsequently showed that the ET rates are governed by the bond structures of the proteins--that the electrons, in effect, tunnel through the bonds comprising the chain structure of the proteins . </P>

What is the chemical species that is transferred through light reactions