while in respiration electrons are transferred from substrate (electron donor) to an electron acceptor, in fermentation part of the substrate molecule itself accepts the electrons. fermentation is therefore a type of disproportionation, and does not involve an overall change in oxidation state of the substrate. most of the fermentative substrates are organic molecules. however, a rare type of fermentation may also involve the disproportionation of inorganic sulfur compounds in certain sulfate-reducing bacteria.