CO and/or H2 can supply the electrons used in the enzymatic reactions.
However, CO and CO2 are used as a carbon source. As per the stoichiometry,
if only CO is used as the sole carbon and energy source then the carbon
conversion efficiency to ethanol will only be 33%, while, 67% of the carbon
are lost as CO2 as per Eq.1. However, if both CO and H2
are utilized then Eq.1 and Eq. 2 are combined into Eq. 5.