The relationship of the biomass production and electron transfer rates. The reducing equivalent supplying rates in the (A) DET and (B) MET modes, and the reducing equivalent consumption rate for cellular use in the (A) DET and (B) MET modes, as functions of biomass yield. The line represents the maximal electron transfer rates and biomass production rate, while any point within the pink area represents all allowable electron transfer rates and biomass production rates. The blue area represents the total reducing equivalent-consuming flux for normal cellular function. The distance between the two lines across the pink and blue areas represents the total available reducing equivalent flux in the cell at a metabolic state related to a specific biomass production rate; inset, enlargement of the boxed area. The reducing equivalent represents NADH in the MET mode, whereas it denotes an assumed product of the reaction catalysed by cytochrome c reductase for the DET mode. Stages I-II (1-2) are indicated by green braces and arrows, and Types I-II (1-2) by orange braces and arrows. The points are simulated by bi-objective optimization involving varying coefficients (λ) assigned for the growth and DET or MET maximizations. λ for DET or MET gradually increased from 0 to near 1; the two extreme points produced by the lowest λ and the highest λ are indicated in the plot. A more detailed discussion of the significance of λ is given in the Methods section. Briefly, it is a parameter that continuously adjusts the metabolic state of the cell through a range stretching from pure growth without the extraction of current (at one extreme) to a state in which all metabolic resources are taken up by supplying electric current and no growth, at the other extreme.