Figure . A model of an open reaction network (ORN) used for computer simulation. where kdi is the rate constant for the inflow or outflow of Ri and 𝐶 𝑖 ex and 𝐶 𝑖 in are the concentrations of Ri in the exterior and in the interior of an ORN, respectively. (The superscript "in" for 𝐶 𝑖 in is omitted later when no confusion is caused by the omission.) The rate of the concentration change resulting from the diffusion of organic molecules in an aqueous solution is, in general, much higher than that resulting from reaction; thus, we have here assumed that the concentrations of chemical substances in an ORN are spatially uniform.