FIGURE 2.

Stoichiometric matrix and the system's boundary. (A) A system with internal reactions, v_i, and metabolites, x_i, is closed to the environment and represented by S_int. (B) When the internal metabolites are allowed to be exchanged with the environment, the stoichiometric matrix includes exchange reactions, b_i, and the system becomes “open”. (C) Once the external metabolites (x_oi) are included, the system becomes “closed” and the stoichiometric matrix contains all the reactions and metabolites, S_tot. (int, internal; exch, exchang; tot, total).