Validation of reactions. A. First-order irreversible reaction. Mean for 1000 iterations of STEPS (points with error bars showing the sd) matches the analytical solution (full line, broken lines are the predicted sd). B. First-order reversible reaction. The STEPS simulation (mean of 100 iterations) evolves correctly to the predicted steady state concentrations. C, D. Second order irreversible reaction. With equal reactant concentrations the evolution in time of the inverse of the concentration of one of the source species in comparison to the analytical solution is shown (C) and with unequal reactant concentrations the comparison of the source species ratio with the analytical solution is shown (D).