Control equations can also be derived by considering the effect of perturbations on the system. Consider that reaction rates formula_40 and formula_41 are determined by two enzymes formula_42 and formula_43 respectively. Changing either enzyme will result in a change to the steady state level of formula_44 and the steady state reaction rates formula_45. Consider a small change in formula_42 of magnitude formula_47. This will have a number of effects, it will increase formula_40 which in turn will increase formula_44 which in turn will increase formula_41. Eventually the system will settle to a new steady state. We can describe these changes by focusing on the change in formula_40 and formula_41. The change in formula_41, which we designate formula_54, came about as a result of the change formula_55. Because we are only considering small changes we can express the change formula_54 in terms of formula_55 using the relation: