Fig. 3. Hypothetical plot of substrate (S) and product (P and Q) concentrations as a function of distance, d, from two sequential pathway enzymes, E₁ and E₂. In this sequence, S is converted to P and then to Q, in reactions catalyzed by E₁ and E₂, respectively. Curves denoting changes in [S], [P] and [Q] as a function of d are purely hypothetical.