Past exam of the biological physics course of the University of Oxford / 2015 / 5 / b / Solution

Conservation of total probability gives $A(t)=1-B(t)$. The two-state master equation is thereforeSolving this linear ordinary differential equation givesThe middle panel in d sketches this solution for the explicitly illustrative ratio $k_f/(k_f+k_b)=0.6$. The curve starts at zero with slope $k_f$, is increasing and concave down, and approaches the horizontal limit $B_{\infty }=k_f/(k_f+k_b)$ on time constant $(k_f+k_b{)}^{-1}$. Its initial tangent meets the asymptote at that time constant. The complementary limit is $A_{\infty }=k_b/(k_f+k_b)$, so $B_{\infty }/A_{\infty }=k_f/k_b=e^{-\Delta G_{AB}/(k_{B}T)}$, the Boltzmann distribution ratio. These are occupancy probabilities; $B(t)$ is not the cumulative first-passage probability when reverse transitions remain possible.