Markov chain representations of the one-step and two-step models. (A) One-step model. The docking site is either empty (left, dotted SV) or occupied (right, continuous SV), and transitions between the two states occur with forward rate R and backward rate P. Transition probabilities r and p during an inter-AP interval Δt are related to R and P as explained in Materials and methods. (B) Two-step model. Here four different states occur, depending of the occupancy of the docking site (bottom, black) and of its associated replacement site (top, red; in this representation, δ is the summed probability of states with continuous black SVs, and ρ is the summed probability of states with continuous red SVs). States are linked with transition rates as indicated. Transition probabilities r′ (docking) and s (replenishment from reserve pool) are related to rate constants R′ and S as explained in Materials and methods. All transitions are assumed unidirectional for simplicity, but a more general model would include backward rates for each transition.