Figure 2: Exemplification of our system: donor liposomes (dark shaded) and acceptor liposomes (light shaded) reside in an aqueous space of volume ; each liposome can carry at most drug molecules (black bullets). For the displayed hypothetical snapshot (taken at a certain time ), the distribution function of drug molecules among the donor liposomes is , , , , leading to a total of drug molecules residing in donor liposomes. Analogously for the acceptor liposomes, the distribution function is , , , , implying .