Figure 1. Schematic Representation of the Receptor–Ligand Binding and Internalization Model.

The reactant species in the model are the ligand L, free receptor R, and receptor–ligand complex C. Ligand is produced at a rate f(t) by a source S and enters the extracellular space with volume V. Ligand reversibly binds free receptors with a forward rate k_on and a reverse rate k_off to yield receptor–ligand complexes. Free receptors are synthesized by the cell at a rate Q_R. Free receptors and receptor–ligand complexes are internalized with the rates k_t and k_e, respectively. The input to the above system is the external ligand stimulus f(t), and the output is the concentration of surface complexes C.