Stoichiometric models for secondary-active transport. Mechanistic descriptions of transport often depict only the major states and transitions contributing to stochiometric coupled transport of substrate and ion. Here, the transported substrate is represented as a cationic drug (green hexagon), and the driving ion is proton (dark red circle), while the transporter is shown in pale blue. (A) For symport, the transporter binds both drug and proton cooperatively, and alternating access occurs in both this doubly bound state and the apo state. (B) For antiport, drug and proton cannot bind simultaneously, and alternating access occurs only when the transporter is bound to either substrate (drug) or ion (proton).