Figure 3.

Sequential (ping pong) vs simultaneous transport mechanisms. A) Cl⁻/HCO₃ exchange via a sequential mechanism. In this mechanism, the transporter (T) accepts only one ion substrate at a time at each membrane surface. T_i and T_o represent the transporter in the inward- and outward-facing states, respectively. B) Na⁺/alanine cotransport via a simultaneous mechanism. Both Na⁺ and alanine (Ala) bind to the transporter before translocation occurs. The tertiary complex T·Na⁺·Ala undergoes a conformational change. Releasing the two substrates on the opposite side of the membrane causes the transporter to be returned to the original state.