Thermodynamic measurements are required to confirm whether cloned transport-associated proteins in a membrane truly constitute a functional transport system. Symport or antiport, catalyzed by native systems or by cloned proteins in membranes, can lead to steady-state intracellular accumulation of solute when the electrochemical potentials of activator ion and solute are energetically coupled. Secondary active transport can occur if an appropriate physical coupling mechanism exists in the membrane. Driving forces for secondary active transport are ultimately established by primary active transport or respiration. Steep steady-state gradients of solute can be maintained when the ion:solute coupling ratio is greater than one and/or when coupling includes an electrical component. Although the steady-state accumulation of substrate is independent of the exact physical mechanism of transport, non-equilibrium and equilibrium transport kinetics aid in interpreting the rate, direction (symport versus antiport) and control of ion-coupled flux across a membrane. In some cases, the activator ion's chemical gradient alone is energetically adequate to maintain steady-state intracellular accumulation of solute, as demonstrated in invertebrate epithelial cells. To interpret accumulation ratios accurately, it is necessary to measure the intracellular activity coefficients for ions. For example, liquid ion-exchange microelectrode measurements demonstrate that over 30% of intracellular Na+ can be sequestered in epithelial cells.

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