Energetics of Coupled Transport Treatment of Active Transport in Terms of Thermodynamics of Irreversible Processes (TIP)

Abstract

To treat coupled transport in terms of TIP we can proceed along two different paths: Firstly, we can start from the TIP equations for uncoupled transport, as developed in the previous section for free and facilitated diffusion, and expand them to include the coupling effects. This is the conventional and simpler way, which gives us the flows in terms of driving forces and phenomenological coefficients only, but tells us little about the mechanistic relationships. Secondly, we can start from the kinetic equations of the coupled processes, as derived in the previous section for primary and secondary active transport in terms of the LMA, and try to transform them into the corresponding thermodynamic equations. For reasons given in the previous section this transformation can be carried out only by the use of certain approximations. The final equations arrived at should be identical with those obtained by the first procedure, but the second procedure enables us to interpret the phenomenological coefficients in terms of mechanistic and kinetic details of the underlying model. This has the advantage that the range of linearity for the phenomenological coefficients can be predicted, provided that the underlying mechanistic model is in its essentials close enough to the real system.