Are mitochondria mesoscopic?

Systems whose length scales are small enough to be near the threshold where macroscopic thermodynamic descriptions fail are called mesoscopic. This is particularly appropriate for studies of mitochondrial function since basic and important ideas such as protomotive force, and models finding common applications, such as the Gouy-Chapman theory describing the diffuse ionic charge layer near the inner membrane, depend on the macroscopic thermodynamic notion of chemical potential. This paper shows that the length scales in Gouy-Chapman theory are not consistent with the premises upon which Gouy-Chapman theory was based, a failure typical of mesoscopic systems. Furthermore, while there are good reasons for a protomotive force to be a valid description for a collection of mitochondria, it is a marginal description even for an entire individual mitochondrion for similar reasons. Mitochondria are, therefore, physically mesoscopic systems.