de Groot and Mazur [1] explained how classical non‐equilibrium thermodynamics can be extended to deal with internal variables; i.e. variables that are not controlled externally. The extension, called mesocopic non‐equilibrium thermodynamics, can capture the non‐linear nature of activated transport processes, and can also describe the coupling among mass and heat flows and chemical reactions. I show what it means for active ion transport by the Ca2+‐ATPase in sarcoplasmic reticulum, and discuss the premises of the method.
