  The thermodynamic (TD) properties of biological membranes play a central role
for living systems. It has been suggested, for instance, that nonlinear pulses
such as action potentials (APs) can only exist if the membrane state is in
vicinity of a TD transition. Herein, two membrane properties - excitability and
AP velocity - are investigated for a broad spectrum of conditions in living
systems (temperature (T), 3D-pressure (p) and pH dependence). Based on these
data we predict parameter ranges in which a transition of the membrane is
located (15-35{\deg}C below growth temperature; 1-3 pH units below pH 7; at
~800 atm) and propose the corresponding phase diagrams. The latter explain: (i)
changes of AP velocity with T, p and pH. (ii) The existence and origin of two
qualitatively different forms of loss of nonlinear excitability ("nerve
blockage", anesthesia). (iii) The type and quantity of parameter changes that
trigger APs. Finally, a quantitative comparison between the TD behaviour of
2D-lipid model membranes with living systems is attempted. The typical shifts
in transition temperature with pH and p of model membranes agree closely with
values obtained from cell physiological measurements (excitability and
propagation velocity). Taken together, these results strongly suggest that it
is not specific molecules that control the excitability of living systems but
rather the TD properties of the quasi 2D-membrane interface. The approach as
proposed herein can be extended to other quantities (surface potential, calcium
concentration, etc.) and makes clearly falsifiable predictions, for example,
that a transition exists within the specified parameter ranges in excitable
cells.
