Exact Calculations of Membrane Areas with Simple Models

The distinction between the true total area and the projected area is elucidated with soluble models which represent the membrane as a self-avoiding string on a plane. Constraining the total area to a predetermined value changes the averages very significantly. The latter are calculated exactly from the generating functions of self-avoiding walks and are shown as functions of activities $q$ and $r$ related to temperature $T=\pm 1/\log (q)$ and lateral force $f=-\log (r)$. The constraint makes the partition functions and averages valid for all $q,r >0$ and reduces the ratio of $A_{tot}$ to the projected area $L$. High temperature divergences are supressed. Possible applications to simulated bilayers/membranes are discussed.