Biological membranes are typically not in thermal
equilibrium. Experiments on lipid-protein model membranes
have revealed that protein activity influences the
mechanical properties of the membrane. A possible
explanation for this alteration is given by a theoretical
model in which the active proteins act as force-dipoles on
the surrounding medium. A prediction of this model is that
the protein activity will modify the tension of the
membrane. However, one has to be careful when one looks at
the consequences of this tension modification for the
fluctuation spectrum of the membrane shape.