The scale free structure p(k)~k^{-gamma} of protein-protein interaction
networks can be reproduced by a static physical model in simulation. We inspect
the model theoretically, and find the key reason for the model to generate
apparent scale free degree distributions. This explanation provides a generic
mechanism of "scale free" networks. Moreover, we predict the dependence of
gamma on experimental protein concentrations or other sensitivity factors in
detecting interactions, and find experimental evidence to support the
prediction.Comment: organization improved, and experimental evidence of predicted
dependence on sensitivity is addresse