  The increasing interest in photoactivated proteins as natural replacement of
standard inorganic materials in photocells drives to the compared analysis of
bacteriorhodopsin and proteorhodopsin, two widely diffused proteins belonging
to the family of \textit{type-1} opsins. These proteins share similar
behaviours but exhibit relevant differences in the sequential chain of the
amino acids constituting their tertiary structure. The use of an impedance
network analogue to model the protein main features provides a microscopic
interpretation of a set of experiments on their photoconductance properties. In
particular, this model links the protein electrical responses to the tertiary
structure and to the interactions among neighbouring amino acids. The same
model is also used to predict the small-signal response in terms of the Nyquist
plot. Interesting enough, these rhodopsins are found to behave like a wide gap
semiconductor with intrinsic conductivities of the order of $10^{-7}$ S/cm.
