  Cells continuously sense their surroundings to detect modifications and
generate responses. Very often changes in extracellular concentrations initiate
signaling cascades that eventually result in changes in gene expression.
Increasing stimulus strengths can be encoded in increasing concentration
amplitudes or increasing activation frequencies of intermediaries of the
pathway. In this paper we show that the different way in which amplitude and
frequency encoding map environmental changes endow cells with qualitatively
different information transmission capabilities. While amplitude encoding is
optimal for a limited range of stimuli strengths, frequency encoding can
transmit information with equal reliability over much broader ranges. The
qualitative difference between the two strategies stems from the scale
invariant discriminating power of the first transducing step in frequency
codification. The apparently redundant combination of both strategies in some
cell types may then serve the purpose of expanding the span over which stimulus
strengths can be reliably discriminated. In this paper we discuss a possible
example of this mechanism in yeast.
