In contemporary molecular and cell biology, signal transduction research has remarkably contributed to a major paradigm shift in biology in which biology is now seen as a "science of sensing". Once we recognise that sensing is one of the necessary properties of life, we cannot do without considering semiotic logic in order to construct our understanding of living phenomena. Given the central integrating role of signal transduction in physiological and ecological studies, this chapter outlines its semiotic implications.
The multi-modality and modularity of signal molecules and relative "infrastructure" components poses one of the central problems for understanding metabolic codes: the occurrence of different instances of "cross-talk", "redundancy" and "categorial sensing" at different hierarchical levels. The term "categorial sensing" captures very well the essence of the "outstanding question(s)" in signal transduction; i.e.: how specificity is determined, how ubiquitous signals or messengers convey specific information, how undesired cross-talk is avoided, how redundancy integrates the system. This chapter proposes a basic conceptual toolbox for interpreting empirical data that deals with such puzzling phenomena from a biosemiotic perspective.