Molecular Communication (MC), i.e., the exchange of information through the emission, propagation, and reception of molecules, is a promising paradigm for the interconnection of autonomous nanoscale devices, known as nanomachines. Synthetic biology techniques, and in particular the engineering of biological circuits, are enabling research towards the programming of functions within biological cells, thus paving the way for the realization of biological nanomachines. The design of MC systems built upon biological circuits is particularly interesting since cells naturally employ the MC paradigm in their interactions, and possess many of the elements required to realize this type of communication. This paper focuses on the identification and systems-theoretic modeling of a minimal subset of biological circuit elements necessary to be included in an MC system design where the message-bearing molecules are propagated via free di?usion between two cells. The system-theoretic models are here detailed in terms of transfer functions, from which analytical expressions are derived for the attenuation and the delay experienced by an information signal through the MC system. Numerical results are presented to evaluate the attenuation and delay expressions as functions of realistic biological parameters.