  One of the primary computational requirements of a cellular system is the
ability to transfer information between spatially separated components. To
accomplish this, biology uses diverse physical channels including production or
release of second-messengers molecules and electrical depolarization of the
plasma membrane. To send reliable information, these processes must dissipate
energy to compete with thermal noise, in some cases consuming a substantial
fraction of the cellular energy budget. Here we bound the energetic efficiency
of several physical strategies for communication, using tools from information
theory and the fluctuation dissipation relations to quantify communication
through a channel corrupted by thermal noise. We find a minimum energetic cost,
in $k_B$T/bit for sending information as a function of the size of the sender
and receiver, their spatial separation, and the communication latency. From
these calculations construct a phase diagram indicating where each strategy is
most efficient. In addition, these calculations provide an estimate for the
energy costs associated with information processing arising from the physical
constraints of the cellular environment.
