ABSTRACT
This chapter aims to explore the sensory systems of cells, building on bacterial chemotaxis. It examines the remarkable ability of certain biological circuits to respond to relative changes in signal, instead of absolute changes. The absolute number of photons added is the same, one candle's worth, but the relative change is very different. Response to relative changes was described in human senses by Weber in the nineteenth century. Many sensory systems of cells also show the universal features – exact adaptation and sensing of relative changes. For cells, as well as for animals, sensing relative changes is important in order to be robust to noise in the input. The chemotaxis system of Escherichia coli can sense relative changes across several orders of magnitude of background signal. Fold-change detection may be an instance where biological circuits evolved to “learn” a scaling symmetry of the physical world: the multiplicative nature of ambient light, protein levels or chemotaxis source strengths.
