In the new Nature paper, Sarpeshkar, Lu and colleagues have done the reverse — mapping analog electronic circuits onto cells. Sarpeshkar has long advocated analog computing as a more efficient alternative to digital computation at the moderate precision of computation seen in biology. These analog circuits are efficient because they can take in a continuous range of inputs, and they exploit the natural continuous computing functions that are already present in cells. In the case of cells, that continuous input might be the amount of glucose present. In transistors, it’s a range of continuous input currents or voltages.

Digital circuits, meanwhile, represent every value as zero or one, ignoring the range of possibilities in between. This can be useful for creating circuits that perform logic functions such as AND, NOT and OR inside cells, which many synthetic biologists have done. These circuits can reveal whether or not a threshold level of a certain molecule is present, but not the exact amount of it.

Digital circuits also require many more parts, which can drain the energy of the cell hosting them. “If you build too many parts to make some function, the cell is not going to have the energy to keep making those proteins,” Sarpeshkar says.

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