  Life is a nonequilibrium phenomenon: metabolism provides a continuous supply
of energy that drives nearly all cellular processes. However, very little is
known about how much energy different cellular processes use, i.e. their
energetic costs. The most direct experimental measurements of these costs
involve modulating the activity of cellular processes and determining the
resulting changes in energetic fluxes. In this review, we present a flux
balance framework to aid in the design and interpretation of such experiments,
and discuss the challenges associated with measuring the relevant metabolic
fluxes. We then describe selected techniques that enable measurement of these
fluxes. Finally, we review prior experimental and theoretical work that has
employed techniques from biochemistry and nonequilibrium physics to determine
the energetic costs of cellular processes.
