Isoprene is the most important volatile organic compound (VOC) emitted by plants. Morfopoulos et al. hypothesize that NADPH availability for isoprene biosynthesis depends on the balance of electron supply and Calvin cycle demand. A simple model based on this hypothesis explains many features of the observed response of isoprene emissions to environmental factors. The decoupling between carbon assimilation and isoprene emission (e.g. opposite responses to CO2) is accounted for, whereas previous models resorted to empirical corrections. This work suggests a way forward to global-scale VOC emission models that will be both simpler, and more robust, than those currently favoured.