A quantitative description of recombination processes in nanostructured semiconductor photocatalysts - one that distinguishes between bulk (charge transport) and surface (chemical reaction) losses - is critical for advancing solar-to-fuel technologies. Here we present an in situ experimental framework that determines the bias-dependent quantum yield for ultrafast carrier transport to the reactive interface by simultaneously measuring the electrical characteristics and the subpicosecond charge dynamics of a heterostructured photoanode. Together with direct measurements of the overall incident-photon-to-current efficiency, we illustrate how subtle structural modifications drastically affect the overall photocatalytic quantum yield. We reveal how charge carrier recombination losses occurring on ultrafast timescales - as high as 37% in our model system - limit the overall efficiency, even in nanostructures with dimensions smaller than the minority carrier diffusion length. Our results demonstrate the efficacy of multifunctional designs where high overall efficiency is achieved by maximizing surface transport yield to near unity and utilizing surface layers with enhanced activity.