A parallel diffusion model is presented to explain an apparent transition in uptake mechanism seen in experimental observations of protein uptake into porous adsorbents. While such models have been invoked previously, this mesoscopic description is augmented hereby miscroscopic models for representing surface diffusion by "hopping" and adsorption within a Gibbs surface excess formulation. These contributions lead to a relation for the apparent protein diffusivity as a function of adsorption conditions, which can be used predictively with knowledge of a few readily measured physical quantities. The approach can be useful in seeking optimal conditions for preparative protein chromatography separations.