Mixing normal and anomalous diffusion

In the densely filled biological cells often subdiffusion is observed, where the average squared displacement increases slower than linear with the length of the observation interval. One reason for such subdiffusive behavior is attractive interactions between the diffusing particles that lead to temporary complex formation. Here, we show that such transient binding is not an average state of the particles but that intervals of free diffusion alternate with slower displacement when bound to neighboring particles. The observed macroscopic behavior is then the weighted average of these two contributions. Interestingly, even at very high concentrations, the unbound fraction still exhibits essentially normal diffusion.