  Based on quantum fluctuations in momentum and of the proton into meson-baryon
pairs, we develop a physical model for the non-perturbative x-shape of parton
density functions in the proton. The model describes the proton structure
function and gives a natural explanation of observed quark asymmetries, such as
the difference between the anti-up and anti-down sea quark distributions and
between the up and down valence distributions. An asymmetry in the momentum
distribution of strange and anti-strange quarks in the nucleon is found to
reduce the NuTeV anomaly to a level which does not give a significant
indication of physics beyond the standard model.
