  We present a physical model for polarized and unpolarized structure functions
and parton density functions (PDFs) of the proton and the neutron. It
reproduces the data on F_2^p(x,Q^2) for 0.00001<x<1 and 2.5<Q^2<5000 GeV^2,
F_2^p(x)-F_2^n(x), F_2^n(x)/F_2^p(x), xg(x), dbar(x)-ubar(x), d(x)/u(x), the
Gottfried sum, the fractional momentum of charged partons and the polarized
structure functions g_1^{p,n}(x), at various Q^2. We present for the first
time, proton and neutron PDFs which do not assume charge symmetry. Contrary to
the common practice, we explain polarized and unpolarized data with a single
model.
