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), d(x)-u(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.