  The chiral quark model with a nonet of Goldstone bosons can yield an adequate
description of the observed proton flavor and spin structure. In a previous
publication we have compared the results of a SU(3) symmetric calculation with
the phenomenological findings based on experimental measurements and SU(3)
symmetry relations. In this paper we discuss their SU(3) and axial-U(1)
breaking corrections. With two parameters, we obtain a very satisfactory fit to
the F/D ratios for the octet baryon masses and for their axial vector
couplings, as well as the different quark flavor contributions to the proton
spin. The result also can account for, not only the light u and d quark
asymmetry, but also the strange quark content, of the proton sea. SU(3)
breaking is the key in reconciling the strange quark fraction as measured in
the neutrino charm production and that as deduced from the pion nuleon sigma
term.
