Parton and valon distributions in the nucleon

Structure functions of the nucleon are analyzed in the valon model in which a nucleon is assumed to be a bound state of three valence quark clusters (valons). At high $Q^2$ the structure of the valons is described by leading-order results in the perturbative quantum chromodynamics. From the experimental data on deep-inelastic scattering off protons and neutrons, the flavor-dependent valon distributions in the nucleon are determined. Predictions for the parton distributions are then made for high $Q^2$ without guesses concerning the quark and gluon distributions at low $Q^2$. The sea-quark and gluon distributions are found to have a sharp peak at very small $x$. Convenient parametrization is provided which interpolates between different numbers of flavors.